Hello everyone,

Check the scheme bellow.
I used two filtering capacitors of 2200 MFD, each one supports 400V DC, and they're connected in series. So the capacitor bank ( the two in series) can support up to 750V.
I was applying only 535V DC on the capacitor bank. The PWM sampling frequency (sawtooth frequency ) was 1000Hz.
The phase current was only 1.5A, so it didn't reach the max current supported by the motor which is 2.75A.
The motor was making some weird noise, but i was controlling it's speed with no problem.
After a while of functioning, about 2 minutes, the two capacitors exploded.

So, can anyone tell me why did the capacitors explode?

Thanks a lot in advance for your answers.

.

 

Capacitors explode because of too much charge. Usually because of too much voltage. Put a voltmeter on your dc bus.........and watch when you lower the speed. You might be getting a regenerative effect.
Increase the cap voltage....and increasing the capacitance will help too. I assume you have a current limiting charge circuit for the diodes.

 

Another common problem when adding high value caps in series, is the leakage current imbalance thru each cap.
This causes a voltage across one of the caps to exceed its rating.
Adding balancing resistors across the caps is one way to to prevent this problem.
E

http://electronics.stackexchange.co...lancing-resistor-values-for-series-capacitors

 

Another common problem when adding high value caps in series, is the leakage current imbalance thru each cap.
This causes a voltage across one of the caps to exceed its rating.
Adding balancing resistors across the caps is one way to to prevent this problem.
E

http://electronics.stackexchange.co...lancing-resistor-values-for-series-capacitors

Actually, each capacitor has a resistor R=22K ohm connected to it in parallel.

 

Could be high ESR.
I have replaced exploded caps in Huge A/C's. They are in series across a 3 phase bridge.

 

Capacitors explode because of too much charge. Usually because of too much voltage. Put a voltmeter on your dc bus.........and watch when you lower the speed. You might be getting a regenerative effect.
Increase the cap voltage....and increasing the capacitance will help too. I assume you have a current limiting charge circuit for the diodes.

Could be high ESR.
I have replaced exploded caps in Huge A/C's. They are in series across a 3 phase bridge.

Well, i didn't use this kind of capacitors, ... i used aluminum electrolytic ones.

 

Well, i didn't use this kind of capacitors, ... i used aluminum electrolytic ones.

Excessive ESR for the application would cause them to overheat and blow.

 

This is basically poor design. Although the capacitors have voltage dividing resistors all it takes is one capacitor to develop a higher than normal leakage current and the other capacitor gets too much voltage and blows.

Much better to have a single capacitor correctly rated.

Actually to handle the ripple current better to have a number (at least two) capacitors in parallel. Aluminium electrolytic capacitors are not reliable at high voltages and high ripple currents. High-value non-electrolytic capacitors are available but are expensive so not used where cost matters!

 

A smoothing cap at 800VDC.
Good luck on that.