Understanding Variable Frequency Drives

[sloanelectric]

A variable frequency drive (VFD) is a type of motor that controls the levels of frequency and voltage that alter the speed of an electric motor. The drive alters torque and motor speed and can be used with any sized, three phase electric motor. You’ll notice that VFDs are used in everything from small appliances to giant compressors. Here we’ll take a look at the makeup of a VFD system, the benefits of using this type of system, and the specs of a Mitsubishi VFD system.

The Makeup of a VFD System
AC Motor
The AC Motor is typically a three-phase induction motor or a motor designed for fixed-speed operation. However, if the VFD is going to place an elevated amount of voltage on the motor, then a definite-purpose, inverter-fed duty designed motor is a better choice.

Controller
The VFD controller is made up of three sub-systems, the converter or rectifier stage to convert AC voltage to DC voltage, a DC capacitor assembly, and an inverter assembly to create the three phase voltage. Since the introduction of the insulated gate bipolar transistor in 1983, progress in power devices and computer processors has reduced VFD costs and increased amperage ratings.

Operator Interface
VFDs use an operator interface to start and stop the motor, as well as adjust the speed of operation. An operator interface also has the capability to reverse and switch between automatic and manual controls. The VFD can be controlled through the control panel on the operator interface, or through a computer communications network or some sort of external wiring of analog and digital controls.

Drive Operation
There are three different types of drive operations – single-quadrant, two-quadrant, and four-quadrant:
• Quadrant I – The most common; used for driving and motoring; positive speed and torque
• Quadrant II – Used for generating or braking; positive speed and negative torque
• Quadrant IV – Common for high-performance applications; used for generating or braking; negative speed and positive torque

Variable Frequency Drive Benefits

VFDs are Energy Efficient.
Not all applications have to be run at their default speed. Slowing down the motor allows you to limit energy consumption as well as energy costs. It is estimated that up to 65% of all U.S. electrical energy is used by power motors. With that amount of saving potential, someone who purchases a VFD can see a return on their investment in as little as 6 months.

VFDs Optimize Performance.
When you go with fixed-speed options, motors may have to deliver more power than what is required due to the inability to vary the load speed. This results in poor process control. VFDs, on the other hand, allow for infinite variability so that the energy consumed makes sense for what is needed – reducing errors, increasing production, and maximizing revenue.

VFDs Save on Maintenance.
Because VFD errors are so rare, they require little to no maintenance. The VFD can protect your motor from electro thermal overloads, phase protection, under voltage, over voltage, and more. This means less downtime and reduced spending on repairs – increasing your bottom line.

Mitsubishi VFDs
Mitsubishi has released a FREQROL FR-A800 series of variable speed drives that are compatible with existing models like the FR-A700, FR-A500 and FR-A200. These VFDs are made as small as 0.4 kW and as large as 500 kW. The upgraded VFD series takes advantage of power conversion and open network communications. All applications can use the FR-A800 model but it is especially effective for difficult applications like centrifuges, winding machines and high-bay warehousing systems. It can connect with a variety of networks that include CC-Link, Profibus, SSCNETIII and Modbus. There is an energy saving mode that allows for the reduction of motor loss.

The FR-F800 is designed for general purpose industrial and commercial applications where the performance features of the FR-A800 VFD is not needed.

By understanding exactly what function you hope to accomplish, you can select the right VFD for the job at hand.