School me on capacitors

Don-Wi · Jun 20, 2015

So we've got a few motors that are either in use or want to be put into service, but they need a capacitor. One is a 5HP that we want to put on the vacuum pump so we can put the 3HP that's on it onto the bale chopper Dad picked up. The 5 is just too big to fit.

I opened up the 5 and the mice got into the box so 1 of the 4 capacitors is shot. Can't really read much on it except Mallory. The 3HP has 2 and they are a little smaller in both height and diameter, but it's also a much newer motor.

The other is a 1/2 HP 220V motor we've got on the bale elevator. I stole a capacitor off another 1/2 HP motor, but again the capacitor was illegible but the size was the same. Plug it in and it works now without having to risk fingers in the belt...

We've got a Graingers near by as well as a few other places, so we just gotta go get a few to replace the ones we need. Would be nice to get the bale chopper working so we can use up some of the straw bales in the mow- The manure auger doesn't like the long stuff.

Donovan from Wisconsin

Geo-TH,In · Jun 21, 2015

I'm not aware of knowing what size cap you need from just the motor size. All motors
are different. Some capacitors are start and some are run. Some motors have both

Start capacitors are only used for starting only. If cap is too small, motor won't
start. If cap is too large there may not be too big of an issue.

Run capacitors are used all the time there is power to motor. If it's too small motor
may not work like it should but it will run. If it's too large, motor will over heat.

Recently I had a portable 1 ½ hp air compressor having difficulty starting. The cap
was a 60 mic. Unusual size for a run capacitor. So I took motor apart, no end switch,
it was a run capacitor. I had to order one. So like I said, difficult to tell by just
motor size.

If you are lucky to still have a motor shop locally, take motors there. Or go to the
manufactor of the motor and ask there. Good luck.

Don-Wi · Jun 21, 2015

We've got a few good motor shops in town but it costs almost as much as a new motor to have them fixed up. Figured I'll look into the motor again and see if one of the other capacitors has better markings on it.

Donovan from Wisconsin

M-MAN · Jun 21, 2015

There is a simple formula to determine capacitor size but it's been a good while since I've used it. Common in the hvac field. Type "how to determine capacitor size in your google search".

guido · Jun 21, 2015

Hello Don-WI

Here is the formula for determining a proper size capacitor for a 60 Cycle application.

2650 X Amps / volts. Make sure that the instruments you are using to measure the voltage and the Amps are fairly accurate. 10% either way won't matter.......

Guido

guido · Jun 21, 2015

Hello Don-WI,

The 10% applies only on the capacitor size, if you knew the value, not the instruments. Typing way to fast then my brain!

Guido.

buickanddeere · Jun 21, 2015

(quoted from post at 23:42:41 06/20/15) So we've got a few motors that are either in use or want to be put into service, but they need a capacitor. One is a 5HP that we want to put on the vacuum pump so we can put the 3HP that's on it onto the bale chopper Dad picked up. The 5 is just too big to fit.

I opened up the 5 and the mice got into the box so 1 of the 4 capacitors is shot. Can't really read much on it except Mallory. The 3HP has 2 and they are a little smaller in both height and diameter, but it's also a much newer motor.

The other is a 1/2 HP 220V motor we've got on the bale elevator. I stole a capacitor off another 1/2 HP motor, but again the capacitor was illegible but the size was the same. Plug it in and it works now without having to risk fingers in the belt...

We've got a Graingers near by as well as a few other places, so we just gotta go get a few to replace the ones we need. Would be nice to get the bale chopper working so we can use up some of the straw bales in the mow- The manure auger doesn't like the long stuff.

Donovan from Wisconsin

Start caps and run caps are two different types of caps, don't mix.
Simple rule of thumb. Installed the highest mcfd , 370 volt caps that will fit under the cover. You can't over cap a motor that way no matter what somebody may try to tell you.

Steve@Advance · Jun 21, 2015

From what I've observed of caps, the black Bakelite cased ones are start caps. The sealed aluminum case ones
are run caps.

Start caps are not as critical to size, go big on the mfds, and go over on the motor voltage and it will
work. Also might open the end of the motor, take a look at the centrifugal switch contacts and wiring,
especially if it's been siting out and mice have been in it.

Texasmark1 · Jun 25, 2015

The start cap is to setup a quadrature phase shift to maximize the motor's torque; used for high load starts. The motor's inductance
makes the current lag the voltage and the capacitor brings the current back up in phase with the voltage improving the motor
torque. Limited duty cycle.

The run capacitor is a different value as it's job is to get the power factor nearer to 1 (voltage to current relationship in phase with
one another) while the motor is running at the nominal running current.......so the power company gets paid for the volt-amps they
supply you, unlike the start cap where you are dealing with "locked rotor" currents in the motor. This C is rated for continuous
running.

So, as others have said, you need to know the motor to size the C. Just shotgunning is just that......capacitive reactance has to equal
inductive reactance to get a PF of 1 at the line frequency....acts like a resistor to the power line and that produces watts out of volt-
amps and the power company charges/gets paid, for kilowatt hours.

School me on capacitors

Don-Wi

Well-known Member

Geo-TH,In

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Don-Wi

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M-MAN

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guido

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guido

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buickanddeere

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Steve@Advance

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Texasmark1

Well-known Member

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