Permanent-magnet (PM) motors are
available in fractional and low integral-horsepower sizes. They have
several advantages over field-wound types.
Excitation power supplies and
associated wiring are not needed. Reliability is improved, since
there are no exciting field coils to fail, and there is no likelihood
of overspeed due to loss of field.
Efficiency and cooling are improved by
elimination of power loss in an exciting field. And the torque
versus-current characteristic is more nearly linear. Finally a PM
motor may be used where a totally enclosed motor is required for a
continuous-excitation duty cycle.
Temperature effects depend on the kind
of magnet material used. Integral-horsepower motors with Alnico-type
magnets are affected less by temperature than those with ceramic
magnets because flux is constant.
Ceramic magnets ordinarily used in
fractional-horsepower motors have characteristics that vary about as
much with temperature as do the shunt fields of excited machines.
Disadvantages are the absence of field
control and special speed-torque characteristics. Overloads may cause
partial demagnetization that changes motor speed and torque
characteristics until magnetization is fully restored.
Generally, an integral-horsepower PM
motor is somewhat larger and more expensive than an equivalent
shunt-wound motor, but total system cost may be less.
A PM motor is a compromise between
compound-wound and series-wound motors. It has better starting
torque, but approximately half the no-load speed of a series motor.
In applications where compound motors
are traditionally used, the PM motor could be considered where
slightly higher efficiency and greater overload capacity are needed.
In series-motor applications, cost consideration may influence the
decision to switch.
For example, in frame sizes under 5-in
diameter the series motor is more economical. But in sizes larger
than 5 in, the series motor costs more in high volumes. And the PM
motor in these larger sizes challenges the series motor with its high
torques and low no-load speed.
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