PERMANENT MAGNET BRUSHLESS MOTORS/RELUCTANCE MOTOR

PERMANENT MAGNET BRUSHLESS MOTORS/RELUCTANCE MOTOR 

CONSTRUCTION AND TYPES OF PERMANENT MAGNET

SO THERE ARE MANY MOTOR USED AND THERE IS A COMMONLY USED SYNCHRONOUS MOTORS ARE:-

1. WOUND FIELD (CYLINDRICAL/NON-SALIENT) MOTOR

2. PERMANENT MAGNET (SALIENT POLE) MOTOR

3. RELUCTANCE MOTOR

4. HYSTERESIS MOTOR

ALL THESE MOTORS HAVE A STATOR WITH A 3- PHASE WINDING, WHICH IS CONNECTED TO AN AC SOURCE.

FRACTIONAL HORSEPOWER- SYNCHRONOUS RELUCTANCE AND HYSTERESIS MOTORS EMPLOY A SINGLE PHASE STATOR.

IN MEDIUM AND SMALL SIZE MOTORS, DC FIELD CAN BE PRODUCED BY PERMANENT MAGNETS. THUS THERE IS NO NEED OF DC SOURCE FOR FIELD WINDING, SLIP RINGS, BRUSHES, AND HENCE RESULTS IN NO FIELD WINDING LOSSES. SUCH MOTORS ARE KNOWN AS PERMANENT MAGNET SYNCHRONOUS MOTOR.

PERMANENT MOTOR SYNCHRONOUS MOTORS

USUALLY, FERRITE MAGNETS ARE EMPLOYED. RARE EARTH (COBALT- SAMARIUM) MAGNETS, ALTHOUGH VERY EXPENSIVE, ARE SAME TIMES USED TO REDUCE THE VOLUME AND WEIGHT OF THE MOTOR.

PERMANENT MAGNET SYNCHRONOUS MOTORS ARE CLASSIFIED AS:-

1. SURFACE MOUNTED   

      A. PROJECTING TYPE

PROJECTING TYPE, IN WHICH MAGNETS PROJECT FROM THE SURFACE OF ROTOR. POLES ARE FIXED ON ROTOR CORE WITH THE HELP OF EPOXY GLUE.

      B. INSET TYPE

IN THIS MAGNETS ARE INSERTED INTO THE ROTOR, PROVIDING A SMOOTH ROTOR SURFACE. EPOXY GLUE IS USED TO FIX THE POLES. WHILE THESE MOTORS ARE EASY TO CONSTRUCT AND ARE LESS EXPENSIVE, THEY ARE LESS ROBUST COMPARED TO INTERIOR TYPE ROTORS AND ARE NOT SUITABLE FOR HIGH-SPEED APPLICATIONS. 

2. INTERIOR TYPE

IN INTERIOR TYPE PERMANENT MAGNET MOTORS, MAGNETS ARE EMBEDDED IN THE INTERIOR OF THE ROTOR.

THE DIFFERENCE BETWEEN WOUND FIELD AND PERMANENT MAGNET MOTORS IS THAT WHEN A BOUND FIELD MOTOR IS STARTED AS AN INDUCTION MOTOR, DC FIELD MOTOR IS KEPT OFF. IN CASE OF PM MOTOR, THE FIELD CAN NOT BE TURNED OFF.

THE PERMANENT SYNCHRONOUS MOTOR IS DESIGNED SO THAT THE BRAKING TORQUE IS VERY SMALL AS COMPARED TO INDUCTION MOTOR TORQUE. BECAUSE OF THE CAPABILITY OF START DIRECT ONLINE, THESE MOTORS ARE CALLED LINE START PERMANENT MAGNET SYNCHRONOUS MOTOR.

RELUCTANCE MOTOR

RELUCTANCE MOTORS OPERATE ON THE FOLLOWING PRINCIPLE.

WHENEVER A PIECE OF FERRO-MAGNETIC MATERIAL IS LOCATED IN A MAGNETIC FIELD, A FORCE IS EXERTED UPON THE MATERIAL, TENDING TO BRING IT INTO THE POSITION OF DENSEST PORTION OF THE FIELD (i.e WHERE MAGNETIC FIELD INTENSITY IS HIGH AND RELUCTANCE IS LAW). THE FORCE TENDS TO ALIGN THE MAGNETIC MATERIAL SO THAT THE RELUCTANCE OF THE MAGNETIC PATH WILL BE MINIMUM.

CONSTRUCTION

A SINGLE-PHASE SYNCHRONOUS RELUCTANCE MOTOR IS BASICALLY THE SAME AS THE SINGLE PHASE CAGE TYPE INDUCTION MOTOR. THE STATOR HAS THE MAIN WINDING AND AUXILIARY WINDING. IN GENERAL, THE STATOR OF A SINGLE PHASE RELUCTANCE MOTOR IS SIMILAR TO THAT OF ANY ONE OF THE SINGLE-PHASE INDUCTION MOTOR.

THE ROTOR OF A RELUCTANCE MOTOR IS BASICALLY A SQUIRREL CAGE WITH SOME ROTOR TEETH REMOVED AT THE APPROPRIATE PLACES SUCH AS TO PROVIDE THE DESIRED NUMBER OF SALIENT ROTOR POLES. THE 4 POLE RELUCTANCE TYPE SYNCHRONOUS MOTOR. HERE TEETH HAVE BEEN REMOVED IN FOUR LOCATIONS TO PRODUCE A 4 POLE SALIENT POLE STRUCTURE. THE ROTOR BARS ARE KEPT INTACT EVEN IN THE SPACES FROM WHERE TEETH ARE REMOVED. THE TWO END-RINGS SHORT CIRCUIT THESE BARS AS IN A CAGE ROTOR.

WORKING

WHEN THE STATOR IS CONNECTED TO A SINGLE PHASE SUPPLY THE MOTOR STARTS AS A SINGLE PHASE INDUCTION MOTOR. AT A SPEED OF ABOUT 75% OF Ns, A CENTRIFUGAL SWITCH DISCONNECTS THE AUXILIARY WINDING AND THE MOTOR CONTINUE TO SPEED UP AS A SINGLE PHASE MOTOR WITH THE MAIN WINDING IN OPERATION. WHEN THE SPEED IS CLOSE TO THE Ns, A RELUCTANCE TORQUE IS PRODUCED DUE TO TENDENCY OF THE ROTOR TO ALIGN TO THE SYNCHRONOUSLY ROTATING FLUX OF THE FORWARD FIELD. THE ROTOR PULLS INTO SYNCHRONISM FOR THIS TO HAPPEN EFFECTIVELY THE LOAD MUST BE MINIMUM OR WITHIN LIMITS SO THAT LOAD INERTIA MAY NOT PULL OUT THE MOTOR FROM SYNCHRONISM.

AFTER PULLING INTO SYNCHRONISM, THE INDUCTION TORQUE DISAPPEARS BUT THE ROTOR REMAINS IN SYNCHRONISM DUE TO SYNCHRONOUS RELUCTANCE TORQUE ALONE. 

WHEN THE SPEED  IS CLOSE TO Ns, THE RELUCTANCE TORQUE DEVELOPED AS SYNCHRONOUS MOTOR PULLS THE ROTOR IN SYNCHRONISM AND ROTOR CONTINUES TO ROTATE AT Ns.

THESE MOTOR HAVE:-

#POOR POWER FACTOR

#LOW EFFICIENCY

#POOR TORQUE

APPLICATIONS

THESE TYPE OF MOTOR WIDELY USED IN CONSTANT SPEED APPLICATIONS SUCH AS:-

#TIMER

#CLOCKS

#RECORDING INSTRUMENTS

#SIGNALLING DEVICES