PRESENTATION ON3 PHASE INDUCTION

MOTOR

NAME:MONIKA SHARMA

BRANCH:EE(6th semester)

ROLL NO.:0821321028

MOTORA Motor is a machine that converts electrical energy into mechanical energy

PRINCIPLE OF MOTOR When a conductor is placed in a magnetic field ,a force is produced on it .

Consider a conductor placed in a slot of armature and suppose that it is acted upon by magnetic field from north pole of the motor. By applying left hand rule it is found that the conductor has the tendency to move to the left hand side .

Principle of induction motor contd….

• Since the conductor is in a slot on the circumference of the rotor ,the force F acts in a tangential direction to the rotor .thus a torque is developed on the rotor. Similar torques are produced on all rotor conductors.

• Since the rotor is free to move, it starts rotating in the anticlockwise direction.

3 PHASE INDUCTION MOTOR

The three phase induction motor is the most popular type of a a.c motor.

USES OF INDUCTION MOTOR:-Commonly used for industrial drive since it is- 1. Cheap2. Robust3. Efficient4. Reliable5. Good speed regulation 6. High starting torque 7. Reasonable overload capacity

CONSTRUCTION

A 3 phase induction motor consists of two part:-

STATOR and ROTOR

STATOR• The stator is the stationary part• It consists of stator frame,stator core,3 phase

winding,two end covers.• STATOR CORE is a stack of cylindrical steel

laminations which are slotted along the inner periphery for housing the 3 phase windings.the stator core fits closely in the cast iron stator frame

• The TWO END COVERS provide mechanical support to stator core

• The stator has slots and 3 PHASE WINDINGS .Three coils aa´ ,bb´, cc´ respectively represent the windings of 3 phases a,b,c respectively.these windings are displaced by 120 degree and may be connected in star or delta.

ROTOR

• The induction motor has two types of rotors.

• 1. SQUIRREL CAGE ROTOR.

• 2. WOUND OR SLIP RING ROTOR

SQUIRREL CAGE ROTOR For squirrel cage rotor, winding consist of uninsulated

conductor in form of copper or aluminium bars embedded in semi-closed slots.

These solid bars are short-circuited at both ends by end rings of same material.

Rotor bars form a uniformly distributed winding in the rotor slots. As the rotor bars are short circuited by two end rings no external resistance can be inserted in the rotor circuit.

ADVANTAGES OF SQUIRREL CAGE

• 1. Robust construction.

• 2. Cheaper.

• 3. The absence of brushes reduces the risk of sparking.

• 4. Less maintenance.

• 5. Higher efficiency

• 6. High power factor.

WOUND OR SLIP RING ROTOR• The Wound rotor consists of slotted armature. Insulated

conductors are put in the slot and connected to form three phase double layers distributed windings similar to the stator winding. The rotor windings are connected in star. The three leads from star connection are then connected to three slip rings mounted on but insulated from the shaft. Carbon brushes pressing on the slip ring allow external resistors to be inserted in series in rotor winding.

ADVANTAGES OF WOUND ROTOR

• 1. High starting torque and low starting current.

• 2. Addition resistance can be connected in rotor circuits to control speed.

PRINCIPLE OF OPERATION• The stator windings of a three phase induction

motor is connected to a three phase balance supply.

• The flow of three phase currents in three phase stator windings produces rotating fields

• The speed of rotating field is synchronous speed NS

=2f1/P r.p.s( synchronous speed is the speed of flux)

• Supply is given to stator, rotor is stationary and flux is rotating hence emf is induced which gives rise to current in rotor conductors.

• The interaction of these rotor currents with rotating flux produces torque and hence rotor begins to rotate. According to lenz’s law effect opposes the cause therefore torque must oppose the flux cutting action.

Principle of operation contd…..• This is possible only if the developed torque forces

the rotor to rotate in the direction of rotating field, and hence the relative speed between the rotating flux and the rotor conductors reduces and therefore the flux cutting action also gets reduced.

• If rotor is assumed to synchronous speed in the direction of rotating field then there will be no flux cutting action, hence no emf in the conductors,no current in rotor bars and no torque.thus the rotor of 3 phase induction motor can never attain its synchronous speed and is hence called ASYNCHRONOUS MOTOR.

SPEED AND SLIP

• The difference between the synchronous speed and the actual rotor speed is called the slip speed.

If Ns = synchronous speed in r.p.m Nr= actual rotor speed in r.p.m. Slip speed = Ns – NrThe slip speed expressed as a fraction of the synchornous speed is called fractional slip or per unit slip. Denoted by s.s = Ns – Nr / NsSlip is always less than 1

ROTOR FREQUENCY

Let the frequency of supply = f. Therefore Ns = 120 f/P

When rotor is stationary frequency of current in rotor conductor is same as that of supply but when rotor rotates at speed at Ns - NrNs – Nr = 120 f´/P

ROTOR CURRENT

• At standstill-• Let E20=emf induced per phase of rotor at standstill• R2=resistance per phase of rotor • X20=reactance per phase of rotor at standstill = 2∏f1L1

• Z20=rotor impedence per phase of rotor• I20=rotor current per phase• Z20=R2+jX20

• I20=E20/Z20

• I20=E20/√(R2+jX20)

ROTOR CURRENT AT SLIP s

• Induced emf per phase in rotor winding at slip s is E2S=sE20

• R2=rotor winding resistance per phase• X2S =rotor winding reactance per phase at

slip sлf1sL2=sX20

• Rotor winding impedence per phase at slip s=Z2S=R2+jX2s=R2jsX20

• Rotor current I2S =E2S/Z2S

• I2S=E2S/√(R2/s)2 +(X2)2

EQUIVALENT CIRCUIT

1.Circuit model of stator

2.Circuit model of rotor

These two circuit models are combined to obtain the equivalent circuit of 3 phase induction motor.

STATOR MODEL• When the voltage V1 is applied to the stator terminals ,a

rotating air gap flux is established• This flux generates counter emf in all 3 phases of stator

winding• Now the stator terminal voltage V1 has to overcome the

counter emf V1’ and stator leakage impedence drop I1(R1+jX1)

• Therefore V1=V1’+I1(R1+jX1) • I1 ,The stator current has 2 components • 1). the load component I2’,which counter acts rotor mmf I2 N2

• 2).exciting current ie,it creates resultant air gap flux ie can be further divided into Ic(core loss component) and Im(magnetising component )

ROTOR CIRCUIT MODEL

• When a 3 phase supply is applied to the stator windings a voltage is induced in the rotor windings of the machine.

• In general greater the relative motion of the rotor and stator magnetic field,greater is the resulting rotor voltage

• The largest relative motion occurs when rotor is stationary

• We know I2s=E2s/Z2s

• I2s=sE20/R2+jsX20=E20/R2/s+jX20

EQUIVALENT CIRCUIT

• as the emf of rotor circit differs from emf of stator circuit by turns ratio therefore rotor voltage must be reffered to stator or primary hence E=N1E2/N2

STARTING OF INDUCTION MOTOR

• When the supply is connected to the stator of three phase motor ,a rotating magnetic field is produced and the rotor starts rotating .thus a three phase induction motor is self stating

• At the time of starting the motor slip is unity and starting current is very large .therefore a starter is used to

• 1)reduce heavy starting current• 2)to provide overload and under voltage

protection

METHODS OF STARTING INDUCTION MOTOR

• The following are commonly used starters for cage motors-

• 1)Direct on-line starter

• 2)Star delta starter

• 3)Autotransformer starter

DIRECT ONLINE STARTER

• As the name suggests this method involve direct switching of polyphase stator on the supply mains.

• If the supply system is of sufficient power capacity and low power factor starting current surges does not cause objectionable voltage dips in supply line voltage, then direct online method should be preferred.

AUTOTRANSFORMER STARTING

• In this method a fraction of supply voltage v1 is applied to stator terminals at the time of starting by means of autotransformer.

• This reduces the motor current and also current drawn from supply.

• After motor has accelerated near to its operating speed,autotransformer is disconnected and full line voltage is applied to induction motor.

STAR DELTA STARTING

• The connections ofa three phase inducton motor with star delta starter is shown

• When the switch is at start position the stator windings are connected in star.when the motor picks up speed 80percent of its rated value,the change over switch (TPDT)is thrown quickly to run position which connects the stator winding in delta

• By connecting the stator windings first in star and then in delta,the line current drawn at starting is reduced to one third as compared to the starting current with windings connected in delta

DETERMINATION OF EFFICIENCY

• The efficiency of the small motors can be determined by direct loading them and by measuring their input and output powers

• For large motors ,it may be difficult to arrange loads for them.moreover ,the power loss will be large with direct loading tests

• Therefore indirect methods are used to determine the efficiency of three phase induction motors

• The following tests are performed on the motor-• 1)NO LOAD TEST• 2)BLOCKED ROTOR TEST

NO LOAD TEST• In this test the motor is uncoupled from its load and

rated voltage at rated frequency is applied to the stator to run the motor without load .

• The input power is measured by two wattmeter method• An ammeter and voltmeter is connected • The ammeter measures the no load current and the

voltmeter gives normal rated supply voltage • Since the no load current is 20-30 percent of the full

load current,the I2R losses in the primary may be neglected as they vary with square of the current

• Hence the power loss is equal to constant iron loss,friction and windage losses of motor

BLOCKED ROTOR TEST• In this test the shaft of the motor is locked so that it

cannot move and rotor windings are short circuited• A reduced voltage at reduced frequency is applied to

the stator through three phase auto transformer so that full load rated current flows in the stator

• Hence the total input power is equal to the sum of copper losses

• This is due to the fact that a reduced voltage is applied to the stator and rotation is not allowed therefore core and mechanical losses are neglegible