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03 presentation
- Three-phase induction motors require control gear like starters to control and protect the motor. - A basic direct-on-line (DOL) starter connects the motor directly to the power supply using contactors. It includes fuses, an isolator, contactor, and overload protection. - More advanced starters like a reversing DOL starter use additional contactors to reverse the motor direction, while a star-delta starter reduces starting current by initially connecting the motor in star before switching to delta.
03 presentation
- 1. Three-Phase AC machines Three-phaseCage Rotor Starters and Control Gear Resource 3
- 2. Three-Phase AC Machines Resource3 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
- 3. Objectives At the endof 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
- 4. Components of aDOL 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 quickly Protect against short circuit currents to earth or between phases Three-Phase AC Machines Resource 3
- 5. Components of aDOL Starter 3 Phase Cage Rotor Starters and Control Gear Isolator Makes circuit dead allowing for maintenance Should be door interlocked and lockable for safety Isolator with integral fuses Three-Phase AC Machines Resource 3
- 6. 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 Coil Main poles Auxiliary contact 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3
- 7. Components of aDOL 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 Gear Three-Phase AC Machines Resource 3
- 8. Components of aDOL 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/O Auxiliary contacts 97 & 98 N/C Auxiliary contacts 95 & 96 Reset button Red pushbutton can be used to reset 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3
- 9. Components of aDOL 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 Gear Three-Phase AC Machines Resource 3
- 10. DOL Starter Power SchematicFuses Isolator Contacto r C1 Overload Unit OL1 Induction Motor Auxiliary contact (retainer) Auxiliary contact (interlock) Safety earth 3-phase supply 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3
- 11. DOL Starter Control Circuit Starting •PressS2 •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 Gear Three-Phase AC Machines Resource 3
- 12. Power Schematic Fuses Isolator Forward Contacto r C2 Overload Unit OL2 Induction Motor Safety earth 3-phase supply Revers e Contact or C3 Phases swapped hereby C3 Mechanical interlock Three-Phase AC Machines Resource 3 3 Phase Cage Rotor Starters and Control Gear
- 13. Reversing DOL 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
- 14. Induction Motor StartingProblems High Starting Current Starting current = 7 x full load current As speed increases, stator current reduces 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3
- 15. Induction Motor StartingProblems Solution:- 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 Gear Three-Phase AC Machines Resource 3
- 16. Induction Motor StartingProblems Solution:- 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 Gear Three-Phase AC Machines Resource 3
- 17. STAR-DELTA Starter Power Schematic Induction Motor STAR Contact or C2 3-phasesupply, fuses, isolator and motor earth not shown Mechanical interlock DELTA Contactor C3 MAIN Contactor C1 Overload Unit OL1 STAR U1 V1W1 U2 V2 W2 U2 U1 V1V2 W1 W2 DELTA DELTA Contactor connects U1 to W2 V1 to U2 W1 to V2 STAR Contactor connects U2 to V2 to W2 3 Phase Cage Rotor Starters and Control Gear Three-Phase AC Machines Resource 3
- 18. Control Circuit Starting •Press S2 •C1coil 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 Gear Three-Phase AC Machines Resource 3