Electronic – BLDC speed and torque control

brushless-dc-motor

I have a question understanding the exact commutation of the sensorless BLDC motors.

My motor has 7 pole pairs. Let us assume that the 3 phases are $U,V and W$ ( so that's \$U+,U-,V+,V-,W+\$ and \$W-\$ from the MOSFETS). Now, the back EMF (BEMF) of the floating phase, is the indicator to be measured. Now, the BLDC motor datasheet states \$1000 \;RP/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 increased, we also increase the current through the motor, thereby increasing the thrust. So, if we want the speed to remain at, say \$3000 \; rpm\$, and increase the thrust from, say \$400\;g\$ to \$500\;g\$, we increase the current through the motor. We do this by increasing the PWM duty cycle. But, this means the speed will also increase from \$3000\;rpm\$ to a higher value, right? How can the speed be maintained while increasing the thrust?

Next, I was operating my BLDC in a stepper motor like action. The disadvantages being that, it loses 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 example, 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.
  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 would 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. 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?

Best Answer

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?

You would only want to increase the torque (thrust) if the mechanical load were threatening to slow the motor down - by increasing the PWM to counter that increase in load torque you are largely keeping the motor at constant speed. Remember a simple DC motor (even a brushless type) has a very clear torque-speed characteristic that means, for a given speed at a given torque, an increase in torque of \$x\$ will reduce the speed largely by \$x\$ also.

It's a not a perfectly linear relationship and changes entirely when field windings are implemented but it's fairly reliable: -

enter image description here

Related Topic