# What's all this about brush timing.



## mattW (Sep 14, 2007)

What voltage were you planning on for the generator?

Brush timing is to do with the commutator that switches the polarity of the motor as it spins so that current only flows in 1 direction. The timing is to do with exactly when the terminals are switched because at high voltage and rpm sparking can occur at the transition point between positive and negative. I have heard it is only a problem at >96V but I haven't found any good resources about it.


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## mattW (Sep 14, 2007)

What voltage were you planning on for the generator?

Brush timing is to do with the commutator that switches the polarity of the motor as it spins so that current only flows in 1 direction. The timing is to do with exactly when the terminals are switched because at high voltage and rpm sparking can occur at the transition point between positive and negative. I have heard it is only a problem at >96V but I haven't found any good resources about it.


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## Dunkard (Jun 1, 2008)

I want to use the highest voltage that is practical with brushed motors, and I'm not sure what that is either. Yes, I realize that it's about when the polarity is switched, but I was surprised to hear that would need to vary with RPM. In an IC engine it's easy to understand why timing varies since the speed of combustion is not always fast enough to keep up with the movement of the piston at high RPM unless you advance the timing. But in an electric motor, I assumed the magnetic fields would build up so fast that timing would stay fixed when RPM changes. On the other hand, my timing won't really need to vary, it just needs to be set correctly for high RPM, since this generator would NEVER be be expected to provide power except after being spun up to the max-power speed of the ICE that drives it.

If brushed motors have this problem more at high voltages, then maybe I should go with using a brushless motor as my generator, but I'm not sure if that works. I thought brushes would be a good low resistance way of rectifying the current for me without the losses of high current diodes, and since this is for a motorcycle experiment, I'm not worried about RF noise in a sound system from the brushes.

Surely the people who design electric motors for a living must have some good books on the subject that they learned from. I thought maybe someone might know of a good one.


Anybody know what would be the highest practical voltage that you can expect to use with brushed motors?

And also: If I use a brushless DC motor, would there be any need to rectify current or use diodes as would be required with an AC motor? I was going to feed the output from the generator directly into a super cap, so pulsating output from the generator was also not really a concern.


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## 3dplane (Feb 27, 2008)

Dunkard!
Yes if you use BLDC it has to be rectified.You will get AC out of the 3 wires of the motor.You would use two rectifiers and paralell the DC output legs.(it will leave an unused AC leg on one of the rectifiers) Barna.


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## Dunkard (Jun 1, 2008)

What about universal AC/DC motors. I think they have brushes. Will they generate electricity if spun by an external power source.


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