Electrical – PWM 12V DC motor using lower supply voltage for lower speed operation

motorpwm

I am using a 12vdc PWM to control the speed of a 12vdc motor that can draw up to 25 amps of current. The speed I usually run the motor is at the very low end of the PWM which makes it very sensitive when controlling the speed. Can I reduce the voltage going into the PWM so the output range is between maybe 0 and 7 or 8 volts instead of 0 to 12 volts? Will this damage the PWM or the motor?

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Can I reduce the voltage going into the PWM so the output range is between maybe 0 and 7 or 8 volts instead of 0 to 12 volts?

Yes, but reducing it to 7 or 8 Volts (a 33-41% reduction) might not be enough if you are running 'at the very low end of the PWM'.

Will this damage the PWM or the motor?

Provided that the PWM controller still works properly on the lower voltage and the motor wasn't overheating on 12V, it should be OK. It might actually be better because now you will be using higher PWM ratios which are more efficient (ratio of peak to average current is lower, so rms heating is less). Dropping the voltage too much might cause the motor to stall at high loading, which could burn out a winding due to the higher than normal current with no cooling.

Ideally you should choose a supply voltage that just gives you the maximum speed required at 100% PWM. If this is well below the motor's rated voltage then it might be better to gear it down, as this will give you more torque and higher efficiency. However if you don't need the best possible performance then running at a lower voltage is fine.

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Control hacked servo motor speed using L293D and PWM

Based on your (further) info, if there is no control wire, I think the reason it is not moving properly is that what you have now is basically a DC motor. To control a standard DC motor with PWM, you simply use a duty cycle between 0% (off) and 100% (full speed)

The current needed will be a lot higher than a servo control wire, as rather than controlling the feedback, you are controlling the actual power to the motor. This is okay as you have the L293D though, which will supply more current than a uC pin can, and exactly what they were designed for.

I suspect that the 2ms is too low to provide enough power to start the motor turning - if you have a 20ms period, 2/20 = 10% of full "power" (roughly - note actual power is I squared R, so at 10% will actually be 1/100th of power at 100% - RMS voltage would perhaps be a better term here) Try a higher value and see where the motor starts reliably. Let us know the results.

Here is a page that explains the differences quite well:

Dc vs Servo

Electronic – arduino – Motor speed control using arduino and quadrature encoders

It looks to me that your control loop is essentially:

if speed is too slow:
    increase PWM duty cycle one unit
if speed is too fast:
    descreate PWM duty cycle one unit

As you have observed, this doesn't work so well. Your control loop cyclically overshoots the target speed.

The canonical solution to this sort of problem is a PID controller. The concept is essentially the same, measure a thing and compare it to a target to calculate an error. Then adjust something (in your case, PWM duty cycle) based on the error.

However, a PID controller has three error terms:

P: the current error
I: the integral of the error
D: the derivative of the error

For each of these terms, the controller has programmed some gain. It then multiplies each term by the respective gain to calculate how much the control input (PWM duty cycle) should be changed. Properly tuned, this allows you to get a feedback loop that at first ramps up the duty cycle, then as your vehicle approaches the target speed, backs off smoothly so you get a minimum of overshoot.

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