# Expectations of PWM for wiper motor



## duke000088 (Sep 1, 2013)

Last season, I tried my first animated prop - and failed. I attempted a simple trash can pop-up using a wiper motor. I purchased the whole sheh-bang from Monster Guts:http://www.monsterguts.com/store/product.php?productid=17761&cat=3&page=1

This is their package with the package with the simple PWM and power supply.

I was under the impression that the PWM would allow me to slow the RPMs while maintaining normal torque to the motor. In my case, I found that the motor torque plummeted when I ran at relatively low RPM (which is what I desired for the pop-up/down effect). At near normal RPM, the motor could lift my mechanism and trash can lid... but at low RPM, it could could not.

Are PWMs very variable in their performance? Do you get what you pay for? Will investing in a higher quality PWM yield high torque at low RPM?

This year I am revising the mechanism to support the rigor of higher RPM but I would be willing to invest in a better PWM device if it would mean I can salvage my wiper motor for this purpose.

Thanks folks


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## DarkOne (Oct 16, 2012)

You're going to lose torque when the motor slows down. The only way around it is using a gearbox or gearmotor. I've experienced the same thing using PWM with a wiper motor.


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## duke000088 (Sep 1, 2013)

Gearmotor... yeah - that's where I'm heading now: https://www.sparkfun.com/products/12154

thoughts?


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## DarkOne (Oct 16, 2012)

I think the torque is there if there specs are accurate. That's a really slow motor! 
And so tiny! Let us know if it works out and please post photos or video of the finished project.


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## skidrow (Feb 21, 2013)

My guess is that the frequency of your PWM controller is too slow, which would manifest itself as lower torque at slower RPMs. I have used other PMW controllers with a windshield motor and a good power supply and I did not notice any loss of torque at low speeds. Does the PWM controller you got come with any specs or manufacturer/model info so we could attempt to find this info? I doubt that a different motor will make much difference as the windshield motor is also a gear motor.


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## duke000088 (Sep 1, 2013)

skidrow - thanks for the input. 
I used the Monster Guts PWM that came with their wiper motor package: 
http://www.monsterguts.com/store/product.php?productid=17741&cat=3&page=1

It is certainly a bare-bones PWM... the economy model. I did not realize there would be such an effect from the PWM frequency. From the specs on Monsterguts.com, I cannot determine what frequency it is running at. But I assume the take home message is: a higher quality PWM will yield better results.

I am starting Take 2 on my wiper motor trash bin prop now. I'm strongly considering coupling the wiper motor to a pinion/spur gear setup to give me a solid boost in torque and a major reduction in RPM. I'm going for a slow "peek-a-boo" pop up out of my recycling bin. I'm shooting for only a few RPM anyway... so maybe gearing the wiper motor down further will get me there. Plus it should make for a more robust lifting strength.

I would love to hear what others recommend for a high quality PWM???


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