Basics of dc motors

The first motors were exclusively dc powered, getting energy from primitive voltaic piles that were unsuitable for most applications. Consequently, pre-Edison motors were scientific curiosities rather than industrial work horses made possible by serious utility generation and distribution systems.

All rotary motors, dc included, operate by the interaction of two magnetic fields, one firmly mounted to the inside of the enclosure, known as the stator, and the other attached to the spinning shaft, known as the rotor.

From time to time, perpetual motion schemes have surfaced whereby the stator and rotor are both equipped with permanent magnets so outside power would not be required. This cannot happen. Such a motor would at best go one-half turn and remain in that position forever. The only way an exclusively permanent-magnet motor could work is by having one or the other of the magnets repeatedly flipped over end-for-end so as to reverse north and south poles. Of course, this fantastic operation would require more energy than provided by the output of the motor.

Either the stator or the rotor can rely on permanent magnets, but not both. At least one set of windings requires a continuous supply of electrical power from an outside source. For continuous rotation to take place, either the rotor or the stator field must reverse in time with the rotary motion. That way, the shaft is pulled along as its magnetic field chases after the stator magnetic field.

In a brush-type dc motor, the usual scenario is for dc power to be connected to the stator, providing a non-rotating magnetic field. DC power is also connected to the rotor. If the supply wires were to be directly connected to the rotor coils, they would quickly twist and break off. This problem is solved by means of the brush-commutator combination. Brushes, originally copper but now carbon, ride along a commutator attached to the spinning rotor, providing entry for the electrical current. Being segmented, the commutator performs the dual function of reversing both electrical power polarity and the polarity of the rotor magnetic field.

In contrast, the brushless dc motor has a rotor equipped with a permanent magnet. Commutation of the power supplied to the stator is accomplished externally, by means of an electronic or mechanical oscillator.

The brush-type dc motor has some advantages and is still used in our ac era, even when rectification is required. Speed control and direction reversal can be accomplished by varying the power supply. Brush replacement is easy and inexpensive, but it must take place in a timely fashion to prevent costly damage to the commutator.