Torque. Reducing rpm in an universal motor

Thread Starter

Rufinus

Joined Apr 29, 2020
106
Hello.

I have got an universal motor from a washing machine. It´s rated for 220V and at this voltage gives 20.000 rpm. I want to use it in a DIY belt sander and I need to reduce the rpm. For example whit a dimmer.

Te question is: If I reduce the RPM, will torque be affected? And, what would be the best way to reduce the rpm in that motor?

Thank you

Best regards
 

AlbertHall

Joined Jun 4, 2014
12,023
Unless there is speed feedback from the motor to the controller then reducing the speed will also reduce the torque.

A simple method, if the motor supply is AC, is to add a diode in series. The motor will see a half wave rectified signal. The torque will be reduced as well as the speed. I have a drill which uses this method to give a lower speed option.
 

shortbus

Joined Sep 30, 2009
9,372
If I reduce the RPM, will torque be affected?
Yes. This is the reason a good belt sander uses pulleys and belts, not a variable speed controller. As a life long, machinist and fabricator the only belt sanders I've ever seen only have one speed, and that is figured in "feet per minute" of the belt. Doing that makes the belts last much longer than running them slow, since that is the way the belt makers figure the bond of the grains of abrasive to the belt.
 

MaxHeadRoom

Joined Jul 18, 2013
25,212
My DeWalt disc sander is Universal motor.
Probabally not the ideal for a belt sander which should run slower.
Typically a 4pole induction motor is used.
 

Thread Starter

Rufinus

Joined Apr 29, 2020
106
Thank for your answers

Yes. I´m not very happy using the universal motor. I have this 2 motors

mot1.jpg

A is the universal motor. Around 400/500watt 20.000 rpm I prefer not to use pulleys to make the sander small and less complex

B is an induction motor. 2900 rpm. With a disc/roller of 10cm diameter I´d get the perfec speed, but it´s only 130watt, and I think is not enougth for a sand belt of 2x27inch
 

MaxHeadRoom

Joined Jul 18, 2013
25,212
B is an induction motor. 2900 rpm. With a disc/roller of 10cm diameter I´d get the perfec speed, but it´s only 130watt, and I think is not enough for a sand belt of 2x27inch
Mine does not have any larger frame size motor , and that is a 4X24 model.
If you do build with this motor and it turns out to weak, then look for a larger model.
 

strantor

Joined Oct 3, 2010
5,967
You can use a "router speed controller" Search Harbor Freight website or amazon. It is basically a robust dimmer for universal motors. This will enable you to slow down the motor. But a univeral motor's RPM is entirely dependent upon the load you put on it. If you dial in the speed you want and then drive your workpiece into the sander, it will slow down. So maybe you dial it in to be a little faster than you want, and then drive the workpiece in, and it slows down to the speed you desire. It could be a workable solution if you'll be feeding the workpiece in by hand. Or it could be a disaster. I've never tried.
 

Audiotrack

Joined Jan 6, 2022
18
If you can work with a 2800-3000rpm motor speed (depending on mains frequency in your area) you might want to consider an old pool pump or spa pump motor. I suppose it depends on where you live but around here people throw them out all the time... I found one on the side of the road, just needed a new bearing on the output end of the armature. They are almost always a 2 pole AC induction motor rated at around 750-1000W so be careful, they won’t be forgiving and probably overkill for your project. Sometimes they come with a mains plug installed which simplifies things if you aren’t experienced with mains wiring.
 

Thread Starter

Rufinus

Joined Apr 29, 2020
106
Thanks for your answers. I´ll try with the 130w induction motor. If it doesn´t work I´ll buy a 3000 rpm induction motor
 
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