Hard-starting electric motor diagnosis & repair:

What causes hard-starting compressor motors, fan motors, or other electric motors?

Understanding 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. Understanding why a motor is hard starting also tells us if it makes sense to install a hard-start capacitor on the equipment.

Causes of Hard-Starting Air Conditioners, Refrigerators, Freezers, and other Electric Motors

A compressor or other motor may have trouble starting for any of a variety of reasons such as:

• Low line voltage supplied to the equipment or excessive power drop on a long circuit wire
• Un-balanced cooling or refrigeration systems - such as a compressor motor that is having trouble starting when the refrigerant pressures are high on one side and low on the other side of the cooling system. This hard starting condition happens when a compressor is turned off in the middle of an "on" run cycle.
  
  You can guess that you have this condition on an air conditioner if the system starts just fine when it's been shut down for a half hour or more (refrigerant pressures are equalized on both sides of the compressor) but starts with difficulty (the compressor motor "hums" and does not start quickly or may even trip the circuit breaker) when trying to restart a few minutes after having been shut off.
• Old, aging compressor motors or other electrical motors that are at or near the end of their life may have trouble starting and may be able to function for some additional time given the "help" provided by a starting capacitor.
  
  The underlying trouble might be failing motor bearings.
  
  See HARD STARTING COMPRESSOR MOTORS and also
  
  TIGHT or SEIZED AC COMPRESSORS for more details about old or failed compressor motors.
• Troubles with an electric motor such as used on an air conditioner air handler, condensing coil fan, or a well water pump:
  
  A loose or bad connector or a motor winding that's damaged, perhaps opening or shorting, can cause motor start trouble as can a defective motor control or switch.
  
  see ELECTRIC MOTOR OVERLOAD RESET SWITCH
• Bad or failed starting capacitor: the air conditioner compressor (and some other electric motors) may already have a starting capacitor installed, but the starting capacitor might have failed, causing the air conditioner compressor to start with difficulty or not at all.
  
  A bad starter capacitor can also disable the fan in the outdoor compressor/condenser, or the blower fan in the indoor air handler unit.
  
  Capacitors are electric devices that get an electric motor running at start-up or that help keep a motor running once it has started. If the capacitor has failed the symptom is that the motor won't start. You may hear it humming or observe that it's getting hot. If you observe this we suggest that to avoid damage you turn off the system while waiting for repairs.
  
  We give more detail about failed electric motor starting capacitors, and we explain possible visual diagnosis of a failed starter capacitor (bulged ends) without having to perform electrical testing,
  
  at HARD STARTING COMPRESSOR MOTORS.
  
  Thanks to reader George Fazio for this air conditioner start-up diagnosis suggestion and for the photo (above left).

Starting capacitors or electric motor starting capacitors (or motor start boosters) are often present on large single phase air conditioning compressors, as found on home air conditioning units, or on occasion on blower motors or even fan motors.

Electric motor starting capacitors are only very rarely present on small refrigeration compressors, such as those in refrigerators, and as far as we know, never present on 3-phase power systems.

• Bad centrifugal switch in the electric motor: Thanks to a reader, Paul J. Ste. Marie we can add that a bad centrifugal switch can also keep an electric motor from starting or running as it should.
  
  See details
  
  at ELECTRIC MOTOR CENTRIFUGAL SWITCH or PTC PRD

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-11-29 by Inspectapedia Com Moderator (mod) - diagnose a motor that won't start: check other parts

@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 caused by the machine it's driving.

Your prior notes point to a bad motor, perhaps sizing - burning up the capacitor, though a short in wiring of the cap 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, when we disconnect the part of the circuit that contains the cap, 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 described this same point:

"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.

On 2021-11-29 by Bo

@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 havn'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.

The start winding is left connected in series with the run capacitor. i don't know if this is explained on this site, I came across a good explanation on https://electronics.stackexchange.com/questions/368512/how-is-a-higher-torque-achieved-in-a-single-phase-cap-start-cap-run-motor

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

On 2021-11-26 by Inspectapedia Com Moderator (mod)

@Bo,

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

On 2021-11-25 by Bo - I noticed smoke coming out of the run capacitor cover

@Inspectapedia Com Moderator, Thanks for responding. 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 2021-11-23 by Inspectapedia Com Moderator (mod) - bricksaw motorwon't start.

@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.

On 2021-11-23 by bo

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 2021-05-06 - 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 short head wire or switch or motor winding

On 2021-05-06 by Michael Kent

My 1.5 HP table saw mtor hesitate on start and blows switch in panel

On 2021-04-23 - 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.

On 2021-04-23 by Wayne

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 2020-09-14 - by (mod) -

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

On 2020-09-14 by ron

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 fixers

On 2020-06-29 - 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

On 2020-06-29 by jayrad

how to troubleshoot when this kind of freezer is harving a hard start

On 2019-08-30 - by (mod) -

Khalid

Thank you very much for your generous comment. We work hard to provide researched, accurate information without conflict of interest or as you put it, stuffing. So I am particularly pleased when a reader finds our information useful.

Reader questions, criticism, or content suggestions are also quite welcome as they help us see where more work is needed.

Daniel

On 2019-08-30 by Khalid al-Aqied - this is one of the best websites I have ever seen regarding repair and maintenance.

Honestly, this is one of the best websites I have ever seen regarding repair and maintenance. It goes directly in to the point without all the unnecessary stuffing. It is clear and direct to the point, easy to understand and giving real life demonstrations, illustrations, and samples.

I am not a technician, rather a do it yourselfer, and I can sense that all the contents and contributions of the website are genuine and are coming from real life professionals.
Thank you very much for this useful and helpful website. Keep it up. Best regards. Sincerely.

On 2018-09-22 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

On 2018-09-22 by Ortega

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 2017-11-24 by (mod) - why would they call the capacitor a dual run if there seems to be a problem in the starter winding circut.?

Thanks for asking, I agree that these capacitor types and names can be confusing.

Electric motors and how they run and use capacitors vary:

1. Some electric motors use no capacitors at all. We'll skip those for now.

2. Other electric motors require a capacitor to get the motor spinning (the start capacitor) and another capacitor keep the motor spinning after it's up to speed (the run capacitor).

So first off we have

Start capacitor - start the motor spinning - we need more of a kick to start up a motor that's not moving. Typical start capacitors on equipment you'll typically see are in the 70 to 120 uF or microfarads. It's giving the motor a big kick in the patuchka to get it started up.

Run capacitor - keep the motor spinning - A typical run capacitor doesn't need so much oomph as the motor is already spinning, so it'll be in the 7-9 uF range. It's giving the motor just a little extra torque force to keep the motor spinning.

The term "dual capacitor" can be confusing because there are two kinds of "dual capacitors":

Dual Capacitors - three- lead capacitors to combine "start" function and "run" function in a single device. So a dual capacitor for a fan motor will have three leads,

Common - incoming power


Start - power used just while the motor is starting up

Run - power used while the motor is running


I like to call thse "dual function" capacitors as they combine two functions: start, and run, in one physical device. They would not work any differently than if two physically separate capacitors were installed, one for starting and one for running, but in a single device less space is needed.

Dual Run Capacitors - three lead capacitors to combine the "run" function for two physically separate motors in a single device. So a dual capacitor for an air conditioning condenser unit will have three leads

Common - incoming electrical power


F - for Fan - to run the fan motor


H - for HERMetically sealed compressor motor - to run that big compressor motor


I like to call these dual motor capacitors since the same cap runs two different motors.

On 2017-11-24 by matthew

my question is a simple one but i just cant find an answer anywhere. a lot of online videos show a compressor motor or fan motor not starting.

in every single video i see them changing what they say is the dual run capacitor. one part for compressor, one part is for fan.

But why would they call the capacitor a dual run if there seems to be a problem in the starter winding circut.? i would think that the capacitor is a starting, not run capacitor..

On 2017-08-29 by (mod) - bad motor speed switch on spa pump

I'm puzzled too, and have to suspect a bad switch.

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.

On 2017-08-28 by David O

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 2017-07-06 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.

On 2017-07-02 by Paquito

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 2015-01-10 by (mod) - bad centrifugal switch can cause an electric motor to fail to start

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.

On 2015-01-10 by Paul J. Ste. Marie

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.

Question: is it possible to rebuild a submersible well pump

(Feb 21, 2014) Anonymous said:

is it possible to rebuild a submersible well pump

Reply:

Yes ... maybe; it depends on the condition of the pump casing, parts, an in my opinion, an accurate diagnosis of the trouble; at some point rebuilding is not cost effective. Indeed there are specialists (H Shreck in Poughkeepsie used to be one of them) who rebuild electric motors of all kinds.

There are also many water pumps whose design makes it quite reasonable to replace pump impellers.

So to answer your question, I dunno - it depends on what's broken.

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

(June 1, 2014) Anonymous said:

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.

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

(June 7, 2014) Joel said:

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.

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 HOW a STARTING CAPACITOR WORKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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• BASIC ELECTRICAL TESTS for BURNED OUT COMPRESSOR MOTORS
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  • TEST a MOTOR START or RUN CAPACITOR
• 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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CAUSES of HARD STARTING ELECTRIC MOTORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

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