Well, your demonstration is very systematic and well designed. Your hard work and interest in teaching make the listeners benefitted, wish you good health and good life.
Hi I like this simple and very good explanation of Common Mode Current Source. If there is an interest, I can give some Information for suppressing Common Mode Currents by Nanocrystalline Cores (CoolBlue) which are working in the Motorcable as Inductive Absorbers.
Hi Hans-Joachim, thanks for your reaction. We do supplyh high-frequency common mode cores that reduce electromagnetic interference and protect against bearing currents.They are light blue coloured. Is that the ones you are referring to? Rgds Hans
Yes, they are already an Option for your drives and I helped to design them in Denmark ;=) Some background regarding Impedances, Absorption the improved design is available. We do have now improved versions of it they do have a (+) at the Number. If you are interested we can have a phone talk about the possible Improvements together with NALa which are line Absorbers for reducing the ringing of dU/dt of the IGBT´s. BR Hans-Joachim (Alias Jochen ;=)
Okay Jochen, I am not very much into this (as a simple digital marketer), so I don't know much about it. I am also not really sure who in our organization is dealing with this. Are you able to give me your contact details. Then I will try to find and inform a couple of people and advise them to get in contact with you. Rgds Hans
At this time the development was done with Norbert Hanigovszki, my contact is hpoess@magnetec.de. If there is some interest, feel free to contact me or to share my contact data. more info´s under www.magnetec.de/fileadmin/pdf/pb_cb.pdf Rgds Jochen
at time 11:30, it says that if power source is not grounded like ship, those Y high frequency caps are not allowed to be used or they will cause problems. Any explanation why? And how to reduce the common mode voltage seen from DC link +/- terminals to ground or from line A/B/C to ground at input of rectifier?
The problem is that the pwm outputs are not edequately filtered to reduce cost and vfd drive volume (size). With filtering the high dv/dt waveforms and edges are greatly reduced limiting the capacitive currents. Older drives had better filtering but were bigger and a bit more expensive.
Hello Roger. It is true that older drives had slower voltage rise times on the PWM pulses. And the issues with common mode and also voltage stress between winding layers were less with this older drives. However, the slower voltage rise times produced more heat losses on IGBT's. The drive needed more cooling for same motor power, thus bigger size drive and bigger losses. As evolution of drives are towards smaller physical size and less heat losses, the IGBT technology is pushed for higher voltage rise times. A modern drive with low losses will actually stress the motor and cabling with PWM voltage stress way more than an old drive. Thus the issues with both common mode and PWM voltage stress are more important to cope with today than previous years. Also modifying the IGBT switching pattern to produce a more balanced common mode increase the losses and decrease the efficiency of the drive. Which makes the drive less value for the money when valued only for power, size and efficiency. Simplified You could say that to produce a better drive we push more issues over to the motor. More need for robust motors and dU/dt filters, low stray capacitance cabling etc.
Hello Kamil, delta coupling does not change the commond mode issues in the motor much. Simplified You can look at all the copper in the stator windings as one big galvanic lump of copper where the 3 motor phases are connected to the drive. Measuring the DC resistance between motor phases it is small values in the 0,1-0,3 ohms range. So for DC voltage it is like one lump of copper. Impedance for AC current is of course high, otherwise it would be like an short circuit also for the base AC. However, regarding common mode which initially is a pure voltage phenomen, all the copper in the stator windings will be lifted to the sum of voltage in the 3 motor phases. And this sum of voltage keeps jumping around when measured towards PE ground, aka the metal casing of the motor. Regardless of delta or star coupled motor phase windings. To draw a leakage current between copper and PE induced by this common mode voltage the path between is the stray capacitance. Stray capacitance current will be an AC current with the frequency same as the drive switching frequency, typical 3-4 kHz. An AC current of that high frequency emits an magnetic field, EMC noise, that is harmful for other electrical equipment. So we do all kind of measures to reduce the negative effects. Reducing the stray capacitance to a minimum is important. Also keeping the 3 phases and all of its "lumps of copper" inside a faraday cage shielding. The shield, weather it is the cable shield or motor and drives metal casing, need to be bonded together galvanic to get best effect of the faraday cage.
Very informative video. Thank you. Can you please advise on a how common mode chokes installed in the VSD output effects the floating neutral (star) point in the motor? Will that reduce the voltage and current being capactively coupled through the motors frame and bearings? This is in reference to the Coolblue cores or other chokes and how they actually work to protect bearings.
When chokes are connected on the VSD output between drive and motor it is called a dU/dt filter. It is usually a 1,2% impedance choke with also some small RFI caps on the motor side. Purpose is to increase the voltage rise time on the Pulse Width Modulation (PWM). Unfiltered the voltage rise time is about 2000 volts in one micro second. Stray capacitance in cables and motor will react to this fast voltage change and create HF currents with several negative consequences. The dU/dt filter will be a low pass filter for this HF currents. On the motor side this will result in slower/ longer voltage rise time. Maybe to only 200 volts per micro second. Slower increase rate of voltage in each PWM pulse result in lower HF current leaking to stray capacitance. For the neutral star point in the motor it still is a rotating potential. However, with the slower voltage rise time the stray capacitance in the motor casing will not draw that much HF current as without dU/dt. Less currents flowing trough bearings, less stress on insulation, less HF EMC.
Danfoss Drives thank you. On another note. In regards to the floating neutral point in the motor. I assume this occurs with motors connected in star? How about motors in delta? Is there a floating neutral in a delta connected motor that has no star point?
Even though there is no physical star point in Delta there is an artificial one as formed by the summary of voltage potential in the 3 phases. So to speak the summary of voltage in the stator winding is rotating just like when in Star.
We have a installation with TNS system where common mode filter (ferrite core) is melting due to excessive common mode current. The VFD manufacturer is suggesting to convert TNS to IT system. Will it help? Since there is a PE conductor running from VFD to motor which will provide return path for common mode current. Pls. advise. It's an AFE drive.
Hi Ishaan Regardless of the type grid (TN/IT) you are using, a PE and screened cables are always required. Changing the grid type will not solve your problem. About the common mode filter/coils, it is essential that you use coils intended for this purpose. The common mode voltage is 3x Fsw, where Fsw is the switch frequency of the drive. This mean that we are dealing with a high frequency signal. Traditional common mode ferrit cores allow low frequency signals of 50/60 Hz to pass through, but filter out higher frequencies - if exposed to VFD output signal, coils will go into saturation and become really hot (burn). The common mode filter needs to be of nanocrystalline magnetic core type in order to allow high frequency PWM signal pass. More info: www.danfoss.com/en-us/products/ac-drives/dds/vlt-common-mode-filters-mcc-105/
@@danfossdrives Yes, the link shared by you is helpful. How to select the suitable model? In our application, Motor KW = 560 Voltage rating = 690V Cable size = 3 runs x 3C x 185 sq. mm Cable length = 450m (each run) Our VFD manufacturer has installed common mode filter on DC bus. He says common mode filter rating is 50A. They measured the common mode current with filter in place at DC bus as 150A. So we asked him to replace the 50A filter with 150A. But he says 150A filter is not possible to manufacture. Is it true? Can you pls. advise on this and also which model of Danfoss MCC 105 to select for above requirement..
Hi Ishaan I'm really sorry, but we can't offer such detailed support here on RU-vid. Could I ask you to contact your local Danfoss service office? You can find it here: www.danfoss.com/en/contact-us/contacts-list/?filter=type%3Adanfoss-sales-service-center%2Csegments%3ADDS
Hi Thank you very much for your input. For the moment, if you have a question about this topic, please reach out to your local service center: www.danfoss.com/en/contact-us/contacts-list/?filter=type%3Adanfoss-sales-service-center
Hello Nishan, emc filter on motor cables like dU/dt filter please see explanation how it works in one of the comments above here, from ibd1977. Another measure to mitigate negative effects of the PWM voltage rise time is to produce the motor windings with "form windings". Instead of spinning the copper wire randomly around each iron pole, the wire is installed more organized in separate layers. This will not reduce common mode, however it will increase the motor robustness for insulation failures. PWM voltage rise time impose a risk of getting high voltage differences between each strand of copper wire. The insulation is pretty thin on each wire so a high voltage difference can stress the insulation and risk flashing over. Mitigating this a dU/dt filter helps a lot. But an alternative to utilizing a dU/dt filter is to produce the motor with form windings to avoid wires with high voltage differences to be laid close to each other. We see form windings mostly on permanent magnet motors where total drive train efficiency do not like to include the losses of dU/dt filters. Losses on the dU/dt filter is the voltage drop of 1,2% multiplied with the current.
