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Field Weakening for PMSM motors

Rushi Jariwala
Expert 1225 points

Hi,

I see TI has a solution for field weakening along with MTPA. I was trying to understand field weakening in detail. 

As I understand, as the voltage limit exceeds, the id component is increased to reduce the next flux in air gap - that reduces back emf. So, does that mean we are drawing more electrical power to give the same mechanical output power in field weakening region? 

Am I correct to understand that the Id component of current gives no useful output power and is only used to decrease the net flux? Iq component remains the same (limited by the max Is and Id)?

If that is the case, why is flux weakening so common in traction drives. Why we cannot have base speed up to the max required speed. Is it limited by motor limitations?

Also, the document Sensorless-FOC With Flux-Weakening and MTPA for IPMSM Motor Drives mentions that the field weakening algorithm mentioned is applicable to an electric compressor, fan and pump that do not need high dynamics. What happens in traction drives where high-speed dynamics is the case? What care do I need to take to have a good operation in field weakening region for high-speed dynamic operation? 

 

  • 0
    TI__Guru** 115420 points
    1. Correct, Id is used to decrease the flux to improve the maximum running speed of the motor, it can't increase the torque, even limit the output torque. The output power depends on speed and torque.
    2. Right. Iq is limited to maximum current and could be decreased if Id increased during the field weakening control. So the output torque will be decreased and motor worked in constant output power mode.
    3. In most case, to get a high power converter efficiency, traction doesn't work in FWC mode, FWC is just for high speed running without a heavy load. This is a limitation of the motor spec. and the input voltage.
    4. It's a very complex topic, as mentioned above, FWC is just used for some applications that the motor needs a high speed for some time, but the input voltage is not enough. The FWC could be also used in Traction application, but it's very difficult to get a good performance since the speed traction needs to be changed very quickly, the machine could be a motor or a generator, which works in four-quadrant frequently.
  • 0 in reply to Yanming Luo
    Expert 1225 points

    Hi Yanming,

    1. Thanks for answering and solving some of my doubts. There is still one thing that I do not understand. In field weakening, Id becomes negative and decreases flux. At the same time, you suggest decreasing Iq also to maintain the same output power. But is not the torque dependent on both Iq and flux? In this case, both flux and Iq are decreasing (with an increase in speed), still, the output power remains the same? At what rate is the speed increasing and at what rate is the torque reducing. I understand it will be a bit mathematical but for output power to remain same, the rate of decrease and increase in torque and speed respectively should be the same, is my understanding correct?

    2. In the application of high-performance e-bikes, it is likely the speeds will go up to 80-100kmph (49-62 miles per hour) and even more. In that case, what would be the best mode to operate the motorcycle - in FWC or below base speed?    

  • 0 in reply to Rushi Jariwala
    TI__Guru** 115420 points
    1. Not increasing Id, must decrease the Iq at the same time. The torque current (Iq) depends on the speed and load, will be limited to the maximum current of motor and input voltage, Id^2+Iq^2<Imax^2 and Vd^2+Vq^2<Vmax^2. So the Iq will be decreased for the increasing Id in deep filed weakening control.
    2. That depends on your requirement and motor spec. To get a higher power efficiency, the FWC is not used in most e-bike and traction applications since the rated frequency of such motor is enough to provide a high-speed output.
  • 0 in reply to Yanming Luo
    Expert 1225 points

    Sorry Yanming, but I did not understand the first point - what is the rate of torque reducing and the rate of increase in speed?

    Is it right to understand that, in field weakening mode, to give the same mechanical output power, we are using more electrical energy (to weaken the flux)?

  • 0 in reply to Rushi Jariwala
    TI__Guru** 115420 points
    1. That depends on the motor and input voltage. Three operation regions for the IPMSM can be often defined, Constant Torque, Constant Power, Constant Voltage. When current constraint circumference is reached, the torque output is reduced in field weakening operation in constant power region.
    2. Right. The output power will keep constant, speed increase, torque decrease, input power increase, motor energy efficiency decrease.
  • 0 in reply to Yanming Luo
    Expert 2550 points
    That's a good discussion about FWC.