Petbow Cavalier Gen Set

[clivejo]

Anyone have any knowledge of a Petbow Cavailer Gen Set.  Its pretty old and needs looking at.  Unfortunately the control circuits have been fried and need some attention.  Any help would be great!

[camillitech]

Never heard of them Clive but a quick Google makes them look like a quality product using first class prime movers, Rolls Royce, Lister etc. What model/output is it and I'll ask 'one who knows'.

Cheers, Paul

[clivejo]

Its an ex-army job, designed for 24/7 use.  There is a dynamo generating approx 20V as an excitor for the main generator.  Depending on how much of this voltage you use as input to the big genny, you can vary the output voltage 0-300ish volts.  I need some kind of feedback loop to keep the output at 240VAC.

There used to be a big variable resistor on the dash used to increase and decrease the voltage, but it was very badly damaged before we got it.

[ecogen]

A Dynamo? A Dynamo!!! Mouth, soap, water old bean. Far more than a dyno, 60 years of evolution more, you have yourself a long since extinct Magnicon. Commonly known as a member of the rotating cross field amplifier family, the Magnicon was developed and produced by Macfarlane Engineering. It had the advantage of being able to quickly compensate for both resistive and inductive load changes. So far so good I hear you say?

Unfortunately I do not have any drawings or documentation on your actual generator. 
However, on the plus side I do possess some missing mixed up and practically useless memory of how they work.
Very few external parts are required with a Mag exciter the stator poles and windings are the smart bits. Has your model got a plate rectifier? If so it should be between the output AC and the Mag via a wire wound resistor and a fine volt adjust rheostat. The Mag needs to be fed with a small rectified current from the AC output. Small changes in the AC terminal voltage cause a very small change in the control field coils which is greatly amplified by the Mag into the alternators main field. Stability is ensured by damper coils which do something or other with a magnetic squirrel, I fell asleep in that lecture. A pic of the Mag body, terms and any control bits might help.
You might be far better off looking to vintage engine and militaria enthusiasts.

[Philip R]

Petbow are still in Sandwich. Ramsgate Road  CT13 9NE.  I thought they had become part of Cummins, maybe they did not!! 

Petbow generators were known for their excellent stator designs, well designed winding and pitch factors which in turn give good sinusoidal output waveforms and ability to cope with non linear loads.

Another well made machine were the "Dale" machines from Scarborough, again used by the army and sometimes going on Ebay.

PhilipR

[clivejo]

Has your model got a plate rectifier? If so it should be between the output AC and the Mag via a wire wound resistor and a fine volt adjust rheostat. The Mag needs to be fed with a small rectified current from the AC output.

Would that be what the wee square things hanging from the top used to be?

[ecogen]

Would that be what the wee square things hanging from the top used to be?

Very possible. However the same components can have other uses. You need to follow the wires back from the Volt trim rheostat that would have been on the front panel. If one of them goes to the rectifier either directly or via the wire pot then you can be sure you have found the excitation control circuit. Point to note is that many military sets even small ones were equipped for parallel operation. If so then this could complicate your understanding of the wiring out from the Mag. Mag exciters were paralleled by field linking so changes to one sets volt trimmer would be shared by all paralleled sets.
The plate rectifier can be changed for a new silicon rectifier but may require an increase of resistance made to the pot.
It is good practice to meg all windings and if less than 500K consider drying especially before testing a new exciter control circuit.

[clivejo]

Have you got a circuit drawing at all?

Where are you based?