Generator conneted

[Gary]

I was wondering what would happen if you connected a working generator to the mains whilst power was on.

I am asuming it would not be a good thing?? anybody actually know what happens?

Gary

[The Other Mike]

All the smoke that is trapped in the wires would come out, there may be extreme acoustic events, and photons would strike your eyes.

[<me9]

Depends. If it's nearly in phase and roughly the right speed it will synchronise. (I've done it many times). If out of phase BNAG!!!

In the late 70s we were no longer allowed to synchronise to the mains for two reasons.

1, Someone had done it out of phase and BNAG!!!!. 2, It's not nice to have mains back fed onto an otherwise dead supply. You then might get dead electricity board employees as well, and they don't like it uppem! to use a Clive Dunn expression.

[Brian Gaff]

Erm, if you don't synchonise the frequency, it would not be good, and probably illegal. Brian

[Martin Brown]

ITYM a fuel cell using the waste heat to warm water. The Japanese Prime Ministers residence has one such installation - although they have gone very quiet about how well it is working.

Thermopiles are only really cost effective for space flight.

Regards, Martin Brown

[tony sayer]

In article , [e-mail] scribeth thus

Umm ... don't they have fuses? .......

[Ian Jackson]

In message , Gary writes

I have actually done this (at university, in 1963). It was a pretty large three-phase generator (the size of a small car), and the object of the exercise was to connect it to the three-phase mains. The generator was driven by a DC motor.

Before connecting the generator to the mains, I had to adjust the speed of the DC motor so that the generator was driven up to synchronous speed and the correct phase angle (ie each phase of the generator was looking at the correct phase of the mains). The generator DC field current was then adjusted to that there was minimal voltage difference between the generator and the mains.

When I was satisfied, and making sure that my fingers were well and truly crossed, I then threw a big knife switch which connected the generator to the mains.

I then increased the field current so that the generator started generating (ie current started to flow into the mains). At the same time, I had to adjust the field current of the DC motor so that its power output to the drive shaft of the generator also increased (otherwise the generator would have started to lack power, and become a motor).

The experiment then continued with me monitoring and plotting on graphs all the relevant currents and voltages while the generator was behaving as both a generator and a synchronous motor. I recall that, in one condition, the generator can be made to behave as a capacitor to the mains feed, and this used to be one method of providing variable amounts power factor correction.

BTW, don't try this at home! ;o))

[js.b1]

Geothermal all over wales, mendips etc, would be fine in the UK.

[Martin Brown]

You have to be kidding. You can have affordable or efficient thermopiles but not both at the same time. It is very niche market.

I tried to put together a candle powered demo for an Xmas lecture using them (fairly high grade units). I was intending to drive one puny LED from the waste heat of a candle flame (about 100W).

I had a high temperature TEC module and a boring one stacked and even operating it between 250C hot side to a large deep frozen heatsink -18C on the cold side I could barely get enough juice out to do anything useful. I decided that at these temperatures it was too hazardous for a childrens' Xmas science lecture and scrapped the idea.

The same TEC units can move 50W or so from the hot plate to the cold plate when suitably powered. But try to generate power with them with a temperature difference applied and it is a lot less efficient. I would be interested to know if anyone has found a setup that works - ideally enough to power a 1W led (but I'd settle for 10mA and 4v).

Previous toys have included candle powered nodding heat engine and a heat engine that on a good day will run off a cup of fresh coffee.

Regards, Martin Brown

[The Other Mike]

Fifty years on you've been replaced with a box of tricks costing a few thousand quid that does it all for you and gets the breaker operation point spot, including allowance for the mechanical operation of the contacts, to ensure no damage to the circuit breaker or the generator.

In years gone by - well until the early 80's in the UK on small generators they used an incandescent lamp connected between the voltage transformers situated either side of the circuit breaker and tweaked the generator excitation to produce minimum brightness in the lamp to ensure the generator was in phase with the grid system before breaker closure.

They also, and more commonly, used a lamp as above and a synchroscope.

[Ian Jackson]

In message , The Other Mike writes

Surely you tuned the generator for a steady MINIMUM (ideally zero) brightness?

university lecturers were 'ard - 'ard, but fair.

[Andrew Gabriel]

They're even less efficient, but on a spacecraft, they are usually driven by a nuclear reactor, which might present a few problems for home generation. You can pick up many abandoned ones from around the northern coasts of the former USSR, where they were used as things like lighthouse power generators. Most of the people who have tried recovering one of these, enticed by the idea of 1-2kW of free power for years, have died very shortly afterwards though.

[The Other Mike]

Somehow I wrote generator excitation instead of governor valve - I was to bed late and up early!

They've been around almost forever. Google images pictures some real antiques.,

[Andrew Gabriel]

I've used the 50W peltier devices. I found they generated power after you've powered them off and there's still a temperature difference of ~40C across them - enough to continue self-powering their cooling fan and a power-on LED for half a minute. I think these ones were rated up to 250C, but I never ran them over 40-50C differential.

Even as heat pumps, they aren't efficient - it consumes 50W to pump

50W, but as a tiny heat pump, they can be very handy.

[The Natural Philosopher]

until that heat source was exhausted..

[Andrew Gabriel]

I used to know one of the engineers who worked on Earley Power Station (in Reading). They had a phase meter which showed how far out of phase the generator was from the grid. When the needle was moving very slowly, and coming up to zero, that was the point where it was connected in. On the first runup of one of the generators, they were watcing a duplicate phase meter, waiting for the control room to engage with the grid. They watched it go slowly past zero without being switched in, and relaxed as it would be another couple of minutes before it got back into phase. Then there was an almighty bang or earthquake or something, they didn't quite know for a moment. Anyway, turns out the generator was put onto the grid completely out of phase. The control room's phase meter was incorrectly connected up. His wife was shopping in Heelas (now John Lewis) in the middle of Reading, and apparently all the lights browned out. The generator had to be stripped down and checked before it could be used again.

[Steve Firth]

I think you are failing to understand CHP. The point of CHP is to raise the overall efficiency of the system by compromising on the efficiency of components in the system. Slightly less efficient electricity generation and comparatively inefficient heat great ion add to total thermal efficiency higher than a good generator or a good boiler working alone.

For this sort of use a Stirling engine generator is good. There's (some) mechanical output to generate electricity and a fair amount of waste heat that can be used for space heating. It has the advantage that it provides electricity when photovoltaic cannot - winter evenings. It's a great idea for a sensibly managed energy economy.

[Martin Brown]

Yes. They are good individually for 25mV open circuit or 10mV @ 0.2A.

But do you have any idea how many of them I would have needed to generate 1W of electricity at 4v?

A back of the envelope calculation suggests about 400 or so thermocouple junctions to provide the target 1W of output power at 4v and perhaps half that number to get an LED glowing at low current.

The TEC modules at least have a decent number of junctions in series and can source a voltage high enough to use semiconductors.

Regards, Martin Brown

[grimly4]

A friend had one of those in his narrowboat, powering a fan on the stove top. In such a small space it was very effective.

[Graham.]

Especially if you have a nice lump of fissionable material to keep it hot.