High Voltage DC Capacitor

Please describe the outcome of the telephone call that has been made to the original equipment manufacturer, whose name has been withheld from the forum, and the reason the required information could not be determined during that telephone call?

We had 3000V capacitors in store for 20+ years, when needed they worked fine. Its not as if there's much to go wrong with them.

Thank you

What make and part code are the existing caps?

Are they eletrolytics?

What is the voltage rail they are on? It is normal to de-rate electrolytics by at least 50%: depending on ripple etc. So if the rail is 2000V DC you probably need a 4000V cap.

Look for Ixys / westcode

http://www.westcode.com/caps.pdf

I don't understand your questions about losing charge and storage. Think of a capacitor as a reservoir/container, when you need to use it you fill it up then use it to smooth the current demand on the supply, and, to smooth the voltage for the load.

Capacitor should be kept protected from damage, clean and dry....corrosion or damage should be your only concern....

Ideally, that would take a band with in excess of the inverse of the rise and fall times of switching waveform.

The capacitors should not be failing, it indicates the VFD is either of poor quality or it is being used in an environment that it is not designed for (such as high ambient temperature or vibration).

In all my (few) years of VFD servicing in harsh farm environments I never saw a capacitor failure.

Have a good look through the VFD manual and online literature as you should not have to replace VFD main capacitors. You may need to move or alter the VFD protection to remove the cause of the capacitor failure. If it has only happened once, well perhaps the drive was just a dud.

Come . . come, . . a capacitor just does NOT loose its charge. It can fail from overvoltage or overheating.

As it is used in a VFD - which is likely a PWM control, . . . there will be steep spikes of voltage, . . . which will make the capacitor draw spikes of high current . . . these will overheat the capacitor and cause a short.

Where in the circuit is the capacitor located ? You likely need a small inductance to prevent the capacitor from drawing the spiky current .

VFD capacitors are generally electrolytic.

ABB gives a recommended procedure for re-forming electrolytic capacitors that have not been in service. Feel free to google there procedure.

Rockwell has similar procedures.

I recently started a VFD that had been in storage for 9 years. When we started re-forming the capacitors they "leaked" badly, but after 12 hours of re-forming they were back to normal.

HV capacitors can be stored indefinitely or for extended peiod of time in a low humidity clean room With normal temperature..

It is also a good and safe practice to put a shorting link between the terminals. This will prevent the capacitors from accumulating any unwanted energy while in storage, thus preventing a shock hazard condition!

Prior to putting into service, capacitors can regain its rated capacity back by conducting an initial series of slow charge and discharge cycles!