ELE-351Electrical Energy ConversionElectrical Energy Conversion

Chapter 7I d ti M t 1Induction Motors-1

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OutlineOutline

Basic characteristics of induction motors Basic characteristics of induction motors. The induced torque phenomenon. The slip speed and the rotor speed. The equivalent model of an induction The equivalent model of an induction

motor Power & torque in induction motors. Torque x speed characteristicsTorque x speed characteristics.11:52AM 2

Induction MachinesInduction Machines

Usually employed as motors.Usually employed as motors. Advantages: Cheaper, lighter and easier to

maintain than synchronous machines.maintain than synchronous machines. Disadvantages: Speed, for motor operation,

varies with load torque.varies with load torque. Stator: Identical to that of a synchronous

machine.machine. Rotor: No need for a dc field current (cage

rotor or three-phase wound rotor).rotor or three phase wound rotor).

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Stator of a 2 poles ac machine

Generation of the rotating magnetic field

The rotating magnetic field as created by a rotating pair of magnets

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Induction machine rotors Wound rotor: A set of

three-phase windings usually Y-connected, accessible at the stator through slip rings &brushes. Not very common.

WoundrotorIM

Cage rotor (Squirrel Cage): Conducting bars g ) gshort-circuited at both ends. Very rugged. Workhorse of y ggindustry.

SquirrelCageIM11:52AM 5

Induction Machine

WoundrotorIM

Squirrel Cage IM11:52AM 6

Squirrel-Cage IM

Induction motor conceptsh i i fi ld ( S) i d b h The rotating magnetic field (BS) is created by the stator

winding with a speed:

A voltage is induced in the rotor conductors being cut by the rotating magnetic field (BS) (Faradays Law):

The rotor magnetic field (BR) is created by the rotor h l f i d d lcurrent that results from an induced voltage.

The induced torque:The induced torque:

This means that the rotor and the rotating field can not h th d!have the same speed!

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Induction motor concepts

Phenomena related to a moving permanent l d b d i l ddmagnet placed above a conducting ladder:

(a) Voltage induction and (b) force on the conductor.

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The concept of the rotor slipThe concept of the rotor slip

The voltage induced in the rotor depends on theThe voltage induced in the rotor depends on the speed of the rotor relative to the magnetic fields.

Slip speed: p p

Slip:p

Mechanical speed of the rotor.ec a ca speed o t e oto .

Rotor frequency:Rotor frequency:

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ExampleExample

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Equivalent model of an induction motor An induction motor is like a rotating

transformer But the rotor (sec) frequency istransformer. But the rotor (sec) frequency is different from the stator (prim).

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The final equivalent model Referring the rotor to the stator:

These values can be measured by specific tests11:52AM 12

Power flow in induction motors

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Power & torque in induction motors

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Equivalent circuit of an induction motor

Shows the power dissipated/converted at each point of the modelpoint of the model.

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Equivalent circuit of an induction motor Shows the power dissipated/converted at each point of

the model. The magnitude of the currents in the rotor side can be

calculated from this circuit but the frequency is still q ygiven by: fr = s fe

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Derivation of the induced torque equation

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Torque x speed characteristics start > rated Maximum (pullout) (p )

torque. For load torques q

smaller than rated, the slip varies linearly with h l d !the load torque!

ind= 0 for nm =nsyncy11:52AM 18

What is the rotor speed at no-load condition?

Torque x speed characteristics The complete curve.

If the rotor is driven at a speed greater than nsync, the machine becomes a generatorgenerator

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Comments on the IM -n curve 1. Induced torque of motor is zero at syn. speed2. Torque-speed curve is nearly linear between no load and

full load In this range rotor resistance is much larger than itsfull load. In this range rotor resistance is much larger than its reactance so rotor current, rotor magnetic field & induced torque increase linearly with increasing slip

3. There is a maximum possible torque that cannot be exceeded (pullout torque) is 2 to 3 times rated full-load torque of motor (calculated in next section)

4. Starting torque on motor is slightly larger than its full-load torque, so this motor will start carrying any load that it can supply at full powersupply at full power

5. Torque on motor for a given slip varies as square of applied voltage. This is useful in one form of induction motor speed control that will be described.

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Comments on the IM -n curve 6. If rotor of induction motor driven faster than sync.

speed, direction of ind reverses and the machine p , indbecomes a generator converting Pmech to Pelec

7. If motor turning backward relative to direction of gmagnetic fields, induced torque will stop machine very rapidly & will try to rotate it in other direction Since reversing direction of magnetic field rotation is

simply a matter of switching any two stator phases, this fact can be used as a way to very rapidly stop an y y p y pinduction motor. The act of switching two phases in order to stop motor

idl i ll d l ivery rapidly is called plugging.11:52AM 21

Comments on the IM -n curve Power converted to mechanical in an induction motor: Power converted to mechanical in an induction motor:

Pconv = ind m Note: peak power supplied by induction motor occurs at aNote: peak power supplied by induction motor occurs at a

different speed than maximum torque; and of course no power is converted to mechanical form when rotor is at zero speed

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Maximum (Pullout) Torque in IM Since ind=PAG/sync , maximum possible torque occurs when

air gap power is maximum. Si i d i R / i Since air-gap power = power consumed in R2/s maximum

induced torque will occur when power consumed by this resistor is maximum

Maximum power transfer to R2/s will occur when the magnitude of that impedance = source impedance magnitude

Equivalent source impedance: Equivalent source impedance: Zsource=RTH + jXTH + jX2

Maximum power transfer occurs when: Maximum power transfer occurs when:

( )22TH2TH2 XXRR ++=11:52AM 23

( )2THTH XXRs ++

Maximum (Pullout) Torque in IM Maximum power transfer occurs when:

( )22TH2TH2 XXRR ++= Solving for the slip

( )2THTHs2R

Note: smax ~ R2( )22TH2TH

2

XXRs

++=max

Note: smax R2Applying this value of slip to torque equation

( )2

max 222

3

2TH

TH TH TH

V

R R X X

= + + +

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( )22 sync TH TH THR R X X + + +

Maximum (Pullout) Torque in IM

The maximum torque is proportional to the square of the supply voltage andproportional to the square of the supply voltage and inversely related to the size of the stator impedances

and the rotor reactance

The smaller a machines reactance the larger itsThe smaller a machine s reactance the larger its maximum torque

Note: smax ~ R2 , however maximum torque is independent of R2independent of R2

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Maximum (Pullout) Torque in IM

Effect of varying rotor resistance on -n of wound rotor

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Maximum (Pullout) Torque in IM As the value of external resistor connected to rotor

circuit of a wound rotor through slip rings is increased the pullout speed decreased however theincreased the pullout speed decreased, however the maximum torque remains constant.

Advantage can be taken from this characteristic ofAdvantage can be taken from this characteristic of wound-rotor induction motors to start very heavy loads.

If a resistance inserted into rotor circuit, max can be adjusted to occur at starting conditions.

And while load is turning, extra resistance can be removed from circuit, & max move up to near synchronous speed for regular operationsynchronous speed for regular operation.

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Start-up schemes The starting current in IM

is 5-7 times the full loadis 5 7 times the full load current which causes a dip in the system voltage.

It is sometimes required to reduce the starting currentcurrent.

The starting schemes are based on the principle ofbased on the principle of applying a reduced voltage during the start-up

d h f ll lAn autotransformer starter for induction motor

and then full voltage.11:52AM 28

Speed control of induction motors Line frequency variation: To prevent saturation: V/f To prevent saturation: V/f Power derating.

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Speed control of induction motors

Line voltage variation:Line voltage variation:

V = 0 5V V = 0.5Vrated start= 0.25start_rated

Used for small motors only

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Speed control of induction motors

Speed can be controlled pby inserting a resistance in the rotor circuit of

d IMwound rotor IM. This method reduces the

ffi i f th tefficiency of the motor as the rotor resistance increasesincreases

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Determining the circuit model parameters

No-load test: Magnetization current and rotational lossesrotational losses.

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Determining the circuit model parameters

The DC test for stator resistance (R1). No induced voltage no rotor current no induced torque novoltage, no rotor current, no induced torque, no rotation

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Determining the circuit model parameters

The locked-rotor test: Stator and rotor impedances

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