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
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.
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.