Electrical Power and Machines- - Three-Phase Induction Motors (2)

7/26/2019 Electrical Power and Machines- - Three-Phase Induction Motors (2)

1/20

Chapter 13: Three-Phase

Induction Motors (2)


2/20

Squirrel-cage Motors

Typical performance over the power range: 1 kW to 20 MW


3/20


Motor operating at no-load

the no-load current is similar

to the exciting current in a

transformer

composed of a magnetiingcurrent that creates the

revolving flux! "m

small active power component

that supplies the iron! windage!

and frictional losses

considera#le reactive poweris needed to create the

revolving flux

a larger air-gap re$uires greater

amounts of reactive power


4/20


Motor operating under load

active power delivered from

the source increases

proportionally to the

mechanical loado efficiency at full-load is

particularly high

the stator and rotor currents

produce mmfs that are

similar to those found in theprimary and secondary

windings of a transformer

o leakage fluxes are created in

the stator and rotor circuits


5/20


Motor with a locked rotor

the locked-rotor current is % to & times the full-load current

I2R losses are 25 to 36 times higher than normal

the rotor must never remained locked for more than a few seconds

a strong mechanical tor$ue is developed

even though no power is delivered

power factor is low #ecause the leakage fluxes are larger

the stator and rotor windings are not tightly coupled due to the air-gap


6/20

Estiating Motor Currents

The full-load current approximate value #ased on

empirical models

where I = full-load current

Ph = output power in HP

E = rated line voltae

The starting current

% to & times the full-load current

The no-load current

0'( to 0'% times the full-load

current

E!aple estimatethe full-load current!

locked rotor current! and no-load

current of a %00 hp! 2(00 )! (-

phase induction motor

estimatethe apparent powerdrawn under locked-rotor

conditions

statethe nominal rating of the

motor in kW


7/20

E!aple

estimatethe full-load current! locked rotor current!

and no-load current of a %00 hp! 2(00 )! (-phase

induction motor

estimatethe apparent power drawn under locked-

rotor conditions statethe nominal rating of the motor in kW


8/20

"cti#e Po$er %lo$s

*otor power

the power flowing across the

airgap is the product of therotating flux speed and the

magnetic tor$ue

Mechanical power

the developed power is e$ual to

the power transmitted to therotor less the rotor copper losses

and the windage and friction

losses


9/20

"cti#e Po$er %lo$s

*otor power the power flowing across the

airgap is the product of the

rotating flux speed and the

magnetic tor$ue

Mechanical power

the developed power is e$ual to

the power transmitted to the

rotor less the rotor copper lossesand the windage and friction

losses

*otor copper losses

for constant power and speed!

the magnetic tor$ue and

mechanical load tor$ue must #e

e$ual

Motor tor$ue the tor$ue developed at any

speed is


10/20

"cti#e Po$er %lo$s

E!aple

+ (-phase induction motor draws ,0 kW synchronous speed is 1200 rpm

stator copper and iron losses are % kW

windage and friction losses are 2 kW actual speed is 11%2 rpm

calculate

the active rotor power

the rotor copper losses

the developed mechanical power the delivered mechanical power

the motors efficiency


11/20

E!aple

+ (-phase induction motor draws ,0 kW

synchronous speed is 1200 rpm

stator copper and iron losses are % kW

windage and friction losses are 2 kW

actual speed is 11%2 rpm

!alculate

a' the active rotor power

#' the rotor copper losses

c' the developed mechanical power

d' the delivered mechanical power

e' the motors efficiency


12/20

"cti#e Po$er %lo$s

E!aple

+ (-phase! &0 .! ,-pole! s$uirrel-cage! induction motor stator copper and iron losses are % kW and 1 kW! respectively

the motor a#sor#s /0 kW

calculate the tor$ue developed #y the motor

what is the relationship #etween the developed tor$ue and the shaft

speed


13/20

"cti#e Po$er %lo$s

E!aple

+ (-phase! 100 hp! &00 )! induction motor synchronous speed of 1,00 rpm

stator iron loss is 2 kW! windage and friction losses are 1'2 kW

stator copper resistance #etween two terminals is 0'(/ ohms

two-watt-meter reading e$ual to 0 kW and line current of , + at a rotor

speed of 1&( rpm

calculate

a' power supplied to the motor

#' rotor copper losses

c' mechanical power delivered to the load

d' efficiency

e' developed tor$ue


14/20

E!aple: "cti#ePo$er %lo$s

+ (-phase! 100 hp! &00 )! induction motor

o synchronous speed of 1,00 rpm

o stator iron loss is 2 kW! windage and friction

losses are 1'2 kW

o stator copper resistance #etween two

terminals is 0'(/ ohms

o two-watt-meter reading e$ual to 0 kW andline current of , + at a rotor speed of 1&(

rpm

alculate




d' 3fficiency

e' developed tor$ue


15/20

E!aple: "cti#ePo$er %lo$s

+ (-phase! 100 hp! &00 )! induction motor

o synchronous speed of 1,00 rpm

o stator iron loss is 2 kW! windage and friction

losses are 1'2 kW

o stator copper resistance #etween two

terminals is 0'(/ ohms

o two-watt-meter reading e$ual to 0 kW andline current of , + at a rotor speed of 1&(

rpm

alculate




d' 3fficiency

e' developed tor$ue


16/20

Torque-speed Characteristic

The developed tor$ue depends upon the speed

the relationship is more complex than a single e$uation

characteristics are provided #y graphs with tor$ue-speed curves

shown on a per-unit #ase with the full-load tor$ue " as the #ase tor$ue

important points starting

pull-up

#reakdown


17/20

Torque-speed Characteristic

4tarting tor$ue tor$ue at ero speed

typically 1'% times the full-load

tor$ue

5ull-up tor$ue the minimum tor$ue developed #y

the motor while accelerating from

ero speed

greater than full-load tor$ue6 less

than starting tor$ue

7reakdown tor$ue the maximum tor$ue that the motor

can develop

typically 2'% times the full-load

tor$ue

8ormal operation at full-load! the motor runs at n

rpm

rotor speed decreases slightly from

synchronous speed with increasing

load tor$ue motor will stall when the load

tor$ue exceeds the #reakdown

tor$ue

for small motors 91% kW; the

#reakdown tor$ue is a#out ,0< of

the synchronous speed

for large motors 9=1000kW; the

#reakdown tor$ue is a#out >,< of

synchronous speed


18/20

&otor &esistance

The tor$ue-speed characteristics are greatly affected #y the

rotor resistance

the rotor resistance of a s$uirrel-cage rotor is essentially constant

the rotor resistance of a wound rotor can #e modified #y adding an

external three-phase resistance load


19/20

&otor &esistance

+ high rotor resistance produces a high starting tor$ue and a

relatively small starting current

desira#le for motor starting

pro#lems appears as a rapid fall-off in speed with increasing load! a

high slip at rated tor$ue! and large copper losses

?nder running conditions it is prefera#le to have a low rotor

resistance

speed decreases much less with increasing load

slip is small at rated tor$ue efficiency is high


20/20

'oe$or

5ro#lems: 1(-1! 1(-20! 1(-2(! and 1(-2/

Electrical Power and Machines- - Three-Phase Induction Motors (2)

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