CLASSIFICATION OF MOTORS BY SPEED AND SERVICE



Speed classification of motor. Each electric motor possesses an inherent speed characteristic by which it can be classified in one of several groups. The following classification of speed characteristics is that adopted by the National Electrical Manufacturers Association (NEMA).

1. A constant-speed motor is one in which the speed of normal operation is constant or practically constant; for example, a synchronous motor, an induction motor with small slip, or a direct-current shunt-wound motor.

2. An adjustable-speed motor is one in which the speed can be controlled over a defined range, but when once adjusted remains practically unaffected by the load. Examples of adjustable-speed motors are: a direct-current shunt-wound motor with field resistance control designed for a considerable range of speed adjustments, or an alternating current motor controlled by an adjustable frequency power supply.

3. A multispeed motor is one which can be operated at any one of two or more definite speeds, each being practically independent of the load; for example, a direct-current motor with two armature windings or an induction motor with windings capable of various pole groupings. In the case of multispeed permanent-split capacitor and shadedpole motors, the speeds are dependent upon the load.

4. A varying-speed motor is one in which the speed varies with the load, ordinarily decreasing when the load increases, such as a series-wound or repulsion motor.

5. An adjustable varying-speed motor is one in which the speed can be adjusted gradually, but when once adjusted for a given load, will vary in considerable degree with change in load, such as a direct current compound-wound motor adjusted by field control or a wound-rotor induction motor with rheostatic speed control.

6. The base speed of an adjustable-speed motor is the lowest-rated speed obtained at rated load and rated voltage at the temperature rise specified in the rating.

Service classification of motors. Electric motors are classified into two groups, depending upon the type of service for which they are designed. General-purpose motors are those motors designed for general use without restriction to a particular application.

They meet certain specifications as standardized by NEMA. A definite-purpose motor is one which is designed in standard ratings and with standard operating characteristics for use under service conditions other than usual or for use on a particular type of application.

A special-purpose motor is one with special operating characteristics or special mechanical construction, or both, which is designed for a particular application and which does not meet the definition of a general-purpose or a definite-purpose motor.

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