Washer motor brush query.

[Tractorfan]

Hi,
Yesterday I replaced the carbon brushes in our Hotpoint washing machine while I had it in bits to repair a drum suspension spring (good job, as it happened, because they were almost worn away after 22 years!). A thought occurred to me and set me wondering; Viz:
Why do the brushes on a washer motor bear on the commutator at an angle, whereas those on a vacuum cleaner are perpendicular to the surface of the commutator? I would have thought that the opposite would be true as a washer motor runs in both directions and a vacuum does not. If it was because of greater current capacity, then fatter brushes would be the order of the day.
The answer is probably something blindingly obvious that I've missed.
Thanks in advance,
Cheers, Pete.

[Station X]

There's an explanation here:-

http://www.maintenanceworld.com/dc-m...arbon-brushes/

[kalee20]

Interesting! Thanks Graham.

I'll bet Lucien can add to this, too...

[Lucien Nunes]

I would have thought that the opposite would be true as a washer motor runs in both directions and a vacuum does not.

If the motor only runs in one direction, brushes will settle into position on the trailing side of the holders and stay there. During reversal, a radial brush will tend to rock back and forth in the holder and never develop a stable contact face. By angling the holder, part of the spring pressure is applied to the side of the holder to counteract this, allowing the brush to remain in the same position relative to the comm in both directions of rotation. The angle chosen must be large enough that in reaction (facing the oncoming segments) the tangential component of spring force overcomes the friction trying to 'repel' the brush from the holder wall, while not being so large that it gets wedged into the acute angle when trailing. A typical washer motor has an advantage over a general purpose reversible motor in that it only needs to spin in one direction, so the brush can be optimised for all speeds trailing but low speed only in reaction.

Brushes and commutators are fascinating things, there is so much going on there beyond a mere sliding electrical contact, both in mechanical and electrical terms. The rate of comm and brush wear is a complex function of all the electrical parameters of the motor including armature coil inductance, speed, dimensions, eccentricity and vibration, brush composition, temperature, humidity etc. Apparently small mechanical irregularities can disrupt the contact surface enough to have a significant impact on motor performance and life. The trick with small low-cost universal motors in white goods and power tools is to make them last a reasonable time, often at high speed (up to 25,000 rpm), with simple, low precision brushgear.

The film projector I am using for my work at the moment has an interesting PM brush motor of very high torque and low inertial moment (due to an ironless rotor). It accelerates from rest to 2500rpm and brakes to a standstill again under servo control, stopping within 0.1° of its target position, all in a total of 8 milliseconds, up to 60 times per second. It is reversible, but it uses radial brushes in precision holders with very high surface pressure. Needless to say the peak currents are very high and armature inductance is low, so brushes of high metal content are used.