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UNIT II - Electric Machines and Drives

The variable reluctance motor is based on the principle that an unrestrained piece of iron will move to complete a magnetic flux path with minimum reluctance, the magnetic analog of electrical resistance. 

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•     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

MATHEMATICAL ANALYSIS

    Consider an elementary reluctance motor as shown in the Fig. 7.2

•     The variation of the inductance of the windings is sinusoidal with respect to rotor position.

•     The variation of the inductance with respect toO is of double frequency and is given by,
  
  
•     The stator winding is exciteal by a.c. supply hence

                                                 i= Im sint

•     The energy stored is a function of inductance and given by,

•     The flux linkage is given by,

•     Then the torque is given by

                           
    Substituting the values of i and L,

•     If rotor is rotating at an angular velocity corn then finally the torque equation can be expressed interms of ω and ωm as,

•     The above equation gives instantaneous torque produced. The average torque is zero as average of each term in the above equation is zero.
  
  

•     The value of torque is not zero when  ω = ωm and at this condition the magnitude of the average torque is,

•     The speed corresponding to the frequency ω = ωm is nothing but the synchronous speed. The  is a torque angle. The maximum torque occurs at δ = 45º which is termed as pull-out torque.of synchronism.

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.

Synchronous reluctance

If the rotating field of a large synchronous motor with salient poles is de-energized, it will still develop 10 or 15% of synchronous torque. This is due to variable reluctance throughout a rotor revolution. There is no practical application for a large synchronous reluctance motor. However, it is practical in small sizes.
If slots are cut into the conductorless rotor of an induction motor, corresponding to the stator slots, a synchronous reluctance motor results. It starts like an induction motor but runs with a small amount of synchronous torque. The synchronous torque is due to changes in reluctance of the magnetic path from the stator through the rotor as the slots align. This motor is an inexpensive means of developing a moderate synchronous torque. Low power factor, low pull-out torque, and low efficiency are characteristics of the direct power line driven variable reluctance motor. Such was the status of the variable reluctance motor for a century before the development of semiconductor power control.

Switched reluctance

If an iron rotor with poles, but without any conductors, is fitted to a multi-phase stator, a switched reluctance motor, capable of synchronizing with the stator field results. When a stator coil pole pair is energized, the rotor will move to the lowest magnetic reluctance path. (Figure below) A switched reluctance motor is also known as a variable reluctance motor. The reluctance of the rotor to stator flux path varies with the position of the rotor.

Reluctance is a function of rotor position in a variable reluctance motor.
Sequential switching (Figure below) of the stator phases moves the rotor from one position to the next. The mangetic flux seeks the path of least reluctance, the magnetic analog of electric resistance. This is an over simplified rotor and waveforms to illustrate operation.