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

2. 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

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

4. Components of a DOL 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 a DOL 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 a DOL 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 a DOL 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 a DOL 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 Schematic Fuses
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
•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 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
here by
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 Starting Problems
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 Starting Problems
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 Starting Problems
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-phase supply,
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
•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 Gear
Three-Phase AC Machines
Resource 3