There is one thyristor for the positive and one thyristor for the negative half wave.
Using phase angle control and various control methods, the R.M.S. value of the motor voltage is increased from a definable start voltage or start torque to the motor rated voltage within a selectable starting time.
The motor current acts proportional to the voltage applied to the motor. Thus, the starting current is reduced by the factor of the voltage that is applied to the motor. The torque behaves quadratically in relation to the voltage applied to the motor.
The starting torque is thus reduced quadratically based on the voltage applied to the motor.
The following graphs illustrate the behavior of the starting current and torque of auto 3-phase asynchronous motor in combination with a soft starter
Starting
This means that because the electronic soft starter controls the motor voltage during motor start-up, it simultaneously controls the ingoing starting current and the starting torque generated in the motor.
The same principle is also used during the stopping process. The effect is that the torque generated in the motor is slowly reduced, thus enabling soft stopping of the application
During this process, the frequency remains constant and corresponds to the network frequency, contrary to the frequency-controlled starting and stopping of a frequency converter.
Upon completion of motor start-up, the thyristors are fully utilized, resulting in the complete network voltage being applied to the motor terminals. Since no motor voltage control is required during operation, the thyristors are bridged by integrated bypass contacts. This reduces the waste heat developing during continuous operation which is caused by power loss of the thyristor. Therefore, the area around the switching devices heats up less.
The following graph illustrates the mode of operation of the 3RW44 soft starter
No comments:
Post a Comment