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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