capacitor blows in generator 1

[stevesteve]

I have a 6kva generator that blows the capacitor after a few weeks after it is changed. When the capacitor is changed, the out put is perfect 240volts 50hz and power output is normal. After a fews weeks running for a couple of hours every otherday it stops supplying output. When checking I find the capacitor open ciruit.Value of capacitor is 8mmf. 450volts. This has happened about 5 times now. Can any one please give me the reason why this same problem keeps occuring?
A friend also experiences the same problem with a single phase capacitor run motor. It blows a 40mmf capacitor every time.

 

[itsmoked]

Three things typically blow caps.

High temperatures
High ripple currents > High temperatures
Over voltage > punctures the innards.

Measure the cap's temperature with a non contact temp gun.

You can put an ammeter in one cap lead and measure the AC current for excessive ripple.

You can measure the voltage across the the capacitor to see that it's less than 80% of the cap's rating.


Other thoughts:
You could be experiencing excess ripple because of blown diodes.

You could have lost the first cap due to age (they have a finite life) then started faithfully replacing it with the wrong one.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Check the recommended duty of the capacitor.
A capacitor meant for starting duty will probably fail on continuous duty.
On the motor, don't use a starting cap. to replace a run cap.
respectfully

 

[stevesteve]

Thanks for your support. The value of the capacitor is exactly the same as that of the manufacturer. It is not the voltage that blows it because to assure myself I placed two 16mmf in series so that the voltage of the caps will double and still the same thing happened./ As regards to the diodes I also changed them although these were good. I had to make sure of them.

 

[itsmoked]

Okay then check the temp with a non-contact temp gun.

One of the best,(for the money), is a Raytek MT4. I have one - I use it constantly. (You can even exercise tropical fish with it. LOL)

You can use it for almost anything.

It is the only way to measure the temperature of a cap as a probe would give a wrong result.

The link below shows it. Personally I don't like Amazon's service but find one anywhere you like!

http://www.amazon.com/Raytek-Minitemp-Temperature-Laser-sight/dp/B0002SR5NA

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[powerjunx]

i vote to keith, check the temperature.

 

[waross]

The majority of capacitors in stock at your local supplier will be motor starting capacitors. Some starting capacitors will overheat if used as run capacitors or generator capacitors.
respectfully

 

[bogeyman]

Have you checked the capacitor current wave form? Some caps cant stand high frequency ripple currents and will die if exposed to the high levels of third and fifth harmonics which may come out of a small generator with a thyristor or other switching type field drive. The makers of ferro-resonant transformers use very special caps with high harmonic ripple current ratings to handle the funny currents in the tuned circuit of these constant voltage stabilizers. Maybe you have a fault downstream of the cap within the generator that is causing a higher harmonic content in the generators output than it should be. In this scenario the generators output impedance (at the harmonic frequency) and the capacitor will form a low pass filter absorbing the extra power and killing your cap.