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Topic: PMW voltage (Read 2864 times)previous topic - next topic

SouthernAtHeart

Oct 09, 2012, 02:00 pmLast Edit: Oct 09, 2012, 02:45 pm by SouthernAtHeart Reason: 1
Can I run a 12 volt motor off of a 24 volt power supply with adafruits motor shield, as long as I only use PMW from 0 to 50%?  At fifty percent, that would be half voltage, but I know the voltage is really just 'on' or 'off'.

dc42

#1
Oct 09, 2012, 11:10 pm

Can I run a 12 volt motor off of a 24 volt power supply with adafruits motor shield, as long as I only use PMW from 0 to 50%?  At fifty percent, that would be half voltage, but I know the voltage is really just 'on' or 'off'.

If the PWM frequency is high enough, then the inductance of the motor will smooth out the current flow and you won't be overloading the motor. However, the default Arduino PWM frequency is less than 500Hz, which probably isn't high enough to smooth the current out very much. So you may still overheat the motor unless you raise the PWM frequency.
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MarkT

#2
Oct 10, 2012, 12:24 am
Depends on the motor - larger motor can take lower freq PWM, but low freq PWM can lead to mechanical resonances.  Try it and come back if its an issue, the frequency can be changed.

And make sure the controller is up to the task for your motor
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Grumpy_Mike

#3
Oct 11, 2012, 12:37 am
As a motor is not sensitive to voltage you can use PWM on it without exceeding any peak rating. The heat will be spread over the thermal time constant which is long compared to the PWM frequency.
So yes you can.

dc42

#4
Oct 11, 2012, 10:12 am

As a motor is not sensitive to voltage you can use PWM on it without exceeding any peak rating. The heat will be spread over the thermal time constant which is long compared to the PWM frequency.
So yes you can.

I beg to differ. In the absence of substantial current smoothing due to motor inductance, the power dissipation will be greater with PWM compared to a steady voltage.

Consider a 10V motor with a resistance of 10 ohms. When stalled and fed from a steady 10V, the  power dissipation will be 10V*10V/10ohms = 10W. Suppose we now run it from 20V with 50% PWM. For 50% of the time the power dissipated will be 20V*20V/10ohms = 40W. The rest of the time it will be zero. So the average power dissipation has doubled to 20W even though the average torque is unchanged.

Now suppose the motor is running but loaded, so it is producing a back emf of 5V. When driven from a steady 10V, the power dissipated is (10-5)V*(10-5)V/10ohms = 2.5W. When driven from 20V with 50% PWM, for 50% of the time the power dissipated is (20-5)V*(20-5)V/5 = 22.5W. The rest of the time it is zero (the back emf is in the wrong direction for the flyback diode to conduct). So the power dissipation has increased by a factor of 4.5. In fact the motor torque will also be higher, because the average current has increased from 0.5A to 0.75A.

So I maintain that driving a motor with above its rated voltage and using PWM to limit the average voltage is only safe if the combination of motor inductance and PWM frequency is sufficiently high to substantially smooth out the current.
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Grumpy_Mike

#5
Oct 11, 2012, 04:43 pm
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

#6
Oct 11, 2012, 05:02 pm
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 pmLast 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?

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dc42

#8
Oct 11, 2012, 05:19 pmLast 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.

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