Why can’t I hear the radio? VFDs and Interference Issues

When properly installed, variable frequency drives (VFDs) offer many advantages over conventional systems, including constant pressure, soft starting, and the option of a small, space-saving tank. While these advantages far outweigh the chance of ever experience a problem, in certain circumstances, all VFDs have the potential to create electromagnetic interference (EMI). Observing proper installation and grounding procedures greatly reduces the chances of EMI, and this issue of Franklin AID discusses a few simple steps to do just that.

What is EMI?

Also called radio frequency interference (RFI), EMI refers to any kind of an unwanted electrical disturbance that interferes with other electrical devices such as radios, telephones, televisions, etc. In the case of some devices such as radios and televisions, the interference is audible. In some cases, as with devices such as lighting systems, the interference can actually affect proper operation of the device.

EMI can be divided into two categories: radiated and conducted. Radiated emissions, as the name implies, travel through the surrounding air, much like a wireless radio signal. Conducted emissions on the other hand, travel through electrical wiring, where they reach other devices and cause interference. IN addition, conducted emissions can create radiated emissions and vice versa.

What causes EMI?

A small wave of electromagnetic energy is emitted whenever an electrical current is switched on or off. You may notice, for example, that you sometimes hear a short click in a nearby AM radio when a nearby light switch turns on. The energy from this switching action generates the electromagnetic interference. Since nearly all VFDs today use switching transistors that create this action thousands of times a second, nearly all VFDs have the potential to create EMI.

There are few important things to note before we move on. First, besides VFDs, many types of electrical devices can cause electromagnetic disturbances, even fluorescent lights. Second, although it can be a nuisance, EMI does not cause any hard to the VFD, the motor or any other device. Finally, the vast majority of FVD installations will never experience EMI issues caused by the VFD.

How do I reduce or eliminate EMI?

Understanding that EMI is an inherent property of electrical devices in general and VFDs in particular we can focus on minimizing its effects. The solutions may vary depending on whether the interference is radiated or conducted, but all are remarkably simple.


The single most important way to avoid EMI issues with a VFD centers on one factor: grounding. The reason is that the drive is designed to decouple/filter as much electromagnetic energy via the ground wire as possible so that it cannot cause interference without proper grounding, you can almost be guaranteed to experience EMI of some kind. Proper grounding techniques for Franklin’s SubDrIve family can be found in the SubDrive/MonoDrive installation manual, as well as in the product installation guide.

Physical Location

When a drive has been properly grounded and still causes interference, the simplest way to reduce radiated EMI is to physically move the drive away from the device that is experiencing the disturbance (or to move the device away from the drive). Because emitted electromagnetic energy falls off very rapidly with distance, moving the devices even a small distance apart can have a big impact. For example, doubling the distance between devices reduces the electromagnetic energy by four times. Tripling the distance yields a nice-fold improvement, and so forth. When the drive must be placed outside to accomplish this, Franklin’s SubDrive NEMA 4 product offers an ideal solution, since its all-weather rating means it can be mounted anywhere.

Physical Configuration

As mentioned earlier, EMI can be conducted via electrical wiring, particularly through a drive’s input and output leads. Running the input or output leads of a VFD in parallel and in close proximity to electrical or telephone wires may lead to interference. Consequently, often one fo the best ways to eliminate conducted EI issues is to keep the input and output leads physically away from other electrical wiring; the recommended eight inches (minimum) can make a big difference. In addition, where the drive’s leads must cross other wiring, crossing them at a 90-gegree angle should eliminate this type of interference.


Drive manufacturers often include filters inside their drives to minimize the effects of EMI. Franklin Electric’s SubDrive for example, contains a great deal of both input and output filtering inside the drive itself. Even so, in some cases internal filtering may not be able to completely eliminate the effects of EMI. For those cases, VFD manufactures and electrical supply houses offer various types of external filters.

Isolation Transformers

In more difficult situations, an isolation transformer has proven successful at preventing EMI from being conducted back to the rest of the house or facility wiring. Placed between the electrical supply and the VD’s input, an isolation transformer is like any other transformer, but the primary and secondary sides are equal. That is, the output voltage will equal the input voltage. However, and isolation transformer is like any other transformer, but the primary and secondary sides are equal. That is the output voltage will equal the input voltage. However, and isolation transformer features no direct electrical connection between the input and the output, allowing the conducted EMI to be reduced. Make sure the transformer secondary is grounded for safe operation (an electrical code requirement) and to obtain the best noise reduction benefit.

How can I troubleshoot an EMI issue?

When addressing an interference issue, start by working through the following checklist of the first places to look when you encounter EMI.

  1. Was the interference present before the drive was installed? If so, the issue is not with the drive.
  2. Does the interference occur when the drive is not running? If so, the issue is not with the drive.
  3. Is the drive’s input side connected via dedicated cable from the breaker panel that includes a dedicated ground? If not, have a qualified electrician correct this.
  4. Do the drive’s output leads include a dedicated ground wire? If not, have a qualified electrician correct this.
  5. If the drive is connected in a sub-panel, are the ground and neutral separated? If not, have a qualified electrician correct this.

In summary, electromagnetic interference does not have to be a problem with your customer’s variable frequency drive. In fact, with proper grounding, the vast majority of installations never experience an EMI problem. When an issue does occur, however, just a few simple steps can result in a very satisfied customer who loves their new water system.