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Transformer in Generator Head?

Cavalry

Registered
I have a Powr Gard 6.6kw head and was working fine until some mice made a nice conductive home across the main terminal block, now its not generating power, and it appears bad things happened. Unfortunately there is manufacturer support, no parts or prints available.

I have done some generator repairs before but have not encountered a transformer before? Its a asynchronous, with the standard capacitor and rectifier to maintain the field. The transformer clearly has damage. I have to uncouple the head off the engine and remove from the cabinet as its impossible to do any work on it in place, so I do not know what other components took a hit at this point. I will make a print when I can see everything.

-What function does the transformer perform? Does anyone have a generic print showing the purpose of the transformer?
-The transformer has no visible part numbers, so I am a bit lost on how to find a replacement other than do an autopsy and count the individual windings.
 

Joe Romas

Subscriber
Age
76
Last Subscription Date
10/03/2019
This might be no hope at all but does it look anything like this?
If so it's of my Stamford Newage it's on my Boss light tower.
You might look there.
 

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Kevin K

Subscriber
Last Subscription Date
07/12/2019
Pictures would help. I'm guessing it is a voltage regulating (sometimes called current) transformer. The transformer may have one or two primary windings, connected in series with the generator output leads. The full AC output current produced by the generator flows through these primary windings. Increasing or decreasing the current output from the transformer winding produces a corresponding increase or decrease in the current produced in the secondary winding. The secondary winding is connected in series with the circuit that supplies AC current for rectification to DC field current.
 

Joe Romas

Subscriber
Age
76
Last Subscription Date
10/03/2019
Here's a 3 phase version S-N unit, sorry have no model number.
The single phase above has the diodes mounted inside the generator head.
 

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Thaumaturge

In Memory Of
Age
68
Last Subscription Date
07/12/2019
Likely transformer is used to scale down output voltage before comparison to an internal reference voltage, for the AVR to switch field voltage up or down. Could also supply lower voltage to AVR itself.
As to figuring out windings... that could be tough, if transformer is damaged. My suggestion would be to try subbing in another filament transformer and then measuring effect upon main output (no load). If output goes too high, increase secondary voltage of transformer. If too low then decrease secondary. (Output being a response to sample voltage.)
Doc
 

armandh

Sponsor
Last Subscription Date
09/02/2010
simple "transformer " regulation

induction passes dc but "blocks" AC to some degree
speaker crossovers are a good example

so add series induction to a field rectifier input and
it can drop the field voltage quite a bit
but if the main output lead is wound a few turns in opposition to the loss
nothing much happens until there is some output current
then the field supply reduction is lessened
field current goes up to correct for output load
the voltage is kept constant

the transformers are wound to the specific use
the bridge rectifier usually has a resistor or a cap across the field connection to kill spikes
magniciters [the not so simple] use induction on the AC input [no cap induced lag]


older exciter generator regulation.

using a separate exciter generator, an increase in AC load causes an increase in field current
the field current passes through the exciter generator's series field coil increasing the DC output

both methods need a motive power with a constant speed across the output range.
 
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Cavalry

Registered
Thanks all for the information posted. Its a horrible pic but its all I have with me. I need to drag this in the shop and pull the head so I can see what is actually going on and test individual components to make sure the head is salvageable.
 

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Cavalry

Registered
This might be no hope at all but does it look anything like this?
If so it's of my Stamford Newage it's on my Boss light tower.
You might look there.

Thanks, This too is a light tower. I will look into it so see if its similar enough to work from.
 

Cavalry

Registered
Any reason why I cant get a appropriately sized solid state AVR and adapt that to my generator? They are cheap enough I could put one on the shelf in case of a failure.
 

armandh

Sponsor
Last Subscription Date
09/02/2010
possibly yes
depending on maximum field current and exciter [regulator] capacity
 
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Cavalry

Registered
Ok, so I finally dragged this pile into the shop. The bridge rectifier (standard square electronic type) is toast, and joyfully enough there is not a part number on it to cross reference.
Anyone want to take a stab at an appropriate rating for the rectifier?
 

Cavalry

Registered
This should work. You get five 50 amp 1,000 volt bridge rectifiers for $11.88 & free shipping on Amazon. ]

Thanks all for the quick response, I was actually looking at these exact ones earlier. Are there any potential problems if I overrate amps on this?
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Looks like you have elected to keep your transformer compounded generator. "Transformer regulated" is not correct terminology- there is no "regulation".

The transformer and rectifier are what allow an AC generator to be compounded, just like a compound wound DC generator.

This is vastly superior to an AVR in as much as it is elegantly simple, using only simple components. Whereas an AVR is an overly complex electronic solution looking for a problem that for all practical purposes does not exist.

As long as the transformer itself tests good, I would replace the rectifier (the only other part) and reinstate the original arrangement.
 

Joe Romas

Subscriber
Age
76
Last Subscription Date
10/03/2019
"Transformer regulated" is not correct terminology- there is no "regulation".
In publication No BCH-018 STAMFORD NEWAGE "INSTALLATION, SERVICE & MAINTENANCE MANUAL" page 22 calls them
AVR Controlled Generators and
Transformer Controled Generators.

Here's a good read. in Stamford Rating Book Edition E
It's possible to connect 12 lead windings for Single Phase operation in Double Delta or parallel Delta configuration
http://pdf.directindustry.com/pdf/c...tamford-range-ac-generators/12525-193758.html
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
AVR vs Transformer.

Regulated vs Controlled vs Compounded.

It is common to see incorrect terminology, even by manufacturers themselves. Either deliberately for marketing, or merely out of ignorance.

A key difference between the two designs can easily be seen with loads of other than unity power factor, and at speeds other than rated speed.

If a transformer compounded generator, driven at rated speed, is level compounded for a load at unity power factor, it's output voltage will drop below rated voltage when a lagging power factor load is applied, and will climb above rated voltage when a leading power factor load is applied. So "controlled" really isn't the correct term either. These deviations from rated voltage would not be significant under ordinary circumstances, nevertheless, the output voltage is not actually being "controlled".

Likewise, if the driven speed of this generator is changed, so too will it's terminal voltage. Indeed the compounding can be designed to compensate for prime mover governor droop. It can also be designed as compromise between, say, 80% lagging power factor and unity. But in all cases it is not controlling the output, rather it's effect on the output is a fixed consequence of its design. This small, normally inconsequential compromise is what is traded for elegant- and rugged- simplicity.

An AVR on the other hand is without question actively "controlling" the output voltage. It is constantly checking the voltage, and adjusting the exciting current accordingly. It will, within limits, maintain the generator terminal voltage close to rated voltage, regardless of power factor, be it leading or lagging, and, again within limits, regardless of some speed variation. It can be designed to hold the voltage constant as speed drops to a certain point, then allow the voltage to fall off as speed goes lower: Under-Frequency Roll-Off. They'll frequently have user adjustable operating parameters. All of these things are easily made possible by electronics. The trade-off here is something that is several orders of magnitude more complicated and fragile, yet less expensive.

To me the small benefits of an AVR are not worth the drawbacks.
 
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