The complete overhaul of a 1Mva Leroy Somer Alternator

Failure Report

Our client had been called by the equipment user to say the 1Mva generator which was part of a synchronized pair had failed.

They run a large biomass plant in Cumbria and the customer was concerned regards time scale for repair. Our client Pleavin Power attending the site to investigate the issue to find that the varistor unit had burnt out on the rotor, and on further testing that the insulation resistance was very low, around 0.5Megohm to earth.  The machine was taken out of service immediately and the alternator removed from the engine for repair. We received
the call and were asked for a rough time scale which is very common, MD Charles Bellwood suggested that we could carry out the work in 1 week.

The alternator was collected the next day using our own Volvo 26ton rigid lorry that is fitted with a 20Ton/Mtr lorry loader. This vehicle allows us to collect and deliver without inconveniencing the client as we can load and off load without the need of customer assets.

The alternator was brought back to our workshops and unloaded at 10pm ready for a start the next day. At 8am our Works Manager allocated two engineers to undertake the task of a full overhaul of the machine. The first job after and initial assessment on the machine is to remove the coupling that fits into the engines’ flywheel to drive the alternator, these are extremely tight as they purely rely on the shrink fit to the shaft, there is no key or any other locating device.

We removed the flexible rubber drive around the main coupling, then set up a hydraulic pressing rig rated at 25tonnes to attempt the removal. The coupling is put under pressure and then heated with a superheat torch which is a combination of propane and oxygen, the flame burns at an extreme 750deg C to ensure the heat gets into the coupling very quickly.

The coupling didn’t move first go, once the heat travels into the shaft the coupling is sat on, then it needs to cool down, then you start over again. We ended up heating and cooling it 3 times as the coupling was coming about 50mm at a time.

Once the coupling was off the alternator was taken into the workshop and positioned under our overhead crane for dismantling. The rear end of the alternator contains the rotating diode assemblies, the varistor and the permanent magnet generator, these were removed one by one, then the main exciter was removed.

Once the above had been removed and placed ready for testing, the large cast iron endshields were removed with the aid of our overhead crane, this then leaves the rotor sat in the stator, the rotor is also removed using the overhead crane. This is a difficult procedure that needs to be done with pin point accuracy to avoid damaging either the main rotor or stator windings. Once the rotor is out and put into a rotor stand then the initial testing can be carried out. We carry out a full spectrum electrical test on each component, record the results and then after cleaning and stoving we
can make a comparison of improvement. As our client pointed out the main rotor was below 0.5Megohm to earth, it was full of dust contamination and the insulation system was dried out due to heat over the years.

After the tests were carried out we began the job of cleaning all of the windings, we use a mild detergent that is strong enough to remove contamination but doesn’t harm the winding or insulation in any way. Each component was then stoved twice over a period of 8 hours at a temperature of 135DegC in our large stoving oven.

We then turned our attention to the mechanical aspect of the job. The old bearings were removed from the shaft as they are replaced as a matter of course. We then clean the bearing journals and bearing housings with a non-abrasive material so as not to cause damage to the diameters. The bearing journals were fine and well within tolerance, the non-drive end bearing housing was well within tolerance also, but the drive end housing was out of tolerance and out of concentricity. The bearing housing required sleeving and machining back to standard tolerance in a centre lathe. The housing is machined out to make space for an oversize sleeve to be pressed in, the sleeve is also coated with bearing retainer to further strengthen the fit. Once the sleeve is in place the operator machines the sleeve out to the correct diameter, the finished bore is exactly concentric with the endshields mounting spigot ensuring perfect alignment when re-built. We also measure the output shaft diameter and coupling bore inside diameter to ensure no heat stress has taken place and distorted them on removal, both were in tolerance.

Once the above had been removed and placed ready for testing, the large cast iron endshields were removed with the aid of our overhead crane, this then leaves the rotor sat in the stator, the rotor is also removed using the overhead crane. This is a difficult procedure that needs to be done with pin point accuracy to avoid damaging either the main rotor or stator windings. Once the rotor is out and put into a rotor stand then the initial testing can be carried out. We carry out a full spectrum electrical test on each component, record the results and then after cleaning and stoving we
can make a comparison of improvement. As our client pointed out the main rotor was below 0.5Megohm to earth, it was full of dust contamination and the insulation system was dried out due to heat over the years.

After the tests were carried out we began the job of cleaning all of the windings, we use a mild detergent that is strong enough to remove contamination but doesn’t harm the winding or insulation in any way. Each component was then stoved twice over a period of 8 hours at a temperature of 135DegC in our large stoving oven.

We then turned our attention to the mechanical aspect of the job. The old bearings were removed from the shaft as they are replaced as a matter of course. We then clean the bearing journals and bearing housings with a non-abrasive material so as not to cause damage to the diameters. The bearing journals were fine and well within tolerance, the non-drive end bearing housing was well within tolerance also, but the drive end housing was out of tolerance and out of concentricity. The bearing housing required sleeving and machining back to standard tolerance in a centre lathe. The housing is machined out to make space for an oversize sleeve to be pressed in, the sleeve is also coated with bearing retainer to further strengthen the fit. Once the sleeve is in place the operator machines the sleeve out to the correct diameter, the finished bore is exactly concentric with the endshields mounting spigot ensuring perfect alignment when re-built. We also measure the output shaft diameter and coupling bore inside diameter to ensure no heat stress has taken place and distorted them on removal, both were in tolerance.

When the alternator was fully built back, we carry out final tests before fitting a set of new rotating diodes and a new varistor unit to the very rear of the machine.

The tests above are part of our standard range of tests carried out on every repair. The left hand image shows the surge comparison test carried out on the main stator, this tests passes a voltage around the windings and compares each phase with the others, it is looking for insulation faults, turn to tun shorts and open circuit faults which otherwise would be difficult to locate with more traditional instruments. The “trace” shown on the screen is actually 3 separate traces, but because every phase is testing exaclty the same they all appear on top of each other, if there was a fault it would display multiple traces offset with one another. The right hand image shows a high voltage flash test carried out on the main stator, this puts in this case 1800 volts between the windings and earth to test the electrical strength of the insulation.

The test is normally carried out at twice the voltage plus 1000, hence this 400 volt machine is tested at 1800 volts. The lower figure shows at the end of the 60 second test the maximum earth leakage to be 2.492micro amps, this put in decimal terms is 0.000002492amps, very low indeed. Using ohms law this gives the main stator an insulation resistance of 722,891,566ohms, or nearly 723Megohms, this is a very good figure for something this size and age. Now with all reassembly and testing complete, we have one more process to complete, this is the refitting of the coupling. The coupling is heated up on the induction heater until it reaches 300Deg C, it is then suspended off the crane and slid onto the shaft up to its shoulder, the coupling is then allowed to cool back to ambient naturally before refitting the flexible rubber drive.

The image seen above is a thermal image of the coupling seconds after it was fitted, the temperature can be seen at 262DegC. The completed machine was then loaded back onto to our lorry and delivered back to Pleavin Power Limited, the work was carried out within the week as promised. Once refitted to the engine Pleavin Power personnel will carry out vibration tests and a full load run test prior to taking the machine back to site to be put back into service. People often think about the engine side of a generator regards maintenance, but the electrical side is equally important.

When our client had attended site to look at the machine, they could have quite easily fitted a new varistor and set the machine back to work. With the low IR taken into account and the wear to the drive end bearing housing, this would have almost certainly lead to devastating failure, resulting in major work or even a new alternator. This machine can now go back and the client be confident it is ready for another 10,000 hours work.

If you are experiencing any faults or failures with your generators or motors, get in touch with us today and we will be more than happy to repair your equipment accordingly. Call us now on 01429264097.