Star-delta starters are probably the most common starter with reduced voltage. They are used to reduce the starting current applied to the motor during start-up to reduce the interference and interference of the power supply.
Why is star-delta starter preferred for an induction motor?
Star-delta motors are preferred as the starting method for low starting torque applications. The starting current of asynchronous motors is six to seven times the full load current. Starters are used to reduce the starting current.
A star-delta starter reduces the starting current by switching the motor winding to star at the time of starting up. This reduces the voltage on the winding. The voltage at the winding is (1 / Sqrt (3)) * Mains voltage. After the start time set in the timer, the winding is switched to delta. The voltage at the winding corresponds to the mains voltage.
The switching of the winding configuration is done via contactors. Typical applications of Star Delta engines are compressors Because the torque is proportional to the voltage square. A high starting torque is not possible.
What is the purpose of a star-delta starter?
The star-delta starter is used to start asynchronous motors. At the time of engine start, the rotor is stuck and the slip (between the magnetic field of the stator and the rotor) is large, resulting in a large increase in the armature current (which is 6 to 7 times the rated value). This large current can damage the stator windings and burn the motor. To avoid this, we use star-delta starters.
At the time of starting, motor connections (stator connections) are made in star mode, reducing the impressed voltage by 1 / √3 (phase voltage = mains voltage / √3), which reduces the starting current. As soon as the rotor has reached a speed of 80-90%, the centrifugal switch changes the connections from star to delta mode (the full mains voltage is applied).
Why is an induction motor used in star connections?
When an induction motor is started to apply the full voltage (rated voltage), the starting current taken from the motor is 7-10 times the rated current. Such a high current (albeit short-term) leads to a voltage drop in the supply network that is not permitted by the electricity companies. In addition, the engine can be damaged.
To overcome this problem, the motor voltage is gradually raised from zero. or the voltage is gradually increased. When an induction motor is started by a star-delta starter, the motor starts with the star-connected stator winding. Thus, each phase receives a reduced voltage, i. H. A nominal voltage (mains voltage) divided by 1.732.
The motor consumes much less power than the starting current at full voltage. As soon as the motor reaches the speed, the starter changes the connections from star to delta and the motor is supplied with full voltage. The current finally settles to the normal nominal value. By using a star-delta starter motor current does not increase when starting.