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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
ObjectivesAt 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 StarterDOL = 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 StarterContactor
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 magnetPole contacts closes
Auxiliary contact also closes
3 Phase Cage Rotor Starters and Control GearThree-Phase AC Machines Resource 3
Components of a DOL StarterOverload 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 StarterStart 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 StarterPower 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 SchematicFuses
Isolator
ForwardContactorC2
Overload Unit
OL2
Induction MotorSafety 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
U2V2
W2
U2
U1
V1V2W1
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