There are several starting methods to choose from for motors , but regardless of the method, the goal is to better protect the motor and reduce damage. Today, let's look at what ramp voltage starting works and how it operates.
I. Ramp Voltage Starting Principle and Control Strategy
1. Ramp voltage starting principle
During ramp voltage start-up, the stator terminal voltage of the motor gradually increases linearly, so the driving torque generated by the motor also gradually increases. As the voltage increases, the driving torque also increases. When the driving torque is greater than the sum of the damping torque and the load torque, the motor starts to start and eventually reaches stable operation.
Currently, China's national standard stipulates that the overload capacity Tmax/Tn of an asynchronous motor should be greater than 1.8 , meaning that the voltage should be approximately 74.5 % of the rated voltage when the motor is driving a load with rated torque without stopping.
2. Incline start control strategy
The ramp voltage start of the motor adopts dual control. On the one hand, when the soft start current is lower than the set maximum current value, the firing angle changes according to a predetermined change law. On the other hand, if the starting current during soft start is greater than the set maximum current value, the PI regulator will be put into operation to limit the further increase of the current. However, when the current is adjusted to be less than the set maximum current value, the PI regulator will exit the system operation until the start is completed.
II. Current-limited starting principle and control strategy
1. Current-limiting starting principle
The relationship between the starting torque TN and the stator current per phase IN is as follows:
Typically, the starting current (NS) of an induction motor is very small, ranging from 0.01 to 0.05. Therefore, to obtain a larger starting current (Tst) multiple, a larger starting current multiple is required. During current-limited starting, if the current-limiting value I1 is large, the starting torque (Tst) of the motor will also be large.
2 Current-limiting start-up control strategy
During current-limited starting, the starting current of the motor is required to be a constant value. The PI regulation loop is always in operation and automatically adjusts to keep the stator current stable at the given current value during the initial stage of starting, until the thyristor is fully turned on. After starting, the current returns to the stable rated value.
