The working principle of induction generator?

In an induction motor, a moving magnetic field is sent sideways through the walls of a copper cylinder. The field becomes trapped in the copper, and the magnetic field drags the copper along as it moves. However, if the cylinder is able to spin freely, it will coast along at exactly the same speed as the moving magnetic field. In this case the rotating cylinder is not like a motor, since no energy is being used to spin it. It's just coasting frictionlessly.

If we place a friction load upon the motor shaft, (add a washing machine,) then the copper cylinder will lag behind the magnetic field moving through it. In this case the magnetic field is dragging the copper along, and the device becomes a motor. Energy from the magnetic field gets injected into the copper cylinder, and drives it forward.

If instead we *drive* the copper cylinder forward, making it spin at a faster speed than the moving magnetic field, then the copper cylinder will push the field forward, and energy gets injected into the coils.

So for a 2-pole motor running at 60Hz, it becomes a motor if it spins slower than 1800RPM, and it becomes a generator if it's spun faster.

Here's a second way to visualize things: the rotating copper cylinder contains a trapped magnetic field. This means that the copper cylinder is itself a magnet. If this magnet is allowed to spin freely, then it will turn at exactly the same rate as the magnetic field from the coils. Then if we grab the motor shaft to try to slow down the spinning "copper magnet," the poles of the magnet within the copper will lag behind the moving magnetic poles from the coils, and the coils will pull the copper strongly forward. The "copper magnet" doesn't need to slow down, but instead it will suck energy out of the coils as its gets dragged forward by the moving poles from the coils. On the other hand, if we try to spin the "copper magnet," it won't turn faster. Instead its poleswill get ahead of the moving poles of the coils, and it will pull strongly upon them, injecting energy. In other words a moving magnet can generate a current in a nearby coil, *OR* a current in a coil can force a nearby magnet to start moving. All motors are generators and all generators are motors. It just depends on which side is driving forward and which side is being dragged along.

If induction motors and generators make your brain hurt, just remember that there's a good reason for this. They weren't invented by a bunch of engineers. Instead they sprang from the twisted mind of Nikola Tesla, who no doubt could ride along with his mental squirrelcage rotor while seeing how the fields and currents behaved.