Electrical – BLDC motor/PMSM parameters at reduced operating voltage

brushless-dc-motormotorpmsm

I was thinking about nameplate parameters of BLDC motors/PMSM and wondered what happens to those values if I operate the motor at a lower voltage as rated.

Are my following conclusions correct?

  • If one reduces the voltage to the half of the nameplate value, Mr. Ohm tells us that the starting current is also halved. So I expect the starting torque to be halved as well. The same can be expected for the pull-out torque.
  • If the voltage is halved I only can reach half of the rated speed without field weakening.
  • The maximum power output is quartered because the voltage and the current are halved or the maximum speed and torque are halved.

Thanks for your help.

Best Answer

  1. No, as long the driver can supply the rated current, the torque is the same. The resistance is pure loss, the current regulator has to apply more voltage, since the back EMF voltage at zero speed is zero, then it should be no problem to supply the rated current at start (n=0).
  2. Yes, as the V_BEMF rises when motor is turning there is no longer possible to inject the current, when V_BEMF = Vcc
  3. P = M * omega, torque remains as is, the speed (omega) is halved, so the power is halved.