Electronic variable-speed drives (VSDs) adjust the speed and torque of ac motors by varying the frequency and voltage applied to the motor stator. Varying the motor input voltage and frequency this way can provide benefits such as a soft start, lessening the me­chanical and electric stress the motor would otherwise see with an abrupt start-up where in current surges can reach eight times the full-load current. In addition, VSDs can also reduce motor power consumption when applications are at less than a full load. This feature can be helpful, for example, in pumps and fans that don’t work at full load continuously.

In this guide, we’ll review two control techniques of VFDs – Volts-per-Hertz scalar control and field-oriented control (FOC) and difference between the two.

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