Basics on electric motors

  Electric Motors are essentially inverse generators: a current through coils of wire causes some mechanical device to rotate. The core principle underlying motors is electromagnetic induction. By Ampere's law, the current induces a magnetic eld, which can interact with another magnetic eld to produce a force, and that force can cause mechanical motion. A motor is basically a generator run backwards (using current to produce motion rather than motion to produce current), and in fact the modern era of practical motors was initiated by accident when one DC generator was accidentally connected to another in 1873, producing motion and leading Zenobe Gramme to real-ize that his generators could also be used as motors. The rst AC motors (synchronous and then induction) were invented by Tesla in the 1880s.

  Electric motors are estimated to now consume over 25% U.S. electricity use (though some estimates are even higher, to up to 50%, and over 20% of U.S. total primary energy). While large electric motors can be extremely ecient at converting electrical energy to kinetic energy ( > 90%), those eciencies are only achieved when motors are well-matched to their loads. Actual eciencies in normal usage practice in the U.S. are substantially sub- optimal (motors are oversized for the loads they drive). Small electric motors are also inherently less ecient (more like 50%). Motor design and, even more importantly, motor choice and use practices are an important area of potential energy conservation.

This reading is a (very brief) introduction to four most basic types of electric motors:

 Induction

  Like generators, electric motors consist of a stator and a rotor and the three ingredients: electric current, magnetic elds, and something rotating. A basic rule of thumb is that in an AC motor, as in an AC generator used for industrial power production, the magnet is on the rotor and the current