GMIGERMAN-MALAYSIAN INSTITUTE AC ASYNCHRONOUS MOTORGroup member : Syed Ahmad Destar bin Syed Mohd Aziz Sheratul Ashikin bt. Zamarudin Nazirul Hakim bin A. Manap Norallif bin Norijan TTO : Sir Muhairi
OVERVIEW WHAT IS ACASYNCHRONOUS MOTOR? TYPE OF ACASYNCHRONOUS MOTOR WORKING PRINCIPLEBASIC CONSTRUCTIONAND CHARACTERISTICS APPLICATION OFASYNCHRONOUS MOTOR
The induction motor is a common form ofasynchronous motor Power is supplied to the rotor by means of electromagnetic induction The speed and the torque control in various types of AC induction motors The simplest and most rugged electric motor and consists of two basic electrical assemblies: • the stator • the rotor
TYPE OF ASYNCHROUNOUS MOTOR 2 separate winding Squirrel Double poles cage 3-phase Dahl ender Slip-ring ResistorAsynchronous start motor Capacitor start Capacitor Single phase start-run Permanent capacitor Universal Commutator Repulsion
PRINCIPLE OF 3 PHASE INDUCTION MOTORThe current – carrying conductors kept in a magnetic field will tendto create a force.The rotor of the induction motor is not electrically connected to thestator, but induces a voltage / current in the rotor by the transformeraction, as the stator magnetic field sweeps across the rotor.The current in the rotor is not drawn directly from the supply,but is induced by the relative motion of the rotor conductors and themagnetic field produced by the stator currents.The stator produces a rotating magnetic field in the stator core.The rotor of the induction motor may have either •shorted rotor conductors in the forms of a squirrel cage •in the form of a 3 –phase winding to facilities the circulation of current through a closed circuit.
PRINCIPLE OF 3 PHASE INDUCTION MOTOR The induction motor is rotating in a clockwise direction Applying Fleming’s right hand rule, the direction of emf induce in the rotor will be towards the observer.As the rotor conductors have a closed electricpath, due to their shorting, a current will flowthrough them as in a short – circuited secondaryof a transformer.
The magnetic field produced by the rotorcurrents will be in a counter – clockwisedirection according to Maxwell Corkscrew rule.The interaction between the stator magnetic field The rotor follows the stator field in theand the rotor magnetic fields results in a force to same direction by rotating at a speedmove the rotor in the same action as that of the lesser than the synchronous speed ofrotating magnetic field of the stator the stator rotating. As the motor is loaded, the rotorThe rotor attains a speed equal to the speed has to fall to cope up with thesynchronous speed of the rotating magnetic field mechanical force :thereby the relativeof the stator, there be relative motion between speed increased and the induced emfthe stator field and the rotor , no induced emf or and current increase in the rotorcurrent and not be any torque in the rotor. resulting in an increased torque.
To reverse the direction of rotation of the stator as well as the rotor, the phasesequence of the supply is to be changed by changing any two leads connected to thestator.The principle of operation for all three-phase motors is dependant on the presence ofa rotating magnetic field in the stator.There are three factors that cause the magnetic field to rotate. 1.The voltages of the three-phase system are 120° out of phase with each other; 2.The three voltages change polarity at regular intervals; and 3. The arrangement of the stator windings around the inside of the motor.
CONSTRUCTION AND CHARACTERISTICS OF A 3 PHASE INDUCTION MOTOR Three phase induction motors construction consist of: • The Stator • The Rotor.The stator: The squirrel-cage and the wound-rotor induction motors have nearly the same stator construction and winding arrangement. The stator is a three-phase winding placed in the slots of a laminated steel core and formed of three single-phase windings spaced 120 electrical degrees apart. The three single-phase windings are connected in star or delta formation. The three line leads from the three windings are brought out to a terminal box mounted on the frame of the motor. The laminations of the steel core are insulated by varnish or oxide coating, and are slotted in their inner periphery
CONSTRUCTION AND CHARACTERISTICS OF A 3 PHASE INDUCTION MOTORWe have two major types of rotor construction: • Squirrel cage induction motors • Slip ring induction motors.
CONSTRUCTION SQUIRREL CAGE ROTOR The rotor of the squirrel cage motor contains no windingThe rotor is constructed of laminated steel sheets assembled around ashaft. The rotor winding consists of copper or aluminium bars and solderedto two copper end rings.The bars and end rings are all die cast in position without soldering atthe ends. The conductor rotors more rugged. The slots of the rotor are not always parallel to the slots on the stator. Skewed rotors are twisted (skewed). Skew effectively reduces noise, eliminates the magnetic locking of therotor and increases starting torque.Operates by virtue of the eddy currents established in the rotor. The speed performance of a squirrel cage motor is measured in termsof slip.Slip is usually expressed as the percentage by which the speed of therotor falls behind the speed of the rotating synchronous speed of thestator field.
CHARACTERISTICS OF SQUIRREL CAGE ROTOR The most important characteristic of the induction motor is the •speed •torque characteristic which is also called the mechanical characteristicThe characteristic of the squirrel cage induction motor tofind the relationship between load, speed, torque and slip. Speed, Torque and slip characteristic
SYNCHRONOUS SPEEDThe speed at which the magnetic field rotates isknown as the synchronous speed.Methods of speed controlBy variation of frequencyBy variation of poleNs = 120f Ns : synchronous speed in rpm(revolution per minute) P P: number of poles in the stator
ROTOR SPEEDThe induced rotor voltage UR is proportional to the slip s. In thestopped position, it peaks at n = 1 and s = 1This fact is confirmed in real-life applications by thehigh starting current (starting current inrush).The torque also peaks during the stop period at a certain rotorresistance.This behavior can be modified by design variation. However the rotorresistance is not usually used for this purpose.The following formula applies to the rotor speed:
TORQUEThe torque of a motor is the measure of its turning ability(i.e. the traction force multiplied by the radius at which itoperates).If this force is referred to as F and the radius of the pulleyr, then the torque M of the motor is given by:Equation: T = FRWhere:T = Torque, lb-ftF = Force, lbR = Radius, or distance which force is applied from pivot location, ft
SLIP, S•The rotor speed of an Induction machine is different from thespeed of Rotating magnetic field. The % difference of the speedis called slip. ns nr s OR nr ns (1 s) nsWhere; ns = synchronous speed (rpm) nr = mechanical speed of rotor (rpm)under normal operating conditions, s= 0.01 ~ 0.05, which isvery small and the actual speed is very close to synchronousspeed.Note that : s is not negligible
CHARACTERISTICS OF SQUIRREL CAGE ROTORAlways lag behind the synchronous speed of the stator field.The rotor slip is necessary in order to induce the rotor currentsrequired for the motor torqueThe torque will increase in almost direct proportion to the rotorslip The rotor speed will decrease when the slip is increase Increase in slip in turn increases the induced rotor currents, andthe increased rotor current in turn, will produce a higher torque tomeet the increased load.
WORKING SLIP-RING ROTORThe stator-winding of the slip-ring motor is connected to the 3-phasesupply produces a rotating magnetic.The brushes and the star-connected external resistors.The rotor resistance is high enabling the starting current to be lowThe high resistance rotor circuit increases the rotor power factorThe torque developed at the start becomes much higher than the torquedeveloped in squirrel cage motors.The external resistance is slowly reduced, the motor operates with lowslip and high operating efficiencyBy Increasing rotor resistance, the motor’s slip will be greater, the speedregulation poorer and it will have low efficiencyThe design of external resistance varied to change the speed of the slip-ring motor between 50 to 100 percent of the rated speed.The I2R loss in the rotor due to increased resistance is inevitable.
CONSTRUCTION SLIP-RING ROTORVariable speed and high starting torque are prime requirementsThe stator of the slip-ring induction motor is very much thesameStator windings can be either star or delta connected dependingupon the designThe rotor consists of three-phase windings to form the samenumber of poles as in a statorThe rotor winding is connected in star and the open ends areconnected to three slip-rings mounted in the rotor shaft
CHARACTERISTICS OF SLIP RING MOTORo Constant speed-service requiring a heavier starting torque than is obtainablewith squirrel cage type.o Lower starting currento Insertion of higher external resistance alters the starting torque to a higher valueoExternal resistance improves the starting to torque to a higher valueoThe running efficiency of the motor could be achieved by cutting out the externalresistance when the motor picks up its speed.oa higher starting torque and a variables speed control.
APPLICATION OF SLIP RING MOTOR compressor Central air conditoining
ADVANTAGES OF SLIP RINGAND SQUIRREL CAGE MOTOR SQUIRREL CAGE SLIP RINGcheaper and more robust the starting torque is muchslightly higher and the starting current much lowerhigher efficiency and power the speed can be varied byfactor means of external rotorexplosion proof, since the resistorsabsence of slip-rings andbrushes eliminates risk ofsparking.
DOUBLE POLES SQUIRREL CAGE MOTOR??The rotor speed always remains near rotating magnetic fieldsspeed or synchronous speed.To vary the slip of the motor for given loadThe synchronous speed of the machine can be varied are: • changing the electrical frequency which using the frequency inverter • changing the number of poles on the motorThere are 2 types that most marketable double speed squirrelcage motor :• Two separate winding• Dahlender
PRINCIPLE OF DOUBLE POLES MOTORPermit the synchronous speed to be changed by changing thenumber of stator polesThe synchronous speed will be reduced by one halfThe number of stator poles can be changed by changing thedirection of current flow through alternate pairs of poles.Both poles will produce the same magnetic polarity and areessentially one pole.The coils now produce the opposite magnetic polarities andare essentially two different poles
SINGLE PHASE INDUCTION MOTOR The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth. The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly. One is the main and The other is the auxiliary winding or starting winding
SINGLE PHASE INDUCTION MOTOR This “single-phase” Stator with laminated iron core Slots with winding motors are truly two- Rotor with phase machines. Main winding laminated + iron core The motor uses a squirrel cage rotor, + Bars which has a _ laminated iron core _ with slots. Starting winding Ring to short circuit the bars Aluminum bars are molded on the slots and short-circuited at both ends with a Figure 7.42 Single-phase ring. induction motor.
SINGLE PHASE INDUCTION MOTOR Figure 7.10 Squirrel cage rotor
UNIVERSAL MOTOR The operation of a universal motor is much like a series dc motor. Force is created on the armature conductors due to the interaction between the mian field flux The flux created by the current carring armature conductor. Develops unidirectional torque regardless of whether it operated on AC or DC supply
o In AC operation, both field armaturecurrents change their polarities andresulting unidirectional torque. o Change the rotation of the motor by reversing the flow of current through either the armature or the field windings.
CHARACTERISTICS The speed of the motor is inversely proportional to the load.(eg; low at full load) The speed reaches a dangerously high value due to low field flux at no load. Are connected with permanent load or gear trains to avoiding high speeds.
REPULSION MOTORS Figure above show a two pole motor with its magnetic axis vertical. The armature have a commutator which is short circuited through the brushes that placed in the magnetic field. The stator winding connected to AC supply, it produces an alternating magnetic field. A voltage is induced in rotor conductors by the transformer action.
o The direction of current according with Len’z law. Lenz law An induced current flows in a direction to create a magnetic field which will counteract the change in magnetic flux. or An induced electromagnetic force generates a current that induces a counter magnetic field that opposes the magnetic field generating the current.o Created a north pole at the top just below the stator north pole, and a south pole at the bottom just at the top of the stator south pole to oppose the induction action.o No torque developed due to the absence of the tangential component of the torque
CHARACTERISTIC the current in the starting winding due to its capacitor will lead the applied voltage and line current in very small. This result in producing a higher power factor and an excellent starting toque, several times higher than the normal running torque. The running torque adjusts itself with load by varying inversely with respect to speed
APPLICATION Due to the excellent starting torque and easy direction-reversal characteristic. These machines are used in balted fans, blowers, dryers, washing machines, pumps and compressors.
PERMANENT CAPACITOR MOTOR The starting torque is not required to be high at the same time elimination of the centrifugal switch in the motor The capacitor connect in series with the starting winding These capacitor should be of oil-type construction and have continuous duty rating Below is the torque-speed characteristic
CAPACITOR START, CAPACITOR RUN MOTOR Have excellent starting torque. About 300% of the full load torque, power factor starting is high, When its running torque is not good, power factor is low. Lesser efficiency
CHARACTERISTIC The starting torque is 300% of the full load torque The starting current is low , say 2 to 3 times of the running current Highly efficient running Extremely noiseless operation Starting and running P.F are good Load up to 125% of the full-load capaciy