Choosing the DC bus capacitor in a 3 phase motor driver

[emontllo]

Hello!

I am developing a 60 VDC powered 3 phase motor driver with 17 A_RMS phase current (for BLDC and BLAC motors basically)

I was wondering which is the main factor used to choose the DC bus capacitor? In AC powered drives, the bus capacitor is chosen according to the allowed bus voltage ripple. Since the device will be powered from DC power supplies or batteries (depending on the application) I don't know which are the key parameters to make the choice.

Previous designs used:
2x
100 µF 100 VDC Aluminum Electrolytic SMD caps with 330 mΩ ESR and 450 mA_RMS current ripple capacity
... and they get very hot, actually it was the hottest element in the power stage!

Right now I am using
2x
150 µF 100 VDC Electrolytic Low ESR (100 mΩ) , Low ESL TH caps.
These seem to work better.

Power efficiency, electromagnetic compatibility and size are important in the design...

Thank you in advance for your responses and comments!

Ernest

 

[FvM]

You should determine the motor and pwm frequent ripple currents applied to the capacitors. They can be either measured or calculated.

For 3 phase sine pwm, no motor frequent ripple current will occur, because the delivered power is free of ripple. For simplified (e.g. square wave) modulation schemes, you'll face motor frequent ripple currents.

In addition pwm frequent ripple currents will be generated depending on the output inductors and pwm scheme.