Starter of an induction motor

Starter

Starter of 3 Phase Induction motorCONTENT: Necessity of starter Function of starter Methods of starting

NECESSITY OF STARTER

Starting of Induction MotorFor induction motors, the starting torque (LRT) is approximately proportional to the square of the starting current (LRA) drawn from the line. LRT I2This starting current is proportional to the applied voltage (V)Torque can also be considered to be approximately proportional to the applied voltage. LRT V2An 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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Un-Necessary 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 pulleysGears and chainsCouplings and bearings Cavitation in pumps etc.

Un-Necessary Electrical Effects*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 EffectsContinuous maintenance.Greater chance of unscheduled shutdownsOversized mechanical and electrical components to cater for the power surge on start up.Short component life.

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Solution of the problems..

Starter

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

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FUCTION OF STARTER

Function of starter Start and stop the motor. Limit inrush current where necessary.Permit automatic control when requiredProtect motor and other connected equipments from over voltage, no voltage, under voltage, single phasing etc.

Motor Starter Features.Rated by current (amperes) or power (horsepower) Remote ON/OFF control Motor overload protection Starting and stopping (electrical life) Plugging and jogging (rapid making and breaking current)

Types of Starter for 3-Ph Induction motors

For slip-ring induction motors:Rotor rheostat starter

For squirrel cage induction motors:D.O.L starterPrimary resistance starterAuto transformer starter Star delta starter

Other starters:Soft starters

Starting of slip ring induction motor

In case of slip ring induction motors it is possible to add external resistance in rotor phases as wound rotor has 3- -phase star connected winding to limit the starting high current.

Rotor resistance starter

In rotor resistance starter the three terminals of the rotor winding are connected to a variable external resistances through slip rings.Full supply voltage is applied across the stator. Resistances are fully in the circuit at starting , so that the starting current is reduced. The external variable resistance connected in each phase of the rotor circuit not only reduce the current at starting but increases the starting torque also due to improvement in power factor.The rotor circuit resistance is gradually cut out , as the motor speeds up and during normal running condition ,the rotor circuit resistance is completely cut out and the slip rings are short circuited.

D.O.L.(direct on line) starter

D.O.L.(direct on line) starterA Starter Which Connects A Motor Directly Across The Line Is Called D.O.L. Starter.In This Method, The Motor Is Connected By Means Of A Starter Across The Full Supply Voltage.It Is Very Simple, Inexpensive, Easy To Install And Maintain.It Consist - START Button - OFF Button - Electromagnetic Contector - Overload RelaySwitching By This Starter Is Directly From Line Without Any Provision To Control The Starting Current i.e. There Is No Device To Reduce The Starting Current In This Starter.

WIRING DIAGRAM OF D.O.L STARTERL1,L2,L3 - LINESM MAIN CONTACTSM(a) AUXILIARY OR MAINTAINING CONTACTS1 START PUSH BUTTONS2 OFF PUSH BUTTONOLC OVERLOAD RELAY COILOL OVERLOAD RELAY CONTECTC MAGNETIC COIL OR OPERATING COIL

Operation Of D.O.L Starter 1) When START Button Is Pressed When Start(s1) Button is pressed, Path is L1 - S2 - S1 C OL - L2 Coil C Is Energized ,It Closes Contacts M And Connects The Motor Across The Line. Maintaining Contact M(a) Is Used To Keep The Holding Or Operating Coil Energized After The Finger Is Removed From START Push Button (S1).

2) When OFF Button Is Pressed When OFF(s2) Button is pressed, Path is Disconnected. Coil C Is De-Energized, The Main Contacts M Are Opened. Supply To Motor Is Disconnected And Motor Stops.

Basically There Are Two Types Of Protection In This Type of Starter

1)Under Voltage Protection 2)Overload Protection

Under Voltage Protection

When Supply Voltage Is Not Sufficient Or There Is Failure Of Power Supply , The Coil C Is De-Energized And Motor Will Be Disconnected.

2) Overload Protection The Motor Is Protected Against Overload By A Thermal Overload Relay Which Open Circuits the Control Circuit When Overload occure.

The Normally Closed Contacts O.L Is Opened And The Contactor Coil C is De-Energized To Disconnect The Motor From The Supply.

Rate Of Temperature Is Very High So Motor May Be Damaged If The Starting Period Is large. Which May Be Due To Excessive Load Or Excessive Voltage Drop In The Supply Lines.

Only Used For Less Than 5 KW Squirrel Cage Induction Motor.

Components of D.O.L. starter :Components of a DOL Starter

Overload Unit (Thermal type)Main pole terminals 1, 3 & 5When motor overheats due to overload conditions, main poles latch openAuxiliary contacts also latch open and when interlocked within control circuit prevents motor restarting by itself when cool.Main pole terminals 2, 4 & 6N/OAuxiliary contacts97 & 98N/C Auxiliary contacts95 & 96Reset buttonRed pushbutton can be used to reset

Components of a DOL Starter:

Start and Stop pushbuttonsStart button is green and flush mountedStop button is red and protrudingN/O contactN/C contactEmergency Stop button has a red mushroom head which latches in and must be turned to releaseContacts at the back of switches can be either N/O or N/C

Primary resistance starter

Primary resistance starter

In this method of starting of 3-Phase induction motor , primary resistance are connected in all the three phase of the stator winding, as a result the applied voltage across the stator winding at the instant of starting in reduced to a fraction x of the rated voltage of the motor. Therefore the initial high starting current will also reduce by the same fraction.

If x = fraction of voltage (V) reduced by the stator resistors

The torque developed by the motor is directly proportional to the square of applied voltage, so if the voltage applied across the motor terminal is reduced by fraction X ,starting current is reduced by fraction X ,but the starting torque is reduced by a fraction x of the obtainable with direct switching.

The purpose of primary or starting resistors is to drop some voltage and hence reduce the voltage applied across the motor terminals.

Advantages:

High power factor during start. Smooth acceleration. Less expensive than auto-transformer starter in lower output ratings. Closed transition starting.

Disadvantages:

Heat is given of T by the resistors. Expensive resistors are required because starting duration usually exceeds 5 seconds. Low torque efficiency.

Note: This method is suitable for starting of small machines only

Star-delta starter

Star-delta starter

This is very commonly used starter, compared to the other types of the starters.Star-delta starter can be used, provided the stator of the 3- induction motor is designed for delta connection during its normal operation.At starting, the stator winding is connected in star, therefore the applied voltage to each phase of winding is 1/3 of the rated voltage of the motor.When the motor has picked-up the speed(say 70 to 80% of its normal speed ) the phases of the stator winding are connected in delta. Now full supply voltage is applied across the stator windings.

This method is cheap but limited to applications where high starting torque is not necessary e.g., machine tools, pumps, motor-generator sets etc.

The method is unsuitable for motors for voltage exceeding 3000 V because of the excessive number of stator turns needed for delta connection.

Such starters are employed for starting 3-phase squirrel cage induction motors of rating between 4 and 20 k W.

Advantages of Star-Delta starter:

The operation of the star-delta method is simple and ruggedIt is relatively cheap compared to other reduced voltage methods.Good Torque/Current Performance.It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter:

Low Starting Torque, only 33% starting torque Break In Supply Possible TransientsSix Terminal Motor Required (Delta Connected).It requires 2 set of cables from starter to motor.

Auto transformer starter

Auto transformer starter

Circuit and functionAn auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.

In contrast to the star-delta connection, only three motor leads and terminals are required. On starting, the motor is connected to the tapping of the auto-transformer.

SOFT MOTOR STARTING BASICSIf you reduce voltage by 50%, the result is a 75% reduction in motor torque.(.5)2 = .25 or 25% of Locked Rotor Torque180%100%100%0Percentage of Full SpeedFull Voltage TorquePhysics of Reduced Voltage and Motor TorqueReduced Voltage TorquePercentageof Full RatedTorque

SOFT MOTOR STARTING BASICS*Example 100%0Speed -RPMTorque (ftlb)100%72%25%600%510%300%100% Voltage85% Voltage50% VoltageFull LoadTorque required by the load%FLA (amps)CurrentTorque

SOFT MOTOR STARTING BASICS*Example 100%0Speed -RPMTorque (ftlb)100%72%25%600%510%300%100% Voltage85% Voltage50% VoltageFull LoadTorque required by the load%FLA (amps)CurrentTorque

Starting Characteristics:Motor terminal voltage less than line voltage (by transformer ratio). Motor current exceeds line current (by inverse of transformer ratio). Starting torque is reduced by the square of the terminal voltage.

Advantages:

Voltage is reduced by transformation and not by dropping the voltage in resistors, and therefore, the current and power drawn from the supply mains are also reduced in comparison to resistor starting.Adjustment of starting voltage by selection of proper tap on the auto-transformer.The method is suitable for long starting periods.Motor current larger than supply current. This method can be used for starting of star-connected as well as delta-connected motors.

DISADVANTAGES:

Low power factor. Higher cost in case of lower output rating motors.This method is often employed for starting of large cage motors (rating exceeding 20 kW).

Difference between DOL/Star delta /Autotransformer

Sr. DOL Starter Star delta starter Auto transformer starter 1Used up to 5 HPUsed 5 HP to 20HPUsed above 20 HP2Does not decrease the starting currentDecreases the starting current by 1/3 timesDecreases the starting current as required3It is cheapIt is costly It is more costly 4It connects directly the motor with supply for starting as well as for runningIt connects the motor first in star at the time of starting in delta for runningIt connects the motor according to the taping taken out from the auto transformer

Soft Starter

Soft 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. *

Block Diagram*I MTHYRsProtectiveDevicesFiring Pulse Generator-ControllerControl SignalPower SignalSoft Starter3- Supply

*By using six SCRs 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.1Starting Principle of Soft Starter

Features of soft starterSoft start and soft stop Pulse (kick) start Current Control ramps Pump Control characteristics Tacho/Encoder feedbackDual Adjust - start/stop characteristics for varying loads*

Difference between Soft Starter And Autotransformer

Sr. No.Soft StarterAutotransformer1No electrical moving contacts and no oil hence no sparking and hazards.Circuitry consists of several moving parts and hence sparking is unavoidable.2No limitations on the No. of starts and No. of stops per hour.Limited No of starts and stops per hour because of: motor windings getting heated.3In rush (starting) current restricted to 200% of rated full load current (FLC).Inrush current restricted to maximum of 400% of the FLC.4Current limit facility is available which limits the current drawn by the motor plus the load to a desired safe value.This facility is not available

ACT FAST AND INITIATE FIRST

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THANK YOU

Solution ..*******3- supply*Starting principle of soft starter*