Old field generator?

[TheReturnOfHightower]

Was at a county show today and they had this old beauty on display. Old field generator?

Anyway, thought some of you may appreciate me sharing it.

 

[marconi]

As ever LN provides some clues - if only one can spot them! The clue I think I spotted was about the contact only being relevant for the Lister powered genset. Whether it is also relevant to striking and then powering an arc lamp I don't know bit think not because I reckon (no experience of them) arc lamps would have their own control gear to ensure correct current and voltage during striking and running.

If I observed correctly the Gardner engine is started by an electric motor - there are teeth around the flywheel - and as a modern engine for its time would not have relied on an old-fashioned crank handle. What this means is that there is a relatively slow acceleration and ramp up in rotational speed of the engine-generator until the diesel combusts and the rotational speed stabilises.

In contrast the Lister engine has it seems to me a very rapid acceleration from stationary to running speed - an impulse start. What this means is that the magnetic excitation field in the generator rises very quickly and from Faraday's law of induction a very high emf will be induced in the windings. Such high albeit brief excessive over-voltage could cause flash over and damage to insulation within and without the generator. This is not such a problem for the slower rise in speed of the Gardner engine.

What the lower contact does then is connect in a low value high power resistor across the generator during start up to act as a load and prevent an overvoltage. Once running the resistor is switched out of circuit by a trained operator.

Or something like this...

 

[westward10]

Back in the 80s I worked at Pinewood studios and at that time they still used carbon arc spotlights on the outdoor sets. Never paid much attention at the time but you had a stationary electrode with another on a motorised carriage which moved it along as it eroded. Mounted in the top was a small extract fan. Pinewood had its own generator room to provide DC around the lots, connection boxes consisted of three brass bars in a wooden box where you connected them normally live with a small adjustable wrapped with insulation tape.

 

[DPG]

Well even if it's not the right answer, it's a cracking post Marconi.

 

[Lucien Nunes]

Any engine willl accelerate as fast as its maximum power and flywheel inertial moment allow. The Gardner is a 1700 rpm automotive engine with a light flywheel that will be up to sperd faster than the Lister. The generator is self-excited so only builds up once it approaches running speed.

The contact is there to bring something into operation when required. Raising the switch to the upper position connects the supply to the output terminals.

 

[Carl S]

Very nice, I have some pictures of a 1940's Ellison unit somewhere. The oil filled type, it was a work of art it was in a old foundry that we were testing, sadly the greedy owner and spark stripped it out for scrap, it belonged in a museum.
It was in working order when they ripped it out and replaced it with Schneider power board.

It’s criminal that Ellison board was destroyed for scrap. I will say I’ve had some hairy moments switching OCB’s, being able to run fast helps.

 

[marconi]

my last go - for very cold temperature operations a diesel fuel tank warmer element via a held-on relay's NO contact?

 

[davesparks]

The contact is there to bring something into operation when required. Raising the switch to the upper position connects the supply to the output terminals.

This implies that it is something which wouldn't, shouldn't or mustn't be be operated at the same time as the output is connected. Otherwise it would have a seperate switch.

I would guess from this that it is involved in starting or stopping the engine.

Also the contact appears to be quite a bit bigger than and different to the two upper contacts, which makes me think it is designed to take more current or was added after the original manufacture.

Could it be some form of interlock to prevent starting when the load is connected?

Of course it could just be a clip to hold the switch in the off position and stop it flapping loose!

 

[Lucien Nunes]

There's some truth in both of those posts. Marconi is correct that it powers a heater (directly, not via a relay) and Davesparks is correct that it wouldn't be operated while the output is in use.

What does it heat?

 

[DPG]

I've only just realised that it's a contact for the main knife switch - doh! It caught me out I think because there is only a contact for one pole - would there have been one for the other knife switch contact also originally?

 

[davesparks]

There's some truth in both of those posts. Marconi is correct that it powers a heater (directly, not via a relay) and Davesparks is correct that it wouldn't be operated while the output is in use.

What does it heat?

Thinking about it further there wouldn't be power available before the engine starts so won't be involved in starting the engine.

Could it be heating the arc lamp/electrodes prior to striking the arc?

 

[westward10]

Carbon arc electrodes from my memory do not require pre heating. They are touched together then slowly drawn apart to create the arc. I go with Dave's suggestion it is somewhere to park the switch.

 

[DPG]

Carbon arc electrodes from my memory do not require pre heating. They are touched together then slowly drawn apart to create the arc. I go with Dave's suggestion it is somewhere to park the switch.

But what is it heating in the second position?

 

[westward10]

But what is it heating in the second position?

We don't know it does anything. It maybe as Dave suggests just somewhere to park the switch to stop it flapping about.
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I suspect that switch is the the original part it looks like a replacement.

 

[DPG]

We don't know it does anything. It maybe as Dave suggests just somewhere to park the switch to stop it flapping about.
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I suspect that switch is the the original part it looks like a replacement.

Lucien says it heats something though.

 

[Bobster]

I'm guessing an oil heater, as from experience with old gen sets (never anything like this). Heavy viscous oil due to cold weather would stall the engine on starting.