Over excited

[millman56]

Hi,  I`m seeking advice about a recently aquired Allam 10 KVA 415 Volt generator, this is assumed as there is no spec plate and it has a 20HP engine.  It came with a trashed doghouse containing various bits and pieces of what appeared to be the original transformer regulator/rectifier setup,
The alternator has 6 wires emerging from the main stator which I have connected star,  also there are 4 wires  (3 + N )  of what seems to be a three phase auxillary winding, the exciter field is fed by slip rings and brushes, one brush is grounded.   
                                                                                                                                                                                                                                        To liven it up a battery charger was connected  to ground and the + wire of the field, this gave 105 or so volts phase to neutral,  the next move was to fit an AVR feeding from phase to neutral,  the output was dead until the battery charger + was touched on a phase terminal, this livened it up with an output voltage phase to neutral of 235 volts,  all well and good,  it seemed happy boiling a kettle,  albeit with an 8 volt drop,  however when attempting to start a 3hp 3 phase motor, the output faded away completely.   

With the AVR connected the no load voltage measured at the  auxillary output was 82 volts phase to phase, the AVR was then configured 110 volt input and connected to 2 of the auxillary phases, the false Eureka moment came when the thing powered up without DC  assistance,   my hopes were dashed when it was impossible to raise the output volts to more than 215 using the AVRs voltage trimmer and it wouldn`t start the motor.


Next brainwave was to fit a 3 phase rectifier to the auxillary AC output and feed its DC output into the field,  this self exited but a bit too much, 266 volts in fact, dropping the frequency down to 46 HZ gave a perfect 237 volts phase to neutral and it would start the motor no sweat with an 8 volt drop.
So my question is how best to reduce the  current to the field to enable 50 HZ operation at a sensible no load voltage?                                                                                             
I did try putting a big variable resistor in series with the DC output but it didnt make any difference.   

Mark.                                         

[dieselgman]

Mark,

Did you test the DC output after the variable resistor install? It would seem that should be a workable solution if the resistor is sized and adjusted correctly.

dieselgman

[BruceM]

+1 for Dieselgman's comment.  If you saw no change with your dropping resistor, the value is too low.  Try a  higher resistance. 

[buickanddeere]

  You could find an AVR that senses from the main winding output but is supplied form the aux windings.
  This will make for a very robust and regulated output voltage. Voltage will be maintained even if the rpms drop due to be over loaded if starting a high in rush load.

[Blue]

I posted a wiring diagram for a 415v 3 phase Allam generator in the gallery under the album of Ian Cox. It was for a 7.5 kw but hey, it's close !!!

[millman56]

Thanks everyone for the good advice,  I did previously look for Blues wiring diagram in Ian Coxs album but was unable to find it.    I have compromised and run the engine at a lower RPM which gives 46 hz and 237 volts,   my thinking is that under frequency is preferable to overvoltage for most applications. 
                                                                                                                                                                                         Never checked the DC voltage just the AC output which should be proportional to the DC input  also I have no higher value sliding resistors to substitute.                                                                                                                                                                                                                                                                                                                                                                     Some time ago I had a genset with a Leroy Somer alternator fitted and this had an AREPs excitation system which I beliieve sensed off the main stator and fed off the aux output much like Buickanddeeres suggestion, the AVR had 10 wires to it if I remember correctly.

Mark.

[dieselgman]

One thing I have learned over the years... good diagnosis is methodical. Check and re-check each assumption along the way to avoid missing something important. Do not skip any steps.

dieselgman

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[quotesubstitute.                                                                                                                                                                                                                                                                                                                                                                     Some time ago I had a genset with a Leroy Somer alternator fitted and this had an AREPs excitation system which I beliieve sensed off the main stator and fed off the aux output much like Buickanddeeres suggestion, the AVR had 10 wires to it if I remember correctly.

Mark.
[/quote]And what a horrible thing they were,installed four of these on lister ts3's if I remember right and in two years of test runs only all of them failed with open circuit aux windings.

[Blue]

The wiring diagram is still there in the Lister engine gallery, user albums, Ian Cox