Diode or capacitor across terminals of brushed motor

Question

I am currently building a hobby-weight robot (5.44kg) and will be using 2 x 14.4v cordless drill brushed motors to drive my wheels.

I have read somewhere that due to "induced currents" when I turn the motor off (or reverse it presumably?) I should protect it by using a diode or a capacitor across the terminals.

Which should I use (capacitor or diode) and what are the parameters I need to consider for these components (voltage or current)?

Some answers to a similar question discussed capacitors but not diodes. Are diodes relevant?

Would I seriously damage the cordless drill (presumably quite tough) motor if I did nothing?

And don't motor controllers have any form of inbuilt protection for the motors anyway?

Answer 1

The motor's aren't so much at risk here, poorly designed motor drivers are. The purpose of the diodes is to drain the exces energy generated (by disconnecting the motor) to the battery of your robot. When an electric motor is running by means of external power and that power is removed, it starts acting like a generator. If it can't deliver the power it is generating to something, the voltage over the motor terminals will keep rising until something releases it's magic smoke.

If you look at the direction of the diodes in the attached image, you will see that the diodes enable the motor to drain it's exces power to the battery, thus saving your driver. By the way, the round things with the arrow in them, are the switches, they are called MOSFETs.

The cap over the motor terminals is there to short out high frequency noise generated by power supply, the motor itself or something else. Make sure the cap is bipolar if you use one, otherwise it will blow if you reverse the motor. The size doesn't have to be huge, use something like 100nF.

What motor driver are you using? If you could provide me with a type, I can tell you whether or not you need the diodes, a small cap is always a good idea.