Starting capacitor

**Martinco** · 06-Dec-10, 08:33 AM

I have a problem whereby the starting capacitor of a pump motor packed up and I need to replace this.
Specs: 14 Mfd at 400V. I can only locate a 10 Mfd or a 20 Mfd capacitor. Which would be the better replacement and why ?

Thanks

**Justloadit** · 06-Dec-10, 01:47 PM

Not knowing the application, and making a call like this may be risky. If you do a few starts a day, then use the 20MfD. The higher the capacitance value, the higher current that flows, and the motor will have more starting torque.

The period in which the start capacitor remains in the circuit is a second or so, so the extra current flowing will not cause any harm.

However if this motor starts many times an hour, I would suggest you get the correct one. Using the lower capacitance may draw less current in the start winding, but it may also cause the motor to stall due to torque loss, which could cause a burn out.

**Martinco** · 06-Dec-10, 03:16 PM

Originally Posted by Justloadit

The period in which the start capacitor remains in the circuit is a second or so, so the extra current flowing will not cause any harm.

However if this motor starts many times an hour

Many thanks, The motor only starts once a day so the 20 MfD would be fine.

**AndyD** · 06-Dec-10, 10:08 PM

If there is only one cap and it's rated at 400v discharge voltage then it's most likely a run capacitor and not a start capacitor.

Use a clamp meter on the pump whilst under load and pick the capacitor that gives you the lowest running current. You should be able to find a 15ÂµF - 400v cap if you shop around.

**murdock** · 07-Dec-10, 07:46 AM

this sounds like a pool pump motor?

what size is the pump motor?

what is the application...if it is a pool pump motor i can only assume you think the cap has gone because the motor makes a humming sound when you try start it?

**Martinco** · 07-Dec-10, 08:50 AM

Originally Posted by murdock

this sounds like a pool pump motor?

what size is the pump motor?

what is the application...if it is a pool pump motor i can only assume you think the cap has gone because the motor makes a humming sound when you try start it?

Hi Murdock, Andy

Actually a jacuzzi pump motor @ 1/4 hp and yes, it made a humming sound when trying to start. If you spin it by hand it does start and runs fine.
I have put the 20 Mfd cap in and the motor starts 100% and still runs cool. ( I left it running for +- 30 min.)

So, the problem has been canned and my wife can enjoy her jacuzzi again.