Select the Best VFD for Your Application

Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.

Choosing the right variable frequency drive for an application involves several important considerations. For example, based on acceleration requirements, sensorless vector control may be more suitable than volts-per-hertz (V/f) control. While V/f control is effective in dragging logs up a slope, it’s not appropriate for dockside hoists that position 12-ton shipping containers to within inches.

According to the technical experts at Mitsubishi Electric Automation, Vernon Hills, IL, there are several factors to consider when selecting variable-frequency drives (VFDs). Among them:

What is your load type: constant or variable torque?

For a constant-torque load, the torque is independent of speed (ignoring momentary shock loads). Examples include conveyors and hoists. For a variable-torque load, torque varies as a function of speed. Examples include fans and pumps. This primary distinction underlies every decision you’ll make about the type of drive.

What are your acceleration requirements?

Does it matter how fast your load accelerates up to speed? For a fan, probably not. For a centrifuge, almost certainly. In the latter case, you may want to select sensorless vector control, rather than volts-per-hertz (V/f) control. While the V/f approach is effective for many applications, it doesn’t allow a motor to develop near-full torque at near-zero speeds (unlike sensorless vector control). V/f control can be appropriate for dragging logs up a slope, but not for a dockside hoist that needs to position a 12-ton shipping container to within inches.

Controlled deceleration presents its own challenges.

During decelerations, the motor acts as a generator. The resulting energy needs to go somewhere, and is typically dissipated as heat in a braking resistor. Controlled-deceleration capability is a good solution for constant-torque loads, changing loads, or even unbalanced loads.

What is your speed range?

Although a conveyor belt may operate consistently at 60 Hz, for an unspooling module on a printing line, the motor needs to deliver torque as effectively at 0.5 Hz as 60 Hz. This is another application where garden-variety V/f control won’t do the job. Sensorless vector control will (and most VFDs these days include it). Keep in mind, however, that not all offerings are created equal. Be sure to double check specifications against your requirements.

Do you need to optimize energy usage?

Instead of wasting all of the energy harvested when your motor is overhauling, you can apply it to your next move, courtesy of a regenerative VFD. These drives have internal capacitors that temporarily store energy for reuse.

Do you need an encoder?

Not all drives are the same at low speeds. A drive with a 200-to-1 speed range, for example, can provide 100% speed from your motor down to about 1/3 Hz. This might be acceptable for some applications, but not others—in which case you’ll need an encoder, Since not all drives work with encoders, you’ll want to determine your need for this capability in advance. MT

VFD Sizing Matters

Back in the day, an undersized drive—a common situation in plants—would simply trip when it exceeded operating specifications. Today, VFDs are very good at limiting themselves so they don’t trip.

The bad news? While a properly sized drive today might allow the system to complete acceleration in one second, as commanded, one that’s too small for a task will take two or three seconds. In short, an undersized drive will compensate, but the system won’t perform as desired and the compensation could mask the true problem.

Mitsubishi Electric’s technical experts also note that it’s crucial to size a drive based on peak current command—not on horsepower. Because of horsepower’s connection to motor size, it’s easy to focus on it. In reality, though, you want to size a drive so that the maximum current in the worst-case scenario is always within the mode of the unit’s continuous-current rating.

For more information on selecting VFDs, including various design, installation, and operational factors, visit us.mitsubishielectric.com/fa/en.