Drives School (E2) - Common mode explained

Common mode from drives and how to counter measure

[@AcrodesignerLNSNI]

There will be new episodes of Drives School that shows how to do ballasting of DC bus on complex hybrid ships. How to keep battery BMS and PLC control systems on the clean side and force the dirty common mode side of the many drives over to "loose ends" in insulating transformers and motor stator windings where they do less harm. Also the negative impact of stray capacitance on such systems

[@eriklenzing3288]

Under "Category" this very good video is labeled "Sports" just FYI....

[@MohdRizwan-of7ot]

Professor, can you please explain that how the use of an isolation transformer (at the front end) with a neutral point not connected to PE can eliminate the common mode EMI. Also, can you suggest some good reference for the same. Thank you.

[@k.kubendrankubendran4298]

very good explanation

[@AcrodesignerLNSNI]

Thanks.

[@butch2152]

Professor, I have these HF capacitors (Y capacitors) on my DC plus and DC minus lines going to earth. In your explanation I can see why wanting to keep the AC currents flowing away from the grid essential as required in certain regulations. But in my case if it is a HVDC battery, are these Y capacitors required and why?

[@AcrodesignerLNSNI]

Capacitors between the DC+ to PE and DC- to PE is called a ballasted DC system and the normal and right way to do it when there is battery connected to the DC bus. By ballasting the DC bus You will give priority to have a "clean" DC bus side of the drive and "dirty" AC side of the drive. Then it is very important that the AC side of the drive has a transformer with floating star point i the primary winding so the common mode voltages can rotate around freely there. And all RF capacitors must be removed on the AC side. The drive can have RF or ballasting capacitors only on one side. If You put on both sides the weakest of them will be removed in a cload of smoke as the circulating currents will burn it. Simplified it can be said that You have to choose between having a clean AC side or a clean DC side of the drive. You can't have both. The regular Active Front End drives for ship propulsion seen for many years does not need an insulating transformer because the clean side is selected to be AC and ship grid side. And DC bus is dirty but no problem because there is no battery there that to be harmed. Only a drive with a motor in the end that has a insulated start point. Dirty motor then. In case too much stray capacitance in the motor the motor bearings may crash but that is another story...

[@miksava]

Mr. Berntsen, could you explain why AFE drives does not need insulating transformer? Should I use HF capacitors there instead of transformer? If so, how will it work on ships where, as you said, there is floating ground?

[@laurentbouly7214]

Hello, very clear explaination. But you don't talk about common mode filter like ferrite cores, why?

[@AcrodesignerLNSNI]

laurent BOULY Because ferrite cores are not very effective for high power drives. You need so many of them. We do use them but then it takes a lot of space and installation. All 3 motor phases need to pass trough same ferrite core and if this is a 1000 ampere drive it is big copper bars

[@1965fw]

This rotating starpoint that can be measured at the motor, i would like to understand why it can be measured? I didn't expect that and it is different from what i always 'understood' about such floating zero. Then you explain the problems that can follow from capacitive coupling with the earth, like false earth fault detection and EMC etc. All very interesting, but i still don't know what is 'common mode voltage' now. There was no definition of what it means. You just explain some fenomeonon that can be noticed when using frequency drives. Is this the only situation where they use this concept?

[@AcrodesignerLNSNI]

Frank Weytjens Common mode exist in all applications were frequency converters and IGBT (big transistors) are used. Cheap physics from using 6 pcs of IGBT's to produce 3 phase AC voltage in any forms. Same for all vendors, Vacon, Siemens, ABB, Schneider, Danfoss -anyone. Not so many engineers that understand it fully though. I must understand it fully as technical manager and responsibel for all our customers to be succesfull with hybrid ship design. Your knowlegde about 3 phase systems are true for synchronous machine produced 3 phase neuutral star point. But for IGBT produced 3 phase it is not true until before passed trough an isolating transformer where the rotating star point can rotate freely inside the primary stator winding. The voltage between a rotating star point and PE can be a whopping 1500 volts AC.

[@AcrodesignerLNSNI]

If You want to see the star point on a motor on the oscilloscope the stator star point is not physically available. However, You can create a substitute just for measuring by connecting one 1 Mohm resistor to each motor phase and connect the other side of the 3 resistors together in one star point. Then the common mode can be seen by connecting the oscilloscope between this artificial star point and PE. Be sure that Your oscilloscope probes can cope with a voltage of 1500 volts. The cheap ones made only for electronics and 600 volts may blow up :-/

[@SagaSeaCraft]

Professor Berntsen... this is going to be interesting. Does your syllabus include providing examples on motor winding/wiring? Or at least the tools to calculate the windings/wiring? Thanks!

[@AcrodesignerLNSNI]

Maybe, then for 1 MW permanent motors used for ship propulsion and shaft generators. This video is a tester for putting training courses on youtube. Will be a series of this. For our customers, system integrators building hybrid ships. Mostly electrical engineers.

[@chhayaseng6304]

Thank you professor