Most induction motors are started directly on line, but when very large motors are started that
way, they
cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting
current surge, large induction motors are started at reduced voltage and then have full supply voltage
reconnected when they run up to near rotated speed.
This is the reduced voltage starting method. Voltage reduction during star-delta starting
is achieved by physically reconfiguring the motor windings as illustrated in the figure
below. During starting the motor windings are connected in star configuration and this
reduces the voltage across each winding 3. This also reduces the torque by a factor of
three.
After a period of time the winding are reconfigured as delta and the motor runs
normally. Star/Delta starters are probably the most common reduced voltage starters.
They are used in an attempt to reduce the start current applied to the motor during start
as a means of reducing the disturbances and interference on the electrical supply. The
Star/Delta starter is manufactured from three contactors, a timer and a thermal overload.
The contactors are smaller than the single contactor used in a Direct on Line starter as
they are controlling winding currents only. The currents through the winding are 1/root
3 (58%) of the current in the line
There are two contactors that are close during run, often referred to as the main
contractor and the delta contactor. These are AC3 rated at 58% of the current rating of
the motor. The third contactor is the star contactor and that only carries star current
while the motor is connected in star.
The current in star is one third of the current in delta, so this contactor can be AC3 rated
at one third (33%) of the motor rating.
The dynamic circuit of 2 wire start stop diagram is the same as 3 wire start stop
diagram. However, the control circuit of this circuit uses two push buttons instead of
using a switch.
Unlike a two-position switch, when we stop applying force to the push button, its
contact will return to its original position. Therefore, to keep the motor running, the
normally open contact of contactor K is used in parallel with the ON button push button.
When we press the ON button, the normally open contact of contactor K will close. So
the motor will operate until we push the OFF button, and the circuit will return to the
initial state.
In industrial circuits, three-wire control circuits use more than 2 wire control circuit
because of the safety of the 3 wire control circuit. After an incident such as power failure
or overload that circuit will return to the initial state so the motor does not automatically
restart.
This diagram is the most basic circuit that uses a switch to control a motor through a
contactor. When the state switch is open, the motor will not work. When the state switch
is closed, the motor will start to rotate.
2 wire control circuit
+ The dynamic circuit that controls the start and stop of a three-phase motor
includes CB, contactor, and thermal relay. The circuit breaker will be in series with the
main contact of contactor K and thermal relay ORL to go to the motor.
+ The control circuit uses a two-position switch in series with the coil of the K
contactor, and the normally closed contact of the ORL thermal relay.
