why motor power loss is different during motoring and regenerating?

[fhuang]

It was realized that both 30kW induction and 100kW PM motor have different power loss during motoring and regen. It seems that regen has about 10% higher loss. The measurement was carried out such that DC bus voltage and current were controlled at fixed value, for motoring the dc current is out from the source and for regen it is back to the source. The motor shaft was locked at a fixed speed(400rpm), motor torque was measured with torque transducer.

Motor loss = abs(Vdc*Idc - Torque*Speed)

Notes:
1. motor winding thermal effect is not of concern, it is controlled in the same range
2. with open loop control, if 500Nm is commanded, the motor torque is lower than 500Nm when motoring and higher when regen, WHY?

 

[CJCPE]

Calculating electrical power in/out of the motor using Vdc*Idc neglects the power losses in the output section of the drive.

Error in measured vs commanded motor torque is due to error in estimating torque based on electrical measurements in the drive.

 

[fhuang]

CJCPE:
The whole question is about why the loss (motor + inverter)is different for motoring and regen.

For the same commanded torque, why measured torque using torque transducer, not estimated, are different for motoring and regen with open loop control?

 

[CJCPE]

fhuang:

I see your point regarding the difference in motor + inverter loss. Regenerated power results in more current through the antiparallel diodes and less through the transistors, but I think that any resulting difference in losses would be contrary to what you have observed. There might be some increase in inverter losses during regeneration due to operating at a slightly higher bus voltage. What seems most likely is that the motor is more efficient operating as a motor than as a generator. I have heard that is the case, but I don't know why.

Maintaining measured torque equal to commanded torque with open loop control depends on estimating actual torque based on electrical measurements. The error in the estimation is evidently different for motoring vs regenerating. Perhaps that is just a restatement of the original question. The answer probably lies in the details of the measurement and estimation methods.

 

[sreid]

When you are calculating Vdc x Idc are you integrating instantaneous values? The DC current is always chopped in a PWM amplifier and the Bus voltage is much different during motoring and regeneration.

 

[aolalde]

I assume that for your test a second motor was coupled to the motor under test to provide the torque. Normal regeneration takes the kinetic energy from the load (driven machine) and motor mass only.

The main difference I see is a larger electromotive force induced in the armature as generator, this is required to provide the very same line voltage (VL). When the speed is kept constant, higher magnetic flux and an increase of the magnetic loss result.
E = k *Phi*rpm
As Motor: E = VL – Ia*Ra
As Generator: E = VL + Ia*Ra