VFD: Swap it or upgrade?

We live in a time where variable frequency drives (VFDs) are the standard for motor control. As they’re being installed primarily in commercial and industrial buildings, both new and old applications reap the benefits these intelligent devices bring. But just like all electronics, VFDs do not last forever.

For example, a replacement may be needed due to a failure from a power surge, or because a 20-year-old VFD isn’t compatible with the building management system, and remote monitoring is crucial for the application. In either of these two scenarios, replacing a VFD may not be as easy as merely swapping the unit, powering up and walking away.

On the other hand, the task actually can be quite simple when knowing what to look for and how to properly go about it. There are two primary scenarios most people face when replacing a VFD.

Scenario 1: Replace a VFD with the same model

There are a few things worth examining when changing out a VFD with one of the same model, as simple as it sounds. The first priority is matching the model number and the voltage class and current/HP rating on both units. This also verifies the new VFD will fit in the existing physical space. Next, all wiring must be landed on the same terminal designation. This ensures the input/output functionality of the new VFD equates to that of the old. Finally, parameter settings should match those that are key for the application.

There are many forms of backup, especially if the old VFD can still be powered up. If accessed, parameter settings can be written down, stored in a software program, or even saved to the keypad’s memory and transferred to the new VFD. Ideally this should be done before the replacement process takes place. In cases when the VFD is incorporated in a bigger piece of equipment, the original equipment manufacturer may have its own parameter listing. The worst case is the drive will have to be set up manually and optimized for the application by the user.

It may prove beneficial to save a few parts from the old VFD. Depending on the intensity of the failure, certain components such as the keypad, cooling fans and control/terminal board may have been left unscathed and can be reused. The main circuitry parts of the VFD, including capacitors, should not be kept as there may be internal damage or extensive wear.

Scenario 2: Upgrade a VFD with a different or newer model

It is slightly more complicated to upgrade or replace a variable frequency drive with a different model. This time there are a few more factors involved. For starters, sizing and obtaining the correct model should be approached as if the drive is for a new application. Variables such as amperage, voltage, enclosure, de-ratings, and application type need to be taken into account. It’s also worth considering if the original VFD had been sized correctly in the first place.

Even before installation there are a few other considerations, physical sizing being of the highest importance for obvious reasons. Generally, newer VFDs are smaller and more compact than that of the previous generation but that’s not always the case. This also holds true when switching to a different manufacturer. It is always recommended to check a dimensional drawing to verify space requirements.

An existing package or setup also will need to be looked at closely. Most stand-alone VFDs are rated simply for an indoor environment with little to no protection against airborne debris. They are often put inside an enclosure, which offers greater resistance against harmful elements such as dust and water. Replacing the unit as a whole (drive and enclosure) can get pricey, and only the drive itself may have failed. Swapping the VFD may seem logical but components such as circuit breakers, filters, bypass configurations and other electrical equipment need to be examined for compatibility.

Once the above checks are complete, wiring can then be done. Assuming the terminal designations have no match from one VFD to the next, a simple breakdown of the inputs/outputs (I/O) type can help clarify where the wires should be landed. Once the main circuitry wiring is completed, the control wires and terminal designations on the new VFD can be categorized into five types. These are digital inputs, digital outputs, analog inputs, analog outputs and other I/Os. It’s best to reference a wiring schematic to obtain the exact function of the terminals.

As one of the final steps, programming is once again what makes the application. If a parameter list with non-default values can be obtained from the old VFD, then setting up the new model will be less troublesome. On the other hand, the VFD may just need to be configured from the start. In this case, breaking down the basics that a drive needs, such as a frequency and run command, will make setup much easier. The advanced features can be programmed last. Typically, the VFD user manual will have detailed information on specific functions and a complete parameter list. The keypad may even have a setup wizard with a step-by-step procedure.

Other considerations

Repairing the VFD is another option. A well-maintained, repaired unit may last another decade. The ability to accomplish this depends on factors such as availability of parts, model or sizing of the drive and the extent of damage. Spare parts usually are available even for older models and therefore may be an inexpensive option.

Determining the root cause of the VFD failure is an often-overlooked option. Most drives are replaced without this ever being questioned and rarely are VFDs just plain defective. Environmental issues such as humidity, dust, temperature and exposure to corrosion should first be considered. Other failure means are generally external to the VFD itself. These include, but are not limited to, motor problems and input voltage fluctuations which put strain on internal components such as capacitors.

VFD protection and preventive maintenance are crucial and should never be undervalued.

Original content can be found at Plant Engineering.

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