How to check a winding(s)?

[SparkyTris]

We are trying to eliminate some problems with a pair of large and rather ancient dynamos (10kW ish size).

the dynamos are compound-wound:
- series field coil in the power take-off circuit
- shunt coil controlled via rheostat to trim the line Volts.

the behaviour of these ostensibly identical dynamos under load differs (one appears to be behaving under-compound and the other over-compound).

We would like to eliminate the series windings from our investigations. It is practically very difficult to dismantle the dynamos in order to conduct physical examinations of the windings.

of course the series windings are necessarily very low in DC resistance, so comparing the 2 with a Fluke meter doesn't yield much intelligence. (both present about 0.02 ohms)

clearly neither of the windings are open circuit (each dynamo can do hard work on its own). but there may be one or more shorted turns (?) which could feasibly affect the "gain" of the series field arrangement.

are there any "wizard wheezes" to check the physical/electrical characteristics of the windings without dismantling the dynamo?

 

[Jackrae]

It would be helpful if you could advise what the actual operating problems are.

Could it be that one of the series fields (used as feedback to compensate for load) is simply wired in reverse and load current results in excessive droop.

If you can get hold of a low voltage/high current DC power supply (DC welder perhaps) you could pass dynamo rated current through the series windings. You don't state the dynamo output (volts and amps) to give a guide value of series winding current. However, by way of example, if the output voltage was 250VDC the output current (series field current) would be around 40 amps. With rated current flowing the series coils will drop a measurable voltage.

If the dynamos are 'identical' there should be the same voltage across both coils when driven by the same current. If one is substantially lower then it may have a shortened coil (s).

 

[ericmark]

I was many years ago working as an auto electrician and we had a tool called a growler this was a machine which put a magnetic field into the armature and there was two measuring devices one a pair of prods which you placed across to segments of the commutator and a hacksaw blade which you placed over the armature and if there were shorted windings it would be attracted more than if good. It was a bit hit and miss and was more down to the skill of the user.

I never did work out how to test field coils specially those with bucking windings. With large dynamos for example with a bus the output with no field current could be too high so the field was wound with a bucking winding which was designed to remove some of the residual magnetism as the field current was removed. I remember finding some rewound dynamos where this had been missed and it was over charging the batteries.

Also Lucas was the odd one out. Nearly every manufacturer earthed the field to get the dynamo to generate except for Lucas who put a live feed into the winding. This resulted in the opposite procedure when polarising the dynamos. Being honest it was a lot easier with the Lucas method.

Moving to larger units the 400 MW alternator I worked on had a dynamo at the end which powered the lubrication pumps. This had permanent magnets but it was impossible to assemble with these magnetised so there were two rods and a winding to remove and re-instate the magnetism. We actually used BOC welding sets to get the DC required for this. It was rather hit and miss.

 

[Jackrae]

Moving to larger units the 400 MW alternator I worked on had a dynamo at the end which powered the lubrication pumps. This had permanent magnets but it was impossible to assemble with these magnetised so there were two rods and a winding to remove and re-instate the magnetism. We actually used BOC welding sets to get the DC required for this. It was rather hit and miss.

You sure that wasn't the PMG used to provide excitation voltage on the alternator. Lub pumps wouldn't be running until the generator was up to speed if that was its function by which tine the white metal bearings would be wiped out.

 

[ericmark]

There are three lub pumps. One from mains supply, one from battery back up, and one from the PMG. As you point out lub failure can result in rotors exiting through the roof so rather important it does not fail.

 

[bernardgreen]

Measure the impedance of the windings using a known AC voltage and measurement of the current and then calculate.

Using a resistor is series with the AC feed and measuring the voltage drop across the the resistor can give an idea of the power being lost in the coil. A coil with a shorted turn will take more power than a good coil of the same type.