earthing ppt

7/29/2019 earthing ppt

1/18

Three-Phase AC machines

Three-phase Cage Rotor Starters and Control Gear

Resource 3


2/18

Three-Phase AC MachinesResource 3

Aims

3 Phase Cage Rotor Starters and Control Gear

To understand the control gear requirements of a cage rotor induction motor starter

To understand the operation of various types of motor starters


3/18

Objectives At the end of this lesson you should be able to: To be able to describe the control gear components of a DOL starter To be able to describe the operation of a DOL starter To be able to explain how a three-phase motor can be reversed To be able to describe the operation of a reversing DOL starter To be able to explain the starting problems with an induction motor

To be able to describe the operation of a STAR-DELTA starter




4/18

Components of a DOL StarterDOL = Direct On Line

= Direct connection of stator phases to the 3 phase supply


FUSES MCB

Designed to operate very quicklyProtect against short circuit currents toearth or between phases



5/18

Components of a DOL Starter


Isolator

Makes circuit dead allowing for maintenanceShould be door interlocked and lockable for safety

Isolator with integral fuses



6/18

Contactor

Coil

Coil terminals A1 & A2

Main pole terminals 2, 4 & 6

Main poleterminals 1,3 & 5 Auxiliary contact

terminal 13

Auxiliary contactterminal 14

CoilMain poles

Auxiliarycontact

3 Phase Cage Rotor Starters and Control Gear Three-Phase AC MachinesResource 3


7/18

Components of a DOL StarterContactor

Coil

Coil terminals A1 & A2


Main pole

terminals 1,3 & 5 Auxiliary contact

terminal 13

Auxiliary contactterminal 14

When coil is energised it becomes amagnet

Pole contacts closes

Auxiliary contact also closes



8/18

Components of a DOL Starter

Overload Unit (Thermal type)


When motor overheats due to overload

conditions, main poles latch open

Auxiliary contacts also latch open andwhen interlocked within control circuitprevents motor restarting by itself whencool.


N/OAuxiliary

contacts97 & 98

N/CAuxiliarycontacts95 & 96

Resetbutton

Red pushbutton can be used to reset



9/18

Components of a DOL StarterStart and Stop pushbuttons

Start button is green andflush mounted

Stop button is red andprotruding

N/O contact N/C contact

Emergency Stop button has ared mushroom head which

latches in and must be turnedto release

Contacts at theback of

switches canbe either N/O

or N/C



10/18

DOL StarterPower Schematic Fuses

Isolator

Contactor C1

OverloadUnit

OL1

InductionMotor

Auxiliarycontact(retainer)

Auxiliarycontact

(interlock)

Safety earth

3-phasesupply



11/18

DOL Starter

Control Circuit

Starting Press S2 C1 coil energises Contact C1 retains S2 can be released

Stopping Press S1 breaks circuit C1 coil de-energises C1 retaining contactdrops out

Faults Overload causes OL1 to open C1 coil de-energises C1 retaining contact drops out



12/18

Power Schematic

Fuses

Isolator

ForwardContactor C2

Overload

Unit OL2

InductionMotor

Safety earth

3-phasesupply

ReverseContactor

C3

Phasesswappedhere byC3

Mechanicalinterlock




13/18

ReversingDOL Starter

Control Circuit

Starting - Forward Press S2 C2 coil energises Contact C2 retains S2 can be released Electrical interlock withC3

Stopping Press S1 breaks circuit C2 or C3 coil de-energises C2 or C3 retaining contact dropsout

Starting - ReverseFaults As for DOL


Press S3 C3 coil energises Contact C3 retains S3 can be released Electrical interlock with C2



14/18

Induction Motor Starting ProblemsHigh Starting Current

Starting current= 7 x full load current

As speed increases,stator current reduces



15/18


16/18

Induction Motor Starting ProblemsSolution :- Start off in STAR, then run in DELTA

Lower starting torque in STARmeans that load must bechecked to ensure it can beturned in STAR.

Motor torque follows STARcurve. Change over to DELTAcurve after motor reaches 80%of full synchronous speed



17/18

STAR-DELTA

Starter

Power Schematic

InductionMotor

STARContactor C2

3-phase supply,fuses, isolatorand motor earthnot shown

Mechanicalinterlock

DELTAContactorC3

MAINContactor C1 Overload

Unit OL1

STARU1

V1W1

U2V2

W2

U2

U1

V1V2W1

W2

DELTA

DELTAContactorconnectsU1 to W2

V1 to U2W1 to V2

STARContactor connects U2 toV2 to W2



18/18

Control Circuit

Starting

Press S2 C1 coil energises Contact C1 retains S2 can be released Auxiliary contact C1 closes CR timer starts timing C2 coil energises - STAR CR timer finishes timing C2 coil de-energises C3 coil energises - DELTA

STAR-DELTA Starter

Stopping & Faults As for DOL


Documents

earthing ppt