3 Phase Induction MotorA Review

ByBirju Besra

Department : EEE

I. INTRODUCTIONII. CONSTRUCTIONAL DETAILSIII. PRINCIPLE OF OPERATIONIV. TORQUE SLIP CHARACTERISTICSV. TESTS ON THE INDUCTION MOTORVI. SPEED CONTROL OF INDUCTION MOTORVII. CONCLUSIONVIII.REFFERENCE

INTRODUCTION:Induction motor is an energy conversion device that converts electrical energy into useful rotational kinetic energy, it is an application of the Faraday's law of induction.AC Motors are required in many modern adjustable-speed drives; the requirement is for precise and continuous control of speed and torque with long-term stability and high efficiency. The DC motor satisfies most of these requirements, but its mechanical commutator and the sparking are disadvantages because they may be dangerous in some areas of applications, plus regular maintenance is required and cannot be done when the motor is used at inaccessible locations.

A three phase induction motor mainly consists of two main parts:1. Stator2. Rotor

I. CONSTRUCTIONAL DETAILS

1.StatorThe stator consists of a steel frame which enclosesa hollow cylindrical core made up of thin laminations ofsilicon steel to reduce hysteresis and eddy current losses. Anumber of evenly spaced slots are provided on the innerperiphery. The insulated conductors are placed in thestator slots and are connected to form a balanced three phasestar or delta connection circuit.

2.Rotor:The rotor is rotating part of an induction motor. Itis a hollow laminated core having slots on its outer periphery.The rotor may be one the following types.I. Squirrel cage typeII. Wound type or slip ring type

Squirrel cage type: It consists of laminatedcylindrical corehaving parallel slotson its outerperiphery, in theseslots one copper oraluminium bar isplaced. All these barsare connected at eachend by metal rings,called end rings.These bars,therefore, arepermanently shortcircuited andtherefore no external resistance can be connected.

Wound type or slip ring type: These consist of laminated cylindrical core and carries a three phase winding as in the stator. The open ends of the rotor winding is brought out and connected to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. The three brushes are connected to a three phase star connected rheostat. These resistances are included at starting to limit high circulating current in the short circuited rotor winding and to give a large starting torque. These resistances aregradually reduced to zero as the rotor attains its normal speed.

II. PRINCIPLE OF OPERATION• A Rotating magnetic field (RMF) is set up in the stator when a 3 phase supply is given.• The stationary rotor cuts the revolving field and due to electromagnetic induction an emf is induced in the rotor conductors.• As the rotor conductors are short circuited , current starts flowing through them.• It becomes a current carrying conductor in the magnetic field and starts rotating.

Torque-Slip Characteristics:The curve drawn between the torque and the slip for a particular value of rotor resistance is known as torque-slip characteristics.The motor torque under running condition is given by,

22 22

2 2 22 2

K E sRTR s X

The following points may be noted by the torque-slip characteristics:

1.at s=0, T=0 so that torque-slip curve starts from the origin2.at normal speed, slip is small so that is negligible as compared to ,

2sX 2R

2

sTR

Hence torque slip curve is a straight line from zero slip to a slip that corresponds to full load.

3.as the slip increases beyond the full load slip, the torque increases and becomes maximumat . This maximum torque is known as pull out torque or breakdown

torque. It's value is almost twice that of full load torque when the motor is operated at its rated voltage and frequency.

2

2

RsX

4.when the slip increases beyond the maximum torque, the term Increases

very rapidly so that may be neglected as compared to .22R

2 22

sTs X

1s

as is constant2X

Thus the torque is now inversely proportional to slip. Hence the torque slip characteristics is a rectangular parabola.

2 22s X

2 22s X

TESTS ON THE INDUCTION MOTOR

No Load Test:No load test is performed to determine the no load current, no load power factor, windage and friction losses, no load resistance and reactance. Since there is no power output on no load, the power supplied to the stator furnishes its core loss and the friction and windage losses in the rotor. The circuitdiagram for the no load test is shown in Fig. 3.Fig. 3.

Blocked Rotor test:The blocked rotor test is also known as locked rotor test orshort circuit test. This test is employed to find the short circuitcurrent of the rotor circuit. This test is performed at the ratedcurrent of the induction motor. The circuit diagram forblocked rotor test is shown in Fig. 4.

VII. SPEED CONTROL OF INDUCTION MOTOR

Methods of speed control of induction motor:

A. Pole changing method:B. Stator frequency control:C. Line voltage control:

An induction motor essentially runs at constant speed as theload is varied. Some industrial drives, however, requireseveral different speeds or even continually variable speeds

(1 )Nr Ns s

120 (1 )fNr sP

The operating speed of a 3-phasee induction motor isgiven by,

or,

Conclusion:An Induction motor is a constant speed motorAn induction motor is robust in construction, need less maintenance and can operate in hazardous locations .Induction motor have two major parts:

• Stator• Rotor

Test performed on induction motor• No load test• Blocked rotor test

Speed of Induction motor can be controlled bya. Pole changing methodb. Frequency controlc. Line voltage control

References:• Generalized Theory of Electrical Machines By Dr. P.s. Bhimbhra fifth edition 2013• Principal of Electrical Machines By V.K. Mehta & Rohit Mehta• Electrical Technology By B.J. Theraja & A.k. Theraja Volume 2