Why Are Capacitors on Motors? What is Capacitive Reactance and Inductive Reactance? 

THIS IS NOT HOW A CAPACITOR HELPS A MOTOR START. I apologize for using capital letters. My intention is not to yell. Only to draw attention, so maybe you'll reconsider making another video or updating this one, so in the future it doesn't confuse others like it confused me. I watched this video for the first time about a year ago, and after watching it a couple of times, I was confused about how a start capacitor helps a motor start. After taking some classes and learning how capacitors truly help a single phase motor start, I came back to watch your video again to see what caused the confusion the first times I watched it. After watching it, I realized that you don't use a start winding in your presentation. In a single phase capacitor start motor there are 2 windings: start/auxilliary winding and main/run winding. They are physically placed 90 degrees apart from each other inside the motor housing. The start capacitor is put in series with the start winding and a switch (usually a centrifufal switch). When you start the motor, the start capacitor (when sized appropriately) helps increase the phase angle difference between the current going throught the main winding and the current going through start winding to 90 degrees. This 90 degree phase shift in current between the start and main winding, along with the 90 degree physical spacing between start and main winding inside motor housing is what helps the motor start. The current first passes through the start winding and it creates a magnetic field that makes the rotor begin to rotate. Then 90 degrees later current passes through the main winding and it creates its own magnetic field that helps the rotor continue rotating and accelaratring until 90 degrees later when the start winding's magnetic field helps the rotor accelarate even more. This back and forth dance between the start winding and main winding will continue until about 75% motor speed. At this point the centrifugal switch will open and cut power to start winding. I hope you don't take this comment the wrong way. It's only constructive criticism. Nothing else. Thanks for your time!!!

As a HVAC Tech, your content always help me to better understand what I'm working with.

You got the A/C capacitor info wrong. A 45/5 capacitor is for the motor and the fan. It doesn't start at 45 then drop to 5. The 45 is in line the entire time for the compressor and the 5 is for the fan the entire time. Some units actually split those out. Also, and it was not covered in this video, you can have a soft start unit that ADDS a capacitor for a very brief time to increase capacitance, which increases delay, which helps get the compressor motor turning. Then it drops out leaving you with the original 45uF run capacitor. So this teaches us that the 45/5 are dual capacitors running two different motors constantly.

Not sure about others but I feel like this is one of my favorite videos you’ve done as far as simplifying an explanation.

At around 7:00 you gave the most tangible, understandable explanation of lag on capacitors and inductors I've ever heard. It's such a hard-to-explain subject that it helps to picture myself as an electron getting pushed back by a magnet, ala magic school bus, or to visualize the potential of a capacitor filling up and releasing. Another good analogy for a capacitor is a water tower.

Ole ELI the ICE man!!! I learned about ELI & ICE when I went through advanced electronics school in the Navy back in 1985!!! lol

Look into the function of the Start Winding in split-phase motors like PSC, CSIR, and CSCR. (Permanent Split Capacitor, Capacitor Start Induction Run, and Capacitor Start Capacitor Run) Adding this to your already very good explanation will make it more complete.

Fantastic explanation man. My dad was a 40 yr master. Wish I had his knowledge. You do a really great service to up and coming electricians. Thanks for your knowledge to pass on

The problem with starting an AC single phase motor, is there is no rotating magnetic feild as there would be in a 3ph circuit. So we need a second phase with the current out of phase - thats all the capactitor does, provides the start current winding out of phase to the run winding. Very simple. There is no storing or smoothing anything,

My "down in the trenches" lesson in motor capacitors came years ago when I had a customer who bought an old industrial two-phase drill press from a factory in Philly. I went to a motor shop to see about getting a single-phase motor to replace it and the guy asked "Why would you do that? Just add a capacitor!" He explained how a lot of motors we use now actually have two-phase windings and the capacitor makes it work in a single-phase system. Once he explained the basics, I was able to build a timer circuit to pull the cap out once the motor got started and the customer got a really cool drill press to use in his shop.

I've studied electrical theory (basic) and am currently studying for my amateur radio license. I've always understood the basics of WHAT capacitors are, but not so much about HOW they work and WHY. This explanation has helped me greatly, Thanks!

For someone who has paid attention to electricity for over 20 years, you sure have elucidated quite a bit for me. I can say that the stuff you didn't elucidate I had already known. You have good explanations sir.

Love your videos and going into this topic! Comment on A/C condenser capacitor is a bit off: those are dual value run capacitors (around 14:30 in vid) 45/5 or 50/5. The larger value is wired to the compressor, the lower to the fan motor and there will be a common. On occasion you may find an additional capacitor if someone added a hard-starter to extend the life of a failing compressor. Point is, any of those type capacitors in a condenser are run capacitors. There is no such thing as a dual run/start capacitor. Run capacitors stay energized whenever the device they are connected to sees line power. Start capacitors are always switched off after a very short start cycle or they will fail. Your choices are simply run capacitor, dual value run capacitor, or start capacitor. The only exception are these multi-rated capacitors but those are strictly run capacitor that you use jumpers to in effect set the overall capacitance within the ranges available.

One of the best explanations ever for reactance and its relationship to motors! Thanks!

You explained a very complex issue simply and clearly.

Dude! Thank you I am studying for my electrical license and one of the questions I had were on induction/ cap reac. Thanks for going into more depth

Single and two phase need a capacitor to spin the motor in the right direction because even with two phases they’re 180 degrees out of phase so there’s no reference as to whether it’s clockwise or counterclockwise rotation. Don’t need a capacitor to start a 3 phase motor. That’s just my basic understanding of it.

I know it'a a year old, but love this video. Explains this motor story really well. Thank you!

Thanks for the amazing explanation. Love this channel. I'm IBEW 357 and when you going through the apprenticeship, this info is in one and out the other. After being a journeyman for a few years I want to go back and re learn everything. Thanks again for the great work your doing.

Finally... a good expiation of what a motor start and run capacitor does. I always wondered why the need for both (especially on HVAC equipment and anything with larger motors). :)

Great video. I use the power factor function on my meter to check single phase, dual capacitors all the time on residential condensers. It's quicker than killing the power to test it directly or using formulas while under load. Best part it actually tells you that the cap was sized right in the first place.

I think you're the first electrician I have seen that knows the science behind electricity 😅. I'm an HVAC technician. A pleasure to watch

I have inherited a grinder that I have to start by manually spinning the grind wheel in the direction I want it to go (I can decide, up or down). Now I understand why I have to do that. No capacitors. Thanks! edit: it's very old. Has oil wick lube for the bearings.

This is a great video!! You do a great job of explaining inductance and capacitance. Thanks for creating such helpful content for us electrical theory geeks out there.

this really helped. Thank you for spelling it out, Inductive reactance is where the voltage is leading, and current is lagging. In capacitive reactance is where Current leads and Voltage lags. In essence, in an inductive circuit, the amount of magnetic energy keeps things so bound up that it slows down the current flow. However, the voltage is still churning away, but the current is lagging. In a capacitor, when discharged, the positive and negative are just randomly kind of hanging out together. But when charged, those positive charges group together, as do the negatives, in a much more orderly fashion, ready to be discharged to do their work. But with them being so far apart now, current cannot get thru, hence the current LAG in capacitive reactance! In essence, inductive and capacitive are just polar opposites of one another.

My understanding is: power factor (PF) = RMS power of load (kW of actual power used) / avgVolts*avgAmps (kVA) A resistive load of a simple heating element or incandescent light bulb (old filament) has PF = 1 & current rises with voltage together, in constant proportion & peaks at the same time. A PF between 0 &

I really appreciate the CLARITY of this tutorial...THANKS

Just want to thank you for sharing your knowledge in an understandable way. I recently entered a career college for electrical technichian and the ciriculum is a circus show. I come home from school to watch your videos, we need instructors like you, thanks, God Bless.

Nice explanation. Thanks for sharing your knowledge.

Great explanations! These are concepts I know but you added insight and really good hand motions to them. 👍😁

Exactly the explanation I needed, from start to beginning. Cant thank you enough!

Brilliant explanation. I captured ELI the ICE man and can remember the leads and lags in capacitive and inductive circuits for the first time. Thank you so much for this video.

Thanks for all of information. Been messing with my condenser unit all summer to understand the difference between the start and run capacitors and realized the run capacitor was dying and needed to be changed.

Man!!!!! I can listen to you whole day. you answered all my questions in a single video. Thank you so much. And I also hit that subscribe button!!

thank you for making this video, this is one of the best explainations of Xc/Xl on youtube

Awesome video! I learned a ton. Thank you for putting these videos out sir!

Generally speaking, I found your explanation pretty informative. However, there are some things you should consider (possibly Semantics), but just to make sure it is clear: Current "appears" to flow, but doesn't actually flow "thru" the dielectric gap in a capacitor... I only bring this up because it was explained that way on the diagram in the video, so I don't want people to get the wrong idea. So, to be clear, Current doesn't flow thru that gap that you have in the diagram of the capacitor. Instead, current flows into 1 side of the cap (builds up ) and then returns out the same side... but NEVER "thru" the capacitor. AC can get away with "appearing" as current is flowing because of the way the charge moves into and out of "both" the sides of the plates, but in reality, electrons (and thus current) never cross the gaps in the plates (unless) the capacitor is bad or has leakage. Re: your motor diagram (@11:12), I believe your understanding of it is correct, however, it is important to show the second coil (run / start etc) in order for the capacitor explanation to really make sense, because the way it works, the capacitor doesn't work with 1 winding... it takes at least 2. So, I think people will get confused with your 1 winding diagram. By showing the 2nd winding, you can better explain how the phase shift affects the 2nd winding at a different time due to the capacitance, which allows it to push / pull on the motor at a different point in the rotation.

Glad I watched this. Needed to brush up on my engineering education. But electricians need to understand what Capacitors do to understand why you discharge them.

Awesome upload the whole lead lag has been a bore for me but now that you've explained it like you have it's now got me interested. Cheers

Finally i understand power factor now. I've always thought that: why laging or leading cause inefficiency but the electrical charges (energy) are conserved and not lost ? like a small battery as a buffering zone that saves the energy to use it later?! but now i got it, it is about the instantaneous Work done, Not the energy available within the system, because Power(w) needs both potential(v) and Current(A) available at the same time in order to actually do the work.

I wish I'd seen this when I took my engineering courses in electricity. Superb explanation!

Excellent explanation, its taken me years to learn this, not matter how often I've asked.

Such a great video. Takes me back to studying power 45 years ago!

Love how you explained that. Made it easy to understand

This answered my question. Absolutely.

My man is on another level in his teaching method, I just subscribed job well done!

i took EET 23 years ago and i've forgotten a lot of this, but this explanation was excellent!

Finally an intuition about inductance, thank you!

Great teaching and info. Thank you Dustin.

loving this channel - thanks for the info on this one!!!

Bro, you explained it so that I could understand. Oh my word, thank you!

As A EE - or was until I changed careers, you did a great job explaining these concepts. ELI and ICE. I remember those concepts all too well.

Great video for hvac and refrigeration techs

I’m in school right now and the lack of translation to real world application is hard to follow sometimes. Love your videos for breaking it down to a guy that’s good on the tools but at times struggles with the theory behind it.

Brilliant. Simply a brilliant explanation - as always!!!

Iam electric ingeneer for a long time but I never understood the subject as clear as you but it so I like to thank you very much and like to recommend this video to everyone wish to understand the subject thanks a lot ones. Mor. !!!!!

Great video. It's a good idea to look at the starting capacitor as "low gear" to get the motor spinning, and the run capacitor as an "overdrive" gear to improve efficiency.

Given this explanation, I wonder if this was part of what drove the 2004 blackout (untimely losses of generation in combination with heavy overall A/C demand throwing grid phasing out of whack)? I recall this phenomenon being mentioned in an explanation (Practical Engineering, I believe) so I’m curious if it was an unfortunate situation or a regular occurrence during hot summer months.

16:05 Start capacitor is in parallel. Run capacitor are in serie. But usually, there is only a start capacitor.

Good explanation, you keep make me love & feel proud of being technician!! Thanks

Great video, took me right back to advanced theory 👍

Excellent explanation sir. Paper mills are full of inductive loads. They use synchronous AC motors on some refiner’s to do power factor correction.

Such a good video, thank you!

Here is what is wild to think about... If you take an applied voltage and current and run them in a straight conductor from line to ground... You have created a "short" circuit, a fault condition. But if you take that same conductor and bend enough loops into, it there will be enough "pushback" from the magnetic field created to restrict the flow of voltage/current... The magnetic field created by the electricity can be sufficient to "slow the flow" of that same electricity. That "BONG!!!" noise you hear when you energize a transformer is massive amounts of current rushing through the windings because there is no magnetic field present for a few microseconds... As soon as the field builds it begins restricting the flow of current and the transformer starts doing its thing.

Thanks for the work great video

Nicely explained, manny thanks 🙏🏻

this is an awesome video. you have a very good understanding of what’s going on at the microscopic level. comment, though (i’m a physicist): in a capacitor, there charge rearranges via a “displacement current,” not by getting pulled to the plates. it is actually the changing electric field which creates an effective current. beyond linear reactance theory.

Another function of capacitors in parallel with motors is to straighten out the power factor, so that the load on the supply is a resistive load. When the capacitive load balances the inductive load the reading on the kWhr meter is minimal. In simple language a ballast capacitor reduces electricity bills.

Thanks for filling a lot of gaps, Edwin, Western Australia

Best video I have seen on this subject. Thanks

I explained to a coworker why a capacitor is used. I drew a sine wave like you did to show the phase shift. I explained it a little different tho since he hasn’t learned the theory part yet. I simply told him that we apply an external voltage to the motor to create EMF and turn the motor. When the motor is turning it creates a back EMF and the capacitor is there to overcome the back EMF that the motor itself is producing and thus putting it back in phase. And that’s why the voltage is higher at the cap then your incoming power.

You should discuss power correction factor, voltage correction factor, line conditioners, ferrite current filter resistance, buck, and boost methods.

All good info, as I'm doing more with AC induction motors these days, including having to replace capacitors. I knew what the numbers meant, but not what the numbers do. And now I won't be immediately tempted to get a higher uF capacitor when replacing. No longer thinking it's 'better' and sounds like it may make the motor inefficient, therefore making extra heat probably, as wouldn't it risk being out of phase then?

Dude, that was really good. Congrats. You're one hell of a teacher. I wonder, is that how transistors work at the microscopic level, they're a bit like capacitors?

@10:10 Solar panels are often used to add some capacitance to these customers to offset that power factor too. Here in Ontario, the majority of the Canadian Tire (our Hazard Freight) and Home Depots have a solar installation.

Wonderful explanation.. thank you man

Thank you so much, I am currently studying for my Canadian FSR-B ticket for Film and Entertainment Lighting.

13:00 You gotta watch out for those delayed dumps. Gotta keep them regular. 😎

pretty clear, thank you

Nice explanation , keep up the good work .

thanks so much man u teach me alot.

I love you, Dustin. You are by far my favorite youtube source for learning electrical skills. I've learned a ton of incredibly useful stuff from your videos. But your explanation of how and why single phases motors use run and start capacitors is, let's say, lacking. So here's my "more correct" answer...? :) :) :) Single-phase motors have the problem that they can run forward and reverse just as easily. three phase motors have a rotating field created by the three different phases, and will always rotate in the same direction when you start them. But a single-phase motor, with only one winding, doesn't have a rotating field,. It has a pulsing field. It can run in either direction just as easily. Every time you start them, which direction they start would just be random based on where the rotor last stopped and the phase of the power when you hit the ON switch. The ONLY reason we have run and start capacitors on single-phase motors is to force them to always start up spinning in the same direction! It wouldn't be very useful if your fan motor started spinning backward half the time! And if the rotor stopped 90 deg to the windings, it might only start after giving it a little push. So to make a single-phase motor spin the same way every time, a second winding is added to the motor (the start winding), and a capacitor is added in series with the second winding to shift its phase (as you explained in your video correctly). So now we have a two-phase motor, one phase for the main winding, and one for the start winding. But using a capacitor as a cheap way to create a second phase doesn't work very well. It only shifts the phase about 25 degrees in practice. In a three-phase motor, each phase is 120 degrees out of phase. Creating a second phase that is only 25 degrees out of phase is good enough to force the motor to start spinning in the correct rotation, but very poor for providing even power. In a "real" 2-phase motor, we would have the phases 90 degrees out of sync. But this "fake" second phase created with only a capacitor is good for getting it spinning in the right direction. This is why these motors all use a centrifugal switch that turns off the second "fake" phase windings once the motor is spinning correctly. The start winding and start capacitor is only there to make sure it starts correctly and is spinning in the right direction. In a motor with only a start capacitor, the start winding is very small compared to the main winding. It's just large enough, and so arranged, to help the motor start. It is not designed to keep running and will overheat and possibly burn out, if the motor fails to start (is locked up for some reason) or if the centrifugal switch is bad and fails to turn off the start winding. A bad switch that keeps the start capacitor engaged will cause the motor to run very hot (burn your hand if you touch it hot), and very rough. Many (I dare say most) single-phase motors don't have a run capacitor. they only have a start capacitor which is only used for starting the motor. But run capacitors are common in HVAC motors, so as electricians, we see them all the time. The purpose of the run capacitor is to make use of the start winding even after the motor is running. After all, they put all this extra copper into the motor, to make it start correctly, so why not try to make use of it to add a bit more power to the motor all the time? That is the goal of the run capacitor and the design of a single-phase motor that uses run capacitors. In this type of motor, the extra start winding is used all the time. But they use a large capacitance to force the start winding to have more power (and more phase shift) to help it start correctly but then lower the capacitance when running because the start winding can not sustain the high start current for long. But this allows the start winding to help add more power to the motor (which reduces the cost-to-power ratio of the motor) at the cost of greater complexity and maintenance costs (capacitors break faster than motor windings break and must be replaced). In these motors, both the start and run capacitors are connected (in parallel) when starting, but then the start capacitor is disconnected once the motor is spinning fast enough, leaving only the run capacitor to feed power to the start winding. In industrial applications, we just run all the high-power motors off of 3 phases and get rid of all this complexity of start windings and capacitors. Three-phase motors are dead simple and last a lifetime if they are not abused. But for residential applications, like HVAC, where we don't have 3-phase, power, we must play these games with capacitors to force motors to spin the right way every time they start. And we must pay the HVAC techs to come out and replace our burned-out capacitors every few decades.

"CIVIL" is what I was taught to remember. CIV -- capacitor I leads V, VIL -- inductor V leads I.

There is a formula to figure out exactly how much inductance or capacitance needs to be added to cancel out reactive power and make more efficient. As an Installer of course I dont do those calculations, but know of it and love learning.

Super awesome explanation!

Nice! Thanks for the explanation, appreciate that 😁👍

That makes so much sense. What a breathe of fresh air!! That's why the pg and e are making so much profit because theirs so much energy doing half the work thats provided. So we're consistently buying electricity for double the price. I get it now!!!! That's 🔥 knowledge !!! That we need people to wake up.

Oh goodness, apparently I’m in way over my head. 😅 I’ve been looking at an 8.5” diameter edge flow fan (“motorized impeller”?) that I want to build a fixture for to use as a sort of ceiling fan for a room in my house (pulls air from underneath, discharges air radially across ceiling), and I read something about needing a capacitor for it, so off I went to see if I could look up what kind of capacitor I need. Next thing I know I’m learning about run capacitors vs start capacitors and how doing things wrong can make things go boom, and all I want is to hook this little 120VAC, 400mA, 2700rpm edge flow blower up to a light switch in my house (maybe a dimming switch) so I can have a sleek ceiling fixture that quietly moves air about 530cfm.

Not only did you answer my question. You gave me a direction, in how electricity can work and does work. In our fascinating lives!!! Yeee ha !!✊

Thank you, go in depth! I have learned so much from your channel and I really appreciate everything you have put out.

It functions as a switch to start an AC motor. Using a switch has the same effect. On DC motors, connecting the - to ground, speeds it up to higher performance. For toys, capacitors are used for two directions.

That was a fantastic explanation.

The most common circumstance where motor caps are useful is where your supply is single phase power, feeding a motor with two windings (start windings and run windings). Capacitors are not used in three phase power because those motors are wound with three windings, and they automatically have a phase shift built into each of the three. Single phase motors have start windings to give mechanical advantage during the high torque startup phase. Motors built with only run windings would have difficulty starting because the magnetic field doesn't easily create a rotating magnetic field that the permanent magnet built into the rotor is chasing. By having two separate windings, you can use a capacitor to shift the phase of voltage feeding the start windings such that there is added starting torque no matter where the rotor comes to rest. From the perspective of the rotor, having a separate start winding allows the rotor to chase magnetic north, no matter what angle the rotor happens to be stopped at. Typical permanent split capacitor motors in HVAC applications have only a run capacitor in series with the start windings that is in line all the time. However you can increase motor torque and efficiency by using a separate run and start capacitor, both in series with the start windings. In that case they will have some mechanism to reduce current through the start cap after the motor spools up.

Great Job! Minor correction, It's 60 cycles, so voltage drops to zero 120 times in one second.

Nicely done ✔️

Is the run cap tuned to the inductance of the windings to form a balanced tank circuit or do they build for a minor inductance?

Having an Electrical Engg degree I already knew all of this and now after watching this I am confused.

Amazing video ,very important information

Thank you!

Pretty good job! BTW, a great example of a resistive load (where i is in perfect synchronism with v) is a plain ol resistive heating element…