Unit 4 - Asynchronous Machines

UNIT 4 ASYNCHRONOUS

MACHINES

ELECTRICAL MACHINES

22/10/2015

UNIT 3 ASYNCHRONOUS MACHINES

4.1. Constructive characteristics

4.2. Equivalent circuit and power balance

4.3. Torque-speed and current-speed characteristics

4.4. Operation as a generator and as a brake

4.5. Starting methods and speed regulation

4.6. Single-phase motor. Principle of operation and starting

4.7. Shaded pole motor

TABLE OF CONTENTS

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One and three phase

Two types according to how rotor coils are short-circuited:

Squirrel cage

Consisting of equally spaced bars that are shorted

by rings

Bars made of cast aluminium (copper previously )

Wound rotor

Three-phase winding similar to the stator one

It is possible to modify the rotor electrical circuit and to measure

internal magnitudes

Slip rings

Stator

The same in both types

Core made of laminated silicon steel

4.1. CONSTRUCTIVE CHARACTERISTICS


4


Terminal box

Six terminals corresponding to the ends of the three stator phases: U1, V1,

W1, U2, V2 and W2


U1

U2

V1 W1

W2 V2

U1

U2

V1 W1

W2 V2

U1

U2

V1 W1

W2 V2

Delta Star (Wye)


4




4


Speed of the magnetic field caused by the stator:

Rotor speed: n < ns Slip is defined as: (expressed in %)

Equivalent circuit

1. Stator

Let us consider a single stator phase

R1 = stator winding resistance X1 = stator leakage reactance, due to:

Slot leakage flux Causes an induced emf in the

Zigzag leakage flux conductors that corresponds

End winding leakage flux to a voltage drop

4.2. EQUIVALENT CIRCUIT AND POWER BALANCE

160s

fn

p

=

s

s

n ns

n

-

=

1 1 1 1 1( )U E R jX I= + ++1U 1E

1I X1R1


4


The common flux through stator and rotor windings and whose peak value is m, induces emf forces in both windings whose RMS values are:

E1 = 4,44 f1N1mK1E2s = 4,44 f2N2mK2

K1 and K2 winding factors of stator and rotor f2 value:

Let us write:

E2s = sE2 E2 = 4,44 f1N2mK2 (rotor emf when s = 1 rotor is at standstill)


12 1

22

22

s s

s ss s

s

n ns fn

s s f sff pp

w w

pww w w

pw w w

- - = = - = = == - =


4


2. Rotor

R2 = rotor winding resistanceX2s = 2L2 = 2pif2L2 = 2pisf1L2 = sX2

standstill rotor reactance

The rotor circuit operates at different frequency than the stator one. Let us

modify the rotor circuit:

When the rotor is at standstill, current will flow through the secondary if the

rotor resistance is not equal to its natural value but to:


2 2 22

2 2 2 2 2 22 2 2 2 2 2

2

s

s

E s E EIR X R s X R X

s

= = =

+ + +

+sE2

2I X2sR2

f2 = sf1

+2E

2I X2R2/s

f1+

2E

2IX2R2

f1 21

c

sR Rs

-

=

{

22 2

1R sR Rs s

-

= +


4


3. Statorrotor aggregation

A single circuit is achieved carrying out the following transformations:

The relationship between is also considered:

No load m: m = m1N1K1I0Load m: m = m1N1K1I1 m2N2K2I2m1N1K1I1 = m1N1K1I0 + m2N2K2I2


+2E

2I X2R2

f1 ssRR 2c

=

1

2

1

==

KNKN

EE

r2

1

2

1tu

+1U 1E

1I X1R1

22

11

=

KNmKNm

r2

1ti

2 2 2 2

2 2 2 2

' '

' / 'u u i

i u i

t t t

t t t

E E r R R r rI I r X X r r

= =

= =

2 21 0 2 0 2

1 1'

m N KI I I I Im N K

w

w

= + = +2

1


4


Rc = load resistanceThe mechanical power developed by the motor in its shaft equals the power

absorbed by this extra resistance


1U

1I '2IX1 R2X2R1'210 III =

RFe XFeI I'21 EE =s

sRR 2c

=

1'

I0 is larger than in transformers because the circuit reluctance is larger (air gap)


4




4


Energy balance for the machine in the process of converting

electrical energy into mechanical energy

Power absorbed by the motor from the grid

P1 = m1U1I1cos Stator copper loss

PCu1 = m1R1I12

Core loss (only in the stator)

PFe = m1E1IFe m1U1IFe Air gap power (in the rotating field)

Pa = P1 PCu1 PFe Rotor copper loss

PCu2 = m2R2I22 = m2R2I22



4


Internal mechanical power

Pim = Pa PCu2 which is dissipated in Rc

Shaft useful power

Pu = Pim Pm

Efficiency


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21 2 2

1' '

mi

sP m R Is

-

=

1 21

u u

u Cu Fe Cu m

P PP P P P P P

h = =+ + + +

mechanical losses (friction and ventilation)

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Diagram

Useful relationships

if s increases PCu2 increases



22 1 2 2

21 2 2

' '

1 1' '

Cu

mi

P m R I sP s s

m R Is

= =

- -

2 2 22 22 1 2 2 1 2 2 1 2

'1' ' ' ' '

Cua mi Cu

R PsP P P m R I m R I m Is s s

-

= + = + = =

PFePCu2

Pm

Pu

PCu1

air gap rotorstator

PmiPaP1

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https://www.youtube.com/watch?v=a0hihBGMmxU


Torque-speed characteristics

Electromagnetic torque equation

Synchronism speed

Rotor turning speed Short circuit reactance Xcc = X1 + X2

Tis curve For speeds close to synchronism (s 0)

line with slope

4.3. TORQUE-SPEED AND CURRENT-SPEED CHARACTERISTICS


{1

221 2 1 1 2

221 122

1

' '

'

'

mii

Icc

m R U P m RT I

s sRR X

s

= = = W W W + + 1

1 2 60Sn

pw

pW = =

22

22

' 1'

'

i

R sT K Ks RR

s

@ =

2

1 1

CuaP sP

=

W W

=

2 '

KR

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For low speeds (s >> 0)

(equilateral hyperbola)



212 2

' 1 1i

cc cc

RT K K

s sR X@ =

+

Ti

(n = ns)1(n = 0)0

s 0 nss = 0s = 1

n

Ti

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=



Torque at synchronous speed equals zero

The pull-out torque is around 2 to 3 times the full-load (rated) torque

The starting torque is slightly higher than the rated torque



Stability is guaranteed

from C to E

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Torque varies with the square of the applied voltage:

If the rotor resistance is changed, the torque curve is modified as:

The starting torque can be

significantly increased



21iT K U=

Ti

s

U = Un

U = 0,75UnU = 0,5Un

R2i < R2ii < R2iiiTi

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Current-speed characteristics

It is of great practical use mainly for studying motor protection

It is specific of each motor. From this characteristics, the current-time

curve, which depends on the driver, can be obtained



I (% of In)

n (% of ns)

600

100

100

t

In

I

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A. OPERATION AS A GENERATOR

If an induction machine is externally moved making it rotate at a speed higher than the synchronism one then the machine operates as a generator

Limitations:

It needs to absorb reactive power from the grid in order to create the magnetic field. In isolated operation, it needs a capacitor bank providing the reactive power required by both the generator and the load. The output voltage is regulated changing the capacitor bank power. The frequency also depends on the load

Advantages:

Simplicity and cost

Low maintenance

Directly coupling to the grid

The rotation speed is not determined by the mains frequency

It does not suffer stability problems

Applications:

Wind turbines and mini power stations

4.4. OPERATION AS A GENERATOR AND AS A BRAKE


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A. OPERATION AS A GENERATOR



4


B. OPERATION AS A BRAKE

There are different methods of slowing an induction motor

a. Disconnecting the power and waiting until the friction slows the motor to a stop

(Alternatively a friction brake can be used)

b. Leaving it connected and making it (while slows down) produce electric energy

from the its kinetic energy, sending the generated energy back to the grid or to

any element designed for this purpose such as braking grid resistors

c. Making it operate as an electromagnetic brake

c.1. Countercurrent braking

Reversing the direction of the rotating field. Only in wound rotor motors

c.2. DC injection braking

A DC voltage is injected into the stator after the power supply is disconnected. The

stator creates a magnetic field fixed in space that tends to slow the rotor



4


Starting. Process of making a motor rotate

Main characteristics:

Starting torque

Starting current

The value of the starting current is limited by the Reglamento

Electrotcnico de Baja Tensin (R.E.B.T.), which sets limits on the ratio

starting current/full-load current. Starting current must be reduced starters are used

The starting torque must be high enough to move the load from standstill

(required torque is usually high)

Problem: a reduction in the starting current reduces the starting torque

4.5. STARTING METHODS AND SPEED REGULATION


4


Starting methods regulating stator circuit

They are used with squirrel cage motors and consist in reducing the

terminal voltage

Several methods have been historically used, such as series resistors and

autotransformers, which are no longer in use. Currently, both star-delta or

soft starters are employed depending on the load characteristics



4



Star-delta starter starts the motor with a star connection (lower voltage)

and after a short time switches to a delta connection.



4



Star-delta starter

At starting the current is:

If the motor is directly connected to the grid:

The starting torque is 3 times lower

It is used in those cases where the load torque at starting does not exceed 50% of the rated torque fan-type loads

3


3Y GRID

sYcc cc

U UIZ Z

= =

3 3

13

GRID GRIDs s s

cc cc cc

sY

s

U U UI I IZ Z Z

II

DD D= = = =

=

22

2 2

13 3

GRIDsYsY Y

ss GRID

U TT kU kTT kU kU DD D

= = == =


4



Soft starter starts the motor at reduced voltage by regulating the supply

voltage via a power electronics converter consisting of 2 antiparallel

tyristors per phase

The basic operation consist in producing a voltage ramp that

starts with an approximate value of 20% of the mains voltage

and reaches 100% once the set ramp time has passed

The starting current is also limited to a maximum value, so

that the supply voltage momentarily stops increasing when

this value is exceeded



4



Soft starter. It can adjust the motor torque curve to different braking load

torque curves

Pumps (to prevent water hammer)

WRONG WRONG WELL


Acceleration torque

is too high

Initial voltage

is too low


4



Soft starter

Fans. High moment of inertia the starting current remains for a long time

WRONG WRONG WELL



Initial voltage

is too high

Ramp time is

too short

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Soft starter

Mills. The starting torque is very high. The star-delta starter is not appropriate

WRONG WRONG WELL



Breakdown impulse

is too highBreakdown impulse

is too long

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Soft starter

Constant load machines (conveyors, elevators, escalators). Direct starting may

damage people and material due to and sudden starting

Linear load machines. Long starting times high current for a long time



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Soft starter settings:

Start ramp: 0 180 s

Initial voltage: 20 100%

Current limit: 0,5 6 In

Stop ramp

Energy savings (voltage is reduced during no-load periods in order to decrease

iron losses)

Emergency starting

Ambient temperature

Operation detection

Breakdown impulse

Starting frequency limit

Overload protection



4


Starting methods regulating rotor circuit

Rotor resistance starter. Series resistances

(or rheostat) connected to the rotor to

increase the starting torque



4



Double cage

Upper cage high resistivity material (bronze)

Lower cage electrolytic copper

It aims to increase the rotor resistance to raise the torque but only during

starting. Otherwise losses increase

At starting, the frequency of the rotor currents is very high lower cage

reactance is higher than the upper cage one current tends to flow through

the upper cage, which has lower resistance starting torque increases

As motor accelerates the frequency of the rotor currents decreases the cage

reactances are negligible as compared to the resistances current tends to

flow through the lower cage


STATOR

ROTOR

d


4



Deep bar

Slot shapes

At starting, the current is located in the upper part of the bars as the reactance

is lower the effective cross section is lower resistance increases

As motor accelerates frequency of currents decreases the reactance

decreases the current is distributed throughout the section decreasing

resistance



double squirrel cage motor

squirrel cage deep bar motor

squirrel cage motor

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Speed regulation

It is necessary in many industry applications

Traditionally, DC motors have been used when a variable speed driver was

required

However, since induction motors are most robust and require less

maintenance than DC motors, many methods have been developed for

varying their speed

Possibilities of speed variation:

Acting on p Acting on f Acting on s


60(1 ) (1 )s fn n s sp

= - = -


4


Speed regulation varying the number of pole pairs

Dahlander connection. The winding of each

phase consists of two equal parts which can be

connected in series or in parallel, making the

number of poles vary with the ratio 2:1



It is only used in squirrel cage

motors because the number of

rotor poles automatically adapts

to the number of stator poles

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Speed regulation varying the number of pole pairs

Multiple windings

The stator has two or more independent windings. Each one has a different

number of poles so different speeds are achieved depending on which winding

is used

p = 2 (1500 rpm)e.g. two winding motor (two speed motor)

p = 3 (1000 rpm) Motor cost rises

Combining two independent windings with the Dahlander connection, four

speeds can be achieved



4


Speed regulation varying supply frequency

A device located between the power supply and the motor should allow

varying the frequency of the supply voltages

The frequency variation must be done maintaining U1/f1 ratio constant to avoid the machine saturation, as this would vary the magnetization current

At very low frequencies, E1 >> U1 is nolonger satisfied so voltage is not longer

proportionally reduced



11 1 1

1 11 1

1 1

4, 44

const if , conts const

p w

p

E f N KE U

E f f f

f

f

=

= - - = = 1 1E U

2nU

nU

nf2nf

Voltage boost

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Speed regulation varying supply frequency

Varying the frequency and maintaining the U1/f1 ratio constant, the torque curve is modified as follows

Variable frequency drive (VFD)

Voltage control:

Controlling the DC voltage

Controlling the inverter switching



M

RECTIFIER INVERTER

DC

0 n

Tif1f2f3f4

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0 n

TiR1R2R3

Speed regulation margin

Speed regulation varying slip

Supply voltage variation

Application small motors driving fans

Rotor resistance variation

Drawback the rotor resistance increasereduces the motor efficiency this

method is only employed to reduce speed

for shorts periods of time



0 n

Ti u1u2u3

Speed regulation margin

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Squirrel cage motor connected to a single-phase grid rather than

a three-phase

Application: fractional motors (< 1 kW) in domestic installations

(washing machines, fans, etc.)

Operating principle:

Since it only has a single-phase winding, the magnetic field created by the

stator is fixed in space and pulsing in time A single phase induction

motor is not self-starting

Leblancs theorem: A field both sinusoidally oscillating and fixed in space is

the equivalent of two rotating fields that have constant amplitude,

opposite rotation directions and speed equal to the oscillating field

pulsation

4.6. SINGLE-PHASE MOTOR. PRINCIPLE OF OPERATION AND STARTING


4


Torque curve for each of the rotating fields resulting from the Leblancs

theorem

If the motor rotates in the direction of the positive field, the slip with

respect to this field is:

And the slip with respect to the reverse field:



1sds s

n n ns s

n n

-

= = = -

1 2sis s

n n ns s

n n

+= = + = -

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At n = 0, the torque equals nought but if in any way the motor is helped to start movement, then a resulting torque appears. If this torque is higher

than the braking load, the motor will start rotation until the speed

stabilizes

The operating speed is lower than that obtained by a three-phase motor

with the same braking load torque these motors operate with a

relatively high slip



4


Starting

Three-phase distributed windings supplied with currents out of phase

cause a rotating magnetic field with constant amplitude and speed

A two-phase system (two windings 90 out of phase in space) supplied with

currents 90 out of phase in time also causes a rotating magnetic field

If the currents flowing through both winding have different amplitude or

the winding have different number of turns, a rotating field with variable

amplitude is obtained



4


Starting

SPLIT-PHASE INDUCTION MOTOR

An auxiliary winding is placed at 90 electrical to the main winding

The main winding covers 2/3 of the slots so has higher reactance

The auxiliary winding, which is made of smaller wire, has lower reactance and

higher resistance and is disconnected by a centrifugal switch at 3/4 of

synchronous speed

Scheme Tn curve



30 is enough to create a rotating field 0 n

Ti

3/4ns

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Starting

CAPACITOR-START INDUCTION MOTOR

A starting capacitor inserted in series with the start winding, creating an LC circuit which is capable of a much greater phase shift, almost 90, and so, a much greater starting torque

Scheme

The capacitor must have a high capacity, so it is electrolytic, which has a polarity. As a result, it can only stay connected to the AC grid during a short period of time

Once the capacitor is disconnected, the motor operates as a single phase motor using only the main winding



capacitor

4


Starting

PERMANENT-SPLIT CAPACITOR MOTOR (PSC)

It is also known as capacitor start and run motor

The capacitor is smaller (non electrolytic) and remains in the circuit during the

run cycle. The operation is similar to that of a three phase motor

Advantages: needs no centrifugal switch, it is more efficient and its power factor

is better

Drawback: smaller starting torque

A variation is to start the motor with a relatively large capacitor for high starting

torque, but leave a smaller value capacitor in place after starting to improve

running characteristics



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The simplest and smallest single-phase motor

Salient pole stator with concentrated winding and squirrel cage rotor

The auxiliary winding, called shading coil, is composed of a copper ring or

bar surrounding a portion of each pole

A second field is created that is out of phase in space and in time a (elliptic) rotating field is produced

1 through shading coil induces an emf that produces an inductive current Iccthat causes a flux cc

4.7. SHADED POLE MOTOR

( ) acc

dE t

dtf

= -


4


The direction of rotation is from the unshaded side to the shaded (ring)

side of the pole

It has smaller starting torque, higher slip and is less efficient than other

single-phase motors

Power size

It is not reversible unless four coils are used

4.7. SHADED POLE MOTOR

1 kW

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Unit 4 - Asynchronous Machines

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