What is a Variable Frequency Drive?
Variable Frequency Drive
A variable frequency drive (VFD) is a motor controller that saves energy and improves performance by controlling the speed of an electric motor. They can reduce power consumption by up to 50% in many applications, and they can also reduce maintenance and downtime.
VFDs vary the frequency of the electricity that powers an AC motor. This reduces the overall power consumed by the motor, reducing both its power draw and its operating temperature. VFDs also help with control and automation in industrial processes, allowing operators to monitor and adjust the motor’s speed remotely.
The most important component of a variable frequency drives is the converter. The converter converts the alternating current (AC) power from the utility to direct current (DC), adjusts its voltage, and then turns it back into AC. The final stage of the converter converts the DC power into the AC line voltage that feeds an attached motor. The output of the converter stage is typically quasi-sinusoidal, a condition that is critical for driving most types of AC motors.
A VFD can be a stand-alone unit that mounts in an electrical cabinet or can be built into the machine it will control. The VFD can be operated from an internal keypad or externally via a standard keypad or other display. Some drives have an auto-tune feature that sets parameters for a specific application, while others can be programmed with dedicated programming software. Most drives will block programming changes while they are running, but some models allow a small amount of control from the outside of the cabinet to set basic parameters.
What is a Variable Frequency Drive?
Most VFDs use a pulse-width modulation (PWM) circuit to vary the frequency of their input power. This requires switching the drive’s inverter power devices – either transistors or insulated-gate bipolar transistors (IGBTs) – on and off many times, to generate the correct root-mean-square (RMS) voltage levels. The width of these pulses is what determines the VFD’s output frequency and voltage.
There are three major designs of VFDs: PWM, current source inverter, and voltage source inverter. The most popular of these is the PWM design, as it offers excellent reliability and efficiency.
Besides regulating the speed of a motor, a VFD can also be used to control other electrical equipment and automate processes. For example, a water pump can be controlled by a VFD to match flow to system demand. This conserves energy and prevents overflowing the pumps. VFDs are also used on compressors and fans in ventilation systems for large buildings to control the volume of air that is moved, again reducing energy consumption.
Another common use of VFDs is in cranes, where the hoist motor must stop and start frequently and perform braking to lower and raise loads. A VFD can be configured to perform these braking functions automatically and provide a safer, more reliable, and economical alternative to a traditional brake resistor.