# Calculating current draw of drive motors while at constant driving velocity

I’ve been using the JVN custom one speed drive calculator for a gearbox design. I understand the spreadsheet outputs a “pushing” current draw per motor (pushing up against a stationary object). But is there a way to calculate current draw during normal driving, say during constant velocity and periods of acceleration?

Currently working with:
2 gearboxes, 1 CIM motor each, 12.76:1 gear ratio
4" diameter wheel, Coefficient of friction=1
Robot weight 50lbs

Currently giving me a “pushing” current draw of 28.90 Amps. Can I calculate current draw while not up against an immovable object?

To a reasonable first-order approximation, current draw will be torque / Kt, torque will depend on wheel diameter, gear ratio, number of motors (as in the JVN calculator you are using), traction force will be either torque / wheel radius or weight x CoF, whichever is less, and

F = ma + drag

When driving at constant speed, ma = 0. So your question reduces to “how much power should I budget to overcome drag?”

That depends on how efficiently your drivetrain transfers power to the carpet, and on your motors’ free current. Since those two factors have the same effect, you can measure current draw with the wheels up (robot on jack stands) to get a relative metric. The closer that metric is to total free current of all the drive motors, the more efficient your power train.

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