motor problems

[markymark2406]

I have a job where a 3 phase motor blows the fuses on 2 phases, the shaft is spinning fine, is it a winding problem?

PS i havent seen the motor yet, so dont know if its DOL or set in star or delta.

 

[GaryMo]

You'll need to perform continuity tests on each winding and an insulation resistance test between each winding and the casing as a start.

 

[Stoday]

You might also check for a shorted winding using a "growler".

 

[ericmark]

It is common for three phase motors to just blow 2 fuses as unless earth fault no where for current in third phase to go once the other 2 have been disconnected. I have found motors where only two phases are switched.

But likely windings are problem as with no load only other fault is contactor and although they do go wrong the motor is more likely.

However only way is to find out is the test. And the first test is to have a sniff often you know when something is burnt out because to the smell.

Checking resistance to earth and comparing windings resistance is also a good pointer.

But a single shorted turn in a coil of 500 turns is very hard to detect the only way is to induce some current and this is not an on site test.

So if you can't find a fault you still may need to remove and sent to re-wind specialist. But when it is blowing fuses under no load I would expect the fault to be obvious.

The growler was used to test the amateur of dynamos and induced an alternating magnet field into it. Then a volt meter connected to a twin pin test probe was used to test each winding for open circuit and a hack saw blade lightly held above the amateur as you rotated would check for shorted windings. That takes me back some years. Never seen one used on a squirrel cage motor though. I expect one could be made but it would be a different one for each motor size!

I expect you could use one on the rotor but never done it. Hack saw trick may still work.

But unless a big motor to strip and test and repair is often uneconomic so often it's a remove and to re-winder job.

Specials like flyght pumps you may drop the stator and just return to stator for rewind but that is unusual.

But don't forget your noise it is by far the best tool for testing motors.

 

[conny]

After dismantling the motor and removing the rotor, its possible to connect the motor in 'Star' you can then pass a lower voltage through it say 100vac, and check the current readings across the 3 phases. This of course after you have checked the IR and the resistance of each phase is balanced. If there is any discrepancy then the windings are probably shorted between turns and a rewind or replacement is necessary. Make sure you have an earth strap on the motor casing before doing this test.

 

[securespark]

Hardly DIY....

 

[conny]

Neither was yours!

 

[ricicle]

Hardly DIY....

Yes but, Mr Grumpy, sometimes we ask each other questions don't we - not just the DIYers..........

 

[VanSolo]

If it,s a new or replacement motor, are the terminal links correct?