Asynchronous Generators

"The output frequency of a synchronous generator can be more easily regulated to remain at a constant value. Synchronous generators (large ones at least) are more efficient than asynchronous generators. Synchronous generators can more easily accommodate load power factor variations. Synchronous generators can be started by supplying the rotor field excitation from a battery. Permanent magnet synchronous generators require no rotor field excitation.

The construction of asynchronous generators is less complicated than the construction of synchronous generators. Asynchronous generators require no brushes and thus no brush maintenance. Asynchronous generators require relatively complicated electronic controllers. They are usually not started without an energized connection to an electric power grid, unless they are designed to work with a battery bank energy storage system. With an asynchronous generator and an electronic controller, the speed of the generator can be allowed to vary with the speed of the wind. The cost and performance of such a system is generally more attractive than the alternative systems using a synchronous generator."
http://answers.yahoo.com/question/index?qid=20080820222732AA8byfe
Wind turbines generally use asynchronous generators, you might check with a supplier...here on globalspec...

Thanks for that. I'm surprised by "Asynchronous generators require relatively complicated electronic controllers." I thought you could just open the throttle and it would run at speed sufficiently above synchronous to produce power to equal the engine output. In our case the throttle position controlled proportional to biogas holder level, to generate according to gas availability.

Also plenty of synchronous generators are brushless too, at this sort of size I would think the great majority.

I have seen an asynchronous generator some years ago, on biogas from pig slurry digestion in south-west England, but it wasn't 1.5MWe, more like 100kW.

AS can be seen from the diagram, a PM excitation requires a means of field power control to maintain the voltage output when load varies. It cannot run on the PM only. (Picture copied from a site offering the Synchronous Alternator with a permanent magnet excitation). The need for the PM is to improve control of the voltage and the short circuit performance of the alternator (Mainly!).

Your best source might be the suppliers of asynchronous generators for wind energy systems, there's no shortage of them on the web. Just remember that induction generators, unlike induction motors, must run 3-5% above synchronous speed so make sure your prime mover can operate in that range.

I've seen them on the web, but they look like specialised machines. What I'm after is a package with a gas engine driving a conventional induction motor (at above synchronous speed). Apart from being simpler, motors are produced in vast quantities so are much cheaper than a synchronous generator of same rating.

You could simply bolt an induction motor to your prime mover and adjust the governor for a fixed output speed 3-5% above your synchronous speed. What you'll lack is any load tracking capability and voltage control, you'll be at the mercy of your local grid operator to provide you with a reference signal (grid voltage and frequency) and you won't be able to throttle back your prime mover when its hot water output exceeds your requirements.

As an alternative to the induction generator you might consider driving a bunch of 12V alternators to charge batteries and have them drive a DC to AC converter. No speed issues, no control problems.

Further to RAMConsult's DC comments:

You may be aware that variable frequency drives (VFD's) synthesize AC from a DC bus. Talking to some of the big names in VFD technology such as Allen-Bradley or ABB might get you somewhere. If you could simply generate DC then the VFD could do the conversion to AC at whatever output current was necessary. (of course, you don't care about variable frequency but you can set it for 60 or 50 Hz depending on what your local frequency flavor is.) Big DC generators are available...maybe not as cheap as AC motors but you could purchase used/refurbished DC equipment that will run for a long time.

Another angle to consider with VFD's: You could put substandard AC into their input, maybe from the simple AC generators you are interested in, and allow the VFD to do the phase matching and control of the output. Since they would take your input AC and simply convert it to DC then you would not need to worry about anything but the input voltage.

There are also companies who manufacture just the "front end" of the VFD, to convert DC, usually from battery banks, to AC. The ones I am aware of are made for off-grid homes and the like. I don't know how large the units that these manufacturers produce but they may offer right up to commercial wind turbine size (2MW or thereabouts). I also understand that these systems can be ganged together to increase output.

Jon.

Codemaster, I don't think this setup is as complicated as other posters are suggesting. You definitely don't want to use batteries and/or VFD's as these will add complication and expense. I looked into this when we were looking at the feasibility of a 1MW biomass project 2 years ago. You can use an induction motor coupled to a prime mover. If you drive it over synchronous speed the power you force on to the grid will be similar to the power that would be consumed at the same speed below synchronous. The thing with this type of generator is that you will generate power and not vars. This means the utility grid will have to hold up the voltage. This may or may not be an issue with the utility (depends on the characteristics of the grid in your area). That discussion should be Had with the utility sooner than later. If they want vars you could include a smaller traditional gen set in your system and set it to generate virtually all vars and no power or add capacitor banks which can be brought on in stages (common in industrial facilities).

You will also need protection to sense the collapse of the grid on top of the normal over current, imbalance, ground current,etc. You don't want to back-feed onto a damaged grid for safety reasons.

I was thinking of a start sequence. (Caution! my opinion may or may not be omniscient) Install a clutch between the prime mover and the motor. Start the motor with the grid, bring the prime mover up to speed and engage the clutch then bring it up to overspeed/ generating range. If you regulate on speed you will control power at the same time.

Sorry I don't have any good links. Good luck, it will be an awesome project. What region are you in?

Thanks Frank and others who have responded. I believe on these systems at least in the smaller sizes it's normal to use the generator as a motor to start the engine, more than enough cranking power . A breaker would be required to open on mains failure and also on engine failure causing power flow into the generator instead of out (breaker operation bypasssed during cranking).

I'm in Stoke-on-Trent UK, project is in Northern Ireland.

But I've now found something on the web, extract below. On that basis I'm barking up the wrong tree anyway. I'm surprised to read there's no cost advantage with async at this size, but you learn something new every day!

"Synchronous generators with outputs below 200 kWe are usually more expensive than asynchronous units because of the additional control, starting and interfacing equipment that is required.

In general, at outputs of more than 200 kWe, the cost advantages of asynchronous over synchronous types disappear.

There is a trend, however, towards the use of synchronous generators even in cogeneration units with low power outputs."