Run capacitors on old motor starters?

[greenspark1]

Hi,
I'm looking at replacing several old 480/3 constant speed 200HP motors and motor starters for the government. I have an old one line that shows a 45kVAR capacitor on each motor. I have never seen this before and never used them on new motors. I am wondering if it is just for power factor correction or if there is something else going on? It's a large facility so a doubt a couple large motors would throw off their PF that much. They shouldn't be start capacitors since they are three phase. Any help?

 

[GoldDigger]

For a three phase motor, it would have to be a capacitor bank rather than a single capacitor. Other than that, I would agree that it would have to be PF correction rather than start or run phasing.
The only application that I can think of for a single capacitor on a three phase motor would be when it is used as a rotary phase convertor, driving a single phase line to line and deriving the third phase line.

 

[greenspark1]

Well it's shown on a single line diagram so it's not clear if it's three capacitors per motor or one, but I would presume three. These are large freestanding starters in an enclosure so there could be capacitors buried somewhere in them. No photos allowed of course. Any other ideas beyond PF correction? Does anyone use them these days in their 3 phase motor starters?

 

[Jraef]

There is about a 99.999999% chance they are PFC caps, referred to as "At-Load" PFC, meaning the capacitors are located as close to the motor as possible to get the maximum benefit and are only on-line when the motors are, preventing over correction to a leading PF. Depending on how the penalties are meted out by their utility, the fact that they are only on the 200HP motors is far from unusual.

The .0000001% chance of them not being PFC caps might be as either "surge capacitors", a very common practice on much LARGER MV motors that are controlled usually by vacuum contactors, or as "capacitor assisted starting" capacitors, a very old and very difficult method of trying to reduce the impact of Across-the-Line starting. If the original motor starter was a vacuum contactor for some reason, I have seen people add the surge caps simply because they see it done on MV motors, so they believe it is necessary for LV motors (it is not). This is usually the result of not really being properly engineered, but rather assembled based on observation. If they were for cap-assisted starting though, usually they will have another contactor that takes them off-line once the motor gets to 90% speed.