Re: DC motor conversion
Posted: Thu Sep 09, 2021 7:14 pm
The next type of motor speed control to consider is PWM (Pulse Width Modulation). This is a means of controlling motor speed through a series of full power pulses, with varying width and frequency of the pulses. It provides very high torque for a wide range of motor speeds.
Treadmills usually come with some type of PWM controller, but I have a lot of questions about how these controllers operate. Based on what I've found so far, it is unlikely they are producing 100+ volt pulses of varying width at full current. I suspect these controllers effectively derate the motor with lower voltage, current, and limited pulse control. My suspicions are reinforced by the lack of reasonably priced PWM controllers that can manage the high wattage these motors are rated to consume. This includes commercial controllers that cost hundreds of dollars. They all seem to top out in the range of 500-750 watts, which means the motors couldn't be producing more than 1 HP.
There is another 'feature' of treadmill motors that is undesirable to many, a soft start control that doesn't allow the treadmill to start at high speed which can result in the user getting thrown backward at high speed and suing the treadmill company. If you are using a simple potentiometer as the input to the controller to adjust speed the controller will ignore the potentiometer and start at a slow speed. You must dial your speed down on the pot and then bring it back up again. This shouldn't bother Shopsmith users too much since we all dial our speed down to Low before turning off the machine. We do that every single time, right? Anyway, there are workarounds to disable that feature if you want to use one of those controllers.
I do want to point out that this area of electronics is outside of my background and experience so at this point I have made a lot of assumptions that require further investigation. I do have a salvage motor and controller now and tried out a voltage controller. As soon as I can find a 5K pot I'll try out the MC60 motor treadmill controller to see what I get. I will try to find some way to measure the output of this motor with the different controllers. I definitely to work out a way to apply a load to them.
So, depending on how accurate my assumptions are, I think the use of common PWM controller will limit power output of these motors to no more than 1 HP equivalence with an AC induction motor. This is mot much worse than a 1-1/8 HP motor, and probably will be better than the 3/4 HP motor in a Greenie. The high end of the speed range should increase, but I don't know how much. The low end should definitely do much better. I suspect 250 RPM is achieveable without significant further decrease in power, maybe less. This would be something like a derated version of a PowerPro, possibly useful for some.
Even with the limitations of a PWM controller the motors could still be switched over to operation at full voltage and full speed over 6000 RPM, producing I think at least 1-1/2 HP in equivalence to an AC induction motor. What a reasonable voltage and duty cycle is in this mode is unknown. I suspect the motors will overheat even with additional cooling when run full blast like that and voltage and power still need to be diminished for continuous operation.
I see three different ways of using a PWM controller in a Shopsmith with whatever power they can produce:
1) Simply use PWM to run the motor at approximately 3450 RPM. The result is unlikely to have any advantage over a Shopsmith with 1-1/8 HP motor. Still an option if you need an inexpensive motor replacement.
2) Use PWM to control motor speeds in conjunction with the Shopsmith conventional belt and pulley speed control. This should extend the speed range at both the high end and low end, with considerably lower speeds possible.
3) Use a PWM controller for all speed control and remove the conventional belt and pulley speed system. This option leaves more space in the headstock for the electronics. The speed range should extend both higher and lower. Overall performance is an open question.
Any of these options could also have the secondary mode of running the motor at high speeds with full voltage to produce more power.
Treadmills usually come with some type of PWM controller, but I have a lot of questions about how these controllers operate. Based on what I've found so far, it is unlikely they are producing 100+ volt pulses of varying width at full current. I suspect these controllers effectively derate the motor with lower voltage, current, and limited pulse control. My suspicions are reinforced by the lack of reasonably priced PWM controllers that can manage the high wattage these motors are rated to consume. This includes commercial controllers that cost hundreds of dollars. They all seem to top out in the range of 500-750 watts, which means the motors couldn't be producing more than 1 HP.
There is another 'feature' of treadmill motors that is undesirable to many, a soft start control that doesn't allow the treadmill to start at high speed which can result in the user getting thrown backward at high speed and suing the treadmill company. If you are using a simple potentiometer as the input to the controller to adjust speed the controller will ignore the potentiometer and start at a slow speed. You must dial your speed down on the pot and then bring it back up again. This shouldn't bother Shopsmith users too much since we all dial our speed down to Low before turning off the machine. We do that every single time, right? Anyway, there are workarounds to disable that feature if you want to use one of those controllers.
I do want to point out that this area of electronics is outside of my background and experience so at this point I have made a lot of assumptions that require further investigation. I do have a salvage motor and controller now and tried out a voltage controller. As soon as I can find a 5K pot I'll try out the MC60 motor treadmill controller to see what I get. I will try to find some way to measure the output of this motor with the different controllers. I definitely to work out a way to apply a load to them.
So, depending on how accurate my assumptions are, I think the use of common PWM controller will limit power output of these motors to no more than 1 HP equivalence with an AC induction motor. This is mot much worse than a 1-1/8 HP motor, and probably will be better than the 3/4 HP motor in a Greenie. The high end of the speed range should increase, but I don't know how much. The low end should definitely do much better. I suspect 250 RPM is achieveable without significant further decrease in power, maybe less. This would be something like a derated version of a PowerPro, possibly useful for some.
Even with the limitations of a PWM controller the motors could still be switched over to operation at full voltage and full speed over 6000 RPM, producing I think at least 1-1/2 HP in equivalence to an AC induction motor. What a reasonable voltage and duty cycle is in this mode is unknown. I suspect the motors will overheat even with additional cooling when run full blast like that and voltage and power still need to be diminished for continuous operation.
I see three different ways of using a PWM controller in a Shopsmith with whatever power they can produce:
1) Simply use PWM to run the motor at approximately 3450 RPM. The result is unlikely to have any advantage over a Shopsmith with 1-1/8 HP motor. Still an option if you need an inexpensive motor replacement.
2) Use PWM to control motor speeds in conjunction with the Shopsmith conventional belt and pulley speed control. This should extend the speed range at both the high end and low end, with considerably lower speeds possible.
3) Use a PWM controller for all speed control and remove the conventional belt and pulley speed system. This option leaves more space in the headstock for the electronics. The speed range should extend both higher and lower. Overall performance is an open question.
Any of these options could also have the secondary mode of running the motor at high speeds with full voltage to produce more power.