Hysteresis motor

Hysteresis motor

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  HYSTERESIS MOTOR -- NAUSHEEN( CUJ/I/2014/IEE/032 ) -- DIVYA KRITIKA (CUJ/I/2014/IEE/040)
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  INTRODUCTION:  HYSTERESIS MOTOR: Synchronous motor with uniform air gap  Without d.c. excitation  Operate from single phase or 3- phase supply  Torque produced due to hysteresis and eddy current induced in rotor
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  CONSTRUCTIONS:  STATOR CONSTRUCTION: Similar to induction motor  Connected either to single phase or 3- phase supply  Single phase – stator winding is of permanent split capacitor type
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  Smooth cylinder  ROTORCONSTRUCTION  Core : aluminium or other nonmagnetic material  Outer layer : thin rings to form laminated rotor  Smooth cylinder , no windings
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  HYSTERESIS LOOP (B-H LOOP)  Relationship between the induced magnetic flux density (B) and the magnetizing force (H).  A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased.  Point ‘a’ – all the magnetic domains are aligned. Materials has reached to the point of magnetic saturation  H is reduced to zero, the curve will move from point "a" to point "b."Some of the magnetic domains remain aligned but some have lost their alignment.
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   magnetizing forceis reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve  magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point "d")  Reducing H to zero brings the curve to point "e." It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero  the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back to the saturation point where it with complete the loop
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  OPERATION : Rotor poles “induced”by Stator Flux Stator Flux establishes these magnetic poles Rotor poles follow the rotating flux, but lag behind by angle δh  3- Phase or single phase supply is applied to the stator, a rotating magnetic field is produced  Rotating magnetic field induces poles within it  Induced rotor flux lags behind rotating stator flux
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   Angle betweenstator and rotor magnetic field is responsible for production of torque  Angle depends upon shape of hysteresis loop , independendent on frequency  Additional torque due to eddy current
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   Ideal materialwould have rectangular hysteresis loop ( loop 1)  Ordinary steels are not suitable as they resemble to loop 3  Cobalt vanadium type materials are used as they have loop a/c to 2 which approximates to loop 1
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  It is clearthat the torque is proportional to the slip. Therefore, as the speed of the rotor increases the value of Ʈe decreases. As the speed of the motor reaches synchronous speed, the slip becomes zero and torque also become zero. As the electromagnet torque is developed by the motor is because of the hysteresis loss and remains constant at all rotor speed until the breakdown torque. At the synchronous speed, the eddy current torque is zero and only torque due to hysteresis loss is present.
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  TORQUE – SPEEDCHARACTERISTICS  ideal torque speed curve is shown by curve 1 For practical hysteresis motor – curve 2 Deviation for ideal characteristic is due to presence of harmonics Torque is constant at high speed including synchronous speed
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  ADVANTAGES OF HYSTERESISMOTOR:  Smooth , viberation free operation  Low noise  Steady state speed is directly controlled by electrical input frequency  Positive starting ( non- zero torque at rest)  Constant torque
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  DISADVANTAGES OF HYSTERESIS MOTOR: Low efficiency  Low power  Hunting phenomenon
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  APPLICATIONS : Hysteresis motorsare particularly well suited for driving  Electric clocks  Timing devices  Tape – decks  Other precision audio equipment
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  REFERENCE :  www.google.co.in www.wikipidea.org  www.slideshare.com  www.worldbook.com  www.youtube.com  www.circuitglobe.com  Electric Machines ( 2nd edition ) by Ashfaq Husain
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