Stamford Generator with low voltage

[Madness]

I have a Lister TR2 with a Stamford BC164 Generator. It is wired in zigzag for 240 Volt single phase. Trouble is it will not put out anywhere near the correct voltage. I bought this cheap secondhand knowing the generator is faulty but I have no history on it as the guy I got off was selling for a friend who bought it at an auction.

So far I have found the main field to be burnt out and the SX460 AVR was putting out 160 volts continuously into the exciter field. The field has been rewound and I have a new AVR but still not getting correct output. I have been through all the checks in the Stamford service manual. I bought a Megger Meter and checked insulation following the manual. As per the manual with a 12 Volt battery connected to the exciter field I am only getting around 120 volts with no load it should be within 10% of 240 volts. Voltage is = across all windings, I set it up as 3 phase and get 123 volts from neutral to each of the 3 phases. This being consistent and all resistances are correct the stator should not be faulty. Exciter field is 19 ohms as it should be and I measured the resistance of the exciter rotor and it also has equal and within range resistances. I have tested the diodes and they all measure correctly with a multimeter.

I have followed the manual to the letter and still getting low voltage. The only thing I can think is there is a problem with the diodes under load. The field components of the generator have obviously been stressed as the field was burnt put and the AVR putting out 160 volts when it is supposed to have a range up to 90 volts. I tested this by connecting 240 volts mains to the AVR input and measured the output.

Checks that I have done are

Engine speed is correct I can measure 53 HZ at the generator with no load and the under speed LED on the AVR is going off.
Stator is good according to the tests in the manual
Exciter Field   "                "
Exciter Rotor  "                "
Diodes          "                "
Main field has been rewound and measure correctly

I have checked and rechecked the wiring of the Stator terminals and it is correct, also I rewired it to 3 phase and had the same low voltage issue. In Zigzag wiring I am using terminals  U and W and getting around 120 Volts.


Is there anybody that has had a similar problem that can shed some light on this before I take it to a rewinder to diagnose that it !

[Diesel Down-under]

  

 Hi Madness,

the fact that you are getting exactly half of the output you should be, is interesting, but I'm not sure at the moment exactly what that means...

please confirm that 12VDC on field wiring gives 120VAC output

 and check again the manual (that indicated this test ) with respect to model sub numbers etc you may have overlooked something.

Do you have terminals 3 and 4 on the SX460 linked or unlinked?

[Madness]

Gidday,

Yes the half voltage has got me stumped, 3 and 4 on the AVR are not connected. With a 12 Volt battery on the field (so AVR has no effect) I am getting 130 volts across terminals U and W. According to the manual doing this with 10 to 12 volts on the exciter field and  no load should be within 10% of nominal voltage (240). It is wired to the Zigzag configuration as per the sticker under the cover. The numbers are Serial C075546/01 ID 96495044 Frame/Core BC.164B WDG 311.

Thanks

Gary

[Diesel Down-under]

 I've got nothing further at the moment, have been looking at the Stamford manual, it is a fairly complex generator you have there.

[Madness]

Every thing tests ok following the steps in the manual, trouble is it is not possible to measure anything on the rotor while it is running. At this point I thought I would give this forum a try otherwise I will order a set of Diodes. I forgot to mention there is currently no varistor fitted, the manual says it is only there for surge suppression and running without it will not effect the output. I was told by a rewinder that they remove them because they claim they cause more problems than they are worth.

I have been trying to work this out for a few months now, only thing i can think is that there is Diode failing under load. Has anyone had this happen?

[mike90045]

Diodes usually fail, and stay dead.    I've never seen one come back from dead before.

I wonder if flashing the windings/core would help ?   when it winds down, you don't have any load on the terminals do you ?

[Madness]

I had to flash it initially, I am measuring voltages with only the meter on the terminals nothing else is connected. It makes power no problem but won't get up to the right voltage. I was wondering if diodes can fail under load or is a test with the meter enough to guarantee they are right? I am scratching my head been through all the tests in the manual a number of times and still can't find what is wrong.

[Diesel Down-under]

Flashing is not going to help as you have applied 12VDC as a field voltage anyway and output is low

Dont hang any hope on a diode being intermittent, as long as you test the diodes so as they are not in circuit with any other component, the diodes will be either open circuit, short circuit or GOOD

I concur with the comments on the MOV as that would not impede performance

[Madness]

If it is not the diodes I am thinking then maybe a fault in the exciter rotor. The exciter field measures correctly and I have left it running on the battery for some time and there are no hot spots and magnetism is even across all the poles.

[Diesel Down-under]

What about that rewound field coil.....wouldnt be the first time someone has balls up a rewind, just offering suggestions at this stage.

[Madness]

I did it myself, it is wound exactly as per the original with proper nsns poles. Resistance is also within the range in the manual, I bought a meter from Jaycar that is a megger and low resistance ohm meter to test everything, I needed a megger anyway. The original field did work and gave similar result but slightly lower voltage and got rather hot.

[Diesel Down-under]

OK well better rule that out then!!

what about this section of the manual?

"If voltages are balanced but low, there is a fault in
the main excitation windings or rotating diode
assembly."

[Madness]

The diodes all check out okay after unsoldering them and checking with a meter, there are 3 of each type on each side. Field windings and field rotor pass all the tests in the book. looks like I just need to go back and do all the checks again following the book to the exactly. I have done this at least 3 times now, but there may be something I have missed or got arse about face. I have a very busy day tomorrow so may be a few days before I can get to it again.

[Thob]

The manual I downloaded indicates that there is a link on the AVR for 120 vs 240 operation.  Have you set the link properly?  (p20, AVR, no link between 3&4 for 240V)

[Madness]

The manual I downloaded indicates that there is a link on the AVR for 120 vs 240 operation.  Have you set the link properly?  (p20, AVR, no link between 3&4 for 240V)

There is no link on the AVR between 3 & 4, so it is set to work at 240 volts. All testing at this stage is without the AVR being used and a 12 volt battery used to energize the exciter, as per the testing procedures in the manual.