Hi!

I have a question on when to use braking resistors with a brushless motor (and amplifier), or even with a brush motor. The braking resistor is there to absorb energy that would otherwise be absorbed in the motor coil, wiring, pcb traces, and power mosfets? Doesn't the power supply capacitance absorb most of this extra energy? How do you calculate what size resistor is needed, if any?

Say I have a 1kW brushless servo motor, and I use 0.5 ohm resistors in series with each coil. Further suppose the motor is loaded at 10 amps at 100V. This means the resistor is dissipating 50 watts and has a 5 volt drop across it. Let us say the power mosfets have a RDSon of 0.010 ohms. So during a high energy braking condition, the current in the motor coils will go much higher right? Like 50 amps or more for a few hundred ms? At 50 amps, the resistor would have 25 volts across it and instantanious power of 1250 watts, the mosfet would have 0.5 volts across drain/source and have an instantanious power of 25 watts.

I guess it is the impedance of the motor coil that is the issue here? The motor coil is something like R + Z, where Z=2*pi*frequency and R is the DC resistance of the coil, which is probably less than 1 ohm? So in a braking condition, the frequency will go to zero, meaning Z goes to zero, and the motor acts like a low ohm resistor, so we suddenly have a condition where 95 volts is across the motor coil which has stopped, generating a large current and the braking resistor is there to limit this current somewhat?

Have I got any of this correct?

And if these braking resistors are needed, why are they not incorporated into the sense resistor...oh, I guess because the sense resistor needs to be small so that it does not heat up significatly (which would dramatically change it's resistance).

Confused!

Syp