Grid connect Controller for wind turbine driven induction generator (async motor)?
Thanks to your many very helpful answers to my previous questions I was able to find a company that sells such a device. As you may recall, I was intrigued by this device: http://www.clariantechnologies.com/main/page_home.html. It is a vertical axis wind turbine driven async motor that acts as an induction generator that plugs directly into an existing outlet in your home. No inverters, no batteries. Sales price around $500. With an electric meter (a freebie from our power company) than can run backwards, this would allow me to greatly reduce my energy bill all at a very low cost. To make the cost even cheaper, and to increase the power output, I want to make my own wind turbines, and use my own async motors. So all I need is the controller.
I have found a company that sells such a device: http://www.prairieturbines.com for $830. It connects a 5500 watt wind turbine driven async motor directly to the grid. No inverter, no batteries. It uses a "hall-device" to measure the RPM, it provides about 1 AMP to the motor to energize its coils, 50amp solid state "zero-cross relays", capacitors, 30amp breaker/enclosure, an exterior mounted disconnect, and an interior-mounted fused-control voltage switch. This is it!!!
So my question is, does anyone know how to build this controller. This company doesn't sell the plans to this, and $830 is too high for me. I am sure I could build this with components for less that $400. If there is any electrical engineering person out there that can help, this will be a challenge, but it is great knowing that someone already has figured this out and is selling it.
Look at the Prairie Turbines site again. The price of the micro-controller including the hall device is $255. The $45 book provides a complete list of material and instructions to build everything else.
The micro-controller monitors the speed sensor and provides control signals to two solid-state relays. One relay connects the motor to the grid when the speed reaches the motor’s 1800 RPM synchronous speed and disconnects the motor if the speed drops below 1800 RPM. The other relay is energized to release the brake when the controller is energized and de-energized by the controller when the motor speed exceeds the maximum safe operating speed. If this occurs, the controller apparently must be manually reset.
I did not find the maximum safe operating speed stated on the web site. If the full load speed of the motor is 1750 RPM, the full load slip is 1800 minus 1750 or 50 RPM. The speed at which the motor would generate full load current and present full load torque to the turbine is 1800 RPM plus the full load slip or 1850 RPM. With the design described, a three-phase motor is connected to a single phase source with capacitors connected between the source and the third motor phase. That is likely to increase the motor slip somewhat.
I am not sure what happens if the motor is driven above the speed at which it generates full load current. A three-phase motor operating as an induction generator connected to a three-phase supply has a torque vs. speed curve that is something like the mirror image of the motoring torque vs. speed curve. The curve is likely different with the single-phase connection used in this case. I don’t know what the speed vs. current curve looks like in either case, but I will see if I can find it. I am also curious to know if the Alan Plunkett listed on the web site as an author is the Alan Plunkett that is well known author of technical papers on inverter and variable frequency drive subjects.
Added information:
I found information indicating that, for a 3-phase motor connected to a 3-phase supply, the speed vs. current curve for induction generator operation is like a mirror image of the speed vs. current curve for motor operation. That would mean that the maximum speed would need to be limited to the synchronous speed plus the full-load slip. The web site indicates that the full-load speed for the motor that they used is 1725 RPM for 3-phase operation. That would make the maximum speed 1875 RPM plus the additional slip due to the single phase with capacitor arrangement.