A "MEDIA TO GET" ALL DATAS IN ELECTRICAL SCIENCE...!!: SWITCHED RELUCTANCE MOTORS

The structure of a switched reluctance motor is shown below. This is a 4-phase machine with 4 stator-pole pairs and 3 rotor-pole pairs (8/6 motor). The rotor has neither windings nor permanent magnets.

The stator poles have concentrated winding rather than sinusoidal winding. Each stator-pole pair winding is excited by a converter phase, until the corresponding rotor pole-pair is aligned and is then de-energized. The stator-pole pairs are sequentially excited using a rotor position encoder for timing.

CONSTRUCTION

STATOR AND ROTOR ARE SALIENT IN STRUCTURE

STATOR WINDINGS ARE INDEPENDENT CONCENTRATED WINDINGS WHICH ARE EXCITED WITH SWITCHES FROM SOURCE

NO FIELD WINDINGS HENCE SINGLY EXCITED

DIAMETRICALLY OPPOSITE ARMATURE WINDINGS ARE CONNECTED TO FORM A PHASE

FOR BIDERCTIONAL CONTROL AND SELF STARTING,NUM OF ROTOR POLES ARE LESS THAN NUM OF STATOR POLES

M15,M17,M19 FOR HIGH SPEED AND HYPERCO AND VANADIUM PERMENDUR FOR HIGH PERFORMANCE

SINGLE STACK AND MULTI STACK CONSTRUCTION POSSIBLE

WORKING

FIG SHOWS AN 8/6 SINGLE STACK WHICH HAS 4 PHASES A,B,C,D

WHEN PHASE A IS EXCITED RELUCTANCE TORQUE CAUSES ROTOR TO TURN UNTIL IT ALIGNS WITH AXIS OF PHASE A.

EXCITATION IS CHANGED TO B AND A IS DEEXCITED BEFORE ALIGNMENT

ROTATION IS IN DIRECTION OF ENERGISATION

DIRECTION OF ROTATION REVERSED BY REVERSING SEQUENCE OF EXCITATION

SPEED DEPENDS ON MAGNITUDE OF INPUT MICROSTEPPING CAN BE DONE FOR SINGLE STACK ONLY 1 ROTOR AND STATOR

FOR MULTI STACK OPERATION,NUM OF ROTOR AND STATOR DEPENDS ON NUM OF PHASES

POSITION OF MIN RELUCTANCE CHANGED WITH HELP OF POSITION SENSORS

WHEN PHASE IS EXCITED AFTER THE ROTOR PASSES POINT OF MIN RELUCTANCE, REVERSE TORQUE ACTS[REGENERATIVE BRAKING]

The inductance of a stator-pole pair and corresponding phase currents as a function of angular position is shown below.

Applying the stator pulse when the inductance profile has positive slope induces forward motoring torque.

Applying the stator pulse during the time that the inductance profile has negative slope induces regenerative braking torque.

A single phase is excited every 60degree with four consecutive phases excited at 15degree intervals.

The torque is given by:

where m = inductance slope and
i = instantaneous current.

Switched reluctance motors are growing in popularity because of their simple design and robustness of construction. They also offer the advantages of only having to provide positive currents, simplifying the inverter design. Also, shoot-through faults are not an issue because each of the main switching devices is connected in series with a motor winding. However, the drawbacks of this type of motor are the pulsating nature of their torque and they can be acoustically noisy (although improved mechanical design has mitigated this problem.)

ADVANTAGES