Reversing universal motor direction produces unnecessary arcing at brushes

[Kedar264]

Hi
I wanted to reverse the direction of  an angle grinder which uses common old universal motor , so I switched the polarity of exciter coil and armature coil and now direction is successfully changed.

But strangely motor produces slightly less torque and carbon brushes touching commutator produces very nasty arcing sparks ,even a burning smell comes from extremely hot brushes

It would be really helpful if anyone can help me to solve this problem. I am really stuck in this situation

I want reversed angle grinder motor without those nasty sparks at brushes

[Muffins]

I don't know the solution to your problem but I do have a question.

Going by this video:

Does compensation for armature reaction work for both directions?

[Gyro]

The main problem is that the brushes will have bedded in to the normal running direction. There is always some freedom of movement of the brushes in their holders, so they will have tilted slightly, causing a slightly lower leading edge and higher trailing edge.

When you reverse the motor, the brushes tilt the other way and the higher (now leading) edge catches the commutator segments first, causing bouncing and much reduced contact area, hence high current density and arcing.

The brushes will wear to the new direction given time and light loading. If you're impatient, then you could try removing the brushes and rotating them 180'.

In motors that are regularly reversed, the brushes don't have time to form a preferred direction.

[Kedar264]

Thank you very much ,Upon inspecting carefully I can see slightly angled brushes in my motor . hope my problem will be solved I will update soon

[Benta]

This is very difficult to solve. Universal motors have their brushed "timed" for commutation for one direction only. You'll have to experiment with rotating the brush holders to change the commutation angle, which is often not possible.

[Kedar264]

Hello Sir
I have tried to swap the brushes and holder from right to left and left to right  but still the angle of the brushes remain same and problemof arcing still occurs .

I might need to rotate each brush but its extremely hard fix brushes like that,but I will give it a try, if you have any other solution let me know ,by the way thanks for the help

[Kedar264]

I have tried to rotate the the brush holder angle but still arcing is there ,now I am wondering how bidirectional angle grinders place there motor brushes

[johnkenyon]

I have tried to rotate the the brush holder angle but still arcing is there ,now I am wondering how bidirectional angle grinders place there motor brushes

Buy and fit a new set of brushes, and then bed them in by running the motor in one direction, letting things cool down, then run in the reverse direction, let things cool and then repeat this cycle until the brushes are fully bedded in (= minimal arcing in either direction). (So you do forward, cool down, reverse, cool down, forward, cool down, reverse, cool down and repeat)

The old set of brushes have been run in one direction for years, and have had material removed from one side, and left on the other, and will probably take years of running in reverse before this is undone.

If that doesn't work, then you might have to accept that you have a universal motor which either isn't mechanically designed to run in reverse, or has become worn in such a way that it won't run well in reverse.

john
 

[David Hess]

I have tried to rotate the the brush holder angle but still arcing is there ,now I am wondering how bidirectional angle grinders place their motor brushes

They do not advance their timing so forward and reverse use the same timing.

[rstofer]

I think it is a matter of timing.  There are references all over Google.

[fourtytwo42]

Slightly off topic but wont the fastener unscrew and the grinding/cutting disk fly off if you run an angle grinder in reverse ?

[Leiothrix]

Slightly off topic but wont the fastener unscrew and the grinding/cutting disk fly off if you run an angle grinder in reverse ?

Yeah.  The first response probably should have been "Why do you want to reverse the direction of an angle grinder?"

[james_s]

I remember in the days when RC cars used brushed motors we learned to do a break-in process running the motor in the direction it would run in the car.

For the grinder I would be concerned about the gearbox too, most of these use a worm gear so the direction of thrust on the motor shaft will be opposite of what it was designed for, that may or may not be a problem.

[Kedar264]

Slightly off topic but wont the fastener unscrew and the grinding/cutting disk fly off if you run an angle grinder in reverse ?

Yes but you can solve the problem by welding that nut or using negative threaded nut

[amyk]

If the motor is designed for one direction it won't be happy in the other --- as others have mentioned, the timing of the commutation will be optimised for one direction only, so the brush holders have chirality. Motors designed for bidirectional use will have centered brushes, which are a compromise between both directions, or sometimes even a brush holder that can be rotated depending on the direction.

This has been known for over a century:

https://en.wikipedia.org/wiki/File:Dynamo_-_exaggerated_rotating_field_distortion.png

Not to mention the cooling fan probably won't work so well in reverse either.