Maintenance of AC Motors

operating factor. A good rule of thumb is that a temperature too hot for the hand is too high for safety. Next to the temperature, the sound of a motor or generator is the best trouble indicator. When operating properly, it should hum evenly. If it is overloaded it will “grunt." A three phase motor with one lead disconnected will refuse to turn and will “growl." A knocking sound generally indicates a loose armature coil, a shaft out of alignment, or armature dragging because of worn bearings.

In all cases, the inspection and maintenance of all AC motors should be performed in accordance with the applicable manufacturer’s instructions.


Basic Alternators and Classifications

An electrical generator is a machine, which converts mechanical energy into electrical energy by electromagnetic induction. A generator which produces alternating current is referred to as an AC generator and, through combination of the words “alternating" and “generator," the word “alternator" has come into widespread use. In some areas, the word “alternator" is applied only to small AC generators. This text treats the two terms synonymously and uses the term “alternator" to distinguish between AC and DC generators.

The major difference between an alternator and a DC generator is the method of connection to the external circuit; that is, the alternator is connected to the external circuit by slip rings, but the DC generator is connected by a commutator.

Method of Excitation

One means of classification is by the type of excitation system used. In alternators used on aircraft, excitation can be affected by one of the following methods:

  • A direct connected, direct current generator. This system consists of a DC generator fixed on the same shaft with the AC generator. A variation of this system is a type of alternator which uses DC from the battery for excitation, after which the alternator is self-excited.
  • By transformation and rectification from the AC system. This method depends on residual magnetism for initial AC voltage buildup, after which the field is supplied with rectified voltage from the AC generator.
  • Integrated brushless type. This arrangement has a direct current generator on the same shaft with an alternating current generator. The excitation circuit is completed through silicon rectifiers rather than a commutator and brushes. The rectifiers are mounted on the generator shaft and their output is fed directly to the alternating current generator’s main rotating field.

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