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?
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.
Rather than saying "the very low end of the PWM" you should state the RPM or percent of full speed at which you are attempting to operate. Operating a motor continuously at a very low speed can cause it to overheat. The motion of the rotor helps to move air through the motor for cooling. When the rotor is moving very slowly, the cooling is not as effective.
The speed controller will also have difficulty with maintaining a steady speed at a small percentage of full speed without tachometer feedback.
If you never need to operate at full speed, the best solution is to reduce the speed a little bit with a belt and pulleys. If you only need to operate at full speed occasionally, consider a stepped pulley arrangement. If neither of those alternatives is possible, you may need a tachometer, a closed-loop speed regulator and a separate fan or blower to cool the motor.