Variable frequency drives are used in a variety of applications

Variable frequency drives

Variable frequency drives (VFDs) are used in a variety of applications, including pumps, fans, conveyors, machine tools and compressors. They save energy by varying the voltage and frequency supplied to AC motors and reduce inrush current when starting them, protecting mechanical equipment from damage. VFDs also regulate motor speed and provide controlled acceleration and deceleration to help prolong motor life.

Today’s drives are smaller than shoe boxes, reliable and more powerful, with integrated protection and communications technology. They incorporate a microprocessor that communicates with the drive user and PLC via a human-machine interface (HMI) or keypad, oversees drive operation and checks for faults. Power components like a rectifier bridge and capacitor bank filter DC input voltage, invert the DC back to AC, then supply it to the motor at the desired frequency.

There are three primary drive control methods, volts-per-hertz (V/f), vector control and closed-loop vector control. Vector control uses complex algorithms to monitor, interpret and respond to current feedback from the motor. This provides precise motor control without the need for an encoder. Closed-loop vector control is similar to vector control but uses an encoder to monitor current feedback to more precisely regulate the motor’s speed. This provides more fine-tuned control for high-speed applications where accuracy is critical.

Variable frequency drives are used in a variety of applications

A variable frequency drives can make a motor produce more torque than its rated maximum, but only for a short time. At some point, the iron in the motor can only sustain a certain amount of magnetic flux density. If the motor is made to create more than its max torque, it will generate heat that can damage its internal components and cause premature failure. To avoid this, the drive sets a maximum torque limit or FLA in the software.

The VFD’s electronic thermal overload is another setting that can be programmed to protect the drive from excessive electrical load conditions. Enacting the motor’s overload within the drive eliminates the need for a separate mechanical overload device, which saves cost and space in the system, eliminates a potential point of failure and reduces maintenance costs associated with maintaining the integrity of the mechanical overload’s contacts.

Acceleration and deceleration ramp times are a common VFD settings that can be configured for different applications. Most manufacturers, through years of application experience, have default settings already set for typical pump and fan applications that offer optimum energy savings with little to no programming requirements. For other system configurations, it’s important to understand how to adjust these settings to maintain performance in accordance with the specific system design.