Motors Used as Windmill Generators
As we mentioned above, motors and generators are very similar, and are often interchangeable with little or no modification. In fact, motors always generate a voltage when they are turning. This voltage, known as ‘back emf, is less than the supply voltage, so the net effect is that a current still flows from the supply to the motor. As the motor speeds up, the back emf rises, the supply current falls, and so the motor is regulated to a speed dictated by the supply voltage.
The difference between motoring and generating is just a matter of speed. Say you connect a dynamo to a 12 volt battery. There will be a cut-in speed at which that particular machine generates exactly 12 volts. Force it to rotate faster, and it will generate more emf, causing current to flow into the battery. Slow it down, and it will produce less, allowing current to flow from the battery into what is now a motor.
The same principle applies with a.c. motors. In fact, most large wind turbines connected to the grid use induction motors (see more later) as generators. If there is only a light wind, then the machine will tend to wobble to and fro, between motoring and generating, quite smoothly and naturally. The control system of the windmill connects or disconnects it from the grid, at a windspeed chosen for maximum net benefit.