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Bought a genny head
By BeenzMeenzWind, Section Mechanical Posted on Mon May 23, 2005 at 08:32:00 AM MST
Suggestions as to best use for it welcome.

Hi, I was talking about 3 stage brushless alternators a few weeks back. Just bought one!
'New unused Lucas Aerospace fan cooled 3 phase AC generator, 12 KVA, 200V, 400 Hz, Max 8000 rpm. Very high quality MOD spec. Includes controller Manufactured to work in adverse environments. Ideal for home power supply system charging battery banks etc from a wind or water turbine energy source.'
£75 from Ebay.

I'm going to mate it to a 1.9 litre Peugeot non-turbo diesel, I think. They're cheap and easy to get from the breakers. Plus I've had a couple of vehicles with 'em in so I know to fix them / rebuild them.
As you can see, the output is a little wierd (by homepower standards) as it's an aircraft alternator. You can fiddle with the output voltage a little, though, by messing about with the controller.
Any ideas for the best way to use it would be appreciated.

Bought a genny head | 5 comments (5 topical)

Re: Bought a genny head (3.00 / 0) (#1)
by Vernon on Mon May 23, 2005 at 08:47:46 AM MST

It you want 60HZ it scales to 24-30V at 1200 RPM and with the frequency down to 30HZ it would charge 12V batteries at 600 RPM using a 3 phase bridge. Estimated output at that speed would be 720 watts.

The immediate challenge is to identify the field and output wires and build a controller to regulate at the desired voltage at these lower frequencies.

If you use the auto engine you can belt it up to 8K RPM and get the full 12KW.

Re: Bought a genny head (3.00 / 0) (#2)
by BeenzMeenzWind on Mon May 23, 2005 at 09:42:05 AM MST

Hmm...

Assuming it's the same as the ones I used to work on, it doesn't have a conventional field. It uses a PMG to generate an excitation voltage for the field winding, which the controller uses to regulate the output voltage with reference to output volt drop due to load.

In theory, as long as there is torque sufficient to cope with the load (and the PMG is capable of generating sufficient field strength at the given rpm), it should always give an output of 200v. The frequency will of course vary with rpm. In use on a gas turbine engine, a variable speed gearbox is used to control the frequency by providing a consistent rpm. I think it's about 4000 on the ones I've serviced but that may vary with type.

If I remember rightly, there are a couple of potentiometers in the control module which allow the exact output voltage to be adjusted. Hopefully, there is sufficient 'twiddleability' to give me a reasonable range of voltages.

The problem there, though, is like you say. I'd need to run the bugger at it's designed speed to get the full 12KVA, which means the frequency will presumably be WAY higher than the 50Hz I'd like. (I'm in UK, we use 50 instead of 60. No-one knows why? Cos we're strange, I suppose?)

Though the operation of these things is a bit odd, I think you're right in assuming that gearing it for 50Hz will still give me about an eighth of it's rated output, though. I can't imagine the PMG having sufficient power to maintain a field current high enough to generate 200v during what the control pack will see as a major underspeed condition.

I suppose I could use it as is, run the output through a big-a$$ed transformer and use it to charge a 24v or 48v battery bank. Since I'm planning on using solar and wind as our primary generation method when we build the new house, with the genny as low-sun & low-wind backup / high load supplement, maybe that's the simplest course. Now a transformer that can drop 200v to 48v and handle 12KVA becomes the problem. 12 rewound microwave transformers in parallel? lol
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: Bought a genny head (3.00 / 0) (#3)
by Vernon on Mon May 23, 2005 at 01:49:52 PM MST

I think if you assume that the field stregnth is constant, the voltage will vary directly with RPM. Attempting to greatly increase the field in order to supply 200V at low RPM will tend to overheat the field windings and saturate the magnetic frame. If there is no way to connect the field to a different supply the unit may not produce much voltage at low RPM when you consider that the PMG output will also drop with speed. I would use off the shelf 230V to 36V transformers with the generator output at 200V. Output of each winding is then 31.35V and if you connect them in Y you get 54.29V RMS line to line. Since the nominal float on a 48V battery is ~56V you are damn close to what you need, certainly within a tweak of the control box.
Note that with an unfiltered/unregulated rectifier the peak voltage in the above case is close to 77V .... so the battery would charge on the peak of the wave even if the output was much lower.

Note that a 50HZ transformer will work great at 400HZ.

[ Parent ]

Re: Bought a genny head (3.00 / 0) (#4)
by BeenzMeenzWind on Mon May 23, 2005 at 02:32:09 PM MST

Sounds Ok. I assumed the field strength couldn't be altered by all that great a margin. As you say, you'd soon saturate if you tried to generate a really mondo field to make up for low rpm. I think you're probably right about the heating effect too, though the second stage stator windings on the ones I've worked on are pretty serious, so might be designed to take that for short periods. Plus the thing is fan cooled. They drag a fair volume of air through them, too. We always had trouble getting the exhaust tubes to stay on when we ran them on the test rig.

In any case, my understanding is that the variable stator field will only reliably regulate the output when the frequency is close to correct, anyway, which implies that the control unit isn't designed to try and make up for underspeed conditions even if the rest of the genny IS designed to allow it. Nice to have even that rpm-limited degree of self-regulation, though.

I assume, in the aircraft application, that since the vario-gearbox is maintaining the frequency at as close to 400Hz as possible, the controllable field is utilised solely to stabilise the 200v line as loads are switched in and out. That shouldn't require too much variation in the field current compared with trying to make up for critically low rpms. I can't think of any high voltage system on any airplane I've worked on that draws more than about 8 - 10 amps, though one or two systems draw maybe 3 times that at startup. I'd imagine the power supplies of the various systems must be designed with such momentary 'brownouts' in mind though, wouldn't you. Also, it's unlikely that the aircrew would choose to toggle on all their high-drain systems together, so I imagine the problem is fairly small.

Looks like the only way to really suss this thing out is to nail it to a diesel and monitor the outputs. I've got some pretty good test equipment, so I should be able twiddle the thing until it's doing something like what I want.

Helps to organise your thoughts when you talk it over with someone else.
'....If I even knew that I know nothing, that'd be something. But I don't!'
[ Parent ]

Re: Bought a genny head (3.00 / 0) (#5)
by walsdos on Mon May 23, 2005 at 08:43:53 PM MST

Hello Beenz
You might get lucky with an old welder to drop the volts,preferably an oil cooled model since the seem more resistant to abuse. I no further info from Dr.D or ULR try talking to LEEDS TRANSFORMERS ,Used to be at Larchfield road Leeds, they do custom builds.
Good luck

[ Parent ]

Bought a genny head | 5 comments (5 topical)

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