testing motors help

[gullyboy_131]

anyone know what sort of faultfinding/maintenace tests you would carry out on 3phase ac/dc motors and the test instruments you would require,im from a industrial mechanical background and enrolled on a electricians course, i currently take out motors and just change like for like.

thanks any help will be fantastic

 

[Mr Sworld]

The basics are meggering between the phases and earth and testing for continuity. There are plenty of other tests for motors but they require specialist equipment which is a little pricey!

When the wiring fails on the motors you have little choice but to call in a specialist company to re-wind them. Especially as big ones weigh a tonne or more!

I generally find the 'easy faults' i.e. loose connections, knackered isolators, faults in the control panel and repair them. Any faults with the motor rotor or stator then the specialist company is called!

 

[gullyboy_131]

cheers for the reply ,just one thing about testing continuity of windings, would you carry out this across terminal connections for the supply of motor u1,v1, w1

thanks

 

[Mr Sworld]

Yes. Just test between the combinations of U1, V1 and W1 to make sure there isn't a open circuit.

As most three phase motors are squirrel cage types the biggest fault is 'single phasing' (you've lost one phase somewhere).

If you megger it don't expect huge readings! If you get below 2 M ohms then it probably needs checking by a specialist firm as the lacquer on the windings will of started to crack and flake.

 

[bassman]

anyone know what sort of faultfinding/maintenace tests you would carry out on 3phase ac/dc motors and the test instruments you would require,im from a industrial mechanical background and enrolled on a electricians course, i currently take out motors and just change like for like.thanks any help will be fantastic

Get your tutor college trainer to go through it with you if you state it as a question they usually go into the operation and testing of moters as it can pass a lot of time and some people learn for the future.They are different beasts ac and dc electric motors.

I was taught to disconnect the motor from any torque before testing,whether belt driven or coupling. then you could check the phases without doing any harm whilst starting, alot of problems were on the switching gear.Contactors etc.

All the best good fun fault finding.

 

[Evans Electric]

For a DOL , best to disconnect from starter in case there is a problem there, then megger to earth (500v) and across for any breakdown then resitance U1 - V1- W1. Usually looking for all three windings to be the same resistance, usual fault would be one winding different to others or open circuit. Star/Delta windings would be U1-U2 V1-V2 and W1-W2 .

Older motors are marked up A1- B1- C1 BTW .

Deke

 

[gullyboy_131]

For a DOL , best to disconnect from starter in case there is a problem there, then megger to earth (500v) and across for any breakdown then resitance U1 - V1- W1. Usually looking for all three windings to be the same resistance, usual fault would be one winding different to others or open circuit. Star/Delta windings would be U1-U2 V1-V2 and W1-W2 .Older motors are marked up A1- B1- C1 BTW .

Deke

a little bit confused, i was under the impression to measure resistance/continuity i would test u1-v1 u1-w1 v1-w1 by what you saying is u1-u2 v1-v2 w1-w2

sorry to be a pain , but on holiday from college and i am not one for waiting

cheers for the help

 

[Blue Fox]

a little bit confused, i was under the impression to measure resistance/continuity i would test u1-v1 u1-w1 v1-w1 by what you saying is u1-u2 v1-v2 w1-w2 sorry to be a pain , but on holiday from college and i am not one for waiting

cheers for the help

It depends how it is wired, if its star or delta as that makes a difference to the layout of the cables in the box. Bassically imagine it as 3 coils, if they are wired in star then the u1,v1 and w1 are all joined together in the terminal box with a bar (this is the virtual earth and is in theory 0v), then you will have the u2 v2 and w2 opposite them in the box which are connected to the incomers. To see if the motor is "balanced" measure from the bar to each of the phases. on the other side. The actual values vary greatly from motor to motor but the three on the motor you are testing need to be pretty much exactly the same. If they are then the motor is balanced - this is the "winding resistance", the next step is the insulation resistance, to do this I take the connector bar out and test each phase to each phase and each phase to earth with the megger and this then confirms that is ok - you should get quite a high reading.

The motor could be wired in delta this is like a triangle with each side being a winding. So you would check from U1 to U2, V1 to V2 and W1 to W2 but where on a star the link bar went all down oneside of the junction box, this time it goes accross in 3 strips, this means you connections are in a nice simple u1 v1 w1 on one side and u2 v2 w2 on the other side, in this case you will have u1, v1 and w1 on one side but if you imagine the triangle u1 will connect to w2, v1 will connect to u2 and w1 will connect to v2 so your connection will actually go u1 v1 w1 on one side and w2 u2 v2 on the other. To test the winding resitance just go between the u1 - u2 etc again and check they are all balanced again and megger as before. The only other test I have done regularly is a current check with a clamp meter on each phase to make sure its all balance current wise. As others have said if its more then that then you are into specialists. The most common fault is single phasing where you lose 1 phase (should be called double phasing I suppose) and it will sound very sick if it runs at all and will run very slowly, this is usually simple to find. The other fault is insulation resistance breakdown which usually leads to smike and buring so is easy to see and check with a megger if you suspect it.

 

[Evans Electric]

See Blue Foxes post, it explains it better than mine !! :|

Deke