John Stranahan

iTrader: (27)

Gearing an Electric Motor
I thought I would try to explain why it's important to get the gears at the right ratio, on road, or on the oval. First take a look at the left side graph. This is output from the Fantom Dyno, through the Fantom power supply, for a Novak EBX 13.5 brushless motor. The graph plots Amp draw vs RPM. All I want you to notice is the shape. The graph is highest on the left at low RPM. If the motor stays in this range it overheats. At the right the line is low. The motor pulls few amps at high RPM. It therefore runs cool at high RPM. Amp draw is highest with the motor stopped (right at the start of a race).

Next take a look at the right hand graph which plots power vs RPM for the same dyno run. Notice we have a hump shaped curve. Power peaks at mid RPM. The point of carefully gearing a motor is to try and keep it near its power peak a greater percentage of the time. There are sometimes heat considerations that will foil our attempt but you see my point.

If we gear the oval motor to a higher rollout. It is going to run at fewer RPMs. It is going to draw more current. If we are to the right, of the humps top, this is a good move. If we are too the left of the top you will just heat the motor more and produce less effective power.

I was interested to see these brushless motor curves to see if they were dramatically different than brushed. Maybe the hump was broader or something like that, but no, they are remarkably similar. The proper gearing is still going to give you an edge.

The numbers on these graphs are low for reasons I stated previously having to do with the LRP speed control.

Eventually you just develop a gut feel for what a good ratio is by looking at the car accelerate out of the slow corners of the track and onto the straight. For the beginner there is no substitute for advice from someone experienced with their particular combination as a starting point and then lap times after that. A dyno is not needed just a nice thing to toy with.