# Three phase motor driver DC link, DC bus capacitor values

I am designing a motor driver for a three phase brushless motor and have been looking for a capacitor value for the DC bus of this circuit (C1). Most of what I have found when researching deals with smoothing capacitors during rectification of an AC to DC circuit.

What I am looking for is to calculate the capacitor size 1 - to allow the power supply or battery to catch up due to the high current draw. 2 - to suppress the high voltage coming back into the circuit due to induction. From what I know so far is appears that the capacitor size isn't super critical to meet requirement 1, something between 100~1000 uF. Also the inductance values for L1, L2, and L3 is not representative of the actual motor.

This circuit drives a 24 V motor with a peak current of 30 A, lets say with a 20 kHz PWM signal.

• What's your peak load? What's your maximum time duration? What's your minimum voltage? deltaU=It/C – winny Nov 20 '18 at 15:06
• @winny Minimum allowable voltage drop would be to 22v, lets say the duty cycle is 50%, peak load should be 30a – user2180985 Nov 20 '18 at 15:13
• @winny What is the equation name for deltaU=It/C? – user2180985 Nov 20 '18 at 15:23
• It's the capacitor charge balance equation in integrated form. You know that I = C dU/dt? So, you have all the values except time. Do you just want to ride though the PWM duty cycle droop or "battery to catch up due to the high current draw"? The latter is several orders of magnitude higher. – winny Nov 20 '18 at 15:28
• @winny Preferably the latter but would like to see both options – user2180985 Nov 20 '18 at 15:37