Single Phase AC Induction Squirrel Cage Motors - Aerovent

Shaded Pole MotorsThe shaded pole motor differs widely from the othersingle-phase motors which have been discussed. All ofthe other designs contain a main and start winding, differingonly in details of the starting method and correspondingstarting circuitry.The shaded pole motor is the most simply constructedand therefore the least expensive of the single-phasedesigns. It consists of a run winding only plus shadingcoils which take the place of the conventional startwinding. Figure 23 illustrates the construction of a typicalshaded pole motor. The stator is of salient poleconstruction, having one large coil per pole wounddirectly in a single large slot. The shading coils are shortcircuited copper straps which are wrapped around onepole tip of each pole.point “b”. As slight as this shift is, it is sufficient togenerate torque and start the motor.The single coil winding is the crudest possibleapproximation of a rotating magnetic field. Therefore, theshaded pole motor efficiency suffers greatly due to thepresence of winding harmonic content, particularly thethird harmonic which produces a dip in the speed torquecurve at approximately 1/3 synchronous speed (refer toFigure 24). In addition there are losses in the shadingcoils. These factors combine to make the shaded polethe least efficient and noisiest of the single-phasedesigns. It is used mostly in air moving applicationswhere its low starting torque and the third harmonic dipcan be tolerated. Extra main windings can be added toprovide additional speeds in a manner similar to thatused on permanently split capacitor motors. Table 5summarizes the characteristics of a shaded polemotor.Figure 24. General Performance Characteristic(Shaded Pole Motor)100Figure 23. Shaded Pole Motor ConstructionThe shaded pole motor produces a very crude approximationof a rotating stator field through magnetic couplingwhich occurs between the shading coils and thestator winding. The placement and resistance of theshading coil is chosen so that, as the stator magneticfield increases from zero at the beginning of the ACcycle to some positive value, current is induced in theshading coil. As previously noted, this current will createits own magnetic field which opposes the original field.The net effect is that the shaded portion of the pole isweakened and the magnetic center of the entire pole islocated at point “a”. As the flux magnitude becomesnearly constant across the entire pole tip at the top ofthe positive half cycle, the effect of the shading pole isnegligible and the magnetic center of the pole shifts toPercent Synchronous Speed80604020ThirdHarmonicDip0 0 100 200 300 400 500Percent Full-Load TorqueTable 5. Summary Shaded Pole Motor CharacteristicsCHARACTERISTICNOTESPeak Efficiency 20 to 40%Power Factor 50 to 60%Starting Torque 40 to 50% full load torque plusthird harmonic dipNoise & Vibration 120 Hz torque pulsations pluswinding harmonicsComponents No additional components neededOtherCan be used with speed control devices (notpossible with SP & CSIR types)CostCheapest of all single-phase motors9 Fan Engineering FE-1100