Permanent magnet synchronous motor/generator (MG) operation

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Steve Smith
Pico Staff Member
Posts: 1312
Joined: Sun Aug 25, 2013 7:22 am

Permanent magnet synchronous motor/generator (MG) operation

Research into the operation of a 3-phase permanent magnet synchronous motor/generator (MG) fitted to a VW E-Golf raised a number of questions surrounding “typical” operation and circuit characteristics that I would like to share & discuss here.

Before we proceed, safety, certification, training and system knowledge are both essential and paramount as the captures below include “live working”

Regarding “system knowledge” the following Self Study Programs (SSP-530 & SSP-527) from VAG provide an invaluable insight into the description and operation of the high voltage system on board the E-Golf (N.B. SSP’s are no substitute for EV training)

Starting with an overview of the circuits measured; below we have our E-Golf pulling away from rest and then decelerating to a stop with “recuperation” level 3 activated (selectable by the driver)

I.e., the vehicle is decelerating thanks to the “loading” upon the “Rotor” which is functioning as a generator (The Rotor is connected to road wheels via the transmission)
Image 1
During the above test, a question was raised surrounding MG status, “how can we determine from the above captures when the MG is functioning either as a motor or a generator?”

I guess the simple answer has to be the HV battery DC current captured on channel F, here we have captured the precise point where current flow from the HV battery changes from a positive value (powering the Motor) to a negative value thanks to regeneration from our now Generator (see below)
Image 2
Note above how the positive DC current flow from the HV battery peaks at approx. + 91 A to power our motor and drive the vehicle. During deceleration the current flow is reversed thanks to our MG acting as a generator whereby we achieve a peak negative current flow of approx. – 50 A.

Around 12.5 seconds into our capture, another -16 A of current is delivered to the HV battery (total approx. -66.85 A) thanks to the application of the brake pedal providing an additional reduction in rotor speed and hence, increased current generation.

A quick tip here, note how channel F has 4 signal rulers instead of 2!

In order to utilize 4 signal rulers for one waveform, create a reference waveform which automatically hides behind the waveform of interest. The benefits of duplicating a waveform are the addition of 2 more signal rulers providing a means for multiple measurements of the same trace

For more information regarding reference waveforms, refer to the A to Z of PicoScope here https://www.picoauto.com/library/picoscope-in-depth and here https://www.picoauto.com/library/picosc ... -waveforms

To understand the performance of our MG acting as a generator, a deeper dive into the operational characteristics of a 3-phase permanent magnet synchronous motor can be acquired from the PicoScope data above and the theory