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BLDC motor control

Question asked by vinod Karuvat on May 23, 2014
Latest reply on May 28, 2014 by Perla Andrea Moncada Fajardo

Hello,

 

I have a doubt understanding the exact commutation of the BLDC motors. My motor has 7 pole pairs. Now assuming the 3 phases are - U,V and W ( so thats U+,U-,V+,V-,W+ and W- from the MOSFETS). I am using the KL04 controller.

Now, the BEMF of the floating phase , is the indicator to be measured. Now, the BLDC motor property states - 1000RP/V. So, as I increase the voltage the rotation/speed increases.This voltage is increased via the PWM duty cycle. Now, if the PWM duty cycle is increase , we increase also the current through the motor, thereby increasing the thrust.

So, if I want the speed to remain at , say 3000rpm, and increase the thrust from , say 400g to 500g, I increase the current through the motor.

I do this by increasing the PWM duty cycle. But, this means the speed will also increase from 3000rpm to a higher value,right. How can this be maintained then

 

Next, i was moving my BLDC in a stepper motor like action. The disadvantages being that, it looses thrust as I am not firing my MOSFETs at the right interval. Now, I was studying the BEMF waveform for this stepper motor action in the BLDC. The BEMF is measured in the floating phase of the motor. For e.g - say U and V are supplied and W is floating. The duty cycle is 40%. I observed the following points in this stepper motor action -

 

1) The BEMF (at the bottom part of the PWM) on the floating phase shows a distinct sine wave action.

 

2) This sine wave keeps repeating (almost 3 complete cycles) with 3 peaks and 3 valleys.

 

3) The speed is directly proportional to the amplitude of the BEMF. So , from point 2 I understand that the speed keeps rising and falling.

 

4) After a series of rises and falls the control goes into the next state.

 

My doubts -

 

1) in the 1st cycle, when the speed is max (highest amplitude of BEMF), will commutation to next state ensure no loss in speed. I think that wld be fine.

 

2) Regarding thrust. I need the thrust to be constant. Would proper commutation as stated in point 1 above ensure smooth thrust.

 

3) So for greater thrust , I increase the PWM duty cycle. This will increase the speed and hence give me a higher amplitude BEMF. In my code via the ADC, I measure this amplitude and then commutate to next state at the highest ADC value. This will increase speed as the slope and amplitude will be greater with increased PWM duty cycle.

 

Have I understood the points correctly and am I on the right track please.

Kindly advice me pls.

 

Vinod

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