Electric Power Principles, 2nd Edition

15DC (Commutator) Machines

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. In the machines examined so far, the machine is operated from an alternating voltage source. Indeed, this is one of the principal reasons for employing AC in power systems.

Historically, the first electric machines employed a mechanical switch, in the form of a carbon brush/commutator system, to produce a rotating field, that is, a field rotating with respect to the armature, which is on the rotor of these machines. That field is stationary with respect to the stator. The commutator can be seen at the near end of the rotor in Figure 15.1. 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 due to mass production, particularly in small sizes.

Commutator machines are used in a very wide range of applications. The starting motor in all automobiles is a commutator machine. Many of the other electric motors in automobiles, from the little motors that drive outside rear‐view mirrors to the motors that drive windshield wipers, are permanent magnet commutator machines. Many ...