The Device which is used for starting a 3 phase induction motor is called starters. Its function is to limit a starting high current to a safe value. The starter are a safety and a protecting device by which we can control or minimize the starting current in order to prevent the motor winding of the respective motor. AC (alternating current) motor starters are used on electric motors that utilize a start and stop button or switch for the operation.
In
an ac motor, when supply is given to the stator windings, the rotating magnetic
field, and the produced flux in the rotor windings due to the back emf,
causes the motor torque to increase, causing a high rotor current. During the
time between the application of electric supply to the motor and the actual acceleration
of the motor to its full speed, a large amount of current is drawn by the
stator from the supply. This starting current is about 5 to 6 times more than
the full load current. This time duration can be for a few seconds or longer.
This causes the electrical equipment to damage because of the increased voltage
drop in electrical systems due to the flow of larger currents across the cable.
For this reason, a definite method of starting the motor is needed.
The
purpose of a starter is not to just start the motor, but it performs the two
main functions-
● To reduce the heavy starting
current
● To provide overload and under
voltage protection.
The
three-phase induction motor may be started by connecting the motor directly to
the full voltage of the supply. The motor can also be started by applying a
reduced voltage to the motor when the motor is started. The torque of the
induction motor is proportional to the square of the applied voltage. Thus, greater torque is exerted by a motor when it is started on full voltage than
when it is started on the reduced voltage.
Types
of starters
Star-Delta
starter
The
star delta starter method of starting three-phase induction motors is very
common and widely used among all the methods. In this method, the motor runs at
delta connected stator windings. A two-way switch is used to connect the stator
winding in the star while starting and in the delta while running at normal speed. When
the stator winding is star connected, the voltage over each phase in a motor will
be reduced by a factor 1/(sqrt. 3) of that would be for delta connected
winding. The starting torque will be 1/3 times that it will be for delta
connected winding.
Direct
On line starter
It
consists of simple push-button as the controller. When the start button is pressed,
the switch connecting the motor and the main supply is closed and the motor
gets the supply current. In case of overcurrent, the stop button is pressed and
the bypass auxiliary contact is opened.
Autotransformer starters
In
this method, a three-phase autotransformer is connected in series with the
motor. This transformer reduces the voltage applied to the motor and hence the
current. This starter consists of a changeover switch that switches the motor
between reduced voltage and full voltage conditions. When this switch is in the
start position, a reduced voltage is applied to the motor. When the motor
attains 80 percent of its rated speed, the changeover switch is connected to
RUN position automatically using relays. Due to this, a rated voltage is then
applied to this motor. In this method, the motor terminal voltage is higher for
a given starting current on the line side as compared to other reduced voltage
methods. Therefore, this method gives the highest starting torque per line ampere
current.
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