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E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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For internal circulation of BSNL only E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS
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Page 1: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

For internal circulation of BSNL only

E2-E3

ELECTRICAL TECHNICAL

SOFT STARTERS

Page 2: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

For internal circulation of BSNL only

WELCOME

• This is a presentation for the E2-E3 (Electrical) Technical

Module for the Topic: Soft Starter.

• Eligibility: Those who have got the Upgradation from E2

to E3.

• This presentation is last updated on 15-3-2011.

• You can also visit the Digital library of BSNL to see this

topic.

Page 3: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

For internal circulation of BSNL only

Induction Motors

The induction motor has two basic functions to fulfil :

• To accelerate itself and its load to full speed.

• To maintain the load at full speed efficiently and

effectively over the full range of loading.

Page 4: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The motor speed-torque curve

Locked Rotor

Torque (LRT, MA)

0 SSpeed

Pull Up Torque

Pull Out (Peak) Torque

(MK)

Full Load Torque

(FLT, MN)

Torque

Page 5: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The load speed-torque curve

0 SSpeed

Full Load Torque

(FLT)

Torque

0 SSpeed

Load

Torque

Motor

Page 6: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Putting the two curves together we get

Torque

Load

Torque

Motor

Locked Rotor

Torque

Peak Torque

Full Load Torque

(FLT)

S’

The accelerating torque

at any speed S’ is

(T2 - T1)

0 SSpeed

T2

T1

Page 7: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Because the load starting torque is usually near zero, an

induction motor will develop far too much torque when

connected directly to the supply.

• At the instant of start-up, there is an un-necessary heavy

power surge on both the electrical supply and the

mechanical drive components.

What does this mean in practice?

For internal circulation of BSNLonly

Page 8: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The mechanical effects

• The sudden impact at start up on the load, followed by

the rapid acceleration to full speed causes excessive

wear on :

• Belts and pulleys

• Gears and chains

• Couplings and bearings

Page 9: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The electrical effects

• A heavy current surge on the electrical supply which can be severe

enough to cause voltage dips and flickering lights.

Page 10: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The user experiences:

• Continuous maintenance.

• Greater chance of unscheduled shutdowns

• Oversized mechanical and electrical components to cater

for the power surge on start up.

• Short component life.

Page 11: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Primitive, but familiar - Just a simple contactor in circuit

between the motor and supply giving only

unsynchronised on-off control

• No control over inrush currents or torque surge at start

up

• Very rapid acceleration of load with large transient forces

placed on drive and electrical system

Direct-On-Line Starters

Page 12: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Direct-On-Line Starting

CURRENT

TIME

600%

Page 13: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Star-Delta Starters

• Uses three contactors and a timing mechanism to switch

between them.

• Installation costs are greater because of the need for 6

connections between the motor and starter.

• If the transfer occurs at less than 80% of normal speed,

large current and torque surges can arise.

Page 14: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Although an apparent improvement over the direct system,

significant disadvantages still remain.

• The transfer from star to delta momentarily removes the

motor from the supply. During this time the motor is under

the mechanical influence of the rotating load and, at the

instant of disconnection, current will still flow in the rotor

bars due to the time delay necessary for the magnetic flux

to die away.

Page 15: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Because of this, there is a residual flux "frozen" on the

surface of the rotating rotor, which cuts the stator

windings, generating a voltage in the stator winding at a

frequency depending on the rotor speed which has to be

lower than mains frequency.

Page 16: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Although these effects are present for only a very short

period of time, about one fifth of a second, they are

sources of great stress and damage to the whole drive

system, and when frequent starting is necessary, they

bring increased risk of catastrophic failure and high

maintenance costs.

Page 17: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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The Star-Delta Trauma

Page 18: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Star/Delta Start

Page 19: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Semiconductor Motor Controller

• Is based upon the silicon controlled rectifier or

‘Thyristor’.

• By pulsing a Thyristor, it switches from ‘off’ to ‘on’

until the current stops flowing though it - which

occurs every half cycle in an AC supply.

• By controlling electronically the thyristor turn on

point, it is possible to regulate the energy passing

through it.

Page 20: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Thyristor Control

ONE MAINS CYCLE

TIME OR DEGREES

VO

LT

AG

E O

R E

NE

RG

Y

Page 21: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Since the average voltage is the root-mean square of

each individual voltage level at any instant during a

sinusoidal wave, and since the motor torque is

proportional to the square of the voltage applied to the

motor, it follows that the longer the thyristor is switched

off, the less the torque generated by the motor during

that wave.

Page 22: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Semiconductor Motor Controller (cont.....)

• By starting with a large delay angle and gradually

reducing it , the motor terminal voltage is increased from

a low value to full voltage, giving a smooth, stepless,

start.

Page 23: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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A Soft Start compared

CURRENT

6-700%

DIRECT-ON-LINE

STAR-DELTA

SOFT START

100%

TIME

Page 24: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Soft Motor Starter

Soft starter provides a controlled release of power to the

motor, thereby providing smooth, stepless acceleration

and deceleration.

Page 25: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Soft Starter

Figure - Soft Starter

Page 26: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Description of the softstarters

- Design, settings and signals

• Soft starter components :

Printed circuit board (PCB)

Heat sink,

Thyristors,

Fans

Housing (plastic or metal).

• The controlling circuits can be of digital type,

analogue type or a combination of

these.

Page 27: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Printed circuit board is used to control the firing of the

thyristors based on the current and voltage references,

and also for the calculation of different values, for

example the power factor, active power, etc. It can also

be used for storing historical data, the event

log,indicating trends and much more.

Page 28: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Heat sink is used to get rid of the heat in the softstarter

generated by the current during the start and the

continuous run.

The capacity of the heat sink very much reflects the

starting capacity and the operational current of the

softstarter.

Page 29: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Fans are used to increase the cooling capacity of the

heat sink. One, two or several fans can be used

depending on size and design.

• Some smaller softstarters don’t have fans at all and the

number of starts may be limited.

Page 30: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Housing can be made of plastic material,metal or a

combination of these, and its function is to protect the

inside components from mechanical and electrical

damage. It is also used to protect the components from

dust and dirt.

• For total outside protection from dust and dirt a separate

enclosure is often required since the degree of protection

(IP class) of the unit itself is too low.

Page 31: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Thyristors are semi-conducting components connected

in an anti-parallel fashion and placed in two or three

phases of the main circuit. They regulate (by increasing

or decreasing) the level of voltage during start and the

stop ramp, as described in the picture below.

• During a continuous run the thyristors are conducting

fully.

Page 32: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Page 33: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Start: The thyristors let part of the voltage through at the

beginning and then increase it, according to the set ramp

time for the start.

• Stop: The thyristors are fully conducting and when soft

stopping, they decrease the voltage according to the set

ramp time for stop.

Page 34: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Page 35: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• When starting, AC Induction motor develops more

torque than is required at full speed. This stress is

transferred to the mechanical transmission system

resulting in excessive wear and premature failure of

chains, belts, gears, mechanical seals, etc.

• Additionally, rapid acceleration also has a massive

impact on electricity supply charges with high inrush

currents drawing +600% of the normal run current.

Page 36: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Soft starter provides a reliable and economical solution

to these problems by delivering a controlled release of

power to the motor, thereby providing smooth, stepless

acceleration and deceleration.

• Motor life will be extended as damage to windings and

bearings is reduced.

Page 37: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Soft Start & Soft Stop is built into 3 phase units,

providing controlled starting and stopping with a

selection of ramp times and current limit settings to suit

all applications.

Page 38: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Figure - Soft Starter: Starting current, Stress profile during starting

Page 39: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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Advantages of Soft Starter:

• Less mechanical stress

• Lower maximum demand.

• Less mechanical maintenance

• Energy Saving

Page 40: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

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• Another feature of the softstarter is the softstop

function, which is very useful when stopping pumps

where the problem is water hammering in the pipe

system at direct stop as for star-delta starter and direct-

on-line starter.

• The softstop function can also be used when stopping

conveyor belts to prevent material from damage when

the belts stop too quickly.

Page 41: E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS

For internal circulation of BSNL only


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