DC Generator for Diesel Locomotive

[chad44]

I'm not sure if this is the right forum for this question or if I should go to Electric Power Engineering. I am working with a group that is restoring a 1950's vintage Diesel Electric Locomotive. The Auxiliary generator (used for battery charging and controls) does not work. It is a 25 kW, 75 VDC generator. I was told it has a short somewhere. In my garage, using a digital ohmmeter, I could find no short nor any open windings. Was wondering if I should use a megger, and if so, what would be the maximum voltage that I should apply?

 

[ScottyUK]

Chad44,

Is this a permanent magnet generator, or a wound-field generator? If it is a PM machine, it is possible that over time the field magnets have lost their strength, thus causing your problem. If it is a wound-field machine, can you prove you are passing field current? Insert an ammeter in the field winding circuit. No field, no output.

Regarding Meggering the machine, I think 250V would be the sensible limit for an older machine.

 

[chad44]

It is a wound field machine, not a permanent magnet type, so I will try to measure field current as you suggest.

 

[jbartos]

Suggestion: 100V megger is sufficient for the insulation resistance measuring. The commutator also needs a special attention to have it cleaned and no shorts between lamellae. It is a bit of dirty work; however, it may be necessary to do it.

 

[ScottyUK]

Hi jB,

Standard Megger voltages across the pond here are 250/500/1000/2500/5000V, hence the suggestion of 250V. I guess the US market has different equipment for testing to the NEC.

 

[jbartos]

Suggestion: Visit

http://www.inotek.com/Catalog/biddle1pm.html

for megger test voltages from 50V up.
250V level would be too high for very low voltage circuits or voltage levels.

 

[UKpete]

chad44, have you stripped and cleaned the machine? If there is a short, there's a good chance it will be visible regardless of whether it is in the field system or on the armature.

A good megger reading to earth doesn't mean it's ok. There could be a short between armature coils etc, these may need a more sophisticated test e.g. a growler. Or a good eyeball.

Who is the manufacturer of the machine, I'd be interested in any details. Is it belt-driven off the main generator?

 

[chad44]

When Alco first got into the Diesel locomotive business in the 1940's, they partnered with GE. GE provided all of the electrical components. The generator is belt driven off the engine. It is actually two generators in one housing. One end of the unit is the exciter generator, which provides the exciter voltage for the main generator (which drives the traction motors). This end of the unit works okay. The other end is the auxiliary generator which charges the batteries, and provides power for lights and controls. This is the end that does not work. I have not yet completely dissassembled the unit.

 

[aviat]

Older aircraft used DC generators made by GE and sometimes, especially after storage, the field would have to be flashed to get it going. Some systems had to be flashed on every startup. But you need the generator installed to do that.

Other things to check are the brushes. They sometimes get hung up and do not make contact with the commutator. Also there are several components in the circuit like over voltage and over current protection etc. that can cause problems.

 

[Warpspeed]

If it has been laying idle for many years, there may be accumulated moisture in the windings and insulation.

With old transformers and motors, and so on, it might be prudent to dry it in an oven for a day or so. Or alternatively pass a few amps through all the various windings for 24 hours. Just enough so the windings run nice and warm.

The megger readings might typically go from a few tens of K up to tens of Meg, and your worries about something tracking over will be greatly eased.