DC Generator for Diesel Locomotive
DC Generator for Diesel Locomotive
(OP)
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?





RE: DC Generator for Diesel Locomotive
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.
RE: DC Generator for Diesel Locomotive
RE: DC Generator for Diesel Locomotive
RE: DC Generator for Diesel Locomotive
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.
RE: DC Generator for Diesel Locomotive
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.
RE: DC Generator for Diesel Locomotive
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?
RE: DC Generator for Diesel Locomotive
RE: DC Generator for Diesel Locomotive
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.
RE: DC Generator for Diesel Locomotive
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.