Medium voltage drives (also called VFDs or variable frequency drive) are adjustable speed drives used to control the torque and speed of AC motors. The use of VFDs in industrial powertrains is increasing, in part due to a reduction in their cost and size. If you are in the market for a VFD or you need to replace one, there are certain basics that you need to know.
Basis for VFDs
The equation representing the relationship between power, voltage, and currents is extremely simple: P = VI, or power = voltage x current. From this simple relationship we can see that at a given power level, the voltage and current are inversely proportional. In other words, the higher the supply voltage, the lower the current draw will be.
For AC motors used in high-power applications, operating at a low voltage causes the motor to draw a very high current, resulting in higher energy usage, lower efficiency, and higher cost -- and none of that is good. However, based on the power equation we just discussed, we can see that the current draw can be reduced by increasing the voltage supplied -- and that is the basis for medium voltage drives.