Soft starters are electronic devices used to control the voltage applied to three-phase induction motors during start up and shut down. This helps reduce mechanical and electrical stresses. Soft starters gradually increase the voltage from 0% to 100% over an adjustable ramp time, controlling current and torque surges. They provide a smooth, stepless acceleration and deceleration compared to direct-on-line starting. Soft starters limit inrush current, reduce voltage dips, and extend the lifetime of motors and mechanical components in applications like pumps, fans, conveyors, and other machinery.

1. SOFTSOFT--STARTERSTARTER
BY: RAVI SHANKAR SINGH
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2. 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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3. Starting ofStarting 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.
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4. 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 pulleys
Gears and chains
Couplings and bearings
Cavitation in pumps etc.

5. 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.

6. 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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7. 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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8. Block DiagramBlock Diagram
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I MTHYRsProtective
Devices
Firing Pulse
Generator
µ-Controller
Control Signal
Power Signal
Soft Starter
3-ø
Supply

9. 9
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 VFD
Only the voltage and current changes.
Fig.1
Fig.2
Starting Principle of Soft Starter

10. 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

11. 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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12. 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

13. Kick starting featureKick starting feature
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When require breakaway torque
Voltage pulse -80% VRated
Adjustable pulse length- 0-2s
DOLKick Start
400%

14. 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 cavitations
on pumps
Avoid goods tilting on conveyors
Working:
During ramp-down the controller will gradually
reduce the voltage to the motor thus reducing
the torque and current. As a consequence the
motor speed will fade off.
Soft Stopping
Ramp

15. Current limiting featureCurrent limiting feature
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Current Limiting
This mode of soft starting is used
when 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

16. 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

17. 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

18. 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)

19. Standard Duty Soft Starter
400% Motor FLA@6sec
300% Motor FLA@30sec
Medium Duty Soft Starter
400% Motor FLA@12sec
300% Motor FLA@40sec
High Duty Soft Starter
400% Motor FLA@20sec
300% Motor FLA@60sec
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20. AdvantagesAdvantages
Eliminates 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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21. 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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22. 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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23. Some applicationsSome applications
Vacuum 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.

24. Thank YouThank You……..
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