Reluctance 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. 

        ( 4 ) It is self starting.

Reluctance Motor

Principle

• When a magnetic material is placed in the magnetic material, it always aligns in the minimum reluctance path.

Construction

Stator

• The stator of the reluctance motor is similar to that stator of the single phase induction motor. 
• It consists of starting and running winding in the stator slots. 
• This type of motor is also called as Split phase reluctance motor.

Rotor

• The rotor of the reluctance motor is of salient or projecting poles. 
• Let us consider that the rotor of the squirrel cage induction motor consists of 24 copper bars. 
• If the rotor bar 5, 6, 11, 12,17,18,23 and 24 are cut, it is similar to 4 salient poles.

Working

• When a single phase supply is given to the stator winding, a rotating magnetic field is produced in the stator winding. 
• When a salient poles rotor cut this magnetic field, rotor aligns in the minimum reluctance path due to reluctance torque. 
• The reluctance depends upon air – gap between stator and rotor.
• Figure A shows 4 pole salient pole rotor in which direction of four high Permeance and four low Permeance is shown.
• High Permeance means higher magnetic conductivity and higher inductance. Similarly low Permeance means lower magnetic conductivity and lower inductance.
• The reluctance is inverse of Permeance. Low reluctance means higher inductance and vice versa.

        L α N² / S

        Where L = Inductance and

                   S = Reluctance of magnetic path

• Low air – gap means low reluctance and vice versa

        S = L / μ₀μ_ra

        Where L = Length of air – gap

                 μ₀ = Absolute permeability = 4π × 10 ^{– 7} Henry / meter

                 μ_r  = Absolute permeability

                  a  = Area

• There is low reluctance path between stator and salient poles due to small air – gap whereas high reluctance path between stator and inter – polar axis due to large air – gap. 
• The reluctance motor starts as an induction motor. 
• When the rotor rotates at its maximum speed, it aligns with the stator synchronous magnetic field due to reluctance torque.  
• The angle between stator poles and rotor poles of opposite polarity is called as torque angle. 
• As the torque angle increases, the reluctance torque also increases. 
• The maximum reluctance torque attains at torque angle of 45⁰. 
• The load taken by the reluctance motor is only fraction of the load taken by the three phase inductance motor.

Advantages

• Low maintenance
• DC supply not necessary
• Simple construction
• Constant speed characteristic

Disadvantages

• Low efficiency
• Low power factor
• Only fraction of load taken as compared to three phase induction motor

Applications

• Automatic regulator
• Signaling devices
• Recording instruments
• Tele printer
• Timer circuits
• Gramophone

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