Download Automatic Star Delta Starter Using Relays and Adjustable

AUTOMATIC STAR DELTA STARTER USING RELAYS
AND ADJUSTABLE ELECTRONIC TIMER FOR INDUCTION
MOTOR
Project Synopsis
Due to the wide variety characteristics of the induction motor, it plays the premier role in
the industrial sector. Some of those are self starting, rugged construction, high efficiency, good
power factor and ease of control, etc. Squirrel cage type of induction motors are mostly used
widely than the other types.
When the motor is at idle, it just look like short circuited transformer at secondary side
because all the rotor bars are connected together to form a closed path. This will turn a large
current flow through the rotor bars. So when the motor is started, stator draws the high current
which is 8-10 times that of the motor rated current. Before staring the motor, it is necessary
reduce the voltage applied to the motor.
The project is designed to provide low voltage start to induction motors. This is achieved
by using star to delta conversion. Star/Delta starters are probably the most common reduced
voltage starters in the 50Hz industrial motor world. Star delta is used in an attempt to reduce the
start current applied to the motor then after sometime full load current is applied to the motor.
Since in star connection current is same in different phases while line voltage is the root three
times the phase voltage. So the voltage is reduced (results to reduce current) if motor is started as
star. And also in delta connection the voltage is same as that of phase voltage so full voltage is
applied if we run the motor as delta connection. The Star/Delta starter is generally obtained from
three contractors; electromechanical timer and a thermal overload for operating a 3 phase motor
at 440 volt at ac mains supply 50 Hz. The interlocking arrangement of all the contactor coils is
traditionally wired in 440 volt AC.
Block Diagram Explanation
Power supply block contains three single phase transformers with diode bridge rectifier
for each single phase supply to form the three phase supply. The output from the diode bridge
rectifier will drives the timer as well as the relay coils.
Four relays are used to implement this circuit to perform the switching based on the
different conditions. These relays also protect the motor against single phasing.
Timer is used to switch the motor from star to delta connection. Depends on the potential
meter variation we can adjust the time period for the switching transition.
Circuit Explanation
Star – Delta starter generally comprises of contactors and an adjustable timer to enable
starting of the induction motor in star mode and then switchover to delta mode after the preset
time so decided by the timer. The project uses three step-down transformers the primary of
which are connected in star mode while their secondary develop filtered dc after passing through
respective bridge rectifiers & filter capacitors out of the three such dc derived one dc is used for
a timer circuit built around a 555 timer wired in astable mode but operating in monostable mode.
Two other dc supplies operate the two relays. These two are connected in series with the third
relay coil. Mains of the three phases are connected to the third relay. Star to delta conversion is
taken care by fourth relay. This fourth relay is operated by the timer. Single phasing meaning any
one or two phases Y & B missing bring either relay – 1 or relay - 2 to off condition that results in
relay – 3 to switch Off.
At starting due to the connection of the fourth relay in star the motor is started as star
connection. After the time elapsed by the timer operates the fourth relay coil so the relay is
switched to the delta mode. It also gives the indication in which mode the motor is operating by
LED.
Future Scope
Compared to the reduced voltage methods it is the simple and cheapest way of starting
the motor. But it takes large in rush currents during switching operation. Further the project can
be enhanced by using a thyristors in firing angle control principle for soft start of the induction
motor that would overcome all the drawbacks of star delta starter.
Conclusion
This type of starting is used for low to medium voltage and light starting torque motors. It
is the cheapest way to reduce the starting current and it is in the order of 3-4 times that in case of
direct online starting of induction motor. It can be easily implemented by the relays and the timer
circuit. The way of connecting the relays also prevents the motor from the single phasing.
Output Video Explanation
This project uses a system to start a 3 phase motor at 440 volt AC mains supply 50 Hz
with a set of relays in star mode first and then to delta mode by an electronically adjustable
timer. A set of relays are used to shift the motor connections from star to delta with a time delay.
The project is supplied with six lamps instead of a 3 phase motor i.e., two lamps representing
each phase winding of the motor. The interlocking arrangement of the relay coils and the
electronic timer are all wired to low voltage fed from an inbuilt DC power supply for safe
handling of the starter during the operation. It still retains its application for a 3 phase motor
starting with single phasing prevention also.
During star operation the lamps would glow dim indicating the supply voltage across the
coils are reduced by the factor root three. In delta condition after the timer operates the lamps
would glow with full intensity indicating full supply voltage. The timer in mono-stable mode, the
output of which is fed to a relay for changing the mains supply from 3 phase star to delta. This
timer is driven by the DC regulated power supply. Since 3 phase motors get burnt if any one
phase goes missing during running, this project also has the provision of single phasing
protection. The output to the lamps shall be completely cut-off in the event of any phase failure.