Page 149 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 6 Rotating Components 111

AC Power 1. Capacitor Start/Run
Rotor 2. Capacitor Start
3. Split Phase
4. Split Capacitor
Unshaded Centrifugal Switch
5. Shaded Pole
Pole Portion
600
Copper Ring
Stator Winding 500
Shaded Portion 1
400
Figure 6-23 Shaded-Pole Induction Motor Schematic
2
300
3
200
4
100
force. Figure 6-23 shows a stylized schematic representation
% of Full Load Torque
5
of a shaded pole motor. These motors are typically very inex- 0
0 10 20 30 40 50 60 70 80 90 100
pensive and, when they fail, are usually treated as a dispos-
able item. Their efficiency is very poor, usually less than 20%, Motor Speed (%)
and provide extremely low starting torques. Figure 6-24 Figure 6-25 Induction Motor Torque-Speed Curves
shows a typical shaded pole motor. Note the copper rings on
the upper left corner of the core.
maintenance. In addition, the motor rotation can be reversed
by simply switching two of the power legs and the only main-
tenance item is the shaft bearings.
Copper Rings
Laminated Core Because of the winding geometry, three-phase motors are
self starting and do not require any special starting facilities.
Figure 6-26 shows a schematic of a three-phase induction
Rotor
motor. Figure 6-27 shows a typical totally enclosed, fan
cooled (TEFC) three-phase induction motor. TEFC motors
Mount Studs
generally have cooling fins protruding from the frame to pro-
Bearing vide higher cooling efficiency. The back of the motor has a
Housing cooling shroud that encloses a fan which forces air across the
cooling fins.
Coil
120 VAC
Wire Leads
60 Hz

Three-Phase
Figure 6-24 Shaded-Pole Induction Motor AC Power

(Delta Configuration)
Rotor
Induction Starting Torques Windings
Figure 6-26 Three-Phase Induction Motor
Figure 6-25 shows starting torque curves for common induc- Schematic
tion motor types. All induction motors will produce starting
torques above their full load torque. Capacitor start/run and Lifting Ring
capacitor start motors should be selected for applications that Cooling Fins
have high start loads. Split phase motors provide good general Frame
purpose starting torque. Split capacitor and shaded pole motors Frame Bolts
Cooling
should only be used in applications with low starting loads. Key Seat
Shroud
Output Shaft Cover
Three-Phase Induction Motors Bearing Seal Leads Box
End Plate
Three-phase motors are used almost exclusively in industrial Mount
and commercial applications. These motors provide excellent Figure 6-27 Totally Enclosed, Fan Cooled, Three-Phase
starting torque, high efficiency, compact packaging, and low Induction Motor