Hysteresis Motor : Construction & Working

Single Phase unexcited Synchronous Motor

• The characteristic of single phase unexcited synchronous motor in which excitation does not necessary is given below.

       ( 1 ) It operates on single phase supply. 

       ( 2 ) The stator rotating magnetic field rotates at synchronous speed. 

       ( 3 ) It does not require DC Excitation. It is self starting.

Hysteresis Motor

• The operation of hysteresis motor depends upon effect of hysteresis. It is a type of single phase motor. 
• The rotor magnetic field lags behind by the stator magnetizing force in this motor.

Construction

Stator

• The stator slots consists of main winding and auxiliary winding. 
• These two winding generate rotating magnetic field due to single phase supply.
• This type of construction is called as Split phase construction. 
• If the operation of the hysteresis motor depends upon effect of shaded pole, it is called as shaded pole hysteresis motor.

Rotor

• The rotor is made of chrome steel or alnico type hard material. 
• It does not consist any winding. 
• Its hysteresis loop area must be high in order to high hysteresis loss. 
• The rotor of the hysteresis motor is shown in the Figure A. 
• It consists of two or more rings at outer side and cross bars. 
• The rotor is made by heat treatment of hard steel material. 
• When a stator magnetic field cuts rotor, torque produces due to hysteresis effect and rotor starts to rotate. 
• When the rotor speed increases up to synchronous speed, flux gets low reluctance path due to low reluctance. 
• This will result in creation of rotor poles in the rotor.  The rotor continues to rotate at synchronous speed. The rotor is made of smooth cylindrical as shown in the Figure A. 
• The hysteresis ring is made of chrome, cobalt or alnico material. 
• The resistivity of rotor material is kept high in order to reduce eddy current in the rotor. 
• The output of the motor reduces as the thickness of the ring increases.

Working

• When a single phase supply is given to the main winding and running winding, a stator magnetic field is produced. 
• When a rotor cuts stator magnetic field, rotor starts to rotate due to eddy current torque and hysteresis torque. 
• When a rotor speed approaches synchronous speed, stator and rotor magnetic field locked with each other. 
• The eddy current torque becomes zero due to relative speed between stator and rotor magnetic becomes zero.

• The rotor poles create when a rotor rotates at synchronous speed. 
• The rotor poles always lags behind stator poles due to hysteresis effect. 
• When a load is applied, a torque acts on rotor is called as hysteresis torque. 
• This torque remains constant at all speed. The polarity of rotor poles do not change as the resistivity of rotor material is high. 
• The mechanical power developed in the rotor

       P_m = ( 1 – s ) / s ……….( 1 ) 

       Where P_m = Hysteresis loss and

                     s = Slip

Torque

• The torque in the rotor of the hysteresis motor is produced due to hysteresis loss and eddy current loss. The hysteresis loss

        W_h = K_hB_max^1.6f_r ……….. ( 2 )

              = K_h^’B_max^1.6 ( sf )

        Where

             f_r = Rotor frequency

              f = Stator frequency

             s = Slip

       B_max = Maximum flux density

          K_h = Constant

         K_h^’ = Constant

• Hysteresis torque T_h = K_h^’B_max^1.6 ( sf ) / sω_s

                                        = K_h^’B_max^1.6 f / ω_s ….( 3 )

                                        = Constant

                        Where ω_s = 2πN_s = 2π × 120f / P

• We can say that the hysteresis torque does not depends on torque from equation  ( 3 ). 
• The eddy current loss

       W_e = K_eB_max^1.6f_r²…………( 4 )

             = K_eB_max^1.6 ( sf )²     ( As f_r = sf )

             = K_eB_max^1.6 s²f²

       Eddy current torque T_e = W_e / sω_s………( 5 )

                                           = K_eB_max^1.6 s²f² / sω_s

                                           = Ks

                            Where K = K_eB_max^1.6 f² / ω_s

      Eddy current torque α ( Slip )

                 OR

      T_e α s ………….( 6 )

• The eddy current torque is directly proportional to slip. 
• When the hysteresis motor rotates at synchronous speed, slip becomes zero and therefore the eddy current losses also become zero. 
• The eddy current torque assist to start hysteresis motor.

Torque – speed characteristic

• The torque – speed characteristic of the hysteresis motor is shown in the Figure C. The starting torque and running torque becomes equal at starting.

Advantages

• No mechanical vibration due to absence of teeth and winding in the rotor
• Noiseless operation due to no vibration
• Multi speed operation possible due to gear train

Disadvantages

• Low efficiency
• Low power factor
• Low torque
• The output of the hysteresis motor is one fourth that of induction motor of same size.

Applications

• Good quality record player
• Instruments which are used for sound generating
• Sound recording instruments
• Electrical clock
• Tele printer
• Timing devices

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