Lecturer name: ……………..…………………………………………………………………………...….……
Student name : .…………………………………………………………………………………………………..
Registration # : …..……..…………………………………………………………………………………………
September 2014
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1Objective
To perform a soft start for a three phase induction motor by star/delta starting method.
2Theory
The Star Delta starting method is a motor starting mechanism that minimizes the large amount of starting current that motors draw in. The Star Delta, as the name suggests basically involves feeding the motor with 1/√3 (58%) of the full load current until it attains speed then applying the full load current.
It is required three contactors i.e., the Star Contactor (K3), the Delta Contactor (K4) and the Main
Contactor (K1). However for the motor to be started in Star Delta, its internal connection at the terminal box has to be wired in Delta-giving it capability of receiving the full-load current at any instant.
When the power is fed into the circuit, K1 allows current to flow to the motor. Current flows into the motor and out to the K3 which is the star-connected starter. After a specified period defined by the clock delay (usually 5 sec) the K4 (Delta) Closes and K3 opens to allow the motor to receive the full load current and run at delta.
Traditionally, in many regions there was a requirement that all motor connections be fitted with a reduced voltage starter for motors greater than 4KW (5HP).This was to curb the high inrush of starting currents associated with starting induction motors.
The star and delta contactors are mechanically interlocked i.e., if one of them is closed the other cannot close. This is done to avoid dead short circuit in case both the contactors closing simultaneously. Electrical interlocking has also been provided, by using contactors control contacts.
An advantage of this method could be low or reduced cost as compared to other methods.
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3Instruments & Equipments
Three phase induction motor
Main and Sub feeder isolating circuit breakers
(board #5)
Power and control circuits fuses
(board #6)
Three phase electromagnetic contactor with bi-metal overload relay
(board #2)
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Three phase mechanically interlocked contactors
(board #4)
On delay timer
(board #7)
Indicating lamps and Pushbuttons
(board #1)
Multimeter
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4Connection
L1
L2
L3
Circuit Breaker
C.B
Q1
Fuses
F1 F2 F3
Contactor
K1
Contactor
K4
Overload
F4
T1 T2 T3 T3 T2 T1
3φ
IM
Power Circuit
Contactor
K3
Mech. Interlocked
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11
L1
Q2
C.B
Fuse F6
13
Pushbutton
Start
S1
14
11
Pushbutton
Stop
S0
12
Over Load
95
F4
96
A1
13
K1
14
A1
T1
14
T1
12
21
K3
22
K4
21
H1
X1
A1 A1
Contactor
Coil
K4
Contactor
Coil
K3
22
X2
A2 A2
Contactor
Coil K1
X1
X2
H2
A2
N
Fuse F7
Q2
C.B
Control Circuit
T1
Timer
Coil
A2
Power and Control circuits connections diagram
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5Procedure a.
Connect the circuits as shown in the diagram. b.
Switch on the main and sub feeder isolating circuit breakers (Q1, Q2). c.
Use the start and stop pushbuttons for motor operation. d.
Measure the current firstly in star then in delta connections.
6Results
Current in star = ……………………..A
Current in delta= ……………………..A
7Connect the power and control circuits on the following figure
8Add your own comments
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9Use internet resources to provide advantages, disadvantages and then discuss different applications of star/delta motor starter circuit
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