Shorted turns tester

[Bluejets]

Wondered if anyone has a link or details on a diy shorted turns tester I could knock up.
Currently have an old 240v ac Dunlite generator here with little to zero information but I've pieced together a schematic of what I believe it to be.
Gone through every other possibility on why it won't fire up properly and I'm left with suspected shorted turn either in the rotor (output) or the field coils (stator) or both.

I Googled and found many references to LOPT/FBT testers but as far as I am aware these are normally for use in high frequency or high resistance circuits.
I have a four wire low restance meter we use at the hospitals for earth resistance testing in operating rooms and the like but I don't really want to start stripping this thing down and splitting up windings, hell of a job.
Thought there may be a generator man out there who knows more than me and might be able to help.
Primarily need to verify it simply for my own curiousity as it being only a 3KVA unit on the back of an old 6HP Lister diesel, it's hardly worth the effort.
Bloke originally bought it to show at old engine shows and that was the only reason I became involved.

 

[Bluejets]

Edit:- Had the cap shown on the wrong bridge terminals.

 

[crutschow]

Perhaps a Growler tester is what you want.

 

[Bluejets]

Perhaps a Growler tester is what you want.

No. well aware of what a growler is/does.
As mentioned above, do not particularly want to start ripping this thing apart which is what would be required to use a growler either on the stator or the rotor.
Was pointing more towards an item similar to this which I have since found.

https://www.homemade-circuits.com/simple-transformer-winding-tester-circuit/

 

[Minder]

I recall when I was involved in TV trouble shooting and I came across an article for making a non-invasive tester for horizontal output transformers.
It basically 'rang' the winding at the normal frequency and tested the amplitude of the 'ring' and indicated it on a 8 LED indicator IC.
The principle should be able to be used on the lower freq devices.
Not sure however if I still have the circuit?
It could still be out there.

 

[Bluejets]

I think with this circuit above one can compare the ac impedance against dc resistance to get some indication as to whether or not there is any shorted turns.
At least that's how it appears to me.
I'll knock this up, or a version of it, and see what happens I guess.
If you have anything I could look at it would also be appreciated.
Thanks Minder

My take on it.......

 

[Minder]

Came to me, it was originally a circuit put out by Dick Smith, Australia, I build the first version and ran great, It hits the transformer with a pulse and sees if it 'rings' and indicates the decay.
https://www.flippers.com/fbt-main.html

 

[Bluejets]

Yes, had one of those before the flood here.
Found a circuit and details for it as well so can try that.
Been meaning to get a replacement.(8 years )
I had thought about it but as I previously mentioned, thought it may have only worked in high frequency/resistance applications.

 

[debe]

Your circuit is very similar to an old dunlite gen i once had.

 

[Bluejets]

Your circuit is very similar to an old dunlite gen i once had.View attachment 52135

Thought you would eventually show up........

Yes, I have that circuit, very similar.
Someone has attacked the wiring with a pair of chompers so difficult to say whether the front panel rheostat was part of the 240v AC field winding section or the (additional unit) 40V DC generator.
Still, even without it i should get some activity.

Un-assisted, the output is around 10V AC from the rotor.
If I inject 12v DC into the stator with the engine running, I can get around the 20V AC mark.
And injecting 24V DC it comes up to around 70V AC but all above tend to drop off if I remove the series motor as a load ( 4" grinder)
Pretty much convinced there has to be winding short somewhere.

 

[debe]

The last generator i had that failed with shorted turns had plenty of smoke issuing from the windings. & loaded the engine.

 

[Bluejets]

This is the test meter I finally ended up building and thought i'd show it here for anyone interested.

Tested it on the secondary of a small 15v transformer that has a dc resistance of around 7.5ohm as measured with an everyday multimeter.

Shorted the test leads BP-1 and BP-2 together with the switch in the AC direction and adjusted the 20k pot to give maximum deflection on the 50uA panel meter.

Released the short and the reading was 14uA so really obvious difference in the reading.

With the switch in the DC direction, the short gave a panel meter reading of 40uA and when the short was released, 37uA.

Yet to try on other coil windings just to see but seems hopeful.

When the rain clears up, I throw the covers off the Lister and see what I get there as well.