Adding a VFD? How to treat it like a VIP

Be aware of potential power quality problems on both the line and load sides.

By Stu Accola, MTE Corp.

When companies want to improve performance of equipment that’s driven by motors, they often will incorporate a variable frequency drive (VFD) into their system. VFDs control the AC motor speed and torque by varying the motor input frequency and voltage. Although VFDs offer many benefits, such as increased efficiency and savings on electric costs, they also can introduce potential problems.

Some of the problems occur on the line side, between the utility connection point and the VFD; others occur on the load side, between the VFD and the motor. These problems can be especially challenging with VFDs that are retrofit into legacy motors. Let’s look at some of the top issues to watch out for on both the line side and the load side, as well as potential solutions for addressing these issues.

Line-side power quality concerns

On the line side (or input), when you add a VFD, harmonics can be created and directed back toward the utility. Harmonics exist in current waveforms that deviate from the smooth sinusoidal waves normally found in utility power.

This distortion in the power system leads to issues. What you may see is that some components (fuses, transformers, and wire and circuit breakers) can overheat. Also, using a VFD to control the speed of the motor can create heat in other motors across the line.

You also may experience telecommunication interference, such as buzzing in your phone lines. This occurs because of coupling of harmonics from the power line to the phone line and is known as a telephone influence factor, or TIF.

The spikes in electric current high-crest factor also can cause your circuit breakers to start tripping regularly. In addition, you may notice inaccurate measurements on sensors, equipment malfunctions, and even increased utility costs.

Load-side power quality problems

On the load side (or output), once the VFD receives power, it converts it into a new pulse-width-modulated waveform. Disturbances can happen between the VFD and motor power, including power spikes, fast rise times, and common-mode current. These are especially prevalent when there is a long cable distance between the VFD and the motor.

One of the biggest problems is damage to motor bearings. When common-mode current travels through the bearings, it causes microscopic pitting. Because this occurs thousands of times per second, the pitting can be a serious problem, often leading to premature bearing failure. You may also experience ground fault trips, which can cause downtime when your drive trips.

Adding a VFD to your system also can add common-mode voltage, which leads to unusually high current in your ground-fault detection circuit. This can trip the ground-fault sensor, which disables your equipment and stops production.

How to protect your system

To control issues specific to the line side, you can add a harmonic mitigation filter. This will help reduce harmonic distortion in your power system and prevent issues such as overheated components, telecommunication interference, and circuit-breaker trips. When adding a harmonic mitigation filter, you want to make sure your building meets IEEE-519 requirements for harmonic distortion and performs well even at lighter loads.

To control issues on the load side, you can add a dV/dt filter. This will help provide protection from voltage spikes by reducing the rate of voltage and limiting the peak voltage that occurs at the motor terminals. When adding a dV/dt filter, you should make sure that it has the ability to provide common-mode and rise-time reduction as well. This is especially important if there are large distances between your VFD and the motor. dV/dt filters are best suited for distances of 300 to 1,000 feet. For distances more than 1,000 feet, you may need to use a sinewave filter, while at short distances (less than 300 feet), a load-side reactor may work.

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