How to calculate the needed capacitance of a start capacitor for a 12V 10A DC motor?

Question

I want to use a 12V 15A switching power supply for a 12V 10A DC motor which has about 9A start current and 6A no load current, I also want to add a push button switch for on and off function to use it and by that, each time motor stops it’s gonna draw huge current from supply and I’m going to add some caps on to the curcuit; so my questions are:

will this high amount of start current damage the switching power supply with those repeatedly on and off times?
with numerous on and offs will the motor get damaged in long time or be hot more soon than normal using?
if we use cap to provide srarting current, what size and voltage?
where to add the switch? closer to supply or the motor? (in the case we use capacitor/s)

thanks

Brian Drummond · Answer

DC motors don't use start capacitors. You want a reservoir to keep the supply from sagging.
Use dQ = C * dV, or C = dQ/dV.
If you need 10A for 2 seconds, dQ = 20 Coulombs.
If you can tolerate a 2V sag in the supply voltage, dV = 2V.
Then C = 10 Farads.
Double check your specs though : if the no load current is 6A and the rated current is 10A, you probably mean 90A for the start current. Which would mean 90 Farads in the above.
And remember you can't just switch on a supply to a 10 Farad capacitor...

How to calculate the needed capacitance of a start capacitor for a 12V 10A DC motor?

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