Help needed on a Breezy type grid tied induction turbine

[joestue]

I would reduce the voltage to the motor and see if you can start producing power closer to 15mph which is presumably above the 60hz cut in speed as determined by the gearbox and number of poles.

A 750 watt motor will have a no load power draw at nominal line voltage and frequency of at least 200 watts. So you need at least that much shaft power before you get anything out of it. Reducing the line voltage to half of nominal, should reduce the no load power loss to more like 50 watts, and on a 750 watt motor it should work as a generator at half the voltage up to about 100-150 watts output before it runs away.

two speed dual winding motors are readily available from hot tub pump motors. they are on the order of 1-2hp at 3600 rpm and .15-.3hp at 1750 rpm. the last one i pulled apart had 4 separate windings, and two capacitors, so that you can change between the speeds with just a single spdt switch. these motors are nearly ideal for a wind turbine because they are already designed for a cubic load.


Another option for wind turbines (again due to the cubic load) is to take a 2 or 4 pole motor and reverse half the windings to get a 4 or 8 pole motor. The problem is the motor wasn't designed for it and you have to reduce the voltage to the motor to prevent the core from saturating (because there is no room between the coils for the flux to flow where it wasn't originally intended to go). For the cubic power curve of a wind turbine this isn't nearly as much of a problem as it is for say, a lathe or milling machine spindle because at half the rpm you only have 1/8th the wind power, you only need one half the voltage to the motor (An induction motor's torque follows voltage squared.)

Reducing the voltage also reduces the no load power loss.


A lot of motors are wound with 2 wires in hand, so you can rewire it to get the voltage reduction you need at the higher pole count without using transformers. Another option is to find a 120/240v motor and reverse half the coils and connect it for 240, run it on 120vac. for high speed connect it for 120vac. for each coil that needs to be reversed you need a dpdt relay, so you may find the complexity to be undesirable.

Or don't mess with the motor and change the gear box.

another option would be two gear boxes and two motors. i would run a 2:1 gear box off the existing motor to drive another one. a clutch in between is logical

[Kyle]

Okay, that makes sense to me. Are you suggesting something like this?

https://www.menards.com/main/tools-hardware/power-tools-accessories/routers-accessories/router-speed-control/80820/p-1444421810442-c-10087.htm?tid=6330182982034452332&ipos=1

[joestue]

no you need a transformer or variac.

you can use any transformer to reduce the voltage by its nominal voltage. so the cheaper modified sine wave UPS usually have a 16 volt center tapped transformer in them. the pure sine wave units that use a 24v battery usually have a 13 volt transformer in them.

two of those 16 volt transformers can be configured to reduce your 120v line voltage by 8,16,24,32 volts and they are usually good to around 20 amps. a reduction of 32 volts is about 25% reduction and that may be enough to reduce the no load power draw by half.

anyhow if you reduce the voltage to reduce the cut in, you're going to get more power out of the turbine. however once you start seeing real power from that motor you need to increase the voltage, and once you get to around 600 watts power output, any reduction in voltage will decrease efficiency and the motor will be operating at higher temperatures. basically there is an ideal voltage for the power output.

[Kyle]

I like the idea of the variac, less complicated.

What about changing my blades angle from 5 degrees to 3 degrees and making them longer? Wouldn't that catch more wind and cause them to turn with less effort?

[joestue]

I like the idea of the variac, less complicated.

What about changing my blades angle from 5 degrees to 3 degrees and making them longer? Wouldn't that catch more wind and cause them to turn with less effort?

changing the blades changes everything, too many variables. increasing the diameter reduces the rpm for the same windspeed, but changing the angle will compensate for that a little bit.

and what does "turn with less effort" mean to you? torque produced by the blade increases with the diameter cubed for the same blade angle and geometry. the rpm drops linearly with diameter, so power produced follows the square of the diameter, which means you need a higher gear box ratio. this doesn't solve the fundamental problems associated with direct driven induction motors.. which in your case is the low efficiency of a single phase 750 watt motor.

don't most direct grid tie induction motor generators have variable pitch turbine blades? yours seems to be working only across a relatively narrow range of wind speed.

[Kyle]

I don't have the capability to make variable pitch blades.

"Turn with less effort"- I guess a low tech way of saying the blades will apply more torque to the motor.

I understand the inherent efficiency limitations of my 750 watt motor. I was just hoping/trying to maximize what is there. I appreciate all your suggestions and information.