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I observed that every brushless motor controller has a big capacitor in the design. (1000uF to 3300uF). How is this value choosen and what happens if I eliminate the capacitor in my design ? enter image description here

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  • \$\begingroup\$ What is the input power, voltage and current requirements for that inverter? Without a circuit or a decent specification, this is pointless answering. \$\endgroup\$
    – Andy aka
    Jun 5, 2019 at 18:27
  • \$\begingroup\$ 36V,300W and around 5Amps \$\endgroup\$
    – pantarhei
    Jun 5, 2019 at 18:27
  • \$\begingroup\$ That's only 180 watts - maybe link to a data sheet. \$\endgroup\$
    – Andy aka
    Jun 5, 2019 at 18:29
  • \$\begingroup\$ If you don't understand the purpose and choice of this cap, you should not be designing this. A motor rating does not include the typ. 10x current start surge so RdsOn DCR, ESR must be computed with care. \$\endgroup\$ Jun 5, 2019 at 19:01
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    \$\begingroup\$ On such a "cheap Chinese" module as in the photo, the capacitor choice was probably a compromise between: price / works with most motors / price / fits on PCB / price. Yes I know I mentioned price 3 times. So not really a real "design choice". Making a proper design choice isn't even possible without full details of: the motor, load on the motor, supply feeding this module and maybe some other requirements. \$\endgroup\$ Jun 5, 2019 at 19:08

2 Answers 2

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Most motor controllers are switching 10's to 100's of Watts of power. To avoid voltage drops on the line input from wire inductance, it's advisable to have some kind of energy storage on-board to smooth out voltage switching noise.

You can calculate how much filtering is needed

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  • \$\begingroup\$ You mention 2 reasons for a capacitor: to avoid voltage drops and to smooth out noise. But I think you don't use an "energy storage" cap to smooth noise. You would use small low ESL/ESR caps for that. \$\endgroup\$
    – Huisman
    Jun 5, 2019 at 20:08
  • \$\begingroup\$ I wasn't quite clear on that, I wasn't suggesting that an energy storage cap should be used, but that caps are for energy storage as they are one of the two circuit elements the can store energy. We don't have voltage and ripple requirements or how much current is switching. \$\endgroup\$
    – Voltage Spike
    Jun 5, 2019 at 20:13
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In addition to the other answers regarding supply line filtering, the energy storage function of the capacitor also operates if the motor controller is capable of "braking" the motor. In that case, the motor kinetic energy is dumped onto the supply rail.

If there is enough inductance in the supply leads, this energy can drive the input voltage level above the rated operating voltage of the module, and potentially damage the module.

The capacitor absorbs that dumped energy, protecting the module.

Nanotec and Moog, for example, specify such a large capacitor externally on the rails of some of their motor controllers for this purpose.

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