A DOL starter connects the motor terminals directly to the
power supply. Hence, the motor is subjected to the full
voltage of the power supply. Consequently, high starting
current flows through the motor. This type of starting is
suitable for small motors below 5 hp (3.75 kW). Reduced-
voltage starters are employed with motors above 5 hp.
Although DOL motor starters are available for motors less
than 150 kW on 400 V and for motors less than 1 MW on 6.6
kV. Supply reliability and reserve power generation
dictates the use of reduced voltage or not
To reduce the starting current of an induction motor the
voltage across the motor need to be reduced. This can be
done by autotransformer starter, star-delta starter or
resistor starter. Now-a-days VVVF drive used extensively
for speed control serves this purpose also.
In case of Star Delta starter the motor intially will run
in star connection and when 80% of rated speed picks, the
motor change over to delta connection. With this star delta
connection we are able to reduce starting current by 1/3 of
DOL starting current and starting torque also.
Whenever you start a big heavy electric motor, you need to
start it slowly to prevent the rotor overheating and
drawing an enormous current. When the windings of a 3-phase
motor are connected in STAR the current is reduced to only
root three of the normal running current that is taken when
it is connected in DELTA.
In DOL starter full line voltage is fed to motor which will
start motor with a shock, it also draws very high current
than rated current(around 6 times), but if we use star-
delta starter , the motor starts as a star connection later
which is converted in to delta.
using star connection the line voltage is devided in two
phase voltages, also impedence is also increased due to two
star windings which result in low starting current and low
torque which will prevent motor to run with shock.
another difference is DOL starting takes less time while
star-delta takes more time to run at rated speed.
in induction motor the torque is propotional to squre of
supply voltage in case of dol the motor directly connected
to supply so torque is more while in star delta the motor
is firstly connected to star hence voltage is reduced
torque will be less and when motor reaches its 80% of speed
it connects with delta with the help of timer then full
voltage is supplied hence less rating motor should be
started with dol but for high rating it should be with star
What are the difference between these terms ? e.m.f,
voltage, potential, potential difference, current etc?
what are the difference between these terms ?
(electromotive force) e.m.f, voltage, potential, potential
difference, reference potential, reference potential
How can i explain and describe these all terms in single
line each and little more explaination too
A genarator is connected through a 20MVA ,13.8/138 KV
stepdown transformer to a transmission line. at the
reciving end of the line a load is supplied through a step
down transformer of 10MVA,138/69 KV rating .A 0.72 PU load
evaluated on load side transformer ratings as base
values ,is supplied from the above system for system base
values of 10 MVA and 69 KV in load circuit the value of the
load in generator circuit will be ...........