How to solve our problem now - on the alternator voltage?

Question

We just had our Wartsila-Cummings and Leroy Somer generator being serviced – taken out to a workshop and after the repair, reinstalled back at our plant. The repair is for the engine parts only – the alternator parts were only serviced.



The alternator is Leroy Somer LSA 54M75, 2450 KVA, 415 V, 50 Hz, 4 poles. AVR is Leroy Somer R 449 and R 726 (see attached diagrams). After the installation we could not get the voltage ( we get 50 V instead of 415V). Site reps has checked the connections and even testing the AVR - OK.

At this time the contractor rep. did try to use an external battery – he did connect an external battery to the excitation unit for 2 seconds during rated speed and managed to get the 415 VAC as required. But when he put back the R 449 connection back to the excitation unit – we could not get the voltage level of 415 V but manage only get 50 V.



R 449 has connection from an auxilliary winding for power supply, we measured the voltage level of this auxilliary winding during rated speed and get about 88V.



Another guy came in yesterday and brought in external battery 12 V 200AH and connected to the excitation unit (he removed the cables from R 449 to the excitation unit and connected that to the battery). He did not disconnect other cables. After running for 2 minutes to rated speed, the cables from auxilliary winding to the R 449 got burnt and we stopped the test. However during the test we did get the voltage of about 488 V at the terminal.



The differential relay tripped at this time even though the genset was running without any connection to the bus-bar.





Hope to get your assistance.



Thank you

Answer 1

There is residual magnetism in the rotor which is used to generate a voltage upon start up. Even though this is a weak field and produces a small voltage, the polarity should be such that voltage continues to build up. At some point there is enough voltage that the voltage regulator can take over.
When the residual magnetism is in sufficient to allow proper voltage build up "Flashing" the winding is done with a battery. The idea it to remagnetize the rotor so as to produce a voltage build up. So gens have a battery and momentary switch connected so as to allow flashing with the press of a button at startup.

So based upon your description the gen is putting out 50V.
I would say that the polarity of the voltage at startup (50V) is being bucked rather than boosted by the voltage regulator.
When the voltage regulator bucks the magnetic field the field strenght in the rotor is reduced and approaches zero and lowers voltage output. This trend cannot continue past zero as voltage output and subsequently field strenght become zero.

The generator could be flashed (momentarily connect a battery) at this time with a battery to establish a field in the proper direction. (you may have to swap battery polarity)

if this fails then:

Try swapping the field connections to/from the rotor. This will reverse the polarity of the voltage produced by the residual magnetism and allow the voltage regulator to boost the output.

Answer 2

This is kind of an involved question to be asking on Yahoo answers, where mostly we hear from children who want to get their faces pierced. Your 'attached diagrams' do not appear; you'd have to arrange some other way to view them. Having said that, it's pretty obvious that you have a problem with the voltage regulator circuitry, and I don't know why anyone would be horsing with auxiliary excitation a second time once it became obvious that you can excite the thing with an external battery. I'm assuming that the excitation voltage is produced by an extra winding somewhere on the rotor, possibly associated with a permanent magnet, and that there's a diode bridge to do the rectification and an extra pair of slip rings to pull the excitation off of the rotor. So the first thing you'll want to check is the voltage at those slip rings, with and without load. Presumably that excitation current then goes through a relay/resistor circuit or else some sort of active solid-state device like a transistor or SCR. In response to measurement of the output voltage by the regulator circuit, the excitation current is varied to keep the output voltage at the rated value under various load conditions. Are you sure that you're testing the generator properly? It may be that the machine must be tested with the load attached: without a load, the voltage will go too high and the excitation will be throttled back to a low, default level, thus producing your 50 volts. Good luck with it. I believe you've got an appointment with the people who worked on it last, but you can do some preliminary tests yourself. Be careful when applying voltage from external sources: you can burn things out.

Answer 3

1

Answer 4

Check the diode package that feeds the field windings. Bet it's blown.