ELECTRONIC MOTOR STARTER
This motor starter protects singlephase
motors against voltage fluctuations
and overloading. Its salient
feature is a soft on/off electronic switch
for easy operation.
The transformer steps down the AC
voltage from 230V to 15V. Diodes D1 and
D2 rectify the AC voltage to DC. The unregulated
power supply is given to the protection
circuit.
In the protection circuit, transistor T1 is
used to protect the motor from over-voltage.
The over-voltage setting is done using
preset VR1 such that T1 conducts when
voltages goes beyond upper limit (say,
260V). When T1 conducts, it switches off
T2. Transistor T2 works as the under-voltage
protector. The under-voltage setting is
done with the help of preset VR2 such that
T2 stops conducting when voltage is below
lower limit (say, 180V). Zener diodes ZD1
and ZD2 provide base bias to transistors T1
and T2, respectively. Transistors T3 and T4
are connected back to back to form an SCR
configuration, which behaves as an ‘on’/
‘off’ control. Switch S1 is used to turn on
the pump, while switch S2 is used to turn
off the pump.
While making over-/under-voltage setting,
disconnect C2 temporarily. Capacitor
C2 prevents relay chattering due to rapid
voltage fluctuations.
Regulator IC 7809 gives the 9V regulated
supply to soft switch as well as the
relay after filtering by capacitor C4. A suitable
miniature circuit breaker is used for
automatic over-current protection. Green
LED (LED1) indicates that the motor is
‘on’ and red LED (LED2) indicates that
the power is ‘on’. The motor is connected
to the normally-open contact of the relay.
When the relay energises, the motor turns