How to properly size the additional capacitor for 24v13 and 24v21 motor drivers?

Hello,
The description of 24v13 and 24v21 motor drivers states “For good performance, it is very important to install a large capacitor across the motor supply and ground close to the motor driver. We generally recommend using a capacitor of at least a few hundred μF and rated well above the maximum supply voltage; the required capacitance will be greater if the power supply is poor or far (more than about a foot) from the driver, and it will also depend on other factors like motor characteristics and applied PWM frequency.”
Can you please be a bit more specific about “at least a few hundred μF”? In my application (a RC model boat), the battery is not far away, typically about half a foot and is a 6S LiPo (25.2V or 25.8V fully charged).
I have complete freedom about the PWM frequency so which value should I choose in order to minimize the required size for the additional capacitor?
Finally, my load is a MHD (MagnetoHydroDynamic) drive, not a classical brushed motor, and it has no inductive component. Does this “relax” the need for an additional capacitor?
Thanks.
Best regards,
Georges.

Hello, Georges.

If you want to minimize the capacitor size, the best advice we can offer is to look at your system with a scope and let that guide your decision making since you might find out you don’t even need the extra capacitor.

Using a lower PWM frequency will reduce how much capacitance you need, but using a load with no inductance will put more stress on your system since that means there will be little that limits the rate current can change.

- Patrick