What is V/F control?

For asynchronous motors, in order to ensure the motor flux and output force remain unchanged (constant torque). When the motor changes frequency, the ratio of voltage V and frequency F needs to be maintained approximately unchanged.

This method is what we called constant voltage to frequency ratio (VF) control.

VF control is simple, versatile and economical. Often used in occasions where the speed accuracy requirements are not very strict or the load changes are small.

In essence, VF control actually controls the voltage and frequency of three-phase AC power. However, alternating current has three elements. That is, in addition to voltage and frequency, there is also a phase.

VF control does not control the phase of the voltage. This results in a transient change. For example, when a load is suddenly applied, the motor speed will slow down due to impact.

However, the power supply frequency of the motor that the synchronous speed, remains unchanged. So that the asynchronous motor will lose step instantaneously. This causes the torque and speed to oscillate. And after a period of time, they will maintain a balance under a larger slip.

The phase is not controlled during this transient process. So the recovery process is slow. Moreover, the motor speed will vary with the load. Which is the reason why the so-called VF control accuracy is not high and the response is slow.