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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\$\begingroup\$ The concept is perfectly OK, but there could be some chance that the controller needs a certain minimum voltage. Can you provide more detail about how the PWM is generated? Are you using some type of speed controller? Does it have a make and model or manual or datasheet? \$\endgroup\$– user57037Sep 5, 2016 at 17:00
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\$\begingroup\$ I'm using model MXA066 PWM DC Motor Speed Controller (12vdc 30 amp) that I purchased from Bakatronics to control the speed of a PA-17-8-2000 Heavy Duty Linear Actuator that I purchased from Progressive Automation. \$\endgroup\$– LewisSep 5, 2016 at 20:08
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2 Answers
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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.
answered Sep 5, 2016 at 19:15
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\$\begingroup\$ I'm using model MXA066 PWM DC Motor Speed Controller (12vdc 30 amp) that I purchased from Bakatronics to control the speed of a PA-17-8-2000 Heavy Duty Linear Actuator that I purchased from Progressive Automation \$\endgroup\$– LewisSep 5, 2016 at 20:08
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1\$\begingroup\$ The MXA066 is simple analog controller that uses a dual op amp to generate PWM. It is only rated for operation on 12V or 24V, which it regulates down 10V for the PWM generator. The MOSFETs need 10V Gate drive so you can't run it on lower voltage. The easiest way to get smoother control at low speed would be to replace the 10k linear pot with a 10k 'log' pot. This will still give you the full PWM range, but with the lower 10% expanded to about half the pot range en.wikipedia.org/wiki/Potentiometer#Logarithmic_potentiometer \$\endgroup\$ Sep 5, 2016 at 20:57
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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.
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\$\begingroup\$ I'm using model MXA066 PWM DC Motor Speed Controller (12vdc 30 amp) that I purchased from Bakatronics to control the speed of a PA-17-8-2000 Heavy Duty Linear Actuator that I purchased from Progressive Automation \$\endgroup\$– LewisSep 5, 2016 at 20:08
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\$\begingroup\$ I can't find any detailed info for that unit. But it will probably work below 12V. I would try it and see what happens. I think damage is very unlikely. Monitor closely at first (mainly watch out for excessive heat) until it has proved itself to be safe. \$\endgroup\$ Sep 5, 2016 at 20:29