Motor Start Capacitor

[Whiskey Bent]

The motor on my air compressor won t start, the capacitors are real crusty looking so I ordered new ones. It still doesn t start. I don t know how to read a capacitor this is as close as I can find. Is this capacitor wrong? The first picture is a capacitor that came off motor and 2nd picture is new 1. Thank you.

 

[Whiskey Bent]

 

[caterpillar guy]

Look at your starting switch to see if the grease is hard and dry looking. We had a motor with a bad cap and when changed would not start. Cleaned up the switch on the motor so it could move easier and it was fine since then . Did this a few years ago now.

 

[Steve@Advance]

The cap that was in it is the same as the one you bought.

What voltage does the motor run on, 120 or 240?

If it runs on 240, that is not the correct cap, it needs a higher voltage
rating.

You said caps as in plural, does it have more than one?

An easy test for a bad cap is to simply jump the cap wires together and do a
test start. If it starts, the cap was bad. If it still doesn't, then either
the centrifugal switch is not closing and making contact, there is a broken
wire in the start circuit, or the start windings are open.

However, on a compressor application, there can be another factor. A failed
unloader valve, or a leaking check valve or output valve can cause the
compressor to be trying to start under a load (a pressurized cylinder) which
makes the compressor very difficult or impossible to start.

If the compressor will start with a discharged tank, but not when the tank is
up to pressure, look into the unloader or a bad valve.

Add one more factor, wire size. Compressors typically need a lot of starting
amps. Trying to run one on an extension cord is not a good practice.

 

[Whiskey Bent]

Thank you for the quick responses guys. The starter switch is good, I checked with a multimeter. It is 240 and has 2 capacitors. The unloading valve fine, the motor doesn t do anything when I turn switch on doesn t even try. I have power coming in to the switch and power going out of the switch when I turn it on. Motor is hard wired with 10 gauge wiring 3 foot from panel. I bypassed the capacitors and still nothing. I bought the motor from a neighbor who had it on a auger. He said that he was having trouble with the thermal switch snapping it off on him, so he bypassed it. When I put it on my compressor I hooked the thermal switch up, the compressor would run for 45 minutes or so before it would snap off. I do have power through it now though.

This post was edited by Whiskey Bent on 09/30/2022 at 06:17 pm.

 

[Charles in Aus.]

If you disconnect the motor from the compressor by remov8ng the belt you can try to hand start it by turning the pulley quickly. Be careful of your fingers , if it runs a little then part of the windings have failed somewhere inside .

 

[Stan Coryell]

Your old cap is a motor run oil filled
cap, I think. I don't know if the start
cap can be used in place if it. Do you
have better pictures of both old ones? You
may be able to find a cheap meter to
measure mfd. That would be your best bet.

 

[560Dennis]

Probably , got it to hot and thru solder

 

[Whiskey Bent]

The old caps, don t think they are oil filled very light.

 

[wore out]

If the motor doesn't even growl something is wrong besides the start circuit. Even without the start capacitors connected the ''run'' windings should be active, and the motor should make noise when powered

Does it have an overload protector? There isn't a manual reset you are missing, is there?

The capacitors in your photo are non-polarized electrolytic motor starting duty capacitors, BTW.

 

[Whiskey Bent]

It doesn t do anything. I was sandblasting the day it quit, about every 10-15 minutes the thermal switch would snap the motor off. I would wait 5 minutes or so and would reset the thermal switch and it would run 10-15 minutes again. I think Dennis is prolly right and I melted some solder inside. Think I will pull motor off and take it to a motor shop. Thank you all for your help.

 

[buickanddeere]

. Some duel voltage motors operate the start windings and start cap from 120V . One connection to line and the other connection you the centre point between the main windings .

 

[buickanddeere]

Many people get all uptight about motor capacitors . Any motor with a starter switch that takes the start windings out of service during normal operation . Or garage door openers that operate for 15 seconds . Upsized caps are better . Manufactures save $$$ by going with minimum sizes .
The capacitor voltage rating can be anything from original and higher is better . More resident to failure from voltage surges .
The higher the mfd rating the better on the start cap . The motor will have more starting torque and rev up to full rpm sooner . Somebody however is going run around screaming that an upsized start cap will blowup the motor . Those know nothings need to stay quiet .
Going to higher mfd on run caps will improve the power factor , reduce I2R losses on motor supply conductors . Lagging Current demand from the utility will be reduced . Voltage at the motor terminals will increase towards ideal levels due to lower losses . Once again anyone who does the "Chicken Little " claiming the motor will blowup . They again need to stay quiet .