Scalar Control (V/f Control) of Variable Frequency Drive (VFD, variable speed drive, VSD) (part 1)
As you may know, as a professional manufacturer and supplier, we, Shenzhen POWTRAN Technology Co., Ltd., manufacture & export world class cost-effective frequency converters (adjustable speed drives, adjustable frequency drives, ac drives). Below we explain one speed control technique: Scalar Control (V/f Control).
Scalar Control (V/f Control) is one common speed control technique for variable frequency drives (VFDs, frequency changers, frequency inverters) in the industry.
In this type of control (scalar control, v/f Control), the motor is fed with variable frequency signals generated by the Pulse Width Modulation (PWM) control from an inverter, using the feature rich PICmicro microcontroller. Here, the V/f ratio is maintained constant in order to get constant torque over the entire operating range. Since only magnitudes of the input variables - frequency and voltage - are controlled, this is known as "scalar control". Generally, the drives with such a control are without any feedback devices (open-loop control). Hence, a control of this type offers low cost and is an easy-to-implement solution.
In such controls, very little knowledge of the motor is required for frequency control. So, scalar control (v/f Control) is widely used. A disadvantage of scalar control (v/f control) is that the torque developed is load dependent, as it is not controlled directly. Also, the transient response of such a control is not fast due to the predefined switching pattern of the inverter.
However, if there is a continuous block to the rotor rotation, it will lead to heating of the motor regardless of implementation of the overcurrent control loop. By adding a speed/position sensor, the problem relating to the blocked rotor and the load dependent speed can be overcome. However, this will add to the system cost, size and complexity. (to be continued)
