StartersforThreePhaseInductionMotors

StartersforThreePhaseInductionMotors

Whydoyouneedastarterforamotor?

StartersforThreePhaseInductionMotors

ToStarttheMotor?

StartersforThreePhaseInductionMotors

What do you mean bystarting the motor?

StartersforThreePhaseInductionMotors

Startingisaprocessinwhichamotorsrotorisbroughtfromzerospeedtoratedspeed.

StartersforThreePhaseInductionMotors

WhatisthespeedequationofanInductionMotor?

StartersforThreePhaseInductionMotors

120f/P?

StartersforThreePhaseInductionMotors

Whereisvoltageinthisequation?

StartersforThreePhaseInductionMotors

Thenwhyshouldoneapplyvoltagetothemotorforthemotortostartandrun?

StartersforThreePhaseInductionMotors

Whereisthemistake?

StartersforThreePhaseInductionMotors

120f/Pistheequationforthesynchronousspeedofthemotor(i.e.)thespeedoftherotatingmagneticfield,whichisimaginary.

But,forrotatingtherotorwhichisabodywithamass,oneneedstoapplyaforce.RememberNewtonsLawsofInertia?

Abodyatrestwillnotchangeitspositionunlessanexternalforceactsuponit.

StartersforThreePhaseInductionMotors

A rotor is a body with a mass. So, to lift thatmass against gravity and to keep it inmotion, one needs to apply a force. If it islinear movement, it is called force and in thecase of angular movement, as in the case ofa motor, it is called Torque.

StartersforThreePhaseInductionMotors

As the starting torque is proportional to the squareof the applied voltage, the more voltage that isapplied to the motor, the better will be the startingtorque. It is for this reason that rated voltage isapplied to the motor.

Now, we know that for a motor to start, a certainvoltage and a certain frequency have to be appliedto the motor.

StartersforThreePhaseInductionMotors

Apply full rated voltage at rated frequency, at thetime of starting itself. Isnt it?

This method is called Full Voltage Starting orAcross the Line Starting or DirectonLine orD.O.L. Starting.

We will see the schematics of this starter now.

R Y B

SDF Unit

Contactor

Overload Relay

MOTOR

Direct-On-Line Starter Power

Schematic Diagram

Direct-On-Line Starter Control

Schematic DiagramMCB

STOP Push Button

START Push Button

Contactor Coil (240V AC)

Overload Relay95

96

94

53

54

K

K

a1

a2

Phase

Neutral

Control Supply

DirectOnLineStarter

Now,whataretheadvantagesanddisadvantagesofthisstarter?

Advantages:

MostEconomicalStarter Simpletoestablish,operateandmaintain SimpleControlCircuitry Easytounderstandandtroubleshoot

DirectOnLineStarterDisadvantages:

VeryHighStartingCurrent(Typically6to8timesthefullloadcurrentofthemotor).Why?

ThermalStressonthemotor,therebyreducingitslife. Voltagedropinthesystem,affectingothercustomers

connectedtothesamelines. Unnecessaryhighstartingtorque,evenwhennot

requiredbytheload,therebyincreasedmechanicalstressonthemechanicalsystemssuchasrotorshaft,bearings,gearbox,coupling,chaindrive,connectedequipments,etc.leadingtoprematurefailureandplantdowntimes.

StartersforThreePhaseInductionMotorsSo, some method of starting current reduction hasto be adopted. We know that the current is higheither as the voltage is high or as the resistance islow (I = V/R). So, to reduce starting current, oneeither has to reduce the voltage applied to themotor or to increase the resistance of the motor.

The first method is adopted for squirrel cageinduction motors and the second method for slipring induction motors.

StartersforThreePhaseInductionMotorsReducedVoltageStartersforSquirrelCageInductionMotors: StatorResistanceStarter AutoTransformerStarter StarDeltaStarter ManualStarDeltaStarter SemiautomaticStarDeltaStarter FullyautomaticStarDeltaStarter

ClosedTransition UsingPneumaticTimer UsingThermalTimer UsingElectronicOndelaytimer

OpenTransition Usingaspecialelectronicstardeltatimer

StartersforThreePhaseInductionMotors

POWER SCHEMATIC DIAGRAM OF STAR-DELTA STARTER

SDFK1 OLR

R

Y

BK3

K2

A1

B1

C1

MOTORA2

B2

C2