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.
No comments:
Post a Comment