Capacitor sizing for small motors to reduce inductive voltage spikes

Question

I'm working with some small motors for a robot project and I think I had a different component fail due to inductive voltage spikes caused by the motors. The motors are Mabuchi RC-280 which operate up to 8.4V DC and can draw 2-3 amps each. I have 4 of these motors interfaced with a 20A motor driver, 12V battery, and an Arduino Mega2650 microcontroller.

I do not have any capacitors connected across the motor terminals and I need to know what capacitance to use. Other similar motors have small capacitors attached but I don't know their value, and I am unfamiliar with sizing capacitors for this purpose.

Can anyone advise me as to what I might need for this application, or how to determine the size?

Answer 1

Each motor should have a small ceramic capacitor (10-100nF) across it to reduce RF (Radio Frequency) interference caused by brush arcing. A popular configuration is two 100nF capacitors in series, one from each motor terminal to the metal case. This 'grounds' the case to RF to prevent it from radiating interference, but doesn't put a DC voltage on it.

This small capacitance will not prevent inductive voltage spikes. If PWM is applied then you must not use a large capacitor, as this will become charged up and prevent the voltage from dropping between PWM pulses.

Your motor controller should have 'flyback' diodes in it, which suppress inductive voltage spikes by recirculating current through the motor. This is required not only to prevent damage to sensitive electronic components, but also for efficiency and linear PWM response. The diodes should be Schottky type for fast response and low voltage drop.

If the motor is only driven in one direction then a single flyback diode can be wired directly across it, but this is not possible in a bidirectional bridge circuit because in one direction the diode would short out the motor. Therefore a bridge circuit needs 4 diodes, one across each output transistor. MOSFETs already have built-in body diodes, so external diodes are not normally required for them.

You should also have a large low ESR capacitor (100~1000uF) across the controller's power input rails, to suppress voltage spikes caused by inductance of the power supply wires. If the controller does 'active freewheeling' AKA 'synchronous rectification' and/or dynamic braking then this capacitor also helps to slow down the voltage rise caused by the motor pushing current back into the battery as it slows down.

Answer 2

How to design an RC snubber for a solenoid relay driving an inductive load?
- similar question but different voltage range requires different RC snubber

RC snubber are also used on AC relay coils , but RCD snubbers are popular as well for PWM motor drivers.

• This helps to recirculate the currents $ reduce peak commutation voltage
• Too little R increases PWM turn on current into C and raises Q or ringing
• too much R increases back EMF voltage
  • let Cload = 2x (Coss + stray) as a starting point

Snubber purposes

• · Reduce or eliminate voltage or current spikes
• · Limit dI/dt or dV/dt
• · Shape the load line to keep it within the safe operating area (SOA)
• · Transfer power dissipation from the switch to a resistor or a useful load
• · Reduce total losses due to switching

• · Reduce EMI by damping voltage and current ringing

• power in series R that was energy stored in the cap on average;

• balancing these two factors yields critical dampening at the expense of lossy carbon resistors

  • Pd = C V^2 * f. for V [pp] and PWM rate, f
  • Zo = sq.rt(L/C). for motor primary inductance and snubber C

reference

Answer 3

I believe that DC Relays and Contactors which use DC Coils use a diode across the windings to sink the voltage spike. Its called a flyback diode. No capacitor required. This might work for your DC motors. Read this https://en.wikipedia.org/wiki/Flyback_diode.