Volts-per-Hertz method is the simplest drive control. The ratio of the output voltage and output frequency of VFD is constant. Using this method, motor speed can be regulated by varying the supply frequency with a constant motor flux. Therefore a torque vs speed characteristic similar with that of a separately excited DC motor (ideal standard) can be achieved. Most VFDs operates with a constant V/f ratio. V/f control has poor performance at the supply frequency lower than 3 Hz. That is that the motor starting torque is not enough at frequency lower than 3Hz.
But for most cases, the starting torque is acceptable. Thus, this type of drive control is suitable for many simple applications.There are two types of flux vector control: closed loop vector control and open loop vector control.Closed loop vector control is also called flux vector control with speed sensor.In a flux vector drive, a sensor is used. It is installed on the motor shaft. The microprocessor in the VFD gets feedback from the sensor. The exact vector algorithm of voltage and frequency can be processed in the microprocessor. Therefore, the VFD can produce any required speed or torque which solves the deficiency of basic V/f drive.
Open loop vector control is called open loop vector control as well.In this control method, feedback information is obtained from the current transducers in the VFD instead of an external speed sensors. The performance is better than basic V/f control, but not good as closed loop vector control.For an open loop vector drive, the reduced performance will occurs when the frequency is lower than 1Hz.