# How is resistance in brushless motor ESC calculated by the manufacturers?

In hobby market (rc planes, cars, robots) one of the parameters manufacturers of the brushless motor ESC specify is resistance. ESC in this case means "electronic speed controller" and is a device used for commutation and speed control of the electric engine.

Is there a standard (written or informal) how this parameter is characterized? If yes how it is done?

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I suspect the resistance is the resistance of the internal power MOSFET(s). In which case it's an utterly pointless measurement as the internal resistance of the battery, the resistance of copper tracks, and the virtual "resistance" created by charging and discharging the MOSFET's gate will easily overwhelm this tiny Rds(on).

If you want to know the effective resistance of your ESC you can perform an experiment. Set up a motor on your ESC. Wire it in for maximum speed (this would probably involve wiring the throttle input to a battery line.) Turn on the motor, making sure nothing is attached to the rotor. Put an ammeter in series - preferably a clamp-on one - current will easily exceed the 10A rating of most series meters - and measure the current going to the motor.

Then, measure the voltage across the battery, and across the motor. You could try measuring the drop across the ESC as well but it wouldn't work if it employs some virtual ground or has a current shunt built in, as this will skew your readings. Calculate the voltage drop across the ESC and put this into Ohm's law: R = V/I. Note this is only an average resistance and depends on the state of the battery.

For yuks, measure the current from the battery to the ESC and use that to make input-output power figures. The efficiency is the output divided by the input, in percent. This will probably be more useful than any resistance figure.

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BLDC ESCs are going to output some form of three-phase AC, and unless you have a True-RMS meter it might give garbage readings. @Thomas: What is "virtual resistance" from switching MOSFETs? –  Nick T Nov 24 '10 at 16:14
@Nick T Ah, it's a BLDC? Most ESC's I've found just switch a motor on or off using a single high or low side MOSFET. Virtual resistance refers to the transition from low to high. Poor quality ESC's have a slower transition from low to high. This acts like a resistance on the motor and produces heat inside the ESC and wastes energy. –  Thomas O Nov 24 '10 at 16:54
The question specifies BLDC, and unless the controller is very poorly designed or attempting to do some special waveform control, the switching time should be orders of magnitude lower than the on time. –  Nick T Nov 24 '10 at 17:35