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What is different between DOL(Direct on line starter), VSD,
S/S(Soft starter), and A/T(Auto Transformer)
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DOL-Direct on line Starter, here the starting current will
be 6 times of full load current. A bimetallic thermal
overload relay provides protection against the over load.
There will be no reduction of starting torque during
starting.
Star Delta Starter:In order to limit the starting current
of motors this method is followed. Initially motor windings
are connected in star configuration. The applied voltage
across the winding is reduced 1/1.73 times of rated voltage
due to reduced the voltage current also reduced. Once the
motor reaches the rated speed windings are connected in
Delta. There is 66% reduction in starting toque
Auto Transformer Starters:
In this method windings of the motor is permanently
connected in delta. Reduced starting current is achieved by
applying reduced voltage by means of an auto transformer
while starting, The tapping of auto transformer will be
60%,70%,80% after attaining full speed of the auto
transformer is bypass from the circuit and full system
voltage is applied to the winding, there is a proportional
reduction in starting torque depending upon the taping
selected.
Soft Starter :
Soft starters are Thyristor Drives The rms value of
theapplied voltage is reduced by controlling the firing
angle of power Thyristor. The achieving the full speed
Thyristor conducts in full. Here the starting voltage can
be varied from Zero to maximum value and the motor starting
is vary smooth. Similarly soft-stop facility is also
available therby the motor can be stopped by gradually
reducing the speed.
VFD or VSD:
In VFDs the 50Hz AC is rectified into DC and again inverted
as variable frequency supply , the variable frequency is
achieved by thyristor controlled inverter.
the frequency can be varied from 0-200% This makes speed
contol possible in squrrel cage induction motor. At maximum
frequency the voltage will be eaual to maximum supply
voltage
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