Sizing 3 Phase Induction Generator

I'd reconsider the whole idea!

Go back to basics and think about it.

If it's grid connected it's going to cost you a fortune to run as a pump during low rainfall.

It would be arranged such that the grid connection would be cut if the generator dropped below synchronous speed to prevent the possibility of motoring. Squirrel cage motors are commonly used for wind and hydro power generation, it is just the sizing I am not clear about.

I plan to install an induction motor as a grid connected asynchronous generator driven by a water turbine.

If it is asynchronous it wont work as youll pop out of sync and just plane die. Get a rectifier and inverter to interface to the line -grid and you can control the load limits without worry about staying at line frequency on the generator(motor). Woody

Its impossible for this arrangement to pop out of sync, it is excited by the grid connection so it will always be in sync.

I take it you've never seen a motor stall or an overloaded crane run away on lower!
The motor/generator can be forced out of synchronisation by over speed.

PS, the over loaded crane, I was driving it doing trials.

Your "below synchronous speed relay" should be a good protection against motoring. I too would be a bit concerned when lower power is all you can provide as there may not be much left over to regenerate once losses are overcome. I'd expect you may have some downtime. Import / Export metering and controls are also available for protection against import..

If you want to test your concept driving it with an engine and measuring the shaft speed with a tacho should give you some figures to work with. The more above synchronous speed the shaft attempts to run (within reason) the more power will be regenerated.

AC traction systems from high performance cars up to locomotives all make use of this ability of the induction alternator to provide a clean sine waves at applied frequency when over driven (compared with applied frequency) to provide retard effort.

Wind generators sometimes are twin wound to provide greater range to match available power. This could be done in a couple of ways in your case, the simplest being a 2 speed twin wound motor. Use say the 6 pole 15KW winding for low water times and the 4 pole 55KW when there is plenty of flow. If you use two motors (or more) on a common shaft then the possibilities are endless.

If you elect to just suck it and see, please post your results.

Emjay, thanks for your comments. I had considered a dual wound or double motor setup, this will obviously complicate the control system, I have yet to see what range of speed I get from the turbine which will dictate the number of poles and gearing required. As you say there will be some downtime during the dry summer months, although I am fortunate that my site is downstream from a commercial hydro plant which releases water from a dam at peak times so I will benefit from this extra flow also. The plan is to keep the turbine and generator turning continuously and the control system just has to close the relay when Export is possible. What I am really trying to find out is whether or not a 50KW induction generator will still produce useful power when driven at 20KW or less and what effect this has on the power factor.

I think that the power factor will always depend more on the load connected to the generator.

It has been a long time since I used the "Circle Diagram" for Asynchronous Generators" but if you search and refresh, you will get your answers on the behaviour of the Motor in the Generation section of the diagram.

Just stabilize the rpm to synchronous of the grid! What do you understand by motor that is cage three phase? Do you hope that this would work as generator?

Excellent idea! I would do it in this way:

1. take out rotor;
2. Install in the rotor magnets (within aluminum not ferromagnetic lamination)_
3. Make sure the gap between stator core and magnets is small;
4. assemble the junk and;
5. Hook it to grid;
6. Don't forget to place protection and rpm stabilizer on your turbine!
7. synchronize frequency of the newly developed generator with that of the grid
8. Observe money growing in your account!
9. Do not explode the grid. It might work