Published February 2000

MOTORS: A LOST ART

by Bob Perrin

Start • A Few Words on Words • The DC Motor • Polyphase AC Motors • Single-Phase AC Motors • Winding Down • Sources and PDF

A FEW WORDS ON WORDS

One of the first things a person needs to acquire when discussing an unfamiliar technology is a bit of vocabulary.

The rotating portion of a motor or generator is called the rotor. Rotors may be internal or external. External rotors are sometimes recalled outer rotors. Older reel-to-reel tape recorders often have outer rotor motors to drive capstans or belts.

Rotors come in many shapes and sizes. Some have windings, some have salient poles, and some are as simple as a solid iron cylinder. Tak Kenjo’s book on motors is packed with excellent photographs and illustrations of rotors in chapter three. [1] Motors are often classified by their rotor construction. Kenjo’s book does a nice job describing how to classify motors.

a)

b)

Photo 1—(a) This type of stator can be wound as a three-phase stator. (b) A simple bimetal switch provides over-temperature protection.

The stationary portion of a motor is called the stator. Stators, like rotors, come in many shapes and sizes. In most small industrial motors, the stator will resemble those shown in Photo 1

Photo 2—Two windings and some epoxy have been removed to reveal the shaded-pole mechanism.

The outside shell of a motor is referred to as the housing. Housings are differentiated from stators in that housings are purely structural. Stators have some interaction with the rotor, typically magnetic, to cause the rotor to spin.

Some special-purpose motors, such as the ultrasonic motors found in Canon cameras, have rotors and stators that interact by means other than magnetic fields. Kenjo’s book describes the operation of some of these more exotic motors, but for the rest of this article, we will only be concerned with the more traditional magnetic-based motors.

Field windings are those windings used to set up a static magnetic field. Not all motors have field windings. For example, small inexpensive DC motors often use permanent magnets in place of field windings.

The portion of the motor that carries current that interacts with the static magnetic field is called the armature. Therefore, an armature is technically a winding—the armature winding. Not all motors have an armature.

For example, the usual AC induction motor has windings on the stator that set up eddy currents in the rotor. These are sometimes called armature windings, but technically, they are just stator windings.

The classic example of armature windings is the windings found on the rotor of a small DC motor. A permanent magnet on the stator sets up a static magnetic field. Current is passed through the armature windings on the rotor, causing a field that interacts with the permanent magnetic field of the stator. This also causes torque on the rotor.

Rotors are sometimes called armatures. This again is not technically correct. The word armature refers to the armature windings, not the whole rotor.

In simple DC motors with armature windings on the rotor, the mechanism commonly used to deliver current to the windings is called the commutator. The commutator serves two functions. The first is to simply provide current to the armature windings. The second function is to convert DC to AC (or AC to DC depending on the particular motor). In fact, the word "commutate" means to convert DC to AC.

Figure 1 shows the components of a typical DC motor. You can see the armature windings in the slots that run the length of the rotor. The commutator is shown on the end of the rotor.

Figure 1—A simple commutated motor. (click here for larger picture)

In a motor with commutated rotor windings, there will be a pair of brushes mounted to the housing. The brushes will usually be made of a carbon composition and spring loaded to press against the commutator. When the rotor is in motion, the brushes will sequentially contact each of the commutator’s contacts. This will sequentially energize the armature windings on the rotor.

Armed with the words rotor, stator, housing, commutator, brush, field windings, and armature windings we are now ready to discuss motor construction.

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