Current Feedback in BLDC Motor Drive

[dpj]

Hi All,
I am working on a BLDC Motor Drive Circuit. The motor is having hall effect sensor for commutation. and PWM works at frequency of 20Khz

Want to ask, The current count received from Feedback section will be AC or DC? Can I used same count to process further or I need to convert it in RMS? Or Average of current will do??

Little confused in current feedback current sensing section.
Can anyone show some direction to work?

Regards

 

[alec_t]

Have a good read of datasheets for BLDC controllers. They should give you some pointers.

 

[strantor]

I assume you are referring to the current feedback for the controller's semiconductor overcurrent protection circuit? in-pulse current limitng? That will not be truly AC or DC; it will be pulsed, a DC value whlie the semiconductor switch is ON and 0 when it is off, 20,000 times per second.

If you're referring to motor current, I believe that is going to be a trapezoidal waveform above 0V. If you wanted some display of motor current for user reference, you could probably average this and display as a DC value.

 

[lebevti]

u need to take a whole curriculum of control classes...control is not simple

get a Masters in EE with a focus in control

 

[dpj]

Yeah..I have decided to average it and use it for PI Loop control!!
Thank you strantor!!

 

[dpj]

Hi Lebevti,
I have bachelors degree in Electronics and a working professional.
Do you know any online course on this particular subject??
Regards.

 

[strantor]

Yeah..I have decided to average it and use it for PI Loop control!!
Thank you strantor!!

don't forget about current limiting. If you're dealing with a motor that is of any practical size, the windings are likely as a direct short on DCR measurement. Also likely to have very low inductance, so current may shoot towards infinity as soon as your semiconductors switch on. In this scenario, you need to have very fast current measurement and protection logic to switch the semiconductors off, if their current limit is about to be exceeded (regardless of desired PWM 'ON' time).

 

[dpj]

Thanks for feedback strantor.
My current limit is in place.
Motor is started running without any problem, just feedback current sensing part need to fine tuned.