ThreePhase AC machines Resource 3 Threephase Cage Rotor

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 To To be be be able able to to to describe the control gear components of a DOL starter describe the operation of a DOL starter explain how a three-phase motor can be reversed describe the operation of a reversing DOL starter explain the starting problems with an induction motor 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 with integral fuses Makes circuit dead allowing for maintenance Should be door interlocked and lockable for safety

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

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

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

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 Contacts at the back of switches can be either N/O or N/C 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

Three-Phase AC Machines Resource 3 DOL Starter Power Schematic 3 Phase Cage Rotor Starters and Control Gear 3 -phase supply Fuses Isolator Contactor C 1 Auxiliary contact (retainer) Overload Unit OL 1 Auxiliary contact (interlock) Safety earth Induction Motor

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear DOL Starter Control Circuit Starting • Press S 2 • C 1 coil energises • Contact C 1 retains • S 2 can be released Stopping Faults • Press S 1 breaks circuit • C 1 coil de-energises • C 1 retaining contact drops out • Overload causes OL 1 to open • C 1 coil de-energises • C 1 retaining contact drops out

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear 3 -phase supply Fuses Power Schematic Mechanical interlock Isolator Revers e Contact or C 3 Forward Contactor C 2 Overload Unit OL 2 Safety earth Phases swapped here by C 3 Induction Motor

Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear Control Circuit Reversing Starting - Forward • DOL Press S 2 Starter • C 2 coil energises • Contact C 2 retains • S 2 can be released • Electrical interlock with C 3 Starting - Reverse • Press S 3 • C 3 coil energises • Contact C 3 retains • S 3 can be released • Electrical interlock with C 2 Stopping Faults • Press S 1 breaks circuit • C 2 or C 3 coil de-energises • C 2 or C 3 retaining contact drops out • As for DOL

Three-Phase AC Machines Resource 3 High Starting Current 3 Phase Cage Rotor Starters and Control Gear 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 C 1 STAR-DELTA Starter Overload Unit OL 1 3 -phase supply, fuses, isolator and motor earth not shown Power Schematic U 1 STAR W 2 Mechanical interlock U 2 V 2 W 1 V 1 DELTA Contactor C 3 Induction Motor DELTA Contactor connects U 1 to W 2 V 1 to U 2 W 1 to V 2 W 2 U 1 DELTA W 1 V 2 U 2 V 1 STAR Contact or C 2 STAR Contactor connects U 2 to V 2 to W 2

Three-Phase AC Machines Resource 3 STAR-DELTA Starter Control Circuit Starting • Press S 2 • C 1 coil energises • Contact C 1 retains • S 2 can be released • Auxiliary contact C 1 closes • CR timer starts timing • C 2 coil energises - STAR • CR timer finishes timing • C 2 coil de-energises • C 3 coil energises - DELTA Stopping & Faults • As for DOL 3 Phase Cage Rotor Starters and Control Gear