These motors consist of stators and
rotors only, without shafts, end shields, bearings, or conventional
frame (Fig. 20-39). The rotors are mounted directly on a shaft of the
driven machine, which must also include a suitable support for the
stator and a ventilating arrangement.
FIGURE 20-39 Cross section of
shell-type motor.
The motors are built with relatively
small outside diameters but may be slightly longer than standard
machines. Furthermore, horsepower ratings over a rather wide range
are built in each frame diameter, the ratings for the different
diameters overlapping slightly.
Although a great many of the motors
used are for operation at standard commercial frequencies giving
speeds up to 3600 r/min (on 60 Hz), they are frequently supplied for
operation at higher frequencies and correspondingly higher speeds.
Frequencies up to 2000 Hz with a
corresponding 2-pole motor speed of 120,000 r/min have been used, but
the more common “high” frequencies range from 60 to 240 Hz,
giving 2-pole motor speeds up to 14,400 r/min.
Shell-type motors are used principally
on machine tools and woodworking machinery. Their relatively small
physical size facilitates a compact design with maximum flexibility
in arrangement of machine parts.
The small diameter of the motors is of
particular value, since it allows close spacing of spindle shafts.
The wide range of ratings available in each diameter reduces the cost
of providing suitable mountings for the motors.
Motors of similar mechanical
construction but with special insulation are used in hermetically
sealed refrigeration and air-conditioning compressors, where the
motor runs in an atmosphere of refrigerating gas.
The insulation must neither harm nor be
harmed by the refrigerant and, so that the refrigerant may be kept
clean and dry, must not trap moisture or dirt.
Dimensions. NEMA has standardized
mounting dimensions for various types of motors, those standardized
for polyphase induction motors covering ratings from 1 to 125 hp (at
1800 r/min). For convenience each set of standardized dimensions has
been assigned a frame number, and the various ratings of motors have
been assigned frame numbers from the series.
Any motor offered by a manufacturer
having a frame number from this series will have the corresponding
standardized mounting dimensions. These are listed in NEMA Motor and
Generator Standard, Publ. MG1-1987.
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