SPECIAL MACHINES

Introduction


  •     There are numerous practical applications like recording instruments, clocks, teleprinters, timing devices, computer peripherals which need special motors.
  •     The power rating of such motors is very small. Some of them are even fractional horse power motors hence these motors are called fractional kW motors.
Reluctance Motor
  •     The reluctance motor has basically two main parts called stator and rotor.
  •     The stator has a laminated construction, made up of stampings.
  •     The stampings are slotted on its periphery to carry the winding called stator winding. The stator carries only one winding.
  •     This is excited by single phase a.c. supply. The laminated construction keeps iron losses to minimum.
  •     The stampings are made up of material like silicon steel which minimises the hysteresis loss.
  •     The stator winding is wound for certain definite number of poles.
  •     The rotor has a particular shape. Due to its shape, the air gap between stator and rotor is not uniform. No d.c. supply is given to the rotor.
  •     The rotor is free to rotate. The reluctance i.e. resistance of magnetic circuit depends on the air gap.
  •     More the air gap, more is the reluctance and viceversa. Due to variable air gap between stator and rotor, when rotor rotates, reluctance between stator and rotor also changes.
  •     The stator and rotor are designed in such a manner that the variation of the inductance of the windings is sinusoidal with respect to the rotor position.
  •     The construction of the reluctance motor is shown in the Fig. 7.1(a) while the practical rotor of a reluctance motor is shown in the Fig. 7.1(b)



                                                                                      Fig. 7.1 Reluctance motor


WORKING PRINCIPLE

  •     The stator consists of a single winding called main winding. But single winding can not produce rotating magnetic field. So for production of rotating magnetic field, there must be at least two windings separated by certain phase angle.
  •     Hence stator consists of an additional winding called auxiliary winding which consists of capacitor in series with it.
  •     Thus there exists a phase difference between the currents carried by the two windings and corresponding fluxes.
  •     Such two fluxes react to produce the rotating magnetic field. The technique is called split phase technique of production of rotating magnetic field.
  •     The speed of this field is synchronous speed which is decided by the number of poles for which stator winding is wound.
  •     The rotor carries the short circuited copper or aluminium bars and it acts as squirrel cage rotor of an induction motor.
  •     If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically.
  •     Similarly in the reluctance motor, rotor tries to align itself with the axis of rotating magnetic field in a minimum reluctance position.
  •     But due to rotor inertia it is not possible when rotor is standstill. So rotor starts rotating near synchronous speed as a squirrel cage induction motor.
  •     When the rotor speed is about synchronous, stator magnetic field pulls rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked.
  •     Then rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque.
  •     Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very small and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor










TORQUE-SPEED CHARACTERISTICS

  •     The torque-speed characteristic is shown in the Fig. 7.3 The starting torque is highly dependent on the position of the rotor.



Fig. 7.3 Torque-speed characteristics of reluctance motor
ADVANTAGES


  •     The reluctance motor has following advantages,
  •     No d.c. supply is necessary for rotor.
  •     Constant speed characteristics.
  •     Robust construction.
  •     Less maintenance.

LIMITATIONS
 
  •  The reluctance motor has following limitations,
  •     Less efficiency
  •     Poor power factor
  •     Need of very low inertia rotor.
  •     Less capacity to drive the loads.

APPLICATIONS
 
  •  This motor is used in signaling devices, control apparatus, automatic regulators, recording instruments, clocks and all kinds of timing devices, teleprinters, gramophones etc.


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