In direct torque control, motor torque is directly controlled by VFD, while in vector control, motor torque is indirectly controlled by current and flux. The concept of space vector is employed in this control method to analyze mathematic model of induction motor in stator coordinate, and directly control magnetic flux and torque of induction motor. There are no complicated coordinate transformations in his type of control. Compared with flux vector control, DTC is simple, and torque response time is very fast.
In a VFD constant pressure water supply application, varying the motor speed by changing the frequency of power supply to the motor can change the pump speed and operating condition to adapt the capacity and headof the pump to the change of the demand of water and keep the extreme pressure point pressure constant. Therefore, the electricity consumption is reduced.
Centrifugal pump is a type of quadratic torque load. The relationship between flow rate, shaft power and speed is: Q ∝ n, Ps∝ n3.Take for example a centrifugal pump with rated speed n1. The system curve is 1 when the pump runs at full load. The corresponding head and flow rate are H2 and Q1. Figure 2 represents the relationships between flow rate, speed and head.