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Suppose if a windmill is made torotate in clockwise direction and due to air flow if it rotates in anti clockwise direction then does it produce electricity?

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This depends on the specifics of the design... One can certainly create generators that don't care which way they are rotated, and the simplest generator design (just a magnet and a coil) works either way. Although it might take energy out of the grid rather than add it. But I'd have to guess, based on no particular knowledge, that a real windmill would be very unhappy if you forced it to rotate the wrong way. –  BebopButUnsteady Jul 25 '11 at 14:20
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If a windmill were capable of drawing power from the grid when spun backwards, then the grid would drive the blades backwards all the time! Also, the wind direction cannot spin the blades backwards. That would be a pretty poor windmill design. –  Colin K Jul 25 '11 at 14:43
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I do not think this question is correctly framed. The design of windmills is such that they rotate to face the wind and have sails or blades that will absorb the impulse of the wind into rotation. They will always do that, and will turn in the designed clockwise or anticlockwise direction, so there is no way the air flow will force them to rotate against the design, imo. –  anna v Jul 25 '11 at 14:53
    
@anna, well the question says "due to air flow if it rotations in anti clockwise direction" - if air flow was forcing it to rotate the opposite direction then it would clearly be facing in the direction the wind blows to, as opposed to the direction it blows from, which is the design intent. –  Alan Rominger Jul 25 '11 at 15:15
    
@Zassounotsukushi but that is why the question makes no sense. Windmills are designed to turn and face the wind as far as I know. –  anna v Jul 25 '11 at 17:50

2 Answers 2

Wind turbines use induction generators, and most commonly, the jargon of doubly-fed induction generators applies. Now, the terminology I've seen most often in textbooks identifies motors and generators to be the same thing, just operating in different "directions", and both are called machines. The next major item to cover is the fact that a wind turbine produces power for a 3-phase alternating current electrical grid. This is important and I will return to it.

Mechanically, the blades harness the power of the wind through their airfoil shape and orientation. The common designs, if my understanding is correct, actually rotate in order to face the wind at all times. Now, the question can still remain of what would happen if this system was not functioning, someone let it face the opposite direction (face opposite the wind direction) and the gearbox and electrical systems were still connected. In this configuration, the wind would be pushing it to rotate in the opposite direction of what it normally does because the orientation of the blades would be opposite (they actually have control of the orientation of the blades, but turning 180 degrees may or may not be within the engineered range of motion).

Note that we are going from positive rotation, positive torque (torque from the wind) to negative rotation, negative torque. It is a generator as long as the direction of rotation and torque are the same, and it is a motor if the directions are opposite.

In this case, we are still talking about the wind imparting energy to the turbine but in the opposite direction (rotation) than usual. This could work except for one problem, which has to do with the 3-phase power. Academically, as well as practically, the phases may be labeled A, B, and C. One phase "leads" another phase by 120 degree electrical angle. Think of a sinusoid and you won't miss any accuracy in the model. This means that any given phase will have a phase in front of it and a phase behind it. The normal ordering of the phases is ABC, logically. Electrical devices that deliver current in all phases at equal magnitude in that order are said to be delivering positive sequence current. An alternative exists, in fact two do. You may want to read up more here:

http://en.wikipedia.org/wiki/Symmetrical_components

A negative sequence current is where all phases get current in equal amounts and the order is reverse, that is, CBA. Zero sequence is yet another concept where current is delivered through all sequences with no electrical angle between them. If the generator was working in this backwards direction and it was a synchronous machine (it's not btw) then it would be delivering negative sequence current. This is not desirable for the operation of the electric grid.

A problem with this description is that the machine is really an induction generator, and not only would the delivered currents be wrong, but the electronics for energizing the rotor itself would be messed up. I don't know what would happen in this case, but I hope I've at least impressed the importance of the "handedness" in the electrical systems of generators.

In short, it would still produce electricity, but it would be the wrong kind of electricity.

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note that many windmills for domestic use connect a rectifier to the generator and then feed this DC into an inverter to lock into the grids frequency, either for your own use or to sell back energy. so the "sequencing" is not always the problem in this scenario - the generators function itself could be. –  BjornW Jul 25 '11 at 22:40
    
@Bjorn by "domestic" I take it you mean like someone installing on at their home? You are correct that such designs exist, and what I said really applies for the most major design trend in wind turbines right now only. I could get out some papers that talk about the different designs possible (both obsolete and in development) and all of them would have a different result of being turned around like proposed in the question. –  Alan Rominger Jul 25 '11 at 22:55
    
yes, exactly. it's not common practice here but in the US I've heard it's mandated by some law that you are allowed to do this (inject power).. anyway it would be interesting to see some technical specifications on a big "real" windmill and its grid connection! –  BjornW Jul 25 '11 at 23:49

In theory, and taking a very simplistic view, yes: it moves a conductor through a magnetic field, and this would induce an electric current, regardless of whether the rotation is clockwise or anticlockwise.

However, in practice, the electronics, mechanics and blade design of a wind turbine stop all this from happening, and neither the mechanics nor the electronics are designed to cope with it.

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