Go Down

Topic: PMW voltage (Read 1 time) previous topic - next topic

Grumpy_Mike

I don't agree with that analysis.
That assumes that when driven with PWM the peak current is equal to the stall current, that is simply not the case.
It also assumes a motor with negligible inductance, again not the case.

dhenry

Quote
I beg to differ.


Your analysis is correct, if the motor is held motionless. Your analysis is wrong, if the motor is allowed to move.

Your analysis is correct, but you didn't state it, in that running a motor at 10v vs. 20v at 50% pwm does create different motor behaviors. You will find that at 20v % 50% pwm, the motor actually rotates faster than it does at a steady 10v.

dc42

#7
Oct 11, 2012, 05:17 pm Last Edit: Oct 11, 2012, 05:29 pm by dc42 Reason: 1

I don't agree with that analysis.
That assumes that when driven with PWM the peak current is equal to the stall current, that is simply not the case.


If the value of L/R is much less than the PWM cycle time and the motor is not moving, then it is very nearly the case (where L is the motor inductance and R is its resistance).


It also assumes a motor with negligible inductance, again not the case.


Yes, I was assuming negligible inductance. That is why I prefixed my post with "In the absence of substantial current smoothing due to motor inductance". The effect of the motor inductance will be to reduce the power dissipation; but unless it has sufficient inductance to substantially smooth the current, then the power dissipation will still be much greater in the PWM case. The power dissipation is the average value of I^2 R, and the average value of I^2 is greater than the square of the average value of I, unless I is constant.

Perhaps you have data on the values of L/R for typical motors that you would care to share?

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

dc42

#8
Oct 11, 2012, 05:19 pm Last Edit: Oct 11, 2012, 05:25 pm by dc42 Reason: 1

Quote
I beg to differ.


Your analysis is correct, if the motor is held motionless. Your analysis is wrong, if the motor is allowed to move.


I gave two examples: one with the motor held motionless, and one with it rotating at a constant speed and thereby producing a constant back emf. Would you care to explain what you think is wrong with my analysis of the second example? I am assuming negligible inductance (as Mike has pointed out), and I am also assuming that the back emf is constant, whereas in practice it will have a ripple at a frequency proportional to motor speed.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Go Up