You are here:

Electric Motors/single phase motor stops running


I have a single phase electric motor on an air compressor.  Was running fine (compressor is almost new) then it popped the auto shut off switch.  I pushed it back in and seemed to be running fine.  However, it has started causing problems.  When I first start it in the morning, it runs for less than a minute and then starts slowing down and stops. After that when attempting to start the motor it hums and won't start.  I see the centrifugal switch opens and then shuts when the motor shuts off.  The run capacitor was checked and is ok.  I don't know if the centrifugal switch could cause this type of problem, but it does not seem likely.  Either the start or run windings could be bad I guess but would that cause this kind of problem?  Can you tell me how I can test the windings to see if there is a problem there?  Any other ideas or suggestions would be helpful, thanks.


Can you measure the voltage at the motor?   compare to nameplate?

Can you spit the lines and measure the no load current or even the loaded and compare to nameplate?

So it starts   then it runs   and all is fine  but you don't know at what amps when it is running  right?

If your voltage is high or low  that is one issue,    

You say the run cap tests OK   .

Now when it runs a while and stops  what stops it    ?     Does it sit there and hum or does the thermal pop or blow a breaker?    

If it pops the thermal   the thermal can be weak,  

If the voltage is high or low compared to name plate it might start but the current is on the cusp of bad and so it runs but it builds heat and trips the thermal,   so knowing exactly what the running voltage and current compared to name plate is absolute   needed  

That will tell you a lot,    next you are right it is unlikey the switch is doing this   if the windings are covered with dust and contaminates from the enviroment  it acts like a water heater blanket  and could be building heat   GOT SOME DRY COMPRESSED AIR TO BLOW IT OUT?

Maybe the cap checks OK static, but it is also breaking down as it runs,    

How does it coast    it is possible you are adding false load with a bad bearing or housing or both    the voltage current testing will tell you if you are in range or way over or somewhat out of range but you need to know,    

Is it belted,  ?      Are they aligned and are the pulleys worn out?

Is the compressor somehow not unloading   is there a shut off circuit from a storage tank or is setup as a run only compressor no tank?

If there is a tank is it failing  and turning the motor off?   I need to know when it stops what is off,  the overload     the breaker or is it sitting there humming,   ???

Now you have this other clue  once it gets warm  it stops   and once that happens it wont start again   UNTIL WHEN         when it cools down?

Or when the pressure gage bleeds off,    ?

It sounds like the motor is either in overload by the voltage or current    the run cap could be breaking down as it warms up,      

A bearing could be spinning in the housing and warming up   when it gets hot enough it puts so much false load it trips something       

How old is the motor,  did the compressor come with this motor or is the motor just a motor you got a compressor for or the other way around    ???

Down and dirty cheap,     blow out the motor,   

Run the motor uncoupled unbelted from compressor      same thing?

Get a voltage reading and a current reading   

If the motor has any age   5 or so years,    replace the caps   both the start and run     I assume this has a start cap    it has a switch         so if the run cap is good then it has to have a start,    is the run cap on the wires where the start cap should be      did they get swapped ?

Feel each housing   is one hot  real hot    more than the other,?

The real hot one might not be the cause it might be the thrust is too much   or both ends have been loctited or captured      only one end can be captured with a bearing lock plate or loctitied  if it was a bit over, but it both ends are locked  no room for thermal growth

Then it binds and locks up,   but what is tripped when it goes down,  

So run with nothin connected     and blow it out,     but do one at a time so you know which one is the likey issue,  get the current and voltage AT THE MOTOR<   tell me that the data plate says and what your voltage and current are compared to the nameplate

Process of elimation one thing at a time     measure voltage and current  load connected load not connected,  is it over name plae with compressor connected to it belts or whatever

Give it a good blowing out,   make sure the outside shell is clean   and not holding heat,   

Once you do all that  start buying caps  they should be not that much   anbd if the motor is old it will run a little cheaper,       

What HP?   If you have to replace it and it is cap run cap start  I would guess 4 to 5 HP?

If the windings are heating up and breaking down I would have thought it would have smoked by now,   but you never know,       

Consider if the motor turns out to be bad or so costly to repair you need a new one,   get a VFD if the HP is not to big,  not one of these starting devices  that gets a three phase going and then single phases it,  you want one that you can apply 230 volts single phase in and 230 out three phase REAL THREE PHASE      sort of    they are really good now days,   three phase has the overloads ,  the nema ! enclosure,  48 parameters to tune in the motor,  it is so much better than single phase,        

THree phase motors are like 1/3 price of single phase HP TO HP      so you can almost buy the drive and the three phase motor  for the price of one single phase motor,   as you go through the tests write me at my private address and we can re evaluate as we go      get a clamp meter and volt meter so we have a data line to start from  

Electric Motors

All Answers

Answers by Expert:

Ask Experts




Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


30 plus years in the electrical motor and apparatus repair industry. VP level management of repair facilities, current owner of my own specialty repair and consulting firm.

EASA, IBEW [retired], other specialty organizations, Lubrication, Vibration EDI, Tribo-electric Councils

Currently fielding concerns at this site under "Home Electrical"

4 year technical, College level specific courses, EASA repair courses, vibration analysis electronic and electrical trade school.

©2016 All rights reserved.