Three-Phase AC machines

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

2. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Aims • To understand the control gear requirements of a cage rotor induction motor starter • To understand the operation of various types of motor starters

3. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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

5. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Components of a DOL Starter Isolator Isolator with integral fuses Makes circuit dead allowing for maintenance Should be door interlocked and lockable for safety

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

7. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Main pole terminals 1, 3 & 5 Contactor Components of a DOL Starter Auxiliary contact terminal 13 Coil terminals A1 & A2 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

8. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 When motor overheats due to overload conditions, main poles latch open Overload Unit (Thermal type) Components of a DOL Starter 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

9. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Start and Stop pushbuttons Components of a DOL Starter 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

10. 3-phase supply Fuses Isolator Contactor C1 Auxiliary contact (retainer) Auxiliary contact (interlock) Overload Unit OL1 Safety earth Induction Motor 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 DOL Starter Power Schematic

11. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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

12. 3-phase supply Fuses Mechanical interlock Isolator Reverse Contactor C3 Forward Contactor C2 Overload Unit OL2 Phases swapped here by C3 Induction Motor Safety earth 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Power Schematic

13. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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 Stopping Faults • Press S3 • C3 coil energises • Contact C3 retains • S3 can be released • Electrical interlock with C2 • Press S1 breaks circuit • C2 or C3 coil de-energises • C2 or C3 retaining contact drops out • As for DOL

14. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 High Starting Current Induction Motor Starting Problems Starting current = 7 x full load current As speed increases, stator current reduces

15. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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

16. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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.

17. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 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 V1 V2

18. 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3 Control Circuit STAR-DELTA Starter 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