capacitor
- Thread starter jamieb
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For PFC?
If so, what's the motor PF and what and what PF do you want?
not for pfc but for inrush
Cold Fusion
Senior Member
- Location
- way north
Just curious - How does a capacitor lower the motor inrush?
I had not heard of that before.
cf
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
On large MV AC motors, an old trick called "Capacitor Assisted Starting" used to be used as a way to reduce the voltage drop caused by Across-the-Line starting. The cap has a separate contactor that closes an instant before the motor starter so it charges the caps first. Then a fraction of a second later when the motor starter closes, the caps supply vars to the motor when it is at its lowest PF, the few moments it takes to get to slip speed. Since most of the starting kVA is reactive in the first moment because the PF is so low, it technically works to reduce the voltage drop by becoming an alternative source for the kVARs in that instant. You still have the cap charging current and the magnetizing current for the motor, but those are usually so fast as to not be read by utility metering equipment.
Cap Assisted Starting is VERY tricky to implement. In order for the cap to do any good, it must be large enough to supply VARs equivalent to the motor having a PF of .10 or thereabouts. The problem is, as soon as the motor gets to slip speed you must immediately disconnect said cap or else you have a serious over swing in VARs and that can/will wreak havoc on other connected equipment. So to do it effectively, you should have a speed monitor on the motor shaft, a small PLC to take care of the timing of the two contactors and a type of contactor for the caps that can disconnect them very quickly, typically a vacuum contactor.
By the time you get all that done correctly, it would have been cheaper to buy a soft starter. The trouble is, most people don't bother to do it correctly and end up with other problems in an effort to be cheap. They use a simple timer method and try to take out the caps that way. If the acceleration time of the motor is exactly consistent every time that might work, but t never is and problems end up happening.
I have never heard of anyone doing it on anything as small as 125HP and never at low voltage, it's typically used on MV motors at around 500HP plus and even then, it has become extremely rare because of the serious negative consequences of getting it wrong. People are a lot more conscious of power quality issues now and the severe voltage spikes this creates if the timing is off by even milliseconds is something most people will pay big money to avoid.
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