DC Motor Equalizer Resistance values 1

[Ria05]

Hi
Need suggestion regarding DC traction motor Equalizer.
During periodic overhaul, while testing the commutator the bar to bar resistance values are within limit of 0.6 to 0.71 milli ohms, however the equalizer to equalizer resistance values 12 in a row are showing higher resistance values of around 1.4 to 1.59 milli ohms compared to other values which are around 1.15 to 1.29 m-ohms.

As a thumb rule if the resistance values 6 in a row are high then we suggest for rewind. However there are no relevant files on limits and guidance. Looking for suggestions on this.

Thanks
Ria

 

[edison123]

The equalizer winding is connected in parallel to the armature winding. How did you measure the equalizer winding resistance alone?

Allowable bar to bar minimum & maximum resistance variation from average is ± 5%.

Depending on how many equalizers are used, resistances may not be equal in every bar to bar but there will be a repeated pattern in resistance variation. e.g. Every 3rd bar could have a higher resistance and than the previous 2 bars.

Muthu

http://www.edison.co.in

 

[Ria05]

A total of 101 Equalisers have been used in the old design. So the resistance value measured from 1 to 104, 2 to 105, and so on till 101-204.

Thanks
Ria

 

[edison123]

It's 4 pole motor? For 204 segments, you should have 102 equalizers for 100% compensation and equalizer coil pitch should be 1 - 103.

Are the coils brazed or soldered? If they are soldered, then you can try resoldering those high points and most likely you will succeed.

Just because you are measuring resistance across the equalizer coil pitch (which in your case is also wrong), it does not mean you are measuring equalizer coil resistance alone. It is always in parallel with armature winding once it is connected.

Muthu

http://www.edison.co.in

 

[Ria05]

Usually, the equalizers are tig welded and are not accessible until we remove the armature winding. I am trying to see if the motor can be put to use without rewinding, considering bar-to-bar resistance value measurements and equalizer resistance values, standard practice to decide either to rewind or reuse the same.
Since DC motors are a new area to me I am not sure of interpreting these results to decide whether to rewind or not.

Thanks
Ria

 

[waross]

You may try a brush position test.
Apply 120 VAC to the shunt field.
Normally, the brushes are shifted to the nul voltage position.
However, for trouble shooting, connect a voltmeter to the brushes and revolve the motor slowly.
Serious problems will often show up as a varying voltage at the brushes as the armature is turned slowly.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[panter]

Did you have a good reason to check the resistance or was it done in regular inspections. If there is no bad commutation or any problem, it is a weak reason to think about rewinding. What is the motor power?