Please refer to the figure below, which is from R. Krishnan's book on "Electric Motor Drives: Modeling, Analysis, and Control". The question is about the block labeled "Field Weakening". The block takes the measured rotor speed and issues the flux linkage command. It is evident that if the speed is less than or equal to rated, the flux is kept at its rated value. However, for speeds higher than rated, the flux should be reduced to run the machine in a constant-power mode by field weakening.
I am seeking an equation to compute the flux command from measured speed in the flux weakening mode. I am assuming machine ratings or parameters could be required to compute the flux reference above rated speed.
Your help is greatly appreciated. Thank you.
Regards,
Mohamed
Hello Mohamed,
I think that you need to take the machine equations and solve repetitively for the best flux as a function of RPM and torque desired. If you have it available, it is good to include the effects of saturation if possible, as this has a pronounced effect at higher currents. There are numerous papers and patents in this area, and a quick search yielded the following paper: http://ev.fe.uni-lj.si/4-2003/ljusev.pdf
Unfortunately, it is not a straightforward thing to figure out. If you are in an experimental mode and have the machine, a dyno, and the controls available, you can determine it experimentally.
Thank you so much, Roy. I am actually simulating in Simulink/SimPowerSys environment. I don't have the privilege of experimental measurements. The other thing is that I am simulating a flywheel driven by 3-ph asynchronous machine. Therefore, the moment of inertia of the mechanical system is very high, and the load torque requirement is very low. That is what is special about my system.
Thanks a lot for your reply and suggested reference.
The easiest way whether practical or simulation is to measure the no load or magnetising current. This will become the d-axis current reference required to establish the correct stator/rotor flux.
When one enters the field weakening region the d-axis current reference is simply reduced inversely proportional to the speed as is done in a DC machine when in the field weakening region. Thus we have
Wr Ws Id = Imag-no load/Wr
You can do this in a simple equation block where option is based on input speed as per the diagram.
You may also use a look up table but this should do.
Note in practice due to control lag the d-axis current is reduced earlier than Ws for fast rotor transients.
Hope this helps.
Fref = Fr when Wrwb
where Fref=reference flux
Fr=rated flux
wr=rotor speed
wb=base speed
Dear Awadallah,
Did you found any answers to your questions on Research gate ? I'm also working on a IM control under high speed operation, and I can't manage to find the right equations to simulate the field weakening bloc. Any help Please ?
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