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Electrical Theory & Applications for HVACR
Chapter 3: Motors
Page 57
motor running at full load actually rotates at a speed about 4 to 5 percent below synchronous. This
difference in motor speed is called slip.
SHADED POLE
Shaded pole motors are the simplest type of inducƟon motors. All single‐phase motors require a
means of producing a second magneƟc field for starƟng. In a shaded pole motor, the face of each
stator pole carries a copper ring called a shading coil. Currents in this coil delay the magneƟc flux in
part of the pole to provide a second rotaƟng field. This produces a low starƟng torque compared to
other classes of single‐phase motors. Shaded pole motors have only one winding and no capacitor or
starƟng switch, making them economical and reliable. Their low starƟng torque is best suited to
driving fans or other loads that are easily started and require under one‐half horsepower. Shaded
pole motors rotate toward the shading band.
The photo in Figure 3‐7a shows a common C‐frame motor. With the shading coils posiƟoned as
shown, this motor will start in a clockwise direcƟon as viewed from the shaŌ end.
NUMBER
OF POLES
SYNCHRONOUS
SPEED
ACTUAL
SPEED
Two‐pole
Motor
7200
2 = 3600
RPM
3450 RPM
Four‐pole
Motor
7200
4 = 1800
RPM
1725 RPM
Six‐pole
Motor
7200
6 = 1200
RPM
1150 RPM
MOTOR SPEEDS
Fig. 3‐6
Fig 3‐7b: Shaded pole motor
Fig 3‐7a: Common C‐frame motor