7.1 Introduction

Virtually all electric machines and all practical electric machines employ some form of rotating or alternating field/current system to produce torque. While it is possible to produce a true DC machine (e.g. the Faraday Disk ), for practical reasons such machines have not reached application and are not likely to. The AC machine is operated from an alternating voltage source. Indeed, this is one of the principal reasons for employing AC in power systems.

The first electric machines employed a mechanical switch, in the form of a carbon brush/commutator system, to produce this rotating field. While the widespread use of power electronics is making brushless motors (which are really just synchronous machines) more popular and common, commutator machines are still economically very important. They are relatively cheap, particularly in small sizes, and they tend to be rugged and simple.

Commutator machines are found in a very wide range of applications. The starting motor on all automobiles is a commutator machine. Many of the other electric motors in automobiles, from the little motors that drive the outside rear-view mirrors to the motors that drive the windshield wipers are permanent magnet commutator machines. The large traction motors that drive subway trains and diesel/electric locomotives are DC commutator machines (although induction machines are making some inroads here). Many common appliances use universal motors: series connected commutator motors adapted to AC.

A schematic picture ( cartoon ) of a commutator type machine is shown in Fig. 7.1. The armature of...