Induction motors

 The induction motor was the last great invention in Electric Motors before the age of silicon and semiconductors that gave us the brushless DC motor. Like so many other inventions of the 1880s, it was a product of Tesla's amazing brain. An induction motor is in some sense the simplest motor of all. The drive current is passed through coils on the stator, just as in the synchronous AC motor. But in an induction motor, the rotor holds nothing except for some windings of wire. No magnet, no current sent through an electromagnet. How does it work then? The drive current in the stator in-

duces a magnetic weld in the stator, and the stator weld in turn induces one in the rotor wires, and the two welds interact to allow the rotor to be pushed around and exectively convert electrical energy to mechanical energy.

Of all the motor types, the induction motor can seem the most mysterious or even impossible because it seems like it creates motion out of nothing, as though it were driving itself. Really, however, it is not very di_erent from the other types. All work by the interaction of magnetic _elds in stator and rotor. In the induction motor case, the production of one of those magnetic welds is just very indirect. But if designed right, this induction works very well, and the resulting motor is about as simple as possible. Its ruggedness and simplicity means that it is the dominant electric motor type for industrial use.

  Induction motors do diamter in behavior from synchronous motors in two important ways. First, they do not rotate at exactly the same frequency as that of the alternating drive current. Instead there is some \slip" - the rotor turns slightly slower than the AC current frequency. Second, the torque of the induction motor is actually proportional to that slip. This means that torque is not zero at no zero speed, but is actually maximum, because the slip is maximum: 60 Hz minus zero. While a synchronous AC motor requires some engineering to get to start at all, an induction motor requires some

engineering to keep from starting too fast.

 Finally, in case you are wondering how the large variety of electric motors now have been designed.... the design o fwindings on an electric motor is as much art as it is science. Once you've designed a motor winding, you can model the elects. But you can't write a computer program that itself designs the optimal motor. Motors and their interacting electric and magnetic welds are complex enough that the human brain is still needed to explore new designs, and there is some room for performance improvement and hence energy savings still.