I built a classroom model of a two pole DC motor, 1.5v, ~1A.

The problem I am having is small arcs pitting the commutator.
I have a freewheel diode, and across that, a 1uf/1.5ohm snubber.
I'm still getting arcing, almost no difference with snubber, but
the diode did help somewhat.

The arcing seems worse at the positive brush, but it runs a little looser,
so that might be it.

Any help?

 

How did you decide on values for the snubber?

 

How did you decide on values for the snubber?

Rule of thumb for snubbers.

I don't think the diode is going to work for a motor. I might next try
putting the snubber on each pole, at the rotor/armature.

 

Have you considered advancing the brush timing? If done correctly, it will reduce brush arcing and the motor will run much more efficiently. The easiest way to set the timing correctly is by rotating the brushes to get minimum armature current.

https://pphaneuf.github.io/rccars/articles/motor-timing-theory/

 

Have you considered advancing the brush timing? If done correctly, it will reduce brush arcing and the motor will run much more efficiently. The easiest way to set the timing correctly is by rotating the brushes to get minimum armature current.

https://pphaneuf.github.io/rccars/articles/motor-timing-theory/

I will try it, as it is easy, but I doubt it will make much difference as my rpm is very low, say 300-500.

 

I will try it, as it is easy, but I doubt it will make much difference as my rpm is very low, say 300-500.

Nope, no difference. Let me note, that I am using copper for my commutator, and brass for my brushes.
After polishing out the pitting, I placed a drop of 63/37 solder on the brushes, and although
arcing is not suppressed, I am no longer getting pitting, but the solder is transferring to the commutator, which bothers me little, as it adds a little lubricity.

 

Brushand commutator type motors are going to have a bit of sparking no matter what. Breaking contact slowly while current is flowing will make that happen. Proper adjustment of the brush position (brush timing) is able to reduce the arcing and boost efficiency, but you will always have sparks.

 

TWO POLE? Wouldn't that mean that twice in every rotation the brushes are touching common commutators? Unless the gap between the commutators is larger than the face of the brushes. But then there's a dead spot in the rotation where the motor can be stalled and won't start again unless acted upon by another force (someone spinning it).

[edit]
With two poles, the brushes (rectangular) contact both commutators every time the gap crosses the face of the brushes.

 

Yes, I have a gap, and yes, you have to start it by hand.
It's an interactive teaching tool. I'm already learning a lot.

 

TWO POLE? Wouldn't that mean that twice in every rotation the brushes are touching common commutators? Unless the gap between the commutators is larger than the face of the brushes. But then there's a dead spot in the rotation where the motor can be stalled and won't start again unless acted upon by another force (someone spinning it).

[edit]
With two poles, the brushes (rectangular) contact both commutators every time the gap crosses the face of the brushes.
View attachment 272140

The ones I have don't have the commutator parts that close together. By having a gap close to the size of the brushes you don''t see as many or even no sparking. The inertia of the moving rotor allows it to work with a gap.

 

The ones I have don't have the commutator parts that close together. By having a gap close to the size of the brushes you don''t see as many or even no sparking. The inertia of the moving rotor allows it to work with a gap.

That's what I would expect.

If I recall correctly, a friend of mine, back when in Junior High, had a model airplane with an electric motor to spin the prop. You could stop the prop and it would stay stopped if you stopped it in the right place. And the battery would not drain. But give it a little spin and it would run on and on. It was just a model, not an RC or String Control model airplane, just a toy.

 

A non-electrical solution is to place a drop of Wahl hair clipper oil on the commutators. This may be appropriate if the lesson is an introductory one and the addition of diodes and snubbers complicates the lesson.

This is often done in model railroading to reduce pitting on the rails and maintain electrical connectivity. Plus, it keeps the rails clean! It’s counter-intuitive but it works. As the wheels roll, their trailing edge arcs. A thin layer of oil is displaced allowing electrical contact. The displaced oil fills the narrow space preventing arcing. The contacts don’t pit and collect dirt.