FRC Blogged: - Motor Controllers

[EricVanWyk]

Originally Posted by AlexH

i thought it was (voltage1*amps1=total watts=voltage2*amps2)

but seeing as that doesn't apply to h-bridges....

That formula is close to what you want, but you have it in a form usually used for power conversion.

Total watts is still volts * amps, but you are thinking of the wrong volts. The voltage that matters is the voltage developed across the bridge elements themselves, not the total voltage applied across the entire bridge.

The switching elements (FETs) have a small resistance when they are conducting. The voltage is then the product of the current times that resistance. You end up with Power = (Current)^2 * (Resistance).

This is only dependent on the motor current, and is unrelated to the applied voltage.

There are a few other heating effects related to that applied voltage, but they are largely dominated by the I^2R losses and can be ignored for a first pass glance at the numbers.