ELECTRICAL MOTOR WORKING PRINCIPLE

WHAT IS AN ELECTRIC MOTOR?
ELECTRIC MOTOR IS A MACHINE THAT CONVERTS ELECTRICAL ENERGY INTO  MECHANICAL ENERGY. ELECTRICAL ENERGY IS SUPPLIED BY DC POWER FOR DC MOTORS AND WITH AC POWER FOR AC MOTORS.

PRINCIPLE OF ELECTRIC MOTOR -

WE NEED TO GIVE DOUBLE EXCITATION TO MAKE A DC MOTOR TO ROTATE. IN THE DC MOTOR, WE GIVE ONE SUPPLY TO THE STATOR AND ANOTHER TO THE ROTOR THROUGH BRUSH ARRANGEMENT. BUT IN INDUCTION MOTOR, WE GIVE ONLY ONE SUPPLY. IT IS SIMPLE, FROM THE NAME ITSELF WE CAN UNDERSTAND THAT HERE, THE INDUCTION PROCESS IS INVOLVED. WHEN WE GIVE THE SUPPLY TO THE STATOR WINDING, A MAGNETIC FLUX GETS PRODUCED IN THE STATOR DUE TO THE FLOW OF CURRENT IN THE COIL. THE ROTOR WINDING IS SO ARRANGED THAT EACH COIL BECOMES SHORT-CIRCUITED THUS IS FORMS THE CLOSED CIRCUIT .

THE FLUX FROM THE STATOR CUTS THE SHORT-CIRCUITED COIL IN THE ROTOR. AS THE ROTOR COILS ARE SHORT-CIRCUITED, ACCORDING TO FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION, THE CURRENT WILL START FLOWING THROUGH THE COIL OF THE ROTOR. WHEN THE CURRENT THROUGH THE ROTOR COILS FLOWS, ANOTHER FLUX GETS GENERATED IN THE ROTOR. NOW THERE ARE TWO FLUXES, ONE IS STATOR FLUX, AND ANOTHER IS ROTOR FLUX. THE ROTOR FLUX WILL BE LAGGING IN RESPECT OF THE STATOR FLUX. BECAUSE OF THAT, THE ROTOR WILL EXPERIENCE A TORQUE WHICH WILL MAKE THE ROTOR TO ROTATE IN THE DIRECTION OF THE ROTATING MAGNETIC FIELD. THIS IS THE WORKING PRINCIPLE OF BOTH SINGLE AND THREE PHASE INDUCTION MOTORS.

THE DIRECTIONS OF THIS INDUCED EMF, CURRENT FLOW AND ROTATIONAL TORQUE CAN BE DETERMINED WITH THE HELP OF FLEMING'S LEFT HAND RULE.FLEMING'S LEFT HAND RULE, SHOWS THE DIRECTION OF CURRENT, FORCE/TORQUE &  MAGNETIC FIELD. THIS RULE ONLY SHOWS THE DIRECTION OF EITHER ONE IF OTHER TWO DIRECTIONS ARE KNOWN. HOWEVER, IT DOES NOT SPECIFIES THE MAGNITUDE OF THESE PARAMETERS.

FLEMING'S LEFT HAND RULE DIRECTIONS

TO APPLY FLEMING'S LEFT HAND RULE WE WILL USE 3 FINGERS OF LEFT HAND AND HELD THEM PERPENDICULAR TO EACH OTHER SUCH THAT, THE THUMB WILL SHOW DIRECTION OF FORCE/MOTION OF CONDUCTOR, FOREFINGER WILL SHOW THE DIRECTION OF MAGNETIC FIELD, MIDDLE FINGER WILL SHOW DIRECTION OF CURRENT.THIS RULE IS APPLICABLE FOR ELECTRIC MOTORS.

UNDERSTANDING WORKING OF ELECTRICAL MOTOR -

AS MENTIONED ABOVE DC MOTOR CAN BE STARTED AND RUN WITH THE USE OF DOUBLE EXCITATION. HOWEVER IN CASE OF AC MOTOR WE ONLY GIVE ONE SUPPLY TO STATOR WINDINGS ONLY BUT THIS IS NOT ENOUGH FOR ROTATING THE SINGLE PHASE AC MOTOR. AS WE KNOW, IN ORDER TO FOR ROTOR TO ROTATE WE NEED ROTATIONAL TORQUE BUT IN SINGLE PHASE AC MOTORS ACCORDING TO THE NATURE OF SUPPLY THE POLARITIES ARE KEEP ON CHANGING AND THUS IT CAUSES TO PRODUCE OPPOSITE TORQUE TO PRODUCE IN ROTOR WHICH WILL STOP THE ROTATION. THUS, IN ORDER TO ROTATE WE NEED TO PROVIDE THE SUPPLY TO SINGLE PHASE AC MOTOR IN SUCH A WAY THAT IT WILL PROVIDE A ROTATIONAL TORQUE IN UNI-DIRECTION THUS MOTOR WILL ROTATE IN THE SAME DIRECTION AS TORQUE AND WILL NOT STOP UNTIL THE INTERRUPTION OF SUPPLY.

WORKING OF DC MOTOR - 

WORKING PRINCIPLE OF DC MOTOR MAINLY DEPENDS UPON FLEMING LEFT HAND RULE. IN A BASIC DC MOTOR, AN ARMATURE IS PLACED IN BETWEEN MAGNETIC POLES. IF THE ARMATURE WINDING IS SUPPLIED BY AN EXTERNAL DC SOURCE, CURRENT STARTS FLOWING THROUGH THE ARMATURE CONDUCTORS. AS THE CONDUCTORS ARE CARRYING CURRENT INSIDE A MAGNETIC FIELD, THEY WILL EXPERIENCE A FORCE WHICH TENDS TO ROTATE THE ARMATURE. SUPPOSE ARMATURE CONDUCTORS UNDER N POLES OF THE FIELD MAGNET, ARE CARRYING CURRENT DOWNWARDS AND THOSE UNDER S POLES ARE CARRYING CURRENT UPWARDS. BY APPLYING FLEMING’S LEFT HAND RULE, THE DIRECTION OF FORCE F, EXPERIENCED BY THE CONDUCTOR UNDER N POLES AND THE FORCE EXPERIENCED BY THE CONDUCTORS UNDER S-POLES CAN BE DETERMINED. IT IS FOUND THAT AT ANY INSTANT THE FORCES EXPERIENCED BY THE CONDUCTORS ARE IN SUCH A DIRECTION THAT THEY TEND TO ROTATE THE ARMATURE.

(MINIMUM TORQUE)
NO CURRENT WILL FLOW THROUGH CONDUCTORS

AGAIN, DUE THIS ROTATION THE CONDUCTORS UNDER N-POLES COME UNDER S-POLE AND THE CONDUCTORS UNDER S-POLES COME UNDER N-POLE. WHILE THE CONDUCTORS GO FORM N-POLES TO S-POLE AND S-POLES TO N-POLE, THE DIRECTION OF CURRENT THROUGH THEM, IS REVERSED BY MEANS OF COMMUTATOR. DUE TO THIS REVERSAL OF CURRENT, ALL THE CONDUCTORS COME UNDER N-POLES CARRY CURRENT IN DOWNWARD DIRECTION AND ALL THE CONDUCTORS COME UNDER S-POLES CARRY CURRENT IN UPWARD DIRECTION AS SHOWN IN THE FIGURE. HENCE, EVERY CONDUCTOR COMES UNDER N-POLE EXPERIENCES FORCE IN SAME DIRECTION AND SAME IS TRUE FOR THE CONDUCTORS COME UNDER S-POLES. THIS PHENOMENON HELPS TO DEVELOP CONTINUOUS AND UNIDIRECTIONAL TORQUE.

(MAXIMUM TORQUE)
MAXIMUM CURRENT WILL FLOW THROUGH THE CONDUCTORS

WORKING OF AC MOTOR -

WORKING OF ELECTRIC MOTOR IN THE CASE OF INDUCTION MOTOR IS LITTLE BIT DIFFERENT FROM DC MOTOR. IN SINGLE PHASE INDUCTION MOTOR, WHEN A SINGLE PHASE SUPPLY IS GIVEN TO THE STATOR WINDING, A PULSATING MAGNETIC FIELD IS PRODUCED AND IN A THREE PHASE INDUCTION MOTOR, WHEN THREE PHASE SUPPLY IS GIVEN TO THREE PHASE STATOR WINDING, A ROTATING MAGNETIC FIELD IS PRODUCED. THE ROTOR OF AN INDUCTION MOTOR IS EITHER WOUND TYPE OR SQUIRREL CADGE TYPE. WHATEVER MAY BE THE TYPE OF ROTOR, THE CONDUCTORS ON IT ARE SHORTED AT END TO FORM CLOSED LOOP. DUE TO ROTATING MAGNETIC FIELD, THE FLUX PASSES THROUGH THE AIR GAP BETWEEN ROTOR AND STATOR, SWEEPS PAST THE ROTOR SURFACE AND SO CUTS THE ROTOR CONDUCTOR. HENCE ACCORDING TO FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION, THERE WOULD BE A INDUCED CURRENT CIRCULATING IN THE CLOSED ROTOR CONDUCTORS. THE AMOUNT OF INDUCED CURRENT IS PROPORTIONAL TO THE RATE OF CHANGE OF FLUX LINKAGE WITH RESPECT TO TIME. AGAIN THIS RATE OF CHANGE OF FLUX LINKAGE IS PROPORTIONAL TO THE RELATIVE SPEED BETWEEN ROTOR AND ROTATING MAGNETIC FIELD. AS PER LENZ'S LAW THE ROTOR WILL TRY TO REDUCE THE EVERY CAUSE OF PRODUCING CURRENT IN IT. HENCE THE ROTOR ROTATES AND TRIES TO ACHIEVE THE SPEED OF ROTATING MAGNETIC FIELD TO REDUCE THE RELATIVE SPEED BETWEEN ROTOR AND ROTATING MAGNETIC FIELD.

ARE SINGLE PHASE AC MOTORS SELF STARTING?

THE ANSWER IS "NO";

WE KNOW THAT THE AC SUPPLY IS A SINUSOIDAL WAVE AND IT PRODUCES A PULSATING MAGNETIC FIELD IN THE UNIFORMLY DISTRIBUTED STATOR WINDING.

SINCE WE CAN ASSUME THE PULSATING MAGNETIC FIELD AS TWO OPPOSITELY ROTATING MAGNETIC FIELDS, THERE WILL BE NO RESULTANT TORQUE PRODUCED AT THE STARTING, AND HENCE THE MOTOR DOES NOT RUN. AFTER GIVING THE SUPPLY, IF THE ROTOR IS MADE TO ROTATE IN EITHER DIRECTION BY AN EXTERNAL FORCE, THEN THE MOTOR WILL START TO RUN. WE CAN SOLVE THIS PROBLEM BY MAKING THE STATOR WINDING INTO TWO WINDING – ONE IS THE MAIN WINDING, AND ANOTHER IS AUXILIARY WINDING.

WE CONNECT ONE CAPACITOR IN SERIES WITH THE AUXILIARY WINDING. THE CAPACITOR WILL MAKE A PHASE DIFFERENCE WHEN CURRENT FLOWS THROUGH BOTH COILS. WHEN THERE IS A PHASE DIFFERENCE, THE ROTOR WILL GENERATE A STARTING TORQUE, AND IT WILL START TO ROTATE. PRACTICALLY WE CAN SEE THAT THE FAN DOES NOT ROTATE WHEN THE CAPACITOR GETS DISCONNECTED FROM THE MOTOR, BUT IF WE ROTATE WITH THE HAND, IT WILL START ROTATING. THAT IS WHY WE USE A CAPACITOR IN THE SINGLE-PHASE INDUCTION MOTOR.

SINGLE PHASE AC INDUCTION MOTOR CIRCUIT

ARE THREE PHASE AC MOTORS SELF STARTING?

THE ANSWER IS "YES";

UNLIKE SINGLE PHASE, IN A THREE PHASE SYSTEM, THERE ARE THREE SINGLE PHASE LINES WITH A 120° PHASE DIFFERENCE. SO THE ROTATING MAGNETIC FIELD HAS THE SAME PHASE DIFFERENCE WHICH WILL MAKE THE ROTOR TO MOVE. IF WE CONSIDER THREE PHASES A, B, AND C WHEN PHASE A GETS MAGNETIZED, THE ROTOR WILL MOVE TOWARDS THE PHASE A WINDING A, IN THE NEXT MOMENT PHASE B WILL GET MAGNETIZED AND IT WILL ATTRACT THE ROTOR AND THEN PHASE C. SO THE ROTOR WILL CONTINUE TO ROTATE.

WORKING OF SYNCHRONOUS MOTOR - 

IN SYNCHRONOUS MOTOR, WHEN BALANCED THREE PHASE SUPPLY IS GIVEN TO THE STATIONARY THREE PHASE STATOR WINDING, A ROTATING MAGNETIC FIELD IS PRODUCED WHICH ROTATES AT SYNCHRONOUS SPEED. NOW IF AN ELECTROMAGNET IS PLACED INSIDE THIS ROTATING MAGNETIC FIELD, IT IS MAGNETICALLY LOCKED WITH THE ROTATING MAGNETIC FIELD AND THE ROTOR ROTATES WITH THE ROTATING MAGNETIC FIELD AT SAME SPEED THAT IS AT SYNCHRONOUS SPEED.