Three-Phase AC machines Resource 3 Three-phase Cage Rotor Starters and Control Gear

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Aims To understand the control gear requirements of a cage rotor induction motor starter To understand the operation of various types of motor starters

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear 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 FUSES MCB Designed to operate very quickly Protect against short circuit currents to earth or between phases

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Components of a DOL Starter Isolator Isolator with integral fuses Makes circuit dead allowing for maintenance Should be door interlocked and lockable for safety

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Contactor Main pole terminals 1, 3 & 5 Coil terminals A1 & A2 Auxiliary contact terminal 13 Coil Coil Auxiliary contact Main poles Auxiliary contact terminal 14 Main pole terminals 2, 4 & 6

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Components of a DOL Starter Main pole terminals 1, 3 & 5 Contactor Coil terminals A1 & A2 Auxiliary contact terminal 13 When coil is energised it becomes a magnet Coil Pole contacts closes Auxiliary contact also closes Auxiliary contact terminal 14 Main pole terminals 2, 4 & 6

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Components of a DOL Starter Overload Unit (Thermal type) When motor overheats due to overload conditions, main poles latch open Main pole terminals 1, 3 & 5 Reset button N/C Auxiliary contacts 95 & 96 Auxiliary contacts also latch open and when interlocked within control circuit prevents motor restarting by itself when cool. N/O Auxiliary contacts 97 & 98 Red pushbutton can be used to reset Main pole terminals 2, 4 & 6

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Components of a DOL Starter Start and Stop pushbuttons Start button is green and flush mounted Stop button is red and protruding 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 N/O contact N/C contact

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Fuses Isolator Contactor C1 Overload Unit OL1 Induction Motor Auxiliary contact (retainer) (interlock) Safety earth 3-phase supply DOL Starter Power Schematic

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear DOL Starter Control Circuit Starting Press S2 C1 coil energises Contact C1 retains S2 can be released Stopping Faults Press S1 breaks circuit C1 coil de-energises C1 retaining contact drops out Overload causes OL1 to open C1 coil de-energises C1 retaining contact drops out

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Fuses Isolator Forward Contactor C2 Overload Unit OL2 Induction Motor Safety earth 3-phase supply Reverse C3 Phases swapped here by C3 Mechanical interlock Power Schematic

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Control Circuit Reversing DOL Starter Starting - Forward Press S2 C2 coil energises Contact C2 retains S2 can be released Electrical interlock with C3 Starting - Reverse Press S3 C3 coil energises Contact C3 retains S3 can be released Electrical interlock with C2 Stopping Faults Press S1 breaks circuit C2 or C3 coil de-energises C2 or C3 retaining contact drops out As for DOL

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear High Starting Current Induction Motor Starting Problems Starting current = 7 x full load current As speed increases, stator current reduces

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Solution:- Start off in STAR, then run in DELTA Induction Motor Starting Problems 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

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Solution:- Start off in STAR, then run in DELTA Induction Motor Starting Problems Motor torque follows STAR curve. Change over to DELTA curve after motor reaches 80% of full synchronous speed Lower starting torque in STAR means that load must be checked to ensure it can be turned in STAR.

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear MAIN Contactor C1 3-phase supply, fuses, isolator and motor earth not shown Overload Unit OL1 STAR-DELTA Starter Power Schematic U1 STAR Mechanical interlock W2 U2 V2 Induction Motor DELTA Contactor C3 W1 V1 DELTA Contactor connects U1 to W2 V1 to U2 W1 to V2 W2 U1 DELTA STAR Contactor C2 W1 U2 STAR Contactor connects U2 to V2 to W2 V2 V1

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear STAR-DELTA Starter 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 Stopping & Faults As for DOL