Hi everyone, I’m a bit confused about the currents in my FOC-controlled PMSM AC motor.

At maximum speed and with a load connected, I measured the following currents using a current probe:

• Idc,bus = 1,3 A (current on 48V DC bus)
• Iphase,rms = 2 A (rms current on the phase)
• Iphase,span = [-3,7 .. 3,9] A (rms span/range current on the phase)
• Iq,span = [1 .. 3,9] A (quadrature current, I measured this value in the firmware)

By the term "span" I mean the maximum and minimum values (in square brackets), so roughly the range.

The current in quadrature is not "constant" because my motor has two rollers that press alternately against the load... so we can say that the current (and therefore the torque) is variable, but that shouldn't be the point of my query.

1) If I set Iq,max = 20A via firmware .. will Iphase,max then be 20A? Or are these two independent values?

From my measurements, it seems the answer is "yes, the maximum values are the same" … but I’d like to hear the opinion of someone who knows more about the subject. To confirm this, in the event of a motor stall, one of the three phases has reached exactly the Iq,max value (the others have stopped earlier at different values, as is to be expected)

2) So, if it is true that Iq,max = Iphase,max … does this hold for all other values? Such as the average, the RMS and the minimum values? … for example, for the minimum value, the equality no longer holds because Iq cannot be less than 0, whereas (as you can see from my measurements) Iphase,min is negativ

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

Hi, thanks for your reply.

I’ve added a few more details (in brackets).
I’m asking if, in Field-Oriented Control and PMSM, the quadrature current is the same as the RMS current measured on the phases

 

I think that for the torque to be consistent the currents will need to be the same.

 

Not all of us are conversant with all of the abbreviated terms that you have used in your inquiry.
But it seems that you are asking if the currents in a split phase motor are equal. In theory they should be the same, or at least fairly close.
There is a similar class of motors that are reversable, with one phase powered directly and the second, identical winding, fed thru a series capacitor. Those motors are reversable by simply changing which winding is directly powered.

In a 1ph / split phase cap start motor, under 1/2 HP are likely to posses identical windings, these are capable of using the series cap with either winding , over this size of motor , the start winding is invariably quite higher resistance than the main run winding, and the only one to make use of the start cap.