Help with Squirrel Cage Motor .

[BQW]

Need some advice, for a work College, regarding a small single phase 240V 50 Hz, 270 Watt, Squirrel Cage Motor, it appears to run correctly and at the the right speed, but gets very hot, even with out any load, and according to the reading on a calibrated Clamp Meter pulls 9.5 Amps on start and 7 Amps on run ?
I have not seen or tested the Motor yet, but has any one, a suggestion on the correct resistance reading expected across the Field coils, it apparently has start and run windings connected by a centrifugal switch.
The fact that it runs at the correct speed, gets hot and pulls what I would consider excessive current, makes me think, is there possibly, some leakage to earth in the Coils ?
I intend to get access to the Motor tomorrow and I will then "bell it out" with a Megger, I have also got a Inductance Meter, but have only a very rough idea of what the inductance reading of the Coils might be, any advice on this test would be most appreciated .

 

[Jackrae]

Possible that centrifugal switch isn't opening so start coil remains energised

 

[ericmark]

I remember a submersible pump single phase where the electrician who disconnected it did not realise it was single phase, two windings, both around same resistance one run one start and we got them wrong way around which caused over heating.

 

[Harry Bloomfield]

I agree, the centrifugal start switch is stuck. It stays in the start configuration.

 

[bernardgreen]

It might be shorted turns in one of the windings due to break down of ( or damage to ) the insulation on the winding wire..

Shorted turns will absorb a lot of energy and get very hot without significantly reducing the torque.

 

[Jackrae]

Go for the 'simple' faults first - the CF switch

 

[conny]

As stated, the centrifugal switch, if it has one, is the first port of call. If it has a capacitor(s) this needs to be checked with a capacitance meter, not a basic multimeter, to check they are not failing or failed.

There are no 'correct' figures for resistance readings. Working on motors daily, you get an idea of what readings to expect for various sized motors that run on 3 phase. However, single phase are a different ball game. The 'Start' winding always has a higher resistance than the 'Run' winding. Unfortunately the same sized motors, (I'm talking wattage as opposed to physical size), from different manufacturers can have totally different resistance readings for each winding.
If you can see the 2 sets of windings it may be easy to see if there is any damage to them. Smell the windings. If they are looking dull and flat in appearance it may be a sign of high temperatures. Look for traces of overheating on the enamel coating or melted insulation around the legs of the coils. do any of the coil bindings look brittle and flaky? What is the lead insulation like? Soft and pliable or hard and brittle? If everything looks ok see if it is possible to open the cross connections between the coils on each winding. If it is then break the connection at the half way point and test each half. They should be practically identical. If they are different then the side with the lower resistance is faulty.
WARNING: If the coils are do have shorted turns you do not have the equipment or knowledge to repair them so don't even bother tying. In this case it is a new motor. 270w is not financially repairable.
There is a chance that this may be a salient pole motor. i.e. it has 2 coils opposite each other and a copper strip around the core slots. These may be damaged instead of the coils. Again, not repairable. The fact is, if you cannot find a repairable fault within 30 minutes, it will be far easier and cheaper to buy a new comparable unit.