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Electric motor test & repair diagnostic FAQs:
Questions and answers about diagnosing and fixing electric motors. The page top photo was taken of of an oil burner electric motor not an air conditioning blower fan motor or pump motor, but you'll see that all of these electric motors 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.
At left our photo illustrates the motor as typically found in a direct-drive HVAC blower or air handler assembly. (BLOWER FAN OPERATION & TESTING)
[Click to enlarge any image.]
These questions and answers were posted originally at ELECTRIC MOTOR DIAGNOSTIC GUIDE - topic home. Be sure to check that article.
(Jan 31, 2015) Anonymous said:
heat pump air handler variable speed works, warm air comes out vents. here's the serious problem though. when i go to high speed it runs for 20 seconds, then it goes to low speed for 20 second or so. this cycle continue and lately the high speed act like it wants to quit. any help would be appreciated.
Anon I'm not sure if the problem is in the motor itself - sounds more to me like a control or relay problem.
(Feb 7, 2015) email@example.com said:
Ran my water pump with the live and neutral reversed will his have done any damage to the motor
Some AC motors may work normally when wired backwards; but I don't know what motors and controls (and starting capacitors) your system has installed.
If the pump still runs and no breakers or fuses blew you may be just fine.
For many motors, in the short run no damage may have occurred to the motor but the wiring was unsafe risking electrocution, and also risking damage to the motor controls and switches.
(Feb 9, 2015) Les said:
I have a 3/4hp cap start ac motor which starts fine but trips the breaker after running 30 seconds or so. does not get hot to the touch and capacitor tests within specs. Any ideas?
I'd expect an electrician to look for high current draw which would confirm that the breaker is doing its job and there's a motor or wiring failure. The motor may have a failed run winding or bearing; Or the driven equipment could be binding causing excessive motor loading.
Also check the supply voltage.
Safe would be to leave it off of course.
(Feb 11, 2015) Soham said:
can sometimes condenser be called a capacitor in case of water pump
Yes Soham in some areas people use the term "condenser" where in most articles at InspectApedia we use the term "capacitor"
Synonyms for capacitor include: condenser and electrical-condenser.
Also I'd include
Leyden jar, (and mis-spelled Leiden Jar)
trimmer or trimming capacitor
Capacitors store an electrical charge and release it in a nice big zap.
Other electrical storage devices are various types of batteries or storage cells.
Other "synonyms" posted at some thesaurus websites are nonsense and include very generic terms like CRT AC and Circuit that simply do not pertain.
(May 13, 2015) Solomon said:
My 2horsepower Electric moter for a centrifugal pump at times starts and runs slowly for few seconds and stops. At times when switched on it shotdown other electric motor in the system. what shoud I do?
I'd check the current draw of the motor - it sounds as if a failing motor (or possibly a hard-starting motor due to a bad start/run capacitor) is the trouble.
(June 15, 2015) jim dyson said:
weather king furnice outside unit & fan run continuously inside blower cycles on & off does not cool the house
Sounds like a refrigerant or a control problem. You need an on-site service technician to sort this out, Jim.
(June 16, 2015) Mike said:
I have a 5 hp spa pump motor that fails to self start if the rotor is positioned anywhere within about a 90 Degree segment of the entire 180 degree rotation. In other words if it stops at anywhere from 12:00 to 3:00 it will hang and not begin spinning without a manual assist. Anything outside of this arc and it works fine. We just rebuilt this motor with new bearings, start and run caps. We even hooked another start cap in parallel with the original with no help. Any idea why this is happening?
Do you think it's possible that the motor has an open start winding? Also, double check that the start / run caps are working - that the replacement parts were ok
(June 21, 2015) robert said:
my fan motor on my central heating and air conditioner keep witching back and forth. the air flows out the top to cool the compressor but the next time it come on the motor turn the other way and pull the air in causing the compressor to over heat
This is a control problem not a problem with the electric motor, Robert
(June 25, 2015) Tim said:
3 Phase motor hums and doesn't reach full speed. Is it single phasing or is the fan plugged?
Check for lost power on one of the phases. I think your motor is single phasing but this isn't my expertise.
(July 3, 2015) Mike said:
We rebuilt two identical motors. The other is fine. We swapped all the caps between the motors also with no effect on the problem one. I thought we might have overheated the winding but it looks brand new and has no smell of burning inside. If the start winding was open, why does the motor start as long as it doesn't start from that arc? I was wrong about that angle as it is 90 degree of 360 degree. I suppose we'll tear them down again and check resistance. Could the centrifugal start up switch be at fault?
Very interesting Mike. Could a winding be opening when the motor spins - or some other part moves?
Those defects can be hard to spot by mere visual inspection.
(Sept 29, 2015) Mike said:
I lost the link to here until now. The spa motor with the start problem (July this year). We tore it down again checking all we could and polished the kick out switch contacts with 2000 grit sand paper. This reduced that 90 degree no start arc to about 10 degrees.I think it's resonable to assume that the switch is the problem. Now to figure out where to buy one.
(July 7, 2015) Eric said:
I have a boat lift A/C 1hp leeson motor, 1phase, i control on a barrel switch, when i start and stop the motor i hear a loud pop, sound like arching, the lift seems to work fine other then the noise, any ideas?
Look for a shorting switch or wire - an unsafe condition.
(July 17, 2015) john said:
Single phase motor runs for a min and shuts off i pulled it out of the furnace and hooked it up and does the same thing gets super hot and turns on for 3 secs shuts off for 10 and repeats whats going on here
Could be a failed bearing or winding causing overheating that shuts the motor off via a built-in thermal protection switch.
An electrician would probably check the supply voltage and the current draw.
(July 26, 2015) taffy said:
electric mixer will run if i hold my finger on start button but stop when released also when it does run if load it stops
Sounds like a failing motor or bearing, less likely, low voltage.
(Nov 9, 2015) Mike Bolden said:
Coleman air compressor trips circuit breaker on restart. 5hp belt driven Mod MP0502513. Did not run at all. Caps tested bad -replaced. Runs now. On startup, it reaches cutout and stops. On restart, it reaches cutin and starts but after about 2-3 seconds, it trips the circuit breaker. It has a new pressure switch w/ unloader and a new check valve. I'm thinking motor now. Do these motors have a centrifugal start switch in them? It may be slow to operate on restart holding the high current draw too long and causing breaker trip. Any help is appreciated.
Yes some motors (I don't know yours) include a centrifugal switch to cut out the start cap when the motor gets up to speed.
Do you think the motor itself could be seizing? If it's disconnected how hard is it to spin?
Also check that the air compressor is not trying to start against a high head pressure.
(Nov 21, 2015) Aaron said:
Electric motor overheats on low speed
If your motor is not designed to run at variable speeds then the problem may be
- low voltage
- failed control
- faulty wiring
- internal damage in the motor
- bad start/run capacitor
(Feb 14, 2016) Dan said:
My spa motor doesn't seem to have capacitor, just a small contact panel. Motor hums then shuts off. Is that contact panel a capacitor?
Dan I'm not sure what your system has - not all motors use a starting capacitor. But if the motor hums and then shuts off certainly it's having trouble starting. The problem could be that a bearing is seized or that voltage is abnormally low. I suspect a seized motor bearing as that's common with spa motors exposed to leaks and corrosion.
(Mar 6, 2016) john said:
the motor on my 1'x42' belt sander has a capacitor lately when I turn it on starts an stops 4 or 5 times before running at speed. Is this the capacitor?
It depends on motor type:
A motor may have a start and run cap of which either or both have failed;
A motor may have a centrifugal switch that switches out the start cap when it gets up to speed - also failed.
Or there could be
- a loose wire
- a binding bearing
- a loose or failing switch
- or something else I've not considered
See if you can turn the motor freely by hand when it's not connected to anything else (and power is off) - that may point to or away from a bad bearing.
(Apr 14, 2016) electric motor wont start said:
Electric motor wont start, draws 27.5 amps and wont turn. Motor is free and can spin by hand. It is a 1/2 hp lifttech motor. Any ideas what could be wrong?
I suspect a bad start/run capacitor or an open winding. If the amps draw you cite exceeds the starting amps on the motor data plate then it may be shorted internally. That seems most likely.
(Apr 14, 2016) Doug said:
The data plate has no starting amps listed. The FLA 6.3, so would the 27.5 not be pretty close to the start current?
Doug: you should be able to find the specs of the proper capacitor you need from the wiring diagram or instructions for your lift, and most likely you can find a generic replacement that will work. For **Example** check out the Universal 24889 - R17058571-BH Ballast Capacitor - to see if that matches your specs.
(Apr 26, 2016) Jose said:
BlowerMotor works when thermostat is in "on" position and with switch in the middle of "warm" or "cold" (off position). After a couple of hours I can move the switch "warm-cold" to "cold", leaving the other switch in "on", because if I move it to automatic blower stop, and I have to start all over again.
Forgot to say that already I changed the thermostat and the capacitor.
Sounds as if you are not setting the thermostat fan switch to a normal position. It should be fully in Heat Cool or Off. Not sure what "other" switch you are leaving on nor just what is the question here.
(May 2, 2016) Anonymous said:
Power to elec. Motor on lawn mower but still won't go
See ELECTRIC MOTOR DIAGNOSTIC GUIDE for help in diagnosing and fixing the trouble
(May 28, 2016) Anonymous said:
How to change direction of notoriously
Anon I think your spell checker has tripped us up.
(June 4, 2016) Anonymous said:
ac motor not getting electrial
Check for power at the supply connections. I'd trace the circuit backwards to find the open wire or switch, or forwards from the breaker or fuse.
See ELECTRIC MOTOR DIAGNOSTIC GUIDE for help in diagnosing and fixing the trouble
(Aug 5, 2016) O. Roman said:
Hello. I have a furnace air fan blower with blade housing that continually jerks when it turns off. I have the thermostat set to auto and every time it turns off it jerks. The motor is screwed on tight onto the housing but the motor continues jerking on auto shutoff. Twice the motor has broken one of the three metal supports that attaches it to the housing. And twice I have had to weld this back onto the motor and screw back onto the housing. After the last weld the motor will jerk every now and then. What could it be? An electrical surge right before the thermostat cuts it off? Thanks.
I'd look for a loose fan pulley of motor or fan bearing and for other loose parts, a loose electrical connection, or a stretched blower pulley belt (if your motor is not a direct drive unit). Also a motor drive shaft misalignment could be torquing the system parts at an angle.
(Aug 9, 2016) Anonymous said:
Thank you. I'll check on it and let you know what I find.
(Aug 25, 2016) Ivan said:
Drill press motor starts and runs, then it appears the start switch cuts in and out.
Look for a bad switch or wiring connection first. Check for a difference in behaviour if the drill press is under load.
(Sept 6, 2016) Tonya Kahle said:
What do you do when your ac wiring is burnt up?
Watch out: you are describing a dangerous condition risking electrical shock, death, or fire.
Check for a bad circuit breaker that is not tripping when it should, then check for a short circuit in the elevctrical circuit for the system you are debugging. Search InspectApedia.com for FPE CIRCUIT BREAKER or ZINSCO CIRCUIT BREAKER or BAD CIRCUIT BREAKER to see examples of these failures.
(Oct 6, 2016) ANGEL CERECRR said:
I have a emersión fan motor,got 4 wires,two white,a brown And a orange,how i conecct to work con a convencional fan?
Let's start by identifying the product brand and model, then we can find the wiring diagram for your model that identifies the function and connections of each of the wires you cite.
I stripped and cleaned an electric motor on reassembly it spins backwards Why?
Please see ELECTRIC MOTOR RUN DIRECTION to diagnose and repair an electric motor that is running backwards.
(Oct 27, 2016) Richarf said:
I have a aerator motor that went down and stopped working. I tried to plug it directly into my outside outlet and it trips the outlet as soon as it's plugged in. I checked wiring and motor runs fine with the ground wire disconnected. What's the cause of this?
Watch out: Richard
Sounds as if the motor or its wiring are electrically shorted to ground. DO NOT USE IT as doing so risks death by electrocution. The wiring needs repair - perhaps internal to the motor if you don't see the damage outside.
ELECTRIC MOTOR DIAGNOSTIC GUIDE describes a lot of diagnostic test procedures for A/C electric motors such as those found in air conditioners, heat pumps, washing machines, clothes dryers, condensate pumps, well pumps, even garage door openers. But what about direct current motors such as used by hobbyists and in some automotive or boat applications?
Hobbyist LB Miller has described the design and function of a simple test fixture useful for determining the electrical resistance of DC motors by providing a 1A current to the motor and measuring the voltage drop across the motor, thus giving motor resistance in milli-ohms. Note that his approach is for DC motors. 
(Feb 18, 2014) Anonymous said:
I have a Bosche table saw and the motor has malfunctioned. It is a 120volt motor. When switched on it "hums" but does not spin. What could be the cause of this? Is it repairable?
I would remove the motor from the saw completely - for safer testing.
With the motor out of the saw, see if you can rotate the shaft by hand.
If you can then the shaft is (probably) not bound and the problem may be a bad start capacitor (if there is one). There are also some simple tests in the article above that you might want to try when the motor is out of the saw.
(Apr 7, 2014) Jan said:
My furnace 1/2 hp blower motor starts and runs quietly from cold but will not start and makes a loud buzzing sound when hot until the thermal breaker kicks in. The motor is hot to the touch when this happens. It will go through one heating cycle, turn off and not restart. BTW the motor is only 1 yr old.
Buzzing is often a bad relay, switch, or control board. Hot to touch motor sounds like a failing motor or motor start capacitor, possibly showing up when the system is hot. It's time for a call to your heating service tech. Let us know what s/he finds as it will surely help others.
Sometimes a hot electric motor won't start because the motor shaft is binding in the bearing.
Sometimes a hot electric motor won't start because at the higher temperature a winding or electrical connection has opened.
(May 14, 2014) Jaime (from Huntington WV) said:
Hi! In 2010 we replaced our outside central air unit, without replacing the furnace inside. The forced gas heating inside works great for heat in the winter, but in the summer when it gets humid, or newer AC in combination with our older fan/motor inside the house do not work well together. I have very cold air coming out, but by the end of the day when the thermostat is set on 69, it just keeps going up and up to about 71.
From what I have been told, the humidity makes the air thicker which in turn the old fan/motor in the furnace part can't handle the cool air from the new AC, which in turn my duct work under the house sweats terrible. Can I replace the motor in the house with a newer/more powerful motor without replacing the entire furnace inside? I hope you understand what I meaning. =) Thank you! Email me or comment back BillandJaime2004@gmail.com
would look for an operating problem, possibly improper refrigerant charge, or dirty filter, that is causing the coil to freeze - that could explain the reduction in cooling capacity as the day wears on. Look for frost or ice on the cooling coil.
Another direction of diagnosis: if the system is oversized then it will cool the home too quickly, failing to adequately dehumidify.
(June 16, 2014) Robert holt said:
Hi. I have a cooker hood motor that hums when fan speed is selected but does not turn. I have taken it out from the impeller casing, and if I give ithe shaft a quick turn whilst power is on, the motor spins, but as soon as turned off, does not restart on button. Any help please
It sounds as if the fan motor needs a new start or start-run capacitor.
(July 23, 2014) B said:
Hey guys - I'm having a problem with the blower motor in my carrier air handler. I keep it oiled regularly every yr to 2 yrs as equipment suggests. Started squeaking on me when we turned our ac on this summer. After cleaning / lubricating the motor, restarted ac and it ran fine for 4-6 hours then wouldn't not kick on when tstat showed a higher inside temp then set at. I check ed the motor and all was clear and well lubed and restarted.
Ran OK for a period then same issue. Motor became noisy again and will only run intermittently and then get noisy and shut itself down. Is this a case of bad bearings / windings in the motor or could it be a wiring issue? Any helps would be great. Thanks in advance.
Check first that the blower assembly itself is spinning freely, not binding, and doesn't have worn bearings.
Then consider that the motor may be failing internally, possibly a bad bushing or bearing.
You might extend its life with a hard start capacitor kit.
(July 23, 2014) Jeff Wilmsmeier said:
Can you oil a Emerson K55hxkwa-9803
Jeff I had trouble finding a manual for this fan motor online too - so I don't know the answer.
Look closely at the motor assembly. Oil ports will be obvious, often capped with a hinged cap or a hole facing up marked "OIL" on the steel body of the motor.
Or send us some sharp photos of all sides of the motor and we'll comment further.
See ELECTRIC MOTOR LUBRICATION for detailed advice
(Aug 22, 2014) Blower fan internal heat switch cuts motor after 30 min. said:
Had an HVAC CO provide a free inspection. A/C was working fine for prior 1 yr. Tech added refridg. and change dirty filter in attic. Now Blower Fan only runs 30 min and cuts off. Back on in 20 min. After 4-5 more visits, Tech wanted to sell me a new A/c at $8-$10,000. Found bent blower door and improper/wrong filter. Also, refridg. was 2 lbs low so none had been added by Tech.
What type of Blower Fan filter provides "0" airflow resistance...?
Thank very much,
New motor, cage bearings oiled and spinning freely. New fan belt. I removed the "new" filter and motor is now running cool enough that it is not cutting off from overheating. So I guess I am looking for a filter with no resistance.. any thoughts?
I'm surprised that a tech would add refrigerant if the system was working properly - though we should keep in mind that a skilled HVACR tech may still not be an English major - so may not have explained what s/he was doing or why. Now I'm not sure what's wrong, but that offer of a costly new system is disturbing.
Ask to be shown and told what expensive parts are shot and how that diagnosis was made.
Then call a second HVAC company and ask for a second opinion before we accuse anyone of something further. If your system is an old one it might be at end of life, but working up until the time the tech touched the system is indeed worrisome.
A filter that gave zero air resistance would probably also not filter air passing through it.
A very very low air flow resistance air cleaner that is widely used is an electrostatic air cleaner. However usually we see these used in combination with a more conventional particle filter.
Kudos on getting the new motor going.
There are no zero-resistance air filters - since filtering will always create some resistance. But filters are rated for flow resistance. About the lowest flow resistance type is an electrostatic air cleaner. Typically those are preceded by a washable filter and some by a low-resistance fiberglass or other paper filter.
Take a look at inspectapedia.com/aircond/Best_Air_Filter.php
(Aug 30, 2014) Mike said:
Blower motor in Nordyne heat pump will not turn on. I have tried overriding thermostat but still won't kick on. I checked for voltage at transformer (240v-28vac) and looked good. I wanted to know how to test the blower motor relay. I thought I should see ~240vac across black and white wires of motor.
Check the motor data plate - it more likely is a 120V unit.
A 28VAC transformer output is more likely being used to operate the blower's controls.
(July 18, 2015) Allen said:
Thanks for your prompt reply. After checking the voltage, the main and start windings are within specs. I did however receive a reading suggesting the motor is grounded. Could a damaged wire in the well casing be causing the symptoms?
Double-faults can occur but Occam's razor says we should look first for the simplest explanation as it's most likely.
Is it possible that during the repair a pipe connection simply was not perfectly tightened. For example I've found air leaks into piping (and thus potentially loss of prime) when a hose clamp on plastic piping at joints was not quite tight.
A problem is that air leaking in to the lines may not be so easy to detect unless your system has a clear plastic water filter in which you'd see bubbling and frothing. But you might start by tightening or double-hose-clamping every accessible connection
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