Photograph of the overload reset button on an electric motorMore Electric Motor Diagnostic FAQs
Diagnose & Repair Electric Motors - Q&A Set #2

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Electric motor diagnostic FAQs set #2:

These questions and answers help in diagnosing or repairing electric motors. The page top of an oil burner electric motor points out that many electric motors used on building mechanical systems and appliances look a lot alike.

This article series describes A/C electrical motor troubleshooting: here we provide an electric motor diagnostic table, a troubleshooting guide that helps diagnose and repair most electric motor problems for motors found on HVAC equipment in buildings such as air conditioners, furnace or air handler blower fans, oil burner motors, well pumps, and condensate return pumps.

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A/C Electric Motor Troubleshooting FAQs

Air handler blower assembly(C) Daniel FriedmanAt left our photo illustrates the motor as typically found in a direct-drive HVAC blower or air handler assembly.

[Click to enlarge any image.]

These questions & answers were posted originally at ELECTRIC MOTOR DIAGNOSTIC GUIDE - home.

On 2017-04-23 by (mod) - signs that a pump motor needs a new start or run capacitor

That suggests that the motor has a bad start-run capacitor OR a bad electrical connection or control switch.

A conventional circuit breaker wouldn't know how to send different voltages to different things connected to it.

On 2017-04-23 by Anthony

Thanks very much for your quick response. I have had the motor with the impeller out and everything spins very smoothly by hand. This motor shares a quad circuit breaker with a slightly smaller motor used for the spa booster pump, is it also possible that the breaker is failing to send the right voltage,only to one,circuit or does everythng points to a failing capacitor? Thanls

On 2017-04-23 by (mod) - repair a water pump that can't start


That sounds like a motor or pump impeller that's seizing - you could try a start/run capacitor replacement but first see if the pump motor and impeller will spin freely so you don't waste time on the capacitor. Once a pump starts, even with a bad bearing, it may keep running but when it stops the combination of heat, lost lubricant, or bad bearing will make it hard to re-start - or as you say, maybe we're lucky and it's a bad capacitor.

On 2017-04-22 by Anthony

My waterfall pump motor will only start after 4-5 attempts. It hums the frst 4-5 times amd will then start And run very smoothly fror hours ! the shaft rotates freely. Could it be the capacitor?

On 2017-03-23 by (mod) re: compressor motor sparking badly


You should turn off the compressor unit power right now to avoid a possible electrical fire or shock.

There are companies who re-build electric motors, or IF (which I now doubt) your motor has not been actually damaged by the remains of the brushes that wore out, you may be able to take the motor or remains of its brushes to an electrical supplier to find a matched replacement. If not you'll need to contact the manufacutrer of the motor with the data on the motor's data tag.

On 2017-03-23 by B_Rad

My compressed motor is sparking bad at the contact of the brushes how can I fix this they get red hot and start smokin

On 2017-03-17 by (mod) re: clicking at control relays when a motor doesn't run

Click is probably a relay closing;

I can't guess if the problem is a control board, or the fan motor is shot or there's a bad fan motor start/run capacitor.

If you never heard the fan motor humming and it was never hot (what you never listened to the hummer?) then some testing of the motor is in order. Often if the tech doesn't see an obvious burned or shot component she will just change out the capacitor as a cheap thing to try.

On 2017-03-17 by Tim

I got a problem I need some help on please. I when I turn on my a/c inside the house, everything inside turns on fine but the fan outside will not. All I get is a click (outside). Does that mean that the fan motor is blown?

On 2017-03-03 by (mod) re: signs of a seizing electric motor

Sounds like a seizing motor - that'll draw high current and trip breakers. Could be a bad bearing or if the motor spins freely then I'd expect a motor expert to look for a shorted winding. The amateur quick and dirty test is to replace the start/run cap if the motor uses one.

On 2017-03-03 by Anonymous

Last update. Took the switch assembly off the saw and before testing it decided run motor one more time while on the bench. Well, when I plugged it in it tripped the circuit breaker. Turned off all the lights in the shop and plugged the motor in again. This time the circuit breaker did not trip but the motor only hummed. So, problem appears to be the motor. Any ideas what the issue with the motor might be? I would sure like to solve the problem myself if I can.

On 2017-03-03 by (mod) re: Sears table saw motor repair

Possibly though that's not so common. It could be the motor can't start when under load. I would reinstall the motor in the saw, assemble everything as before, but then bypass the on-off switch temporarily to see what happens. Let me know.

DO NOT cut off any fingers.

On 2017-03-03 by Anonymous

Update on the Sears table saw. Took motor off the saw. Placed on bench and plugged directly into outlet. Motor ran fine. Again, the blade and corresponding pulley/shaft turns freely. The motor has a three prong line that plugs into on/off switch. Switch seems to be the culprit. Can a faulty switch provide line voltage but insufficient amount (110v) to propel the motor thus causing the hum?
I have a Sears table saw that is probably at least 40 years old. Never had a problem until today. When I turn on the switch all I get is a hum. The blade turns easily by hand as does the motor shaft Any ideas?

On 2017-01-29 by Doug Ingram

Just to add one more possibility. Heater blower suddenly stopped working. Turns out the set screw on the pully had gradually loosened over the years and became disengaged with the electric motor shaft. The motor at first appeared to be defective but upon inspection, the shaft was found to be turning properly with power. Solution: Reset the set screw and tightened it up sufficiently. Result: Working better than ever.

On 2017-01-23 by Anonymous

@Rod, it is an 5 hp 120 volt
My motor turn go but if I hold the shaft it don't turn what cause this it off or my. Air compessor

On 2017-01-18 by mike

I have a small saw with a small motor. The motor will run but only if I rotate the armature a little bit. Then it will run. If I stop it ( turn it off with the switch) it will not restart unless I move the armature about one half an inch again. What' might be wrong? the switch is new and works fine.

On 2016-12-23 by (mod) re: industrial coffee grinder motor runs at half speed


Is this a 240V motor? Some 240 or even 3 phase electric motors will still run, but poorly, if they're missing voltage on one of their supply wires.

Some electric motors also have internal jumper connections that will by design run the motor at different speeds.

Take a look at the data tag on your motor for its specifications.

Check also for low voltage to the motor

Use the page top or bottom CONTACT link to send me some photos.

On 2016-12-23 by Jim

I have an electric industrial coffee grinder. It is 1/2hp. I let it get too hot one day and took it to be redone. It came bach running at about half speed. Not enough RPM to throw the ground coffee out. Ideas please ?

On 2016-12-17 by orlin

I need a wireing diaghram for putting new start capactors in a 7.5 motor, this motor does not have ruhning cAPACTORS

On 2016-12-10 by (mod) re: turn OFF smoking electrical equipment right away

TURN OFF the equipment immediately and leave it off, Anon. It's unsafe. If smoke continues or increases after turning off the smoking motor, go outside and from a safe location call your fire department.

On 2016-12-09 by Anonymous

When motor starts to run it started to smoke

On 2016-11-28 by Ed barberry

I recently moved into a house that has a electric motor to draw water from a pond to water the lawn, i was told it hasn't run in over 10 years and of course it won't turn on and any suggestions how to get it going or fix it
Thank you

On 2016-11-28 by Roeks

My house fan motor runs slow. Setting one runs not at all, 2 struggles 3 and 4 runs very slow and I must help the motor to run by hand when switch on.

On 2016-11-27 by Anonymous

I came to work and the spa motor wasn't working so I thought that water had got to the wires cuz the capacitor sits on top of the motor and has exposed wire so I used compressor to blow out the wires and then I gave it power and sparks flew I quickly shut it off and now there is no power even going to it. Can someone plz help me figure this thank you.

On 2016-11-23 by oo

what if my motor does not run .... what is the cause and the solution

On 2016-11-08 81 by (mod)

Don at BIN95
Thanks for the suggestion about the motor troubleshooting article - we've added links to it in the article ELECTRIC MOTOR DIAGNOSTIC GUIDE


Continue reading at ELECTRIC MOTOR DIAGNOSTIC GUIDE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see BLOWER FAN OPERATION & TESTING motors used in air handlers and HVAC systems





Or see ELECTRIC MOTOR DIAGNOSTIC FAQs - set #1 of questions & answers about fixing an electric motor


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