Zilla FAQ25 Feb 2008 10:28 pm

Q: Can I electrically reverse a series motor even if the brushes are advanced?

A: Yes.

If you are only using the reversing function for low speeds then the brush advance for driving (which turns into brush retard for reverse) will actually help you out. Lightly retarded brushes will give higher torque at low voltage, which is just what you need when backing out of a driveway. This would not be wise if you happen to like driving in reverse at 30 MPH, since brush advance is important to brush and commutator life when motor voltage (and speeds) get higher, but that is why the Zilla’s Hairball interface includes a reverse rev limit setting. You can set the reverse voltage and RPM limits low to insure that the motor stays in a safe operating area. I set reverse voltage at 60 Volts and reverse speed at 1500 RPM. I think those are reasonable values for most cars.

On the other hand, most people don’t want to spend hundreds of dollars on a set of reversing contactors (required for electrical reversing) unless they are running a single ratio direct drive transmission. If you are planning that, then be sure to review my faq on “Shift gears, or direct drive?“.

When electrically reversing a motor, I strongly recommend having a speed sensor providing motor speed pulses to the Hairball and having it properly configured in the Hairball. That way the Hairball will keep the driver from switching directions at more than 100 rpm and protect the motor, controller and drivetrain from the damage that would happen if it were reversed at higher speed.

I hope this helps,


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