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# DRV8305: Back EMF Voltage Limit for FOC

Part Number: DRV8305
Other Parts Discussed in Thread: TMS320F28379D

Hello,

My motor has 21V DC Link Voltage from battery.

I want to run it at 4000rpm.

I checked my motor the back EMF waveform rotating it from external and measuring motor phase voltages (phase-phase), here is the plot at 2500rpm:

At 2500rpm peak-peak phase-phase voltage reaches 21V but rms is about 15V..

Does anyone know if FOC algorithm will handle to drive properly the motor above 2500rpm?

I am asking as according to those below equations everything is fine because in therm Ke*we Ke needs to be zero-peak value (not-peak-peak):

Also:

I know I can run motor when voltage difference between battery and motor back-emf is 0A, that's obvious.

But I will have 0A flowing when my motor back-emf RMS value reaches 21V, not peak-peak.

So it seems for me that even the peak-peak (and phase-phase) reaches 21V the FOC Algorythm should work properly (the current is not flowing only on back-emf sinus peaks, the RMS voltage difference is still more than 0V)

Am I right?

I am using classical FOC with SVM modulation and not considering field weakening, I want to ask if this work without field weakening.

Thanks for posting your questions to the motor driver forum!

Today is a holiday for TI in the US, we will aim to provide a response by the end of the week.

Regards,

Anthony

I am not sure if the amplitude of the back-EMF voltage being above the supply voltage would result in the FOC algorithm having issues driving the motor above 2500rpm, but I will check with others on my team to see if I can find an answer. One option if you want to test it out would be to order the DRV8353RSEVM and operate the EVM with the FOC algorithm that is available using the provided GUI. You could try to operate the motor up to 4000rpm to see if the algorithm can handle it.

Regards,

Anthony Lodi

I would appreciate if you check with others.

For now I am using TMS320f28379D+DRV8305N and have trouble at about 3200rpm

• Sounds good, thanks for the background. What sort of trouble are you experiencing at 3200 rpm?

Regards,

Anthony

• No idea, motor just going out of synchronization and stops.

My suspicion is that it could be because the PWM duty cycle might be approaching 100%, which then would result in the low side FET on time becoming too small to be able to sample the phase current. Could you check the duty cycle of the PWMs at 3200rpm to see if this is the case?

Regards,

Anthony Lodi

• Thanks Anthony.

Could you write some more what you mean by "too small sampling time"?

You mean the output capacitance of Current Sense Amplifier (or Input External ADC Capacitnace) does not  have enough time charge up to the steady voltage?

Or tme when low mosfet is on is lower than acquisition time of ADC ?

I'll check the duty cycle at 3200rpm and will get back

Thanks