Brush holder to commutator gap dimension

I am most appreciative of the wealth of knowledge on this forum and y'alls willingness to share it with others.

I am tuning up the big frame Reliance motor on my '41. Before I removed the brush holders I measured all four and
found 0.09" -> 0.120" gap between the brush holders and the commutator. What is the recommended spec for this?

If it is a 250 volt motor, use .075 gap, if a 500 volt motor use .125 gap. Don't forget to lap and stone the brushes and comm. You also need to de-burr the comm and undercut the mica. Simply turning the comm round is just the start...Phil

Thanks Phil. The motor plate is long gone but the armature resistance is about 5 ohm and the field windings are 83 ohm.
If I understand correctly this indicates a 250V motor.

I have the commutator and brush stones on order.

Can you confirm 3-5 psi brush spring pressure?

360 view of commutator - Shared album - Matthew Fiedler - Google Photos

Testing larger picture size.

mjf46 said:

... The motor plate is long gone but the armature resistance is about 5 ohm and the field windings are 83 ohm.
If I understand correctly this indicates a 250V motor. ...

Click to expand...

As far as I know, all of these motors have a 230 VDC armature. Some older motors have 230 VDC fields, the newer motors use a 115 VDC field. With an 83 Ohm measurement on your field, you have the 115 VDC field version. Do you have the big Ohmite rheostats that go with the motor?

Cal
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Cal,

Thanks for the confirmation on the resistance measurements showing I have a 115VDC field. I do have the tandem rheostat. I removed it before taking the video to clean it up a bit. I was getting some spikes in the ohm readings when the wiper was losing contact slightly in some spots. I cleaned the rheo coils with a soft rubber eraser and light application of 600grit sand paper. The resistance measurement is now smoothly changing through the 1/2 revolution of actuation as it should. 480 ohm on the motor rheo and 640 ohm on the generator rheo.

Bill,

Thanks for the heads up on the brush spring pressure white paper on the Helwig Carbon website. I found it was an interesting read. In short there are two main factors affecting brush wear: mechanical friction and electrical erosion. Their study was conducted with 70 A / in^2 between the brush and the commutator. I measured about 0.45 in^2 contact area for the reliance large frame motor brushes and I thought I saw somewhere on this forum the motor pulls about 10A. I imagine those 10 A would be shared by at least two brushes at a time. This would give about 5A / brush.

If this is correct should we bias the brush spring pressure to the low side to minimize the mechanical friction?