Q&A about hard-starting electric motor diagnosis & repair.

These questions and answers address the causes, diagnosis, and repair of hard-starting compressor motors, fan motors, or other electric motors.

This article series explains why an electric motor is having trouble starting or why it won't keep running is an important diagnostic step in keeping an electric motor or air conditioner, heat pump, well pump or other motor working properly.

Hard-Starting Electric Motor FAQs

I noticed smoke coming out of the run capacitor cover

@Inspectapedia Com Moderator, Thanks for responding [to my earlier question]. I had another go at it yesterday. With the belt back on, I had planned to measure the starting current before it fails after 1 second and also when it fails to run at all.

But I didn't get that far, I tried to start it twice, the first time it span for 1 second and the second time it didn't spin at all, both times it was only switched on for 3 secs max.

As I was setting up the ampmeter I noticed smoke coming out of the run capacitor cover. I had previously measured it at 30 microfarad, as per nameplate, now it's slightly deformed and measures 5 microfarad. Perhaps it was dodgy even though it initially measured Ok, will pick up a new cap at the weekend and report back. On 2021-11-25 by Bo -

by Inspectapedia Com Moderator (mod) - Watch for seizing motor

@Bo,

Sounds like a good move, but be alert to the possibility that the motor is seizing and causing the other parts to fail.

We explain at CAUSES of HARD STARTING ELECTRIC MOTORS that besides a bad motor bearing, a bad motor winding can cause the no-start symptoms you've described as can even a simple bad on-off switch or a loose wire connection.

by Bo - New starting capacitor didn't fix the motor

@Inspectapedia Com Moderator, Good advice to be mindful of fault causing other parts to fail. New start cap fitted yesterday, made no difference, still doesn't run.

By luck an almost identical second hand, tested motor came up for sale today on Facebook market place for AU$50.

Bargain snapped it up, fitted like a charm and bricksaw now working

I haven't given up on the old motor, will strip it down and fiddle with it when time permits.

Something I learned through this adventure, the centrifugal switch for capacitor start, capacitor run motor, does not disconnect the start winding, it only disconnects the start cap.

[Yes, as explained at ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD - Ed]

The start winding is left connected in series with the run capacitor. [Shown in the schematic here - Ed.]

I don't know if this is explained on this site, I came across a good explanation at

How is a higher torque achieved in a single phase cap-start cap-run motor? [Website linked to by the illustration just above, red annotations by InspectApedia]

Excerpt:

A permanently-connected auxiliary winding with some capacitance in series makes the motor more like a two-phase motor.

Since the current in the main winding lags the voltage, and adding the capacitance in series with the auxiliary winding can make the auxiliary current lead the voltage, the phase difference between the two can approach 90 degrees.

That improves the overall power factor and efficiency and reduces the torque pulsations that are inherent in single-phase motors.

Thank you for an excellent site, I have created a shortcut for future use.

by Inspectapedia Com Moderator (mod) - diagnose a motor that won't start: when new capacitor doesn't help

@Bo,

Thank you for the important update - this discussion will help other readers troubleshoot a motor versus capacitor problem.

The motor swap rules out any problem in the binding of the motor itself but note that motor binding and hard-starting can be caused by the machine it's driving.

Your prior notes point to a bad motor, perhaps siezing - burning up the capacitor, though a short in wiring of the capacitor itself can do that too, the latter is less common.

Your comment that when the motor gets up to speed the motor disconnects only the start capacitor, not the start winding itself is worth noting and clarifying.

The article you cited is accurate and helpful; if we take a look at the wiring schematic for a starting capacitor, as I understand it, in at least some electric motor and control wiring setups, when we disconnect the part of the circuit that contains the capacitor, we're in essence disconnecting the flow of current to the start winding as well.

At that point the run capacitor remains in the circuit, giving a little kick to the motor to help keep it spinning smoothly while being delivered with alternating current.

As one of the writers at the stack exchange article that you cited described this same point:

Question:

... how this cap-start cap-run arrangement produces a higher torque as opposed to a normal cap start motor.

Answer (part):

"Since the current in the main winding lags the voltage, and adding the capacitance in series with the auxiliary winding can make the auxiliary current lead the voltage, the phase difference between the two can approach 90 degrees.

That improves the overall power factor and efficiency and reduces the torque pulsations that are inherent in single-phase motors."

It's accurate that we'll have a problem with a motor if the centrifugal switch does NOT disconnect when the motor gets up and running: the risks include both failure of the start cap and damage to the motor winding itself.

For example if we overheat a winding with too much current flow we may burn off insulating coating and short out the winding.

Brick saw starts for a second the stops: motor won't start.

My bricksaw won't start. When switched on it will sometimes start for a second and then stop, other times it does nothing, not even a hum, however with the belt removed from the saw blade, it starts and runs every time, and is difficult to stop spinning with a chunk of wood.

I changed the start cap as it measured less than half it's rating, but that has made no difference.

The run cap measures OK.

I removed the motor fan and can visually see and hear the centrifugal switch working and arcing as it opens.

Insulation resistance is OK, winding resistance seems OK, 1 ohm run winding, 2 ohms start winding.

The nameplate is missing but it's about a 1.5HP 230V. Measured start current about 7amps, run current 2.7A (with no load). The motor bearings and saw bearings are good, they both run freely. Now I'm stuck. On 2021-11-23 by bo

Repair suggestions by Inspectapedia Com Moderator (mod) - b

@bo,

Thanks for a real motor start problem puzzler.

You've checked what I'd check. I SPECULATE that there could be a bad start winding that can't spin the motor if it's under load.

At this point it's probably cheaper to replace the motor but you could take it to an electric motor rebuilder if there's one near you as those folks are smarter than us at testing motors and advising if a repair is cost-reasonable.

If you do that, do let me know what you are told as that will help other readers too.

Table saw hesitates on start and trips circuit breaker

My 1.5 HP table saw motor hesitate on start and blows switch in panel - On 2021-05-06 by Michael Kent

Diagnostic tip by (mod) -

@Michael Kent,

With the power disconnected from the saw, see if the blade will spin freely. If it does not then the saw is binding and drawing high current,, tripping the breaker.

The blade spins freely than the problem still maybe a shorted wire or switch or a bad and shorted motor winding

Vornado fan won't start unless Igive the blades a spin

I have a Vornado fan that when I turn it on , I can hear that the motor is on but the fan blades will not turn without some assistance . Any comments or suggestions ? On 2021-04-23 by Wayne

Answer by (mod) -

@Wayne, typically that's a bad starting capacitor - IF your fan motor uses one. (Not all do)

And/or it's a failing fan motor itself.

If with power off you find resistance in spinning the fan blades or see a wobble in the fan motor drive shaft, it's shot.

More sophisticated testing would involve measuring the motor's current draw.

Delta belt disc sander won't start unless pushed

I have delta belt/disc sander 1/3hp 3450 rpm 120 volt 4 amp ph1 hz60 motor will not start, but if i push the sanding belt, it will start and run ok what are the fixes.On 2020-09-14 by ron

Answer by (mod) - Replace the starting capacitor

Ron that does sound like a bad starting capacitor, but possibly also a failing motor. The starting capacitor for a typical Delta disc sander as sold by https://www.ereplacementparts.com/ is
Capacitor Part Number: 1225324

How to troubleshoot a freezer motor that won't start

Can you tell me how to troubleshoot when this kind of freezer is harving a hard start? On 2020-06-29 by jayrad

Tips by (mod) -

I would check the motor current draw for signs of a failing motor or bearing, and I would also be sure that I didn't have a clogged expansion device that was causing the motor to try to start against the high head pressure

Clarke American Wood Floor Sander Motor Won't Start

My Clarke American ez8 wood floor sander won't start ,I'm wondering if is the the motor reset button and if is that where I can located. On 2018-09-22 by Ortega

by (mod) - Clarke American ez8 wood floor sander won't start

Ortega

In this CLARKE AMERICAN WOOD FLOOR SANDER MANUAL on page 11 are the causes and cures given for Clarke American for the EZ8 floor sander if the motor won't run

Spa pump motor won't start at high speed setting

I have a two speed 1.5 HP spa pump. The switch cycles from off to high speed to low speed to off.

When the pump is first turned on, high speed, it fails to start and hums like a bad capacitor. Eventually it trips the circuit.

Pushing the switch twice puts it into low speed and the motor starts normally. Pushing the switch twice more puts it into high speed and it operates normally. Any thoughts? On 2017-08-28 by David O

Suggestions by (mod) - bad motor speed switch on spa pump

I'm puzzled too, and have to suspect a bad centrifugal switch or less likely a bad electric start switch itself.

However some motors have trouble starting at low speed - and need to be turned on first in the high speed position. I've seen this, for example, with some multi-speed ceiling fans.

What causes the HVAC blower motor to run "for no reason" at odd times?

I have Ducane house central hvac. The blower motor inside the house run intermittently at night time.

I directly connect the motor on the door switch and it runs perfectly. What could be the cause of this problem? Hope you can give me the solution. Thank you so much. On 2017-07-02 by Paquito

Reply by (mod) - motor runs when it should not

Paquito

If the motor is running when the system is not calling for heating or cooling, I suspect a loose or shorting thermostat or other control wire or possibly a bad control board in the unit.

A bad centrifugal switch can cause an electric motor to fail to start

Another common reason for a CS or CSCR motor to fail to start is a bad centrifugal switch. If the switch contacts are dirty, the motor can fail to start. If the switch freezes closed, the start capacitor will overheat. On 2015-01-10 by Paul J. Ste. Marie

Reply by (mod) -

Paul

Thank you very much - you're quite right, of course. I have added bad centrifugal switches to our list of motor defects both in the article above - it already appeared in our separate

articles

ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD and at

ELECTRIC MOTOR DIAGNOSTIC GUIDE but it was not explained on this page.

Thanks to you that point appears here. If you want to be further identified in the article let me know by email, or if you prefer to remain anonymous let me know that and I'll remove your name.

Working together makes us smarter.

 

Question: fan will start but won't keep running.

The fan on my Conquest 80 will not stay on (interior fan). It starts and then stops after a few seconds. It was installed in 2005. Help. (June 1, 2014) Anonymous

Reply:

Anon,

The start capacitor is for getting a motor started, not keeping it running.

Often motors have two windings, a start winding and a run winding. Your motor's run winding may be damaged.

Or your fan motor may require a dual capacitor (start and run) or a separate run capacitor to keep it spinning.

Or your system may have a faulty control.

 

Question: will a hard start capacitor reduce current drawn and stop tripping a breaker

I have a commercial hood & exhaust fan (120v) running in a food truck that is used for catering. We occasionally need to run off a generator (3000w) and we've found that the fan cause the generator overload to trip. The fan has been tested and observed to draw 8 amps running at full speed.

Given this, would it be possible to add a hard start kit, such as SUPCO SPP4E? Will need to confirm the motor size, just wondering if this would help.

The total draw of all items is less than 20 amps and the generator is capable of supplying 25 amps constant.

Thanks for the advice. (June 7, 2014) Joel

Reply:

Joel I think the problem lies elsewhere and needs some further diagnostics. I suspect that your total current draw is exceeding the ability of the generator - you may be running more than the fan, such as lighting, a cooler, toasters, other electrical appliances. If it were just the fan, drawing 8A, it has no business tripping the breaker.

Put another way if the problem is the fan and there are NO other appliances running, then there is a failing fan motor drawing high current, or an electrical short circuit or other unsafe condition to find and fix.

A start capacitor or a run capacitor won't change the current drawn by the motor.

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Continue reading  at CAUSES of HARD STARTING ELECTRIC MOTORS - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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• ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD - a bad centrifugal switch causes trouble in electric otors
• ELECTRIC MOTOR DIAGNOSTIC GUIDE for help in testing a bad-behaving electric motor or an electric motor that won't start

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