SOFTSOFT--STARTERSTARTER

BY: RAVI SHANKAR SINGH

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Soft StarterSoft Starter

Soft-Starter is an electronic starter designed to accelerate, deaccelerate three-phase induction motors and to control the voltage applied on the motor during start and stop.

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Starting of Starting of InductionInduction

MotorMotor

For induction motors, the starting torque (LRT) is approximately

proportional to the square of the starting current (LRA) drawn from the

line. LRT � I2

This starting current is proportional to the applied voltage (V)

Torque can also be considered to be approximately proportional to the

applied voltage. LRT � V2

An induction motor will develop far too much torque when connected

directly to the supply.

At the instant of start-up, there are some un-necessary effect on

electrical

and the mechanical

components.3

UnUn--Necessary Mechanical EffectsNecessary Mechanical Effects

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The sudden impact at start up on the load, followed by the rapid acceleration to full speed causes excessive wear on :-

Belts and pulleysGears and chainsCouplings and bearings Cavitation in pumps etc.

UnUn--Necessary Electrical EffectsNecessary Electrical Effects

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A heavy current surge on the electrical supply which

can be severe enough to cause voltage dips and

flickering lights.

Burning of contacts due to high currents which are

many times the motor full-load current.

Problems Arises by these EffectsProblems Arises by these Effects

Continuous maintenance.

Greater chance of unscheduled shutdowns

Oversized mechanical and electrical components

to cater for the power surge on start up.

Short component life.

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Solution of the problemsSolution of the problems……....

Soft Starter

By Adjusting voltage during starting, the current drawn by the motor and the torque produced by the motor can be reduced and controlled.

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Block DiagramBlock Diagram

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I MTHYRsProtectiveDevices

Firing Pulse Generator

µ-Controller

Control Signal

Power Signal

Soft Starter

3-øSupply

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By using six SCR’s in a back to back configuration , the soft starter is able to regulate the voltage applied to the motor during starting from 0 volts up to line voltage.Feedback from the motor to the logic circuit used to control the SCR firing for stabilization of motor acceleration

Frequency do not change as in VFDOnly the voltage and current changes.

Fig.1

Fig.2

Starting Principle of Soft Starter

Characteristics of soft starterCharacteristics of soft starter

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Current/speed curve with Soft start

DOL

Soft start

Torque/speed curve with Soft start

DOL

Soft start

Load Torque

Features of soft starterFeatures of soft starter

Soft start and soft stop

Pulse (kick) start

Current Control ramps

Pump Control characteristics

Tacho/Encoder feedback

Dual Adjust - start/stop characteristics for varying loads

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Soft starting featureSoft starting feature

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Adjustable initial voltage =10 – 50% VRated .Adjustable ramp time = 1– 30 seconds (can be extended )

Current/speed curve with soft starter

Soft start

DOL

Kick starting featureKick starting feature

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When require breakaway torqueVoltage pulse -80% VRatedAdjustable pulse length- 0-2s

DOLKick Start

400%

Soft stopping featureSoft stopping feature

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Often used with high inertia loads where a sudden stop may cause system or product damage.Adjustable stopping time- 1-30s

Avoid hammering and cavitationson pumps

Avoid goods tilting on conveyors

Working:During ramp-down the controller will graduallyreduce the voltage to the motor thus reducingthe torque and current. As a consequence themotor speed will fade off.

Soft Stopping Ramp

Current limiting featureCurrent limiting feature

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Current LimitingThis mode of soft starting is usedwhen it becomes necessary to limit the maximum starting current due to long start times or to protect the motor.

Maximum current of 200- 400% of FLC

rotor current.

Adjustable ramp time =1-30 seconds (can

be extended With requirement).

Current Limiting Curve

Staring voltage-10 -50% VRated Ramp

voltage of the Vrated

Up to 85% VRated for kick start

Voltage/Time Characteristic

Comparison of soft starter with DOL and Y/Comparison of soft starter with DOL and Y/ΔΔ

starterstarter

Soft

Comparison of soft starter with DOL and Y/Comparison of soft starter with DOL and Y/ΔΔ

starterstarter

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6-700%

100%

Soft Starter

Soft start

Current/Time Characteristic Torque/Speed Characteristic

Low voltage soft starter (208Low voltage soft starter (208--575V,9575V,9--900A)900A)208V –(3-200 HP)

230V –(5-200 HP)

460V –(5-700HP)

575V –(7.5-700 HP)

2.3kv –(160 -2400 kw)

3.3 kv –(250-3450 kw)

4.16kv –(340-5000kw)

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Classification of soft starter

Medium voltage starter(1.0-13.8 kv)6.0kv –(590-4500kw)

6.6k –(650-4000 kw)

11kv –(1110-4780 kw)

13.8kv –(1400-6000 kw)

Standard Duty Soft Starter400% Motor FLA@6sec

300% Motor FLA@30sec

Medium Duty Soft Starter400% Motor FLA@12sec

300% Motor FLA@40sec

High Duty Soft Starter400% Motor FLA@20sec

300% Motor FLA@60sec

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AdvantagesAdvantagesEliminates high inrush current and high mechanical torque on startup.Reduction in cable and switch-gear rating in the power supply network.Limits line voltage dip.Soft, stepless acceleration & deceleration.Kick-start function for heavy duty and functional loads with high starting torque.Current limiting characteristic. By-pass contactor increases energy savings.Avoid current and torque peaks, Put less electrical stress on the power supply network and mechanical stress on entire drive.Reduce stress on couplings and other transmission devices (gear boxes, shafts, belts, etc).

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Advantages(contAdvantages(cont……..)..)

Extend lifetime of motor and mechanical components/parts due to reduced mechanical shocks.

Less space required in the switchgear cabinet(Devices are much narrower than other electromechanical motor starters).

Reduction in cost.

Unique starting & stopping characteristics.

Pump control characteristic.

Operation above ambient up to 65°C .

Simple electrical wiring.

Easy operation, programming and maintenance.

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DisadvantagesDisadvantages

Produces harmonics(less than inverter)

Operating speed of motor fixed(unlike VFD, frequency is fixed)

Speed regulation is not possible(used for only starting & stopping

and protection)

Acceleration & deceleration time more depend on load

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Some applicationsSome applicationsVacuum Pumps;

Centrifugal Pumps;

Compressors

Fans / Exhaustors / Blowers

Mixers

Conveyors

Axial Blowers

Propeller Fans

Axial Fans (low inertia – low load)

Pulverisers

Mixer

Ball Mill

Large Pumps

Crushers/Grinders

Ball mills/ Hammer Mills

Escalators

Bottling Lines

23All these and many other applications have been obtained by soft starter.

Thank YouThank You……..

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