Teoria del arranque de motores.ppt

8/10/2019 Teoria del arranque de motores.ppt

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Motor Start Theory

ME00107A


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Induction Motors Have Two

Prime Functions

To convert electrical energy into

mechanical energy in order to

accelerate the motor and load to

operating speedStarting Function

To convert electrical energy into

productive work output from the

machine Work Function


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Motors consist of two major sectionsThe Stator and the Rotor

The stator consists of magnetic poles and stator windings within the

frame of the motor.By variation of winding configuration and the

contour of the stator laminations , the full load characteristicsare

determinedThe motor speedis determined by the number of poles

The rotor consists of a cylindrical short-circuited winding around iron

laminations The rotor design affects starting performance.

The shape, position and material of the rotor bars affect the current

drawn and torque produced during motor starting.

Motor Performance


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Full load characteristics are well

understood with factors such as motor

speed,torque and efficiency being the

typical selection criteria.

A motors start performancecharacteristics are usually the least

understood but set the limits of what

can be achieved with either a full

voltage or reduced voltage starter.

It is especially important to considermotor start characteristics when

seeking to:

- Minimise start current

- Maximise start torque

Motor Performance


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A motors start performance

can be identified by examining

the motor data sheet.

The table details selected

performance data for a rangeof 110kW motors.

Typical Motor DataMotor Speed FLC LRC LRT % FL Torque

(rpm) (amps) (%FLC) (%FLT) Efncy@3xFLC

A 1470 191 600 263 93 65.8

B 1475 184 600 190 93.5 47.5

C 1475 191 570 150 92 41.6

D 1480 187 660 190 94.5 39.2

E 1470 185 550 120 92 36

F 1470 191 670 150 93 30.1

G 1480 190 780 200 94 29.6

H 1475 182 850 220 93.5 27.4

I 1480 190 670 120 94 24

Sample Of Typical 110kW Motors


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The motor performs as a

transformer with currentinduced in the rotor by the flux

in the stator.

Maximum motor start current

under full voltage start

conditions is defined by themotors Locked Rotor Current.

(LRC) This is when the rotor

is stationary

LRC levels vary considerably

between motors

In the example, Motor H will

draw 55% more current at

start than Motor E.

Start Current

Motor Speed FLC LRC LRT % FL Torque


A 1470 191 600 263 93 65.8

B 1475 184 600 190 93.5 47.5

C 1475 191 570 150 92 41.6

D 1480 187 660 190 94.5 39.2

E 1470 185 550 120 92 36

F 1470 191 670 150 93 30.1

G 1480 190 780 200 94 29.6

H 1475 182 850 220 93.5 27.4

I 1480 190 670 120 94 24

LRC ranges from 550% to 850%



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Torque-Speed Characteristic

The Torque Speed Curveshows how themotors torque production varies throughout

the different phases of its operation.

Starting Torque (LRT) is produced by a motor

when it is initially turned on. Starting torque is

the amount required to overcome the inertia

from standstill.

Pull-up Torque is the minimum torque

generated by the motor as it accelerates from

standstill to operating speed. If the motors pull-up torque is less than that required by its

application load , the motor will overheat and

eventually stall.


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Torque-Speed Characteristic

Breakdown Torque is thegreatest amount of torque amotor can attain without stalling.

Full Load Torque isproduced by a motor functioning

at a rated speed andhorsepower.

Synchronous speed is thespeed at which no torque isgenerated by the motor.This

only occurs in motors that runwhile not connected to a load.


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Motor start torque performance

is indicated by the motorsLocked Rotor Torque (LRT)

figure.

This is the measured torque with

the rotor locked and the rated

voltage and frequency applied to

the motor.Torque is a product of

force and the radius at which it

is applied and is measured in

Nm.

LRT levels vary considerably

between motors.

In the example, Motor A

produces twice as much torque

during start as Motor I.

Start Torque



A 1470 191 600 263 93 65.8

B 1475 184 600 190 93.5 47.5

C 1475 191 570 150 92 41.6

D 1480 187 660 190 94.5 39.2

E 1470 185 550 120 92 36

F 1470 191 670 150 93 30.1

G 1480 190 780 200 94 29.6

H 1475 182 850 220 93.5 27.4

I 1480 190 670 120 94 24

LRT ranges from 120% to 263%



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LRC & LRT must be

considered together when

determining a motors start

performance.

The example does this by

ranking the motors according

to the torque produced at

3 x FLC.

A good measure of

comparison between motors is

to divide the LRT% by the

LRC% - the bigger the

number, the better the result

LRC & LRT Work

TogetherMotor Speed FLC LRC LRT % FL Torque


A 1470 191 600 263 93 65.8

B 1475 184 600 190 93.5 47.5

C 1475 191 570 150 92 41.6

D 1480 187 660 190 94.5 39.2

E 1470 185 550 120 92 36

F 1470 191 670 150 93 30.1

G 1480 190 780 200 94 29.6

H 1475 182 850 220 93.5 27.4

I 1480 190 670 120 94 24


Torque developed at 3 x FLC


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A 1470 191 600 263 93 65.8

B 1475 184 600 190 93.5 47.5

C 1475 191 570 150 92 41.6

D 1480 187 660 190 94.5 39.2

E 1470 185 550 120 92 36

F 1470 191 670 150 93 30.1

G 1480 190 780 200 94 29.6

H 1475 182 850 220 93.5 27.4

I 1480 190 670 120 94 24

Torque is reduced by the

square of the current

reduction.Eg:- If you halve the current

the result will be motor

torque

Motors B & G produce almost

the same torque at fullvoltage.

Motor B produces 60% more

start torque at 3 x FLC.

Reduced Voltage

Starting AmplifiesMotor Differences



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Motor LRC LRT TORQUE(%FLC) (%FLT) @ 3 X FLC

A 600 263 65.8

B 600 190

C 570 150

D 660 190

Follow the example and

calculate the start torque at

3 x FLC for motors B, C & D.

How To Calculate

Start TorqueStart Torque = LRT x Start Current

LRC( )

2

( )2

300%

600%65.8% = x263%

47.5

41.5

39.3


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Selecting a motor with low

Locked Rotor Current (LRC)

and high Locked Rotor Torque

(LRT) will:

- Reduce start current.

- Increase start torque.

- Reduce soft starter cost.

Summary


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Current rises instantaneously

to LRC levels. This causes a

current transient that can have

undesirable effects on the

supply.

Current gradually falls as

motor speed increases.

Motor loading affects only the

time taken for acceleration,

not the magnitude of currentwhich is always LRC.

Full Voltage Starting

0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURRENT

(%)


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0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURRENT

(%)

Torque rises instantaneously

to LRT levels. This causes a

torque transient that can be

damaging.

Typical torque falls from LRTto Pull Out Torque before

rising to Breakdown Torque

just before full speed.


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0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURRENT

(%)


Limitations

1. Current transient

1

2. Current magnitude2

3. Torque transient

3 4

4. Torque magnitude

Reduced voltage starting

attempts to overcome these

limitations by applying the

voltage gradually.


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100

80

6040

20

0TIME

% VOLTS

START

Line ContactorOverload

StartRun

Direct on Line


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Electromechanical

-- Primary Resistance

-Auto-transformer

- Star/Delta

Electronic

- Soft Start

Reduced Voltage

Starters


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Primary ResistanceRUN

CONTACTOR

MOTOR

OVERLOAD

THERMALSTART

RESISTORS

LINE

CONTACTOR

3 ~M

Resistors are connected in

series with each phase,

between the isolation

contactor and the motor.

The voltage drop across the

resistors results in a reducedvoltage applied to the motor,

thus reducing start current

and torque.


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Set for 4 x FLC start current.

Primary Resistance

0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTO

RQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)

Limitations:

- Difficult to change

resistance

- Dissipate a lot of heat- Limited number of starts per

hour

- Start characteristics change

between starts if resistors

have not totally cooled

- Hard to start high inertialoads


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Primary Resistance

0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTO

RQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)

Set for 3.5 x FLC start current.

Start voltage is determined by

the resistors used. If the

resistance is too high there

will be insufficient torque to

accelerate the motor to full

speed.

The reduced voltage start

time is controlled by a preset

timer. If the time is too short,the motor will not have

achieved full speed before

the resistors are bridged.


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100

80

6040

20

0TIME

% VOLTS

START

Line Contactor

Run Contactor

ResistorsOverload

Start

Run

Primary

Resistance


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Auto-transformers

ThermalOverload

3 PhaseAuto Transformer

(B) Start Contactor

(A) Start ContactorRun

Contactor

3 ~M

The Auto-transformer Starter

employs an auto-transformer

to reduce the voltage during

the start period. The

transformer has a range of

output voltage taps that canbe used to set the start

voltage.

The motor current is reduced

by the start voltage reduction,

and further reduced by thetransformer action resulting in

a line current less than the

actual motor current.


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0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTO

RQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)

60% Tap

Auto-transformers

Limitations:

- Limited voltage taps

- Limited number of starts per

hour- Torque reduced at all

speeds

- Costly


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0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)

50% Tap

The initial start voltage is set

by tap selection, and the start

time is controlled by a timer.

If the start voltage is too low,or the start time incorrectly

set, the transition to full

voltage will occur with the

motor at less than full speed,

resulting in a high current and

torque step.

Auto-transformers


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100

80

6040

20

0TIME

% VOLTS

START

Line Contactor

Transformer

Contactor

Star Point

Contactor

Overload

Start

Run

Auto-

transformer


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Star/Delta

Motor

3~

Thermal

Overload

Star

Contactor

Delta

Contactor

Main

Contactor

The motor is initially

connected in star

configuration and then,

after a preset time, the

motor is disconnected from

the supply andreconnected in delta

configuration. The current

and torque in the star

configuration are one third

of the full voltage current

and torque when the motoris connected in delta.


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Limitations:

- No adjustment possible.- Open transition switching

between star and delta

causes damaging current

and torque transients.

Star/Delta

0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)

Insufficient torque to

accelerate this load in star

configuration.


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100

80

6040

20

0TIME

% VOLTS

START

Line Contactor

Delta

Contactor

Star Point

Contactor

Overload

Start

Run

Star -

Delta


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Open Transition

Switching

Occurs when the starter goes through an open circuit

stage in the switching sequence. Stage [1] connectionto the reduced voltage; [2] disconnect from the reducedvoltage (open circuit); [3] connect to the full voltage.

Open transition starting causes severe current & torquetransients that can be more detrimental to the supplyand the mechanical equipment than full voltage starting.

When the motor is spinning and then disconnected fromthe supply, it acts as a generator. Output voltage can bethe same amplitude as the supply. At the time of reclosethere can still be significant voltage present at the motorterminals.

Voltage generated by the motor at the instant of reclosemay be equal to the supply voltage but exactly out ofphase. This equates to reclosing with twice the supplyvoltage on the motor. The result is a current of twicelocked rotor current and a torque transient of four timeslocked rotor torque.


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A

Phase Angle Control

Trigger circuit

N
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0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FUL

LLOADTORQUE

(%)

0

100

200

300

400

500

600

700

CURRENT(%)

Reduced Voltage

Starting

IST= LRT xT

ST LRC( )2

Reduces start torque by the

square of the current

reduction.

Current can only be reduced

to the point where the torque

output from the motor exceeds

the torque required by the

load.

Reduces start current.


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Reduced Voltage

Starting

To be effective, a reduced

voltage starter must allow the

motor to accelerate to around

90% speed before applying

full voltage.

Below this speed the current

will step through to almost

LRC levels thus removing any

benefit from the reduced

voltage starter.0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURRENT

(%)

Small Reduction

at 50% speed

Large Reduction

at 95% speed


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Soft Starter

MotorOverloadAC SwitchesContactor

M3 ~

Soft Starters control the

voltage applied to the motor

by the use of solid state AC

switches (SCRs) in series with

the supply to the motor.


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- Minimum possible start

current

- No current steps

- No torque steps

- Good start torque

characteristics

Soft Starter

0

50

100

150

200

250

300

100 90 80 70 60 50 40 30 20 10 0

SLIP (%)

FULLLOADTORQUE(%)

0

100

200

300

400

500

600

700

CURREN

T(%)


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% VOLTS

100

80

60

40

20

0

TIME

Soft StartingSTART

Start

Run


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Motor characteristics set the

limits of what can be achieved

with a soft starter.

Pay special attention to motor

characteristics when:

- it is important to minimise

start current

- it is important to maximise

start torque

- dealing with large motors

(200kW +)

Summary


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Soft start is technically the

best reduced voltage starting

system.

Star/Delta starting is the

cheapest and most commonly

employed reduced voltage

starting system. However its

performance characteristics

are damaging.

Summary


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Because;

they reduce electrical and mechanicalstresses beyond the capabilities of

electro-mechanical reduced voltagestarters.

This further reduces machine downtime,

increasing plant productivity.

Note however, that the level of performance is dependant upon

the design of the soft starter and functionality it offers.

Why Use Soft Starters


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Teoria del arranque de motores.ppt

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