VFDs and VSDs

 There are two primary methodologies for controlling electrical motors:

- Variable Speed Drives (VSDs): These drives change the speed of a motor by changing the input voltage and can be used with both AC and DC motors.

- Variable Frequency Drives (VFDs): Variable Frequency Drives also control the speed of a motor, but they do so by changing the voltage and frequency and can thus only be used with AC motors.

Variable Speed Drives supply specific amperage and voltage to a motor. If the supply power is in AC form, VSDs utilize a rectifier circuit to convert the AC to DC at a specified voltage and amperage that the drive can adjust. Changing the voltage of the DC changes the speed of the motor. A DC motor VSD can consist of a very simple circuit, which makes them generally less expensive than VSDs.

VFDs control the frequency and voltage of the AC signal. Adjusting the frequency of an AC motor controls its speed, while changing the voltage will change the torque provided by the motor. VFDs are also used to control the start-up and slow-down speeds of electric motors and to prevent over current in the motor. These devices are extremely effective at increasing the motor's efficiency in a variety of AC motor applications where speed control is imperative, and may even be required in certain applications where energy conservation is a priority.

Conclusion

Both VSDs and VFDs seek to decrease an electric motor's unnecessary power consumption and optimize the efficiency and operation of systems that utilize electric motors. While both technologies rely on manipulating the power that an electric motor receives, the two drives have critical differences when it comes to application.

A small Variable Frequency Drive