Did I Blow Up the Inverter? 

I am an electrical engineer who specializes in power. The circuit breaker did exactly what it was supposed to do. The spark very likely came from the circuit breaker and this is very normal. The circuit breaker has a very large current flowing through it and they often draw an arc when opening during a fault or large surge. One sure way to tell it came from the circuit breaker is the color of the spark. It was a green blue color indicative of molten copper from an arc blast. The continuity test you performed is not necessarily indicative of a short. The bulk input storage capacitors will act like a short when they are empty and will indicate a false short. The capacitors look like a short when they are discharged. I can tell by the internals of the inverter it was designed to a very tight budget. All the mosfets appear to be lacking any type of snubber or protection circuitry although it's a little hard to tell from the video. The inductive surge from the motor will likely cause an instantaneous voltage spike somewhere in the inverter and I would be willing to bet that spike exceeded the power transistors specification for max voltage causing the transistors to fail short circuit. The saw inrush current is very fast typically few ac cycles so it can't be measured using the method you used. It could be measured using an oscilloscope and it is certainly many times its operating current.

Looking at the picture of inverter, It appears it has nine DC to DC boost blocks converters. To save idle current and improve efficiency at lower power levels, the nine boost blocks are brought online as needed to supply the load demand. Each of the nine DC-DC block can up supply about 1kW. They produce DC voltage output of likely 180 to 200 vdc. The nine transformers are high frequency ferrite, likely running at 25KHz to 50 KHz range. The other side of the inverter is the sinewave PWM switcher to create the 120 vac sinewave output from the HV DC boost supply. Problem with this parallel boost circuit design is the time it takes to engage additional boost blocks, especially during surge demands. At best case, there will be a slump in output voltage. At worse case, the surge overload on a given boost block will saturate the small ferrite transformer causing the MOSFET current to shoot to extremely high level, blowing the MOSFET's out. This happens before additional block can power up and assist in the surge load demand. This type of design needs to have additional circuitry to avoid hard saturating the ferrite cores of the transformers and current limiting on the MOSFET's. It is also possible the surge creates high voltage overshoots on the transformers exceeding the breakdown voltage of the MOSFET's. Surge snubbers are required on MOSFET/transformer to absorb this.

I'm a telecom stand-by/backup power guy. I work with multi-battery string, high current DC power plants and stand-by generators up to 500kw. One thing i noticed that would be critical is that the internal DC distribution leads are not the same length. This does not allow for a balanced dc load across the input stages during surge. In high current discharge conditions, the cables have to be of equal length or the load will be highest on inrush on the shorter cables (and component stages). The existing wiring places more load on the first groups than equal across all the groups. Notice the MOSFETs of the first 4 parallel groups were failed. If one were to replace the DC leads with equal length leads, I'll bet the failure would not occur. I will admit that they seem to have poorly engineered the whole thing "too close to the edge" of the required spec. Now on battery string interconnect cables from lower string on shelf to upper or to the "chandelier" (busbars) overhead, this would not be such an issue, though it can be problematic and cause excess heating at cable attachment points, plates, and crimps which causes "loosening" over time. On component groups, that's a whole different story. If you were able to catch the transient surge on ammeters attached to each group input, you would find that the first and second groups took the brunt of the load and way exceeded their rating by 10+ amps. Remember, surge can be up to twice the continuous, depending on the device. This definitely due to bad engineering, but a fairly simple fix. I would also eliminate the multiple cable per crimp at the input attachment points and stack individual lugs on the bolt of the inputs.

Everybody keeps saying it's a junk inverter. But you forget that the 3000W version worked fine with the saw. So I believe the company got something wrong somewhere in the 8KW version. This happens with even the best name-brand equipment. More rigorous testing is what they need.

I've designed inverters and switching power supplies for years and is now retired. It appears there is no load balancing and over current protection: they just hooked those individuals in parallel. To make it simple, these individuals have a heroic tendency to be "I can do it by myself" and when you hit it with big motors and/or capacitive loads that's pretty much a short circuit for the first few seconds, that fries the hero's mosfets in no time. Stay away from that brand or such design. Very tricky to start loads that are virtually a short circuit for the first few seconds.

A previous comment by another poster pointed out a couple of points I missed. 1. lithium batteries have low internal resistance so can produce very high input surge currents. 2 that if the inverter is only switched on and is in standby / idle mode the inverter protection circuits are not active until a minimum load has been attained. Going from standby to a high load instantly may well be outside the manufactures design parameters. Lack of a useful manual or specifications or operating instructions may be a contributing factor to this failure. When I run inverters I connect a 30watt fan to the load side and point this at the inverter, its a side benefit to keep it cool in 34C (typical Aussie air temperatures) but by using the fan the inverter is already working into a load so is "fully switched on and not in standby". To properly test this inverter a storage oscilloscope would be needed to capture the input current and voltage transitions. Difficult as amps will be in units of hundreds and voltage spikes in tens of thousands. So maybe 2 scopes on different settings would be better. Bench testing of complex electronic equipment by untrained but enthusiastic users is often not effective without a good understanding of electronics and electrical theory. In practice the tests carried out would be typical of most purchasers, in a suck it and see approach. I would do the bench testing as previously posted and get as much info / data on the inverter via specs and the manufacturer so that the testing was designed within the designed working and surge conditions. Black box inverters from China (branded or unbranded) are hit and miss as so many do not have specs, just pics and tempting low prices.

Great analysis very logical, maybe check all of the 9 positive to negative leads one at a time to see which transformer (for lack of a better word) has the short. likely the mosfets are gone.

For taking a video of the amp meter. The meter samples at once per second. If the saw pulls 100 amp for 0.1 second when 1st pull the trigger, would likely miss getting sampled. The 30 amp you measured was while saw was spinning up. When the inverter failed was right away when pulled trigger.

A junk inverter + lithium batteries = smoke. Your batteries have a very low internal resistance (I use the same batteries with a Schneider XW6848 by the way). That means they will provide an immense amount of instantaneous current. Couple that with your meter. You are on the right track but there is no way you captured the full inrush event with that meter. You need something with a LOT more processing speed. The inrush event is a huge spike that lasts for a few milliseconds and quickly tapers down. If the inverter doesn't have some internal processing to limit the current through it when loads like this happen they go boom. The engineers expected you to have slower reacting batteries so they didn't build that into their design. The mosfets shorted due to the spike in current and it was over. Instant boat anchor.

nice video and good logical approach to fault finding. I think you are right at pointing the finger at the inverter having a short circuit, which then caused the breaker to instantaneously open on to a short circuit(causing the spark). The design of that inverter is very questionable, they are relying on paralleling MOSFETS to get the power rating but do NO design effort to ensure good parallel current flow across all MOSFETS on the input side. The differing lengths of blue/red cable show that they have not even thought about that. A MOSFET can turn on in 10's of nanoseconds and at the moment of switch-on of the saw motor the first MOSFETS closest to the input see very little inductance to help with inrush and probably blow since they take ALL the current and effectively short circuiting the first few MOSFETS whilst the devices with the longer blue cables get an easy ride and don't see so much inrush due to the inductance of the longer cables. I doubt any short circuit protection on the MOSFETS such as de-saturation protection exists on their gate drives, so they rely on fuses but this is way to slow for the MOSFET which see the short circuit and die before the fuse even reacts. One solution if you want to continue using such a [bad] inverter is to put a large inrush inductor on the input of the inverter where the batterys connect, this way the MOSFETs closest to the terminals get some inductance to reduce inrush surges, or rewire the blue /red cables so they all have equal and longer lengths to gain some needed series inductance.

The failure mode is definitely shoot through on the low-voltage MOSFETs. My educated guess is this is caused by poor driver circuitry failing during the in rush, basically turning on both sides of the MOSFET bridge shorting the battery input between two or more MOSFETs.

MY Reliable 3000 watt inveters has been running perfectly for two years also. i can run a Hitachi saw an air compressor and have never had a problem.

Just based on the shorted input on the inverter I'm positive that it was junk mosfets in the inverter that shorted thus, causing the breaker to trip. It threw sparks because it was disconnecting the inverter under a direct short condition, house circuit breakers will throw sparks the same way if you have a short in the line.

Hi David,Seems there are already too many comets, so here is another. I design inverters over 40 years now, the first was square wave. Nobody seems to talk about your inverters wave form that is not pure sine wave as power company is and is called a step wave and has THD (Total Harmonic Distortion) rating. Motors or any inductive loads have a reaction to wave forms because a magnetic field is produce around the coil that during zero cross cycle will induce a back EMF (Electro Magnetic Force), This can be helpful if the transformer is tuned to the load but the loads and type vary. The only perfect DC driven inverter that simulate a perfect sine wave is the Ferro Resonance Transformer. Most inverters work fine with resistive loads and the better inverters have protection devices to handle more types of loads. Looking at your video it appears that the inrush and inductive reaction caused a voltage spike across the Drain and Source leads of a MOSFET called Vds (Voltage Drain Source) and on average can be 700 volts and that spike arced across the insulator inside the device and destroyed the MOSFET. (look for pin hole) If it used standard transistor would be the Vce rating (Voltage Collector to Emitter) and because your inverter does not have trans/surge absorbers to clamp and dissipate the spike it shorted allowing DC current not AC to travel through the motor windings and that could easily be 200+ peek amps in the 1/2 second way over taxing and destroying the CB. If you measure your saw's motor winding it will read as a short circuit because DC travels in a loop but with AC its changing direction back and forth and there is never really a short circuit @ 60 Hz so the failure was not normal for the circuit breaker as an overload it was designed to handle. Hope this give you a better in site and toss the breaker/inverter.

Dave, I think the inverter has a fundamental fault in handling surge. As well, I don't think it is the problem of the circuit breaker. You may need to ascertain first if the inverter is really 8000 watts as described. Here is my suggestion: if you are able to repair the inverter into working again, instead of taking it through such surge with the big saw, give it a lower load and keep increasing the loads to see what it can handle (up to how many watt is can handle). This would help you determine if actually it is 8000 watts as rated.

David, Your certainly learning as you go, thats ok. You have to Check the Specification of the Invertors and Big Current Loads, ie your drop saw. The Invertor should come with a Continuous Rating, probably 8000 watts but you need to Find the Surge rating, it may be as low as 10000 watts. Also some invertors have Output Current limit capabilities which can explain the difference between Invertors. Circuit breakers are old reliable technology, hence it is rare that there is a problem is there. If the Breaker is tripping 99 times out of 100 it is doing what it should do. You do have to make sure you have the Max Power supply Current and volts on the 48 V DC Input. Check the Surge Rating on Invertor Dave, if your Drop Saw is 15 amp typical Start up currents are Very Quick but 5 to 7 times full load so 75 amps for 1 second. Good luck.

Hi David. You should understand what to expect from an ohm meter when you try to measure continuity at the input to the inverter. With the on-off switch in the off position on a known good inverter you would expect to see a very low resistance because of the capacitors that are acrossed the input to the electronics. The capacitors are there too try to absorb any spikes that come in on the DC line. Therefore if using an ohmmeter across the DC input do the inverter you would see the ohmmeter trying to load the capacitors. You may or may not have a direct short with the fuses put back in unless you expect to see a solid short across the DC. Leave the ohmmeter on there for a few seconds to observe whether or not if the resistance changes due to the charging of the capacitors. The point that I am trying to make here is that there are capacitors across the DC input to try to absorb any Spike that may occur on the battery side ie the inrush current. Just as a note, my Outback Power inverters, VFXR3524A (2 of them) use a 250 amp Schneider circuit breaker each. Each inverter is a 3500 watt inverter. I run them in a split phase 240 volt system. My system will start and run a 2200 wad 240 volt air conditioning compressor. I decided not to run it through the inverter because of the start current always hammering the inverters on Startup. I do have an air compressor, and a washing machine that does flicker the lights when they start up. I have 3500 ampure hour of battery running my system. If you want a project to spend time on, then go ahead to try and fix the 8000 Watt inverter but do it only as a hobby to prove to yourself that you can fix it on your own or send them both back to them stating that they both failed just moments out of the box! Make us all another video telling us what your decision is. I enjoyed the discussions with the group. I have more tube and transistor theory under my belt than I do with mosfet applications.

Focusing on the breaker is not the issue, voltage sag on input is the problem. With the circuit breaker taking the brunt, but the total resistance of the input circuit (internal resistance if you drew it systemically, a non ideal power source) was high enough to reduce the voltage on the input side, and thus raising the current in the mosfets above the safe area of operation. Like I said on the other video, this is often seen when people are wiring up huge audio amps in vehicles, and cheaping out on the wire. Currents go up, and the mosfets let out the magic smoke. In my estimation, the same thing happened here.

Just a shot in the dark but you can't just use a beeper. You have to check resistance to see if it's a complete short or just some kind of resistant load. It's common to have resistant load across leads.

This is one of the best videos I have watched...A great one for instruction and to share...I'm a former HVAC/ELECTRICAL/MECHANICAL BLDG. systems maintenance foreman....Whew...Try saying that three times fast...I'm a former instructor too, soon to be doing it again...Semi retired, just couldn't stay out of it...I will use this video...The explanations and troubleshooting are detailed, clear, and the best part," understandable " with out talking over a guys or girls heads....I think, pulling one component fuse at a time, while the meter is ringing, may point to the bad circuit, but, reading through all the components, makes a circuit, and it will ring the meter too....You have to isolate the feed from the return and meter each one to ground...Maybe the base of the unit...With the fuses out, if reading the terminals of the socket, where the fuse goes in, one rings to ground, bingo....Where you go from there is determined on how deep you want to go, is it worth your time and /or the bucks, or just send the thing back...This is still a video worth watching and more than once...My thanks to the gent that made it.

I have a 4000w reliable. Powers my entire house. Furnace, 2 freezers, large fridge and Kuerig coffee maker, and also misc. power outlets. Never a problem. Everything is done with 4/0 supply wiring throughout 24 31 series 105 amp hrs 12 volt batteries, and 10 or 14ga house supply wiring. Separate from grid power. 200 amp inline fuses on neg. and positive on battery side. System goes into 60 amp main then to separate breakers 15-20 amp. Inverter is rated at 30 amps. Point is the inverter works very well. Over 3 months now.

One thing to consider is that smaller circuit breakers tend to have higher contact resistance than larger ones. If the resistance of that breaker caused a sag at the moment of the switch on surge the mosfets would have been overloaded for slightly longer than otherwise - possibly enough to bring about that cascade failure. So unfortunately it points back to the smallish breaker being used again. I do agree with other commenters about the differing wire lengths inside the unit possibly contributing to the failure and that would be a design issue of course. The best thing to do when running big motorised appliances from inverters is to use a LOW FREQUENCY TOPOLOGY inverter. These are far more robust when it comes to starting "surgey" items. Interestingly, I watched your previous video a day before I went out and purchased a 1250 watt circular saw, which I intend to run on an old high frequency MSW 3000 watt inverter. Your video made me nervous about popping the thing the first time I spin up the saw so I am putting together a soft-start unit. I have already developed it to the point where I have tested the principle of it with the inverter and the saw. It worked nicely. Could my inverter fire up the saw on it's own? I'll never know, because I am never going to try it that way! The other advantage of the soft-start unit is the saw doesn't kick back the same when it's spun up. Which is a useful feature on a hand held tool.

David, firstly thank you for the recording & insights. Additionally, I really like the way you do your reviews as they are in-depth, clean & clear, and are well weighted in the positive & negative. As an electronic engineer, there are a number of things that you may not be bringing into the discussion. The first being that what you are starting is an AC Motor. AS you have stated there are startup surges that occur, but there are also power feedbacks that occur due to induction fields collapsing with-in the motor that can cause huge power spikes back into the power grid. Normally the power grid is very capable of taking this millisecond surges, but as this is a switch mode power supply/inverter providing the power source, I suspect that the parallel design of the inverter was not designed to take these reverse in-rush power spikes. The way this inverter is designed is also not very inspiring as it relies on a number of Parallel configurations (The MOSFETs, Transformers, Transistors, Switching systems) works with close to the same speeds as each other, which off the shelf electronics without going through a strict matching process, do not do well. So I think you will eventually find 2x things that have caused the cascading fault. 1 is the way the Inverter was designed to leverage low-cost groups of parallel models that seemingly are meant to work as one. 2 the reverse in-rush current that is caused by an induction motor startup within its first 50 milliseconds of operation. Again thank you for the review, the insight and the opportunity for feedback.

you are right on! you are not misreading this chart. the invert has no protection diodes, it has no snubber network. It is totally the designers fault of the inverter. Go to a you tube technician called (Diodes Gone Wild) and his power supply designs always have snubbers and diode protection circuits in every one containing Mos fet drivers

I just got a look at the bottom of the 3000W board. Reliable does use a similar topology to what I described. The transformer secondaries are in series, primaries driven individually. Current sharing isn't an issue.

I agree with Jacob about the inductive starting surge from the motor and the lack of MOSFET protection.

From wikipedia "When an electric motor, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed. For high-power motors, the winding configuration may be changed (wye at start and then delta) during start-up to reduce the current drawn." ... on wikipedia they show a sample graph that this brief "in-rush" current can be like 50x of nominal ...

Hi, I have a Reliable 48v 240vac 3000w inverter which I have carried for a spare. On wiring it up I found it is sensitive to voltages. It decided to not work so taking the top cover of I found the blade fuses. They are only the cheap yellow auto type. Trying to pull these out to check them was nearly impossible. Few of the holders came away from the PCB. My first idea was to throw it in the bin. They have gone up so much in price in Australia that I decided to have a go at replacing the blade fuses with a resettable type blade fuse.

An electric motor is a dead short when you pres the trigger switch and it has not started running yet. A 15 amps rated electric motor may draw in excess of 45 amps at startup. Unfortunately, semiconductors react way much faster that a fuse or a breaker. A breaker may trip after some 50 msec or more. That 35 amps breaker will trip at about 50 amps... If that current is present long enough. A fuse will blow within 10 msec, and faster if the overcurrent is high enough. A semiconductor like a MOSFET will likely blow within 100 microseconds if the absolute maximum rating of the part is exceeded. Furthermore, there is the voltage issue which a breaker or a fuse does not protect against. An electric AC motor is basically a DC motor on which the connections are switched. A coil switching can create voltage spikes over 10 time higher than the driving voltage. So, driving the motor with 120 VAC may have given spikes in excess of 1,000 V. That would certainly exceed the absolute maximum rating of the MOSFETs, blowing them up. In turn, the fuses would blow as MOSFETs often short when they blow up. Conclusion: A strong voltage limiter in the dV/dt domain would need to be used.

To see the inrush current you need an oscilloscope. That way you can see the current waveform in the sub-microsecond time frame. Your meter can barely do tenth of a second.

My guess is that the breaker was to big(80A) and the motor was in short at that particular start and so it was acting as shortcut to the output of the inverter and that make the mossfet+fuses blow . Now if you had a lower value breaker (60A rapid) that may save your inverter (it may cut off the output in real time). In general the motors are surging a lot of current at start. That is why for this power tools applications special inverters are needed.

Circuit breaker like this one has two opening mechanisms. One is bi-metalic thermal based which is what the rated current is based on. It takes some time to heat up and throw its latch opening, by design, to allow surges. The other is a magnetic fast reaction trigger which responds to very high current like a momentary short and trips immediately. A DC breaker also has an arc inhibitor which throws a barrier across the gap as it opens for both bi-metalic rating trip and magnetic fast trip. Problem was not the circuit breaker, it is the inverter design. A good inverter would protect itself against an overload. A circuit breaker is to protect you if the inverter fails, which it did and the circuit breaker did its job.

My guess as an engineer not knowing the circuit, is that the harmonic frequencies caused by the saw starting is triggering the FET circuit causing them to form a short. About five months ago I had to fix the power in a manufacturing plant that converted over to LED lighting that was causing motor soft starts to fail weekly. I attenuated the harmonic frequencies using low pass filters on each power run to the LED lighting. In the office area some of the printers when printing would make some of the LED light flicker when they were printing. I fixed this by adding a extension cords to attenuate the power line noise. A filter would have been better but the extension cords was a much chipper fix. With all this said the working unit must have some type of filter system or a better circuit for firing the FETs and that could be as simple as Ferrite beads.

@davidpoz you put a 80 amp breaker on a 66 amp circuit, 8000W at 120V is the output which pulls ~66.6 amps IF the resistance is ~1 ohm. You should have put a 20A at first then tested with a 40A breaker if the 20A worked. The fuses are the giveaway. Each fuse is rated at 10A which means those Mosfets are rated around 10-15A. You stated the chopsaw pulled 15A which on start-up can be 20-25A depending how long the extension cord is. On that circuit design, in that inverter, it will be possible to pull the whole 20-25A across one 10A circuit. That blows the fuse and the Mosfet. Replace all the Mosfets (you had continuity across the outputs which means the fets are shorted closed) then yank all the fuses and test voltages across across each fet one at a time. If that doesnt fix the inverter then I would need to put a meter across it. The spark coming out of the breaker is normal on an ungrounded/poorly grounded circuit that shorts! I'm no expert but that's what my ohms calculator tells me.

hi David. Great vid. The initial In-rush current is a dead-short that cannot be measured accurately using a meter. You need to use a soft start circuit to protect the inverter.

When I made inverters in my younger days, to change DC to AC, I remember using a high-level MOSFET and a low-level MOSFET to switch one output node to the positive and to the negative. This was used in a half or a full H bridge. The storage capacitor on the DC side had to be very close to the Mosfet as at that current high spikes are generated even in short wires. Well, the firing of the upper and lower Mosfet had to have a delay time as if they switched together that would be a dead short circuit across the storage capacitors. From what I can conclude in this video the fact that the fuses were blown and the MOSFETs were also failed and at the battery end there seems to be a short circuit...... then I am afraid the firing of those Mosfets either to get the high frequency to step up the voltage or to get the sine wave shape due to switched modulation...... well that part of the circuit needs a lot of attention before one markets an inverter.

After reading through most of the comments I just want to throw out a few things which may help. I bought a clamp-on meter with a very cool 'Motor Start Current" capture function. It was about the same price as all of the decent ones. Even the peak hold feature will get you close. Snubbers and Transient Voltage Suppressors (TVS) are not used on DC circuits from a battery source. They are used on utility input circuits to handle Transient Voltage Spikes which are added on the power distribution circuit, aka power lines. The very slight variations of length on the internal power conductors will have zero effect. It's a DC circuit so frequency is zero.The resistance difference would be insignificant but I can calculate. I have to guess at the gauge, maybe 12 ga. so about 1.6 ohms per 1000FT. The longest loop is about 2' longer than the shortest so about 3.2 mohms variance from best to worst. This inverter is not poorly designed. It is value engineered. There is a huge market for this. It's designed to delivery exactly what it claims. It is not designed to survive incorrect installation. The protection systems in the design assume correct installation. Many folks commented that they have seen failures due to undersized conductors. We watched it happen TWICE. This is extremely common in the consumer arena. When hobbying with electrical equipment please read all instructions and every label. Be sure to follow everything on the label exactly. The labels are required by UL to keep you safe and to prevent fires. I have been designing industrial 48VDC power and battery systems, and inverter systems for 3 decades. I have 100s running nationwide. However, I have not designed consumer products. It's fun seeing your innovative ideas here. Please be careful.

Hey David, the only way to measure irush current is with an o-scope. using a clamp meter introduces errors, becauses it has a sampling frequency, so you cant assure it measured exactly the peak, in adition to analize the breaker you need the pulse width of the inrush peak, again, you need the o scope to see it the breaker tripped or not in a given time. Asi you mention, the smaller one has less mosfet in parallel, reducing the paramters i mentioned last video.... the inverter side works as an SMPS, so I gues in a push pull technilogy, if the secondary of the high frequency transformers are not well designed and connected current directions can vary causing an overload and , blowing the mosfets. if the euipment is out of warranty desolder every mosfet , short the three legs and then measure resistance on channel ( Drain to source) look for the datashhet pinout. if you got a short circuit, they mosfet suffered an overcurrent, if its open, it suffered a reverse voltge peak, caused when current stop flowing throw transformers and circuit opens, a big current in transformers will cause a bigger voltage spikes. replace mosfets, and get softstarters, no one of garage tools starts engine with load.

People have said it in other comments and I think they're right. The mosfets may be the problem. Power switching capabilities of mosfets are generally slower than that of igbts. A quick look at the bus layout would mean each mosfets would be taking just less than 1000watt continuous. And peak draw is way higher. But I'd be interested in seeing the part number of the mosfets used. I'm more inclined to think they cheaped out on out of spec fets or regulator caps. If the drive voltage doesn't switch fast enough and gets the fet fried. Looks like bad component choices at first glance but until we know specifics, we're all just talking out of our asses. By the way it started I don't think it has much of a soft start circuit for the PA side either. Another shortcut to make the unit cheaper maybe?

There is a lot of discussion about the "load sharing" between the various sections of this inverter. I haven't worked on a "Reliable Electric" unit but I have worked on a number of large inverters from another manufacturer, and I've seen the bottom of Reliable's 3000W model (in another video on another channel) and seen that it is similar. The sections are NOT parallelled on the output sides at all. The length of the supply conductors isn't hugely relevant, and no section ever sees more current than the other. What you have is nine different, isolated, voltage converters. They are synchronized but otherwise independent. The secondary sides of these converters are in SERIES and produce about 360volts DC, with a center tap (ie: ground, +180VDC, -180VDC). The second board that David here is calling the output board is just exactly that. It uses PWM to generate a somewhat-accurate 60Hz sinewave that swings between the two 180V rails. Since the input sections are in series, they all see the same current. One may output a slightly higher or lower voltage than another, but we're talking a couple of percents here, if even that. There should be enough headroom in the design that a few volts drop on the HVDC rails is irrelevant, and, in any case, the difference will not stress one section more than another. @TrackGeeks also makes an interesting point. MOSFETs can easily be killed by instantaneous current, faster than a fuse can blow. Those fuses are protecting the wiring from dead short current, but they're not really protecting the fets themselves. Most traditional applications consist of a big lead battery. They have scary current capability but it's still "tame" in comparison to lithium batteries. When you try to start up that motor, it's initially a dead short, and the current is limited only by the DC resistance of the motor windings and the impedance of your wiring... ... your wiring is pretty short. There's not a lot of resistance there. The inverter is, for all intents and purposes, seeing a dead short. What would normally happen is that the DC input voltage would sag a bit -- the resistance of the INPUT wiring (and batteries) would limit the actual current through the MOSFETs. It would likely be higher than the fuse value but lower than the value at which the mosfets explode... but you don't have (as much) resistance in your batteries AND the supply wiring is short. Your battery bank is able to provide a much higher (and thus more destructive) instantaneous current than "Reliable Electric" expects of their usual lead-wielding customers.

Yes! Time Delay Circuit breakers. They have a Magnetic trip and a thermal trip. Good Job!

Dude, it's the junk inverter. Nothing to do with your setup... the mosfets failed

My inverter failure was a BAD GFCI receptacle!!!!! Had to take it all apart to replace it. I'm more careful of how much surge current I'm making.

A large part of the problem is current distribution within the inverter. There needs to be a centralized bus system where all the wire lengths from the bus to each bank of MOSFETs are the same, otherwise the ones that are closer to the source will end up doing more work under load. This is apparent in the fact that the MOSFETs closer to the terminals ended up burning out first. One failure mode in transistors/MOSFETs is a short to ground, and it looks like this is exactly what happened. This, in turn, ended up pulling enough current through the circuit breaker for it to do its job and shut the power almost immediately. I wouldn’t be surprised if the traces on the underside of that board are scorched too. I think a centralized power and ground bus inside the inverter, coupled with MOSFETs that are a little bit beefier, will make that inverter run like a raped animal. A few bucks more in parts, and a slight redesign of the internal component layout and power distribution should do some justice.

Duracell bought a company that made a UPS battery backup inverter done the same way as the knot on this. But if the transformers are using wire to thin, or the insulation between the windings are thin then the transformer wires would heat up, melt the insulation, maybe even arc to another winding if they don’t have enough insulation. The power from inductive load (saw) has to be release somewhere and will go back out of the input to the breaker. The transform fuses would blowout as power exits and because of the power coming out it probably aced on the breaker wires. OK, if my theory is correct then the transformer after the last fuse was the fat to go. And the other could also low out and short as the power exits backwards trough the input terminals. Also, not only could this be using sub par transformers, but some heart TVS diodes between the transformers could have stoped most of the damage cased with maybe only pop get a fuse or two. On second thighs back to sub par transformers, maybe on would be damaged even with the diodes since that it’s one is the weak spot and the flaw in the systems under load with inrush current.

Great video I have the 1500w 24v inverter same make I overloaded AC with hoover .... looking to repair mine and your videos inspire me in theses inverters ....hope you do the fix so we can see ......well done that man !!!!!!!!

Likely you have a cascaded failure. Its the same as if you put a lot of LEDs in parallel. Not all LEDs are created equal, meaning some will have less resistance than others. For LEDs this means that some might be brighter (and hotter) than others if they are wired in parallel. For this reason LEDs are usually driven with constant current drivers to give the same power to each LED. In your case having 9 parallel transformer driver circuits, chances are that one of them took a bit more of the startup load than the rest. Potentially because of the gauge and different length of wire used inside the inverter (which in turn gives the path of least resistance). Once one of the nine circuits blows the load is shared between the rest. Then the second one blows until you blow the main external fuse triggers, which stops the cascade. Long story short, it is likely that it does not like the full load being drawn suddenly (due to the design), you might be okay gradually adding load up to 8000w.

I had the same problem with Circuit breakers on large induction loads put in a 300 amp class T fuse, problem solved. DC breakers have arcing problems causing voltage drop and super high ampere rise. Lost nice inverters before I figured it out. If your wondering why your 3000 watt inverter ran it. Look up voltage sag. It’s designed into some inverters to increase start up load for induction motors.

Should find a chop saw with a speed control, so you can slowly spin up the saw and should result in a lower start up current.

My read on this is that the inverter failed first, and it is primarily the fault of the inverter..... but one extra factor is that the Lithium cells you are using are capable of very high current, and their output is very "stiff", it will not sag under sudden high loads... and I think this is where you have defeated the testing that Reliable will have done using Lead Acid batteries!... The two boards in the inverter are literally input, and output... the input board will step up the voltage to around 170vdc (the peak voltage of a 120v sine wave), while the output board will turn it into the 120v(RMS) sine wave. The fact that the input board failed amazes me, since it means that the output board is built like a brick outhouse!... I imagine if Reliable do any redesign work here, it may be to beef up the mosfets on the input board, but they should probably also consider adding some mitigation for a sudden surge current... I don't think the mosfets would fail that fast due to overload, but perhaps the sudden surge caused some high voltage back EMF spike at the primary winding of the transformers... it's possible that the modfets simply need a better snubber array... or perhaps some small filter on the high voltage interconnect between the input and output would help.... It's almost impossible to say without a circuit diagram. I'm also wondering if I've been saved a similar issue because my unit has a 340vdc input board for 240v output.... so the interconnect carries less current.

So, there's something we haven't looked at: The voltage drop across the wires on the DC side. Not saying it was or wasn't the setup, but I didn't see you address that. Why could voltage drop cause the inverter mosfets to fail, you might ask? Because, the output voltage regulation of the inverter causes more current to be drawn from the batteries on the DC side as the battery voltage, or at least the voltage at the inverter terminals, falls. This is of course because to maintain the same power output at a lower voltage, more current is needed. The mosfets don't actually care about the voltage drop, but what they do care about is the current increase. The mosfets can only handle certain amount of current before they fail, usually closed circuit, causing the short you are seeing on your input terminals. For this reason, I'm curious about what would have happened if you had used a larger gauge wires, or simply more of the smaller ones. Again, I realize that you use to the wire supplied, this is clearly reliable electric's fault if my theory is correct as they supplied wires that are just not suitable for the application. However, it definitely seems to me like this would be possible, the voltage falling and the current increasing, triggering the overload of one or more mosfets.

Breakers have two kinds of protection. Short-circuit protection uses EM force. Very high power draw will trigger EM protection and cut power in fraction of second. Second protection is thermal. This one is quite simple, it will pop when it is heated too much. If you are under limit, then it will not heat up. If you exceed limit on breaker, it will start to heat up depending on how much you exceeded this limit. You can run 20A on 16A breaker for quite awhile before it pops. You could run 50A on 16A breaker but it will pop much sooner. For this thing you have classes which determine reaction time versus current. This protection works when it's not short circuit but is too high power draw.

Things to consider in diagnosing this failure. 1. Was the circuit breaker used new or one that had been used in a previous failure. If the circuit breaker had been used previously during a failure it may have suffered internal damage, eg a arc track where a carbon deposit has been formed such that it may fail across that carbon deposit when next used under a high current load. 2. The MOSFET's are vulnerable to punch through of the internal junctions of these devices if they are subjected to a voltage spike. The spark you saw on your video replay would have been a potential source of such a voltage spike. 3. The inverter should have been tested on the bench without any additional components eg circuit breaker. The testing should have been undertaken first with non inductive loads eg heaters, incandescent light bulbs or a variaible non inductive resistive load. After these tests small inductive loads tried next eg fans, then low power motors and finally high power motors. 4. To avoid arc induced voltage spikes the circuit breaker would have better been separated from the inverter by a length of cable rather than being attached directly to the inverter. This cable would have potentially supplied some high voltage spike attenuation. 5. The inverter input ciurcuitry should be checked to see if any high voltage spike surpression has been included in the design (snubbing circuits) these are often just resistors and fast switching diodes. 6. Testing inverters is complex and poses a real danger to life if you get it wrong or the manufacturers design is not up to electrical standards. 7. When the manufacturer rates the equipment the specs quoted need to be clearly understood. eg House mains supply is normally quoted in RMS V and often manufacturers quote peak or surge peak to make their equipment seem to be able to run high loads. 8. Did you re-test your saw with the 3KW inverter and circuit breaker after the first 8KW inverter failure, to determine if the equipment and setup was still OK 9. My guess from the limited test available info, is that there is a fault in the input wiring and components which has created high current arcing and that this has created a high voltage spike ( could be only for a very small fraction of a second) and that this has destroyed the junctions of one or more MOSFETs in the input circuitry of the 8KW inverter. 10. Why this failure in the 8KW inverter did not occur in the 3KW inverter is a good question. a. differences in input ciurcuitry in the inverters, eg protection, imbalances in wiring length, differences in test conditions, differences in components / wiring used or mismatched components or re-used damaged components. I hope this helps you in tracking down the issue. A failure of 2 brand new inverters with the "same" setup means either a design fault in the inverter or a testing setup fault. I would think that the manufacturer has tested this inverter on many inductive loads, but I wounder if these were RMS or "peak" transitory loads. Your saw has a very high peak current at start up, and this may well be beyond the designed operating parameters of the inverter.

You need an oscilloscope or a peak recording meter to see the actual peak power the saw draws. If a meter does not specify that it records peak current, it's not fast enough to capture the peak (which often lasts less than 0.1 second). I'd guess that the saw's startup current is somewhat higher than your measured 39.5 amps, possibly as much as 2+ times higher. Depending on the type of motor and its design, the start current can be 10 or more times the running current. If the running current is 10 amps, the start current might be 100 amps or more. 100 amps * 120 volts = 12,000 watts which is 250 amps at 48 volts - only for a fraction of a second but possibly long enough to damage the inverter. You won't get a definitive answer until you have access to a peak reading amp or watt meter or an oscilloscope that can show the actual peak power the saw draws at startup. When you get into the Outback level of inverters (possibly 5 to 7 times the price of the inverter you're using) they actually specify the continuous watts, the peak (30 minutes) watts, the surge (5 second motor start) watts and the very short instantaneous power (0.1 second) watts. Some electronic equipment has a very high instantaneous inrush current but it only lasts perhaps a cycle or two of the AC waveform. I'm aware that the Outback inverters list all four ratings but I've not seen that from anyone else - there may be others but I've not seen documentation on it. Remember that part of what you pay for in electronic devices is the manufacturer's guess on how long the device will last. The Outback has a 5 year warranty. I don't remember the Reliable Electric warranty being more than maybe 90 days. I have two 2000 watt pure sine wave inverters from Reliable Electric and they have been working fine for several years. However, I checked the average power of each device that I power with the inverter and either measured or did a generous estimate of the start power of each device. The loads are limited to 80% of the inverter's rated continuous capacity which means we don;t use the "1100 watt" microwave whose measured input power is 15.77 amps at 120 volts or 1892 watts. If you listen when this microwave oven starts up, it powers the fan, then low power on the magnetron (that generates the microwaves) then full power on the magnetron. If you set the timer for 10 seconds, you actually get 8 seconds of heating.

Has the breaker been looked at in a forensic manner? An EE with specialization in DC circuits. An example of AC -DC to AC is the NB Power interconnect with Quebec Power's Grid. Now that is a grand scale. The advice I got from an installer - with 20 years experience - was to double the size of the breakers. He had been into fix a lot of somebody else's messes - and that was his advice. Also - go look for an old Metered Amp clamp - The instantaneous spikes are seen on dials. Not so much with electronic meters. They do not react quick enough. The trigger timing of the Meter - is much less than dial meters. Cheers

The inrush current spike kills the inverter. Weird that the smaller unit takes it but maybe it just can't provide enough amps to destroy the mosfets. For this application you might need to add a soft starter.

I think mostly is a problem on the conversion side of the inverter. You can say pure sinus wave when you output it but if you don't engineer unphasing it's junked quickly. A resistive load is in phase, but a motor is inductive load, and on heavy side, 90° unphasing the current with voltage(tension) and for simple basic calculation, consider your start inrush is 6x to 10x the nominal current. It plays on milliseconds, and with that unphasing shifting, if the inverter have no back calculation quick enough you can have spikes not compensated by the electronics up to thousands volts. To be more simple consider your motor as a really fast charging battery but with alternative components harden the calculations. When you want to start it, you have nearly infinite rush of curent. Still in milliseconds once charged up your motor will give back on the circuit with Fourier law some of it's stored energy but at different frequency. If your inverten don't see it and by the way don't care about, the current will have nowhere to go, and by the way, the voltage will raise up to insane levels. Once the voltage is high enough, bam, component destroyed and sparks on your breker. I really hope that I have been clear enough. You can ask me uestions if something still unclear, and well I can provide calculations if you want as well.

Continuity test doesn't necessarily mean a short circuit. capacitors behave like a short circuit to AC, but another way to look at it is that they will resist a change in the input voltage. your multimeter is attempting to apply a small current through the device under test (the inputs to the inverter) and looking for a low voltage proportional to the current that it determines to represent a short circuit. the capacitors resist this change in voltage, so it will be very low for several seconds (or longer) in which the multimeter will register continuity. you should test in resistance mode, and look for the resistance to either climb, representing charging capacitors, or stay steady, representing a short circuit (after several seconds to a minute)

Awesome- the time delay chart tells the entire story ... :-)

Were you using that breaker on the battery side or output side of the inverter? Because none of those chart values apply about average rating if you are using it for AC output. It’s not AC rated. The same gauges of wires will have different ratings depending on AC or DC applications because DC heats wires up much more quickly than AC, thus DC wiring has to have much larger gauges for the same amperage.

You referenced a part as a transformer at time index 6:30. I think you are right, because they sure do look like that. Transformers are AC devices and if you power them with DC they will act as a short, especially if you have a bunch of them wired together to share a load. If I didn't know any better, I'd guess that that inverter was designed to accept 48V AC as an input. Just a stab in the dark. Hope you figure out the problem. Good vid.

David your clamp meter doesnt have a peak function so what you saw may not have been the peak value. I know ac motors usually have a starting current of 8 X their normal but the universal motor is also a series dc motor and the can run upto 12X their normal current when starting so I hate to say it but your 39Amps peak for starting isnt even halfway to the true starting current. Roughly times your output amps by 2 .3 as 110V / 48V gives you 2.3 as a ratio. So I saw 10amps no load running. 12 X 10A = 120A peak 120A peak X 2.3 = 270A on your batteries. As for the multiple Transformers that is fine but why the load wouldnt be shared evenly is because the the leads to each transformer are different lengths. They should be matched. That means that the load would be shared more evenly.

That amp-meter only samples at maybe 2hz - you need a scope to see the massive current spike that only lasted for a few milliseconds. Inductive loads require 5-7 times their rated wattage from the inverter. A low frequency inverter like the Aims has a massive transformer that can absorb huge inductive loads three times its own rating for up to 20 seconds for example - these cheap high frequency inverters let out the smoke from a single current spike like the one that killed yours from the saw.

The typical in rush current for a motor can be 20 times the full load current, the in rush only lasts a few mil seconds. It is due to the motor presenting a short circuit until the magnetic field in the coils produces enough back emf to resist the current flow. I believe you may have hit the cause of the failure in the 8kW over the 3kW as being the way the dc is distributed within the 8kW unit; the wires from the dc in terminals should all be the same length hence the same resistance therefore the current flow will be equal, its possible that one or two of the inverters are grabbing most of the current going short circuit blowing their fuses and this cascades along the string. Its quite possible that manufacture has not designed their inverters to handle motor starts of the rating you are attempting, its worth asking them (I an electrical engineer in the UK :) )

Did you increase the input wire wire size or triple them from your DC buss to the circuit breaker and inverter? I think someone mentioned that in the comments. It looked like you only used one of the supplied wires which would not supply enough current from the battery to the inverter to handle the surge and if you were to run a prolong draw of over 3k watts the wire would most likely melt. Hense, the 3 wires. A sudden drop of voltage at the input to the inverter would most likely cause a sudden current current overload to the mosfets' causing them to short and blow there fuses.

If you look in your 3000W inverter you will probbably find 4 wires of almost equal length goiing to the four component groupsIn the 8000W inverter are 5 differenth lengths of cable powering the component groups - the difference msy be small but in no way can these 5 differenth lenghts distribute the requested power evenly between the groups.one group trying to deliver all the requested power because of a timelag between the next group - ah well the result was spectacular

if you are splitting the load across individual mofsets, without some sort of individual feedback to balance them; and they are all just individually clustered/phased and tied together at the outputs; an inrush of current could go through and short out by overloading one mofset more than the rest; then they cascade in failure due to a short. Another possible thing; is all the mofset's gates are controlled by an ic; sometimes one for all; but typically one mofset controller per-cluster and each (cluster)phase will have a mofset for - and one for +per and most controllers can control both + and - mofsets; [this is how they do it in most buck/boost converters nowadays]; a simple small ic can take the place of many control circuits and logic... so it could be an array of transistors and capacitors/resistors.... i have only seen the top of that circuit board; not the underside. Many converters have inrush/outrush(if added to the AC output instead) limiters to prevent there are common components that are meant to specifically limit inrush current to prevent these very types of issues. I also personally believe that these types of motors should not be used on modified sine wave inverters (use an oscilliscope to see the fake squared sine wave they make and you'll see what i mean [you can look up YT videos as well if you dont have access to an oscilliscope, AvE has a good video], especially if you scope it under load) as they produce strange issues because of things like power factor dynamics as the motor ramps up, etc. I know I spat out a mouthful, but it should help you in your process of figuring it out. But if you can source all mofsets test them, source to drain, drain to gate, gate to source with the diode mode on your meter (I bet all mofsets for + and - rails will be bad on the input [DC] side), as well as check in diode mode on the pins of what controls the gates for shorts (could be a circuit cluster, or an ic as I mentioned above); and it will lead you to other potential components to replace.... but in theory; just replacing the mofsets and if necessary the controllers should give you back a functioning unit. Just check out a few tutorials on YT about using diode mode on a multimemter to diagnose shorts in transistors. if the fuses blew; it probably protected the rest of the device's electronics. Hope the info helps; but I think it all is from the surge that particular type of wound motor draws when it's first juiced.... its a high torque relatively low RPM....

My question is warranty/customer service will they replace how long would it take.

The inverters with big transformers are Low frequency Inverters (basic is a low AC voltage inserted in a secondary Trafo and you get the desired AC Voltage in the primary). Your inverter is a High Frequency Inverter.

You could be a good chap and send me the one that you don't want to fix. In all reality if they say that is a 800 It's probably safe to run at 400 half of what the rated. I love what you have created lifestyle independence

check on google that 8kw inverter can take up to 16kw surge and the surge would probably last no more than 500ms so the inverter would give out 133 amps say for half a second or less but the immediate draw from the batteries @48 volts would be UP TO 770 amps. dont forget the load is inductive and not resistive. you can get special breakers for high inductive start loads. check the small print with the breakers they should tell you how many start up amps they will stand perhaps try a contactor with thermal cutouts but needs to be wired in a single phase "easy when you know how lol" I hope this helps, Paul UK

The current limiting is off or too high, higher than what the mosfets and or igbts can handle. A simple flaw in choices of parameters in the psw current sensor modul. So normally, when an unbearable surge rush in, the current sensing modul will easily turn off the inverter, no harm done, and after a few moment, will power back up *with the normal gradual voltage climb* , and if we keep on pressing the power button of the miter saw, it will run with that slow start very easily amd smoothly this time, without any in rush I hope they can fix that current sensing fault for their future products

I wonder if the ground supplying the inverter is under gauge? I know from car stereo days that if the power can't flow back out the ground fast enough it will blow mosfets.... If the amplifier is under a big load and you were to disconnect the ground supply it will immediately fry the amp. Perhaps the ground wire supply is too thin to allow the voltage to "flow" properly.... I'm just a guy though who knows.

Your inverter had a cascade failure. What it does is splits the voltage into a positive and negative DC then creates a 120V 60hz output from digital pulses in the upper kilohertz range. The reason it failed is one of the FETs shorted out, then stressed the other ones till they shorted out.

That clamp meter isn't fast enough for the measurement you're trying to make. The number you're seeing on the display is more like an average value with a time constant around a second or so. If there was a breaker-popping surge lasting a few milliseconds, the meter would be blind to it.

I presume the 80 amp circuit breaker is on the 48V input to the inverter. So you have a circuit breaker rated for 80x48 or 3840 volt/amps. Your inverter is rated for 8000watts output? Something doesn't work. Also, you can't measure inrush current on a digital meter unless you have a peak hold function that stays active while looking for the peak. I suspect your peak was probably close to 50 to 60 amps and could be more. Most thermal circuit breakers will take up to 200% before tripping because of the delay in heating. You tripped in less than .2 seconds. Based on the internals of the inverter, I'd say it was cascade failure. When you are using wire, instead of a solid metallic bus, to connect several parallel load bearing circuits together, the one with the shortest wire bears the load first. then the next, the next and on to the last stage, having the longest wire, bearing the least amount of the load due to the higher wire resistance. When the OEM tested the inverter for load bearing capability, they probably brought the load up in stages. Not all at once. And in the even they put a surge load on it, it was probably quite controlled. However, I suspect they didn't. For what it's worth, if the wires inside weren't bigger than AWG 8, I'd say they should have used a copper bus. Again, using wire, they all need to be the same length to ensure all circuits share the load equally. Have a great day.

Just a thought... I wonder if they made all the internal DC cables supplying the MOSFETs the same length, that might allay the problem? I would expect the shorter run cables nearer the input to provide significantly greater current than the far end MOSFETs.

Just a thing. When you check continuity where there are caps you'll find a short for a certain time until they charge.

You need to hold the DMM across the input terminals for more than a second to determine if there is a short or not. The large capacitors on the DC (+12V) input will look like a low impedance (short) until they charge up to small voltage (0.1V ?) the DMM is applying across the input.

Hi, You are now going into Ampere Interrupting Capacity (AIC). All breakers have an AIC rating and must be understood to know how to build the system. For instance the home panels are rated at 10,000 AIC, and commercials are rated at 22,000 AIC. The AIC is for panel, breaker, meter socket, the whole electrical system. Surges are based upon AIC rating. delay are not AIC. When working AC breakers in a solar application you do not watn too much of a delay as it will cause a fire or explosion, Spark. The best solution is to use normal electrical breakers and 10,000 AIC max as any higher will be not good, a High AIC system will cause the system to explode if you use a low AIC rating, like put a 10,000 AIC to a 100,000 AIC system. The Inverter over powering the Saw and causing the breaker to Spark, but not trip as the breaker is a delay, not AIC type. Hope the s explains a bit. lazaro

Hi your correct to a degree regarding the circuit Breaker, but circuit breakers are designed for different applications, inrush of motors require a different type of circuit breaker, compared to a circuit breaker protecting a lighting circuit,

Ya AvE did a video a while back he was out on a sail boat, wanting to make coffee and using an inverter to do so the dam coffee maker wouldn't work right. So he had to put a small inductive load on the inverter ( a mixer just running on low) that small load chopped up the waves enough to then let the coffee maker to run correctly. Now my memory fails me sometimes, I'm pretty sure he mentioned something about this. I'm just going from memory but he has an elegant way of esplaining it. Before I get the hate comments I'm just a guy in his living room trying to help another guy in his garage or shop. I did go back and look for the video to link it but I couldn't find it, he also did some house cleaning a few months back. He removed some videos. Having thought about it now and reading some other comments, yah know I wonder if you made 9 sets of blue and red wires ( the same length) and wired them up if that wouldn't fix the problem. Seems to me the closest bank gets the brunt of it. While the banks further down the line are delayed. Probably a screw up between design and manufacturing those dam manufacturing boys are always trying to stretch a dime out of a penny. So cutting those blue and red wires makes a neat tight product they could be screwing up the works So to speak.

What are the specs on your saw, does it have ratings on it anywhere, and like some others have mentioned about your battery bank, can it supply the amps (factoring in the burst load starting the saw) for that 8kw inverter., if not the volts will lower which then requires more amps. And saw's make horrible noise on the power lines, maybe even enough to upset the inner working of the switching inside the inverter like altering the switching time of one or more of the input stages which can put them out of sync, I wonder how it would go if you had an isolation transformer after the inverter (just curious, maybe someone can give some info how they would work.). I think by design that inverter can handle most regular house appliances up to the 8kw but no so great with power tools, but I would at least expect it to cope with a home AC system. Im surprised the makers of that inverter havn't asked you to ship that saw to them so they can test in house before too many more Blown Inverter videos get posted to youtube. Thanks for sharing, we all learn from this stuff. Look forward to more updated.

We didn't use a digital clamp-on ammeter when I was in the field because of the slow response time. We used an analog clamp-on ammeter because you could see the peak that a digital unit would not detect. I know most people believe a digital clamp-on ammeter such as the one you used in the video to draw your conclusions would be better than an analog clamp-on ammeter, but I know from experience, they're wrong. A storage oscilloscope is an ultimate device to prove the point, but the analog ammeter will show you a lot. A quality clamp-on ammeter was used at Western Electric/Lucent Technologies/AVAYA. Technicians such as myself were given an analog clamp-on ammeter device to draw their conclusions in the workplace. We all shared the same device because the digital equivalent showed inaccurate amperage. We all shared the same device for simplicity and affordability; the analog clamp-on ammeter was continuously checked for calibration because we were an ISO 9001 facility. Now, obviously, if the smaller unit was able to start the testing piece of equipment, but the larger (8000-watt) piece of equipment could not, then there seems to be a problem with the 8000-watt inverter. I'm just saying there is a potential problem with a digital clamp-on ammeter as shown.

It would be interesting to know what the manufacturer recommends for a protective device on the input to the inverter: circuit breaker or fuse, and what current rating. My unenlightened skills say rated load of 8,000 Volt-Amps should draw more than 166 Amps on the input bus (remember efficiency is less than 100%). The transient capability of similar inverters is twice rated load, which suggests input could be briefly 325 Amps or more. It seems like the 80 Amp breaker is much too low rated for this application. Probably a 200 Amp DC rated breaker would be more appropriate. (but ask a certified electrician, don’t take my word as final) The transient start-up current of the miter saw very well could be six or seven times its normal operating current for a few hundred milliseconds, which would make the inverter try to put out its full 16,000 Volt-Amp output. That may be enough to magnetically trip the 80 Amp breaker. The breaker opening may actually contribute to the failure because the stored energy in the inverter’s magnetic components will dissipate across the now open circuit on the input side. A very high negative voltage pulse across the input could result due to the very fast change of current from a few hundred amps to nearly zero nearly instantaneously. The large negative pulse could destroy the MOSFETs. Without the circuit diagram, this is speculation. The design of the inverter should have metal oxide varistors (MOVs) across the inputs of the nine DC-DC converters to help, as MOVs would allow a temporary current path when the input connection is lost, thus limiting the magnitude of the negative voltage pulse. The other thing I noticed that several other viewers also commented is that the wires from the input bus bars to the various DC-DC converters is unequal length, and thus the load will not be shared equally if there are not ballast resistors to compensate for the difference in wire length. The ballast could be provided by a printed circuit board trace, so it may not be visually obvious that resistors are there. Without a mechanism to share the load equally, when overloaded, the stages could fail in a cascade with the stage with the shortest wire blowing its fuse, followed by next shortest, etc. My suspicion is it is a weak design of the inverter that does not adequately protect itself when the input goes open circuit and that flaw was uncovered by the under size input breaker. Had the input breaker been larger, the output stage probably would have limited current flow to the saw, and the saw would have started just a little slower than it could. If you want to measure transient inrush currents, Amprobe makes a pretty good true RMS AC-DC clamp meter that will catch very short peaks.

To hard to measure now. But low voltage output from invertor could have been a factor. The focus has been on current from what I have read so far. Can the invertor handle the instant current draw and maintain a steady voltage?

As one other poster commented most common circuit breakers have two methods of trip. They are not very sensitive. First method is thermal. There is a strip in the breaker that applies stress to the contact. If a certain amount of current is applied for a certain period of time enough thermal energy will then cause the strip to pull open the contact and trip the breaker. This is what gives a breaker a delay so to speak. The second method is magnetic. This method is designed to open under very high short circuit currents. These types of faults generate a very high magnetic current which forces the contact open and the breaker into trip. The fault you had when the inverter failed appears to be a magnetic trip. Therefore a very high fault current.

The answer is yes you did destroy the Inverter by overloading it. The inverter design is not totally bad as I have seen worst, and it is not the midnight solar circuit breaker fault. The MOSFETS just could not handle the huge current surge and they SHORTED out. I am sure the inverter can be repaired but the cost could be close to a new one so it may not be economical to repair. I am an electronics technician and would have repaired it if it were mine and upgraded the MOSFETS to handle a larger current. Great video.

You are correct, now check the transistors on the heatsink and you will find a bunch shorted. You should get around .6v base to collector, and base to emitter on a diode check. You may have to unsolder them for accurate reading but the shorted ones will be obvious without desoldering. Not sure why this failed other than a crappy inverter.

uk electrician here. A CB has 2 forms of protection on AC. A bi metallic strip for overload. (prolonged) and a coil pushing a projectile for short circuit. (almost instantaneous) forgive me but it's late here. but I cannot think of a way to get the latter form of protection to working a DC supply.

So.. I don't know if this has been stated but if the current draw into the inverter is so high that your voltage drops in the feed lines then who knows how much current the inverter is trying to draw. The transient inrush of current into the Inverter likely results in very low voltage presented to the inverter. You would think the inverter would have protection for this..

I just saw Part 1 and Part 2. Did you finally find out what happen to those two Inverters that the circuit breaker from these two videos you just uploaded in these problems. Is there a Part 3. ?

After reading a lot of the comments and being a previous integrated Circuit Switching power supply designer, I don't agree with many of the arguments below. The Saw's motor load at start looks like an extremely high AC load. The inductors used in the AC generation to the saw's motors are probably saturating and thereby drawing maybe 10 times the current through the AC output Mosfets. An inductor only looks inductive to a certain current level. Exceeding this level causes the inductance to dramatically reduce and then looking more like a short circuit than an inductor (probably much less than 1 Ohm after saturation). When the inductance falls, the current increases dramatically, almost instantly frying (shorting out the Mosfets due to heat) the Mosfets OR opening the Mosfets by frying their internal bond wires resulting in a HUGE high voltage spike caused by the Saw's inductive motor windings acting as a generator once the input AC power is interrupted. The inductive kickback of the saw's motor can generate thousands of volts, far exceeding the Mosfets absolute voltage maximum ratings. Mosfets are actually much more sensitive to Junction breakdown voltage that overcurrent situations. The Mosfets probably have a rating of 600-800V which is much to low to handle inductive kickback spikes. I am a retired EE as well and admit that I do not know what actually caused the fault without being able to look at the schematic diagram of the circuit used.

Interesting bits.... I agree with your assessment that the inverter is to blame. I don't think you were drawing 135% though. Based purely on the time it took to spark and die, it seemed closer to 600%. The inverter is overly technical. It's running 3 phase power in 3 parallel feeds to a single toroid inductor for smoothing purposes. The amount of capacitors alone guarantees component failure. Go for a large transformer and immerse it in baby oil.

I agree. It was the inverter. You hooked it up the same as the smaller one that could run that saw. Because of the sparks I would replace the breaker just for good measure.

The fault is on both sides. Wrong installation for powering Inductive loads. & Inverter does not have High Voltage protection at the output. High Voltage protection has to be installed on both sides of the automatic circuit breaker. If you had 600W heater already running on the Inverter output before starting up the saw, It would help as well. The inverter was blown up by the high voltage of the Inductive load and not by Current (Amps) or Power (watts). I would like to thank you for making the stream about the issue. Even professionals are learning.

I went to the website and noticed the spec page on all their inverters shared a common rating. stated watts and surge watts. the surge was always 2x of the stated rated watts, so a 5K rated inverter was rated for a 10K surge. so if you apply a power formula ......volts X amps =P your 120 volts X 15 amps X 2 (for surge) should have taken up to 30 amps for more than a second. by the way the best way to meter test transistors with a meter is when they are removed from the circuit. also when you went line to line and had continuity you were done right there. the fuses didn't need to be replaced until a fault was discovered and repaired. also, if a 3K ran the saw all your time considering the amp surge on motor start was a waste on an 8K supply. the 8 is garbage. Cheers

I think it is reasonable to expect the manufacturers of this kind of equipment to make sure that the fuses work correctly under massive inductive overload conditions, if it was fused and designed correctly, the fuses would have popped, protecting the equipment.

There may be capacitors in parallel of the battery input of the on the inverter. That can give you a false reading that it is a short.

I bought two chinese made inverters. The first one was a 5000 watt and when I turned it on the fans and alarm came on without any load. After about ten seconds the fans started reducing their speed. Then there were sounds of small explosions or firecrackers and smoke until the inverter went dead. Upon further investigation, I found the Positive and Negative terminals were both bridged with the copper strip. So that was a manufacturing fault. The second inverter was 3000 watts. I hooked it up, turned it on and explosions again. I opened it up and found the positive and negative were bridged. So I would suggest that you check the terminals to see if there are bridged. I will not be buying any more chinese made inverters again. A technician who repairs inverters told me inverters made in Korea are very good.