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Capacitor bank

Capacitor bank

Capacitor bank

(OP)
Hi,

We are in the process of renewing the electrolytic capacitors on a 900 kW 400 V variable speed drive. The VSD is now 12 years old. The nameplate rating of each individual capacitor is 3600 µF. We were expecting the capacity value to degrade over time i.e ,to be much less than 3600 µF but most of them measured around 4200-4600 µF.The same instrument recorded 3600 µF with the new capacitors. We usually should have expected the di-electric constant of the electrolyte to degrade over time which obviously should have resulted in a lower capacity value.
The opinion of members are welcome.

Guardiano
 

RE: Capacitor bank

Electrolytics usually have a huge tolerance - -10% to +50% isn't unheard of. I'd expect the new ones to be a little high rather than exactly nominal, but they meet the spec. Out of interest, what are you using to measure the capacitance? You really need to put working volts onto the cap before taking a measurement.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Capacitor bank

I would not expect the capacitance to drop with age.  I don't recall ever seeing that in data sheets.

Capacitance change over time is generally given as +/-xx%.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Capacitor bank

The typical problem with electrolytics aging is increase in ESR, not reduced capacitance.

RE: Capacitor bank

Certain suppliers of VFDs have made a business out of the aging phenomenon and offer expensive periodic check-ups. I have been asked to check capacitance several times because people tend to think what they are told by the suppliers and never, actually, found a bad capacitor on drives younger than about 15 years.

I check them using the mains voltage plus a rectifier and a current limiting resistor to bring them up to around 300 V DC.

I then record voltage as I put a heavy load on the capacitor. The initial drop shows ESR and the discharge curve shows capacitance. Simple and very close to actual operation.

WARNING!
Please note that this is dangerous practice and you need to understand exactly what you are doing. Lethal voltages are always involved when working with VFDs and make sure that you work with the correct fusing between you and the capacitors. You need fused probes for the voltage measurements and shall use adequately rated fuses for the "thick current". Mesure voltage across the bare capacitors so that the fuse's voltage drop doesn't fool you.

Another way of checking the capacitors is to run the drive heavily loaded close to 50 or 60 Hz, depending on your mains frequency. If you can move a few percent in speed from just under to just above mains frequency without motor current getting unruly, then your capacitors are OK. Otherwise, you will notice slightly unsteady current in the motor. It is caused by the output switches "sampling" the DC link ripple so that an alias frequency is created if the ripple is high. The alias goes to zero when output frequency is exactly equal to mains frequency.  

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

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