RELUCTANCE MOTOR

RELUCTANCE MOTOR

• A reluctance motor is a type of electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Rotor does not have any windings. Torque is generated through the phenomenon of magnetic reluctance.

The most important characteristics of this technology can be summarized as follows:

Efficiency: 

• The efficiency of SR motors is comparable to permanent magnet motors in all 4 quadrants. The values reach more than 85% in a wide range of operation. Compared to conventional AC induction motors the SR motors often have an advantage (especially in the middle speed range).

Controllability: 

• The SR motor can be operated in all 4 quadrants. Due to the unipolar design the driving electronics is quite simple. To achieve less torque ripple some more sophisticated design is necessary.

Robustness and Reliability: 

• The SR motor is as robust as conventional AC induction motors, since the rotor is pure iron. In case of a winding fault these motors are better protected than other motor technologies.

Audible Noise: 

• The audible noise during operation is as good as other technologies; new lamination designs and driving algorithms promise further improvement.

Speed Range:

•  The SR motor can be operated over a wide speed range. In this characteristic it is similar to a vector controlled AC induction motor with constant torque at low speed and constant power at high speed.

Peak Torque:

•  The SR motor offers a by far higher peak torque than an induction motor of the same size; but it cannot reach the value of a similar sized permanent magnet motor.
• Power to Weight Ratio: 
• Also here the SR motor has advantages over the AC induction motor. For a given power the SR motor is about 40% smaller and lower weight than the AC induction motor. There is no physical limitation for the power of SR motors; SR motors have been developed for a power range of 50 W to 5 MW.

The Torque Speed Characteristics:

• The starting torque depends upon the rotor position. The value of the starting torque varies between 300 to 400 % of its full load torque. As we know that as motor attains speed nearly of synchronous speed the auxiliary winding is disconnected and the rotor continues to rotate at the synchronous speed.
• The motor operates at a constant speed up to a little over than 200% of its full load torque. If the loading of the motor is increased above the value of the pull out torque, the motor loose synchronism but continues to run as a single phase induction motor up to over 500% of its rated torque. At the starting the motor is subjected to Cogging. This can be reduced by skewing the rotor bars and by having the rotor slots not exact multiples of the number of poles.
• The rotor of a Reluctance Motor is unexcited, therefore, the power factor is low as compared to the induction motor. As the motor has no DC field excitation so the output of a reluctance motor is reduced. Hence, the size of the motor is large as compared to synchronous motor.

                                          

Types of reluctance motor:

• Synchronous reluctance motor

• Variable reluctance motor

• Switched reluctance motor

• Variable reluctance stepping motor.

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 conductor less rotor of an induction motor, corresponding to the stator slots, asynchronous 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. 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.

                                             

Advantages:

• Simple construction- no brushes, commutator , or permanent magnets, no Cu or Al in the rotor.

• High efficiency and reliability compared to conventional AC or DC motors.

• High starting torque.

• Cost effective compared to bushless DC motor in high volumes.

• Adaptable to very high ambient temperature.

• Low cost accurate speed control possible if volume is high enough.

Disadvantages:

• Current versus torque is highly nonlinear

• Phase switching must be precise to minimize ripple torque

• Phase current must be controlled to minimize ripple torque

• Acoustic and electrical noise

• Not applicable to low volumes due to complex control issues

Applications:

It is used in,

• ·         computer  plotters and printers,
• ·         Industrial control,
• ·         Numerically controlled machine tools,
• ·         Robotic equipment,
• ·         Clocks and watches