Three-Phase AC machines

Three-phase Cage Rotor Starters and Control Gear

Resource 3

Three-Phase AC Machines Resource 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

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

Three-Phase AC Machines Resource 3

3 Phase Cage Rotor Starters and Control Gear

Components of a DOL Starter

DOL = Direct On Line = Direct connection of stator phases to the 3 phase supply

3 Phase Cage Rotor Starters and Control Gear

FUSES MCB

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

Three-Phase AC Machines Resource 3

Components of a DOL Starter

3 Phase Cage Rotor Starters and Control Gear

Isolator

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

Isolator with integral fuses

Three-Phase AC Machines Resource 3

Contactor

Coil

Coil terminals A1 & A2

Main pole terminals 2, 4 & 6

Main pole terminals 1, 3 & 5 Auxiliary contact

terminal 13

Auxiliary contact terminal 14

CoilMain poles

Auxiliary contact

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

Components of a DOL Starter

Contactor

Coil

Coil terminals A1 & A2

Main pole terminals 2, 4 & 6

Main pole terminals 1, 3 & 5 Auxiliary contact

terminal 13

Auxiliary contact terminal 14

When coil is energised it becomes a magnet

Pole contacts closes

Auxiliary contact also closes

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

Components of a DOL Starter

Overload Unit (Thermal type)

Main pole terminals 1, 3 & 5

When motor overheats due to overload conditions, main poles latch open

Auxiliary contacts also latch open and when interlocked within control circuit prevents motor restarting by itself when cool.

Main pole terminals 2, 4 & 6

N/OAuxiliary contacts97 & 98

N/C Auxiliary contacts95 & 96

Reset button

Red pushbutton can be used to reset

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

Components of a DOL Starter

Start and Stop pushbuttons

Start button is green and flush mounted

Stop button is red and protruding

N/O contact N/C contact

Emergency Stop button has a red mushroom head which

latches in and must be turned to release

Contacts at the back of

switches can be either N/O

or N/C

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

DOL Starter

Power Schematic Fuses

Isolator

ContactorC1

Overload UnitOL1

Induction Motor

Auxiliary contact(retainer)

Auxiliary contact(interlock)

Safety earth

3-phase supply

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

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 contact drops out

Faults

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

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

Power Schematic

Fuses

Isolator

ForwardContactorC2

Overload Unit

OL2

Induction Motor

Safety earth

3-phase supply

ReverseContactor

C3

Phases swapped here by C3

Mechanical interlock

Three-Phase AC Machines Resource 3

3 Phase Cage Rotor Starters and Control Gear

ReversingDOL Starter

Control Circuit

Starting - Forward•Press S2•C2 coil energises•Contact C2 retains•S2 can be released•Electrical interlock with C3

Stopping

•Press S1 breaks circuit•C2 or C3 coil de-energises•C2 or C3 retaining contact drops out

Starting - Reverse Faults

•As for DOL

3 Phase Cage Rotor Starters and Control Gear

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

Three-Phase AC Machines Resource 3

Induction Motor Starting ProblemsHigh Starting Current

Starting current = 7 x full load current

As speed increases, stator current reduces

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

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

Motor current follows STAR curve. Change over to DELTA curve after motor reaches 80% of full synchronous speed

Change over can be done using a timer or centrifugal switch

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

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

Lower starting torque in STAR means that load must be checked to ensure it can be turned in STAR.

Motor torque follows STAR curve. Change over to DELTA curve after motor reaches 80% of full synchronous speed

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

STAR-DELTAStarter

Power Schematic

Induction Motor

STARContactor C2

3-phase supply, fuses, isolator and motor earth not shown

Mechanical interlock

DELTAContactor C3

MAINContactor C1 Overload

Unit OL1

STARU1

V1W1

U2

V2

W2

U2

U1

V1V2

W1

W2DELTA

DELTAContactor connectsU1 to W2V1 to U2W1 to V2

STARContactor connects U2 to V2 to W2

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3

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

3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3