3 phase generator but no output.

foxy farmer

Hi all, I have a 50kva gennie here that has suddenly given up. Its only used to power 7.5hp compressor and large plasma cutter. Its started maybe every 2-3 weeks and runs for maybe 30 mins under load. No single phase loads are ran from it. When running voltage between phases is down to 41.3v and 23.9v between any phase and neutral. Any ideas whats wrong? Would exciter circuit be at fault or AVR? Will post up pics if I can.

Irish Steve

Before you go too far, check that the connections to the AVR are all good, if screws, check they are tight, if push on spade connectors, remove them and replace them one at a time in case there is corrosion inside the connection.

Check to see if there are any fuses on the AVR board. There may also be some externally mounted components that are used to provide fine control over the voltage, if there are any, make sure that none of the connections on those components have corrosion on them, and that no wires have broken off with vibration over time,

There may be a set of diodes mounted on the rotating body of the alternator, which should be checked, to make sure that they are all OK, after that, the prime suspect has to be that a component has failed on the AVR. The Diodes are part of the excitation circuit, if one has failed, strange things can happen.

If there is some voltages present, then the excitation side of things as such is operating, but not getting to the correct voltage.

Depending on the age and make, there may be slip ring brushes on the alternator, if there are brushes, then they need to be checked to make sure that they haven't worn beyond operating limits.

Yes, pics would help, or even the make of the alternator, and any details of the AVR, there are several models out there, and they are slightly different in their connections.

foxy farmer

Irish Steve wrote: »

Before you go too far, check that the connections to the AVR are all good, if screws, check they are tight, if push on spade connectors, remove them and replace them one at a time in case there is corrosion inside the connection.

Check to see if there are any fuses on the AVR board. There may also be some externally mounted components that are used to provide fine control over the voltage, if there are any, make sure that none of the connections on those components have corrosion on them, and that no wires have broken off with vibration over time,

There may be a set of diodes mounted on the rotating body of the alternator, which should be checked, to make sure that they are all OK, after that, the prime suspect has to be that a component has failed on the AVR. The Diodes are part of the excitation circuit, if one has failed, strange things can happen.

If there is some voltages present, then the excitation side of things as such is operating, but not getting to the correct voltage.

Depending on the age and make, there may be slip ring brushes on the alternator, if there are brushes, then they need to be checked to make sure that they haven't worn beyond operating limits.

Yes, pics would help, or even the make of the alternator, and any details of the AVR, there are several models out there, and they are slightly different in their connections.

Could I PM you as I cant upload pics from here

Irish Steve

foxy farmer wrote: »

Could I PM you as I cant upload pics from here

Yes, that's fine, go ahead

foxy farmer

Ive changed a diode and thyristor that were faulty. A 1A fuse had blown( end cap blown off). Replaced the fuse but still no luck. I tried flashing the field coil but that didnt solve anything
If i disconnect the AVR and apply a 12v supply to the field coil and run the engine i have an output in the region of 255v. Is the residual magnetism gone from the rotor or is it still the AVR.

foxy farmer

Here are a few pics

foxy farmer

Don't know alternator manufacturer but unit buit by MIG Group.

Irish Steve

OK, if 12 v is getting the thing to partially activate, then the likely issue is that there's a problem somewhere on the AVR board that's either losing a voltage, or locking it down to a level that's not high enough to activate the unit.

It might be a case of getting a generator supplier to check over the AVR and see if it's performing.

foxy farmer

Irish Steve wrote: »

OK, if 12 v is getting the thing to partially activate, then the likely issue is that there's a problem somewhere on the AVR board that's either losing a voltage, or locking it down to a level that's not high enough to activate the unit.

It might be a case of getting a generator supplier to check over the AVR and see if it's performing.

Have sent a few emails with pics to a few rewinds cos. to identify alternator manufacturer. Spec plate is long gone. AVR is obsolete anyway.
Actually alternator is running fine 255v phase to neutral and 425v phase to phase across all 3 windings when fed 12v.
I suspect AVR is shot.

Irish Steve

HAve to admit, it looks that way.

The OLD AVR may be obsolete, but in most cases, they are interchangeable, and can be swapped around, there may be some work to do to sort out the connections, but it's not impossible.

Best of luck with it, I'd be lost without mine when the ESB goes out.