Variable-frequency drives and electromagnetic interference

How do you address electromagnetic interference when designing a motion system using variable-frequency drives?

Tim Lusha, senior EMC engineer at DLS Electronic Systems: Controlling emissions from a VFD is very challenging. VFD efficiency comes at the expense of EMI performance. Attempting to control the EMI performance of the VFD drive often decreases efficiency and increases heat generation. Placing filter components on the output can and will affect the operation. Placing too much capacitance or inductance can shift the voltage-to-current relationship and affect the motor operation. This can either render it useless or create too much heat and prematurely damage or destroy the solid-state drivers or the motor. Most often a combination of filtering and shielded cables are necessary. Other methods such as switching the drive frequency can shift the resulting harmonics to where they meet the desired limit.

Steve Perreault, industrial drive product management at Yaskawa: A byproduct of VFDs is electromagnetic (EM) radiation. In many industrial applications, the typical amount of EM radiation causes very few problems with other equipment; however, in U.S. commercial applications such as HVAC, EMC (electromagnetic compatibility ) levels are specified by part 15 of the FCC (Federal Communications Commission). The European Union has defined EMC limits for most environments (industrial included), according to IEC 61800-3. Manufacturers provide a variety of solutions for different levels of EMC, ranging from drives with fully integrated filters, to completely separate filters connected in series between a drive and its power source.

Tim Albers, director of marketing and product management at Nidec Motor: I think you will get two different sets of answers to this question referring to radiation of EMI and immunity to EMI. I am not sure which interests you the most. There are standards for both depending on the application. The European standards for radiation for industrial are more stringent than the North American. The US residential standards are the most stringent and hardest to meet for radiation. Immunity can vary as well, but most base VFDs are designed to be fairly resilient as far as EMI immunity. From my perspective the biggest issue with VFD installations is really around cabling EMI radiation. Absolute care must be taken to make sure that power cables are separated from or insulated from control wiring. Standards around the world exist to prevent interference issues, but caution, even over caution, is reasonable. The cables will radiate and the control wiring will pick it up if nearby. That issue is certainly not new and is well documented.

Nisha Chandrasekharan, portfolio manager for low-voltage drives at Rockwell Automation: Electromagnetic interference (EMI), also called electrical noise, are the unwanted signals generated by electrical and electronic equipment. EMI VFD issues can range from corrupted data transmission to electric-motor drive damage. Modern VFDs using IGBT switches for motor frequency control are very efficient because of their high switching speed. Unfortunately the high-speed switching also results in much higher EMI being generated. Typical noise suppression solutions include ensuring all components are grounded appropriately; using a shielded power cable to connect the variable frequency drive to the motor and correctly connecting the shield to drive and motor ground locations; using a built-in or external EMI filter; using twisted control wiring leads to provide a balanced capacitive coupling; and running power and control wiring separately.

Jack Zurick, senior control systems engineer at Thermo Systems, a Control System Integrators Association (CSIA) member: Electromagnetic interference can be most easily be mitigated by increasing the distance between the systems. Filters as well as ferrite cores can be used to further assist in this matter.

Chip McDaniel, technical marketing at AutomationDirect: A properly or adequately sized power supply is key because if the upstream transformer is too large for the drives, the impedance will be too low causing a number of issues. A well-designed grounding system helps provide noise mitigation. The use of Isolation transformers or input line reactors, along with the EMI and RF filters shown in the attached diagram, are critical to addressing interference associated with VFDs. Isolation transformers (with a 10 to 1 reduction in transmitted noise) or input line reactors mitigate power line disturbances and reduce the power line harmonics caused by AC drives, which can affect other equipment operating on the same power supply. Input EMI filters reduce electromagnetic interference or noise on the input side of the AC drive. They are required for CE compliance and recommended for installations prone to or sensitive to electromagnetic interference. RF filters reduce the radio frequency interference or noise when installed on the input and/or the output side of the VFD.

Chuck Fernandez, enclosed low voltage product manager at Siemens: The best way to address EMI issues is applying recommended installation practices. This involves, for example, defining EMC zones, establishing which types of cables are to be used (Shielded VFD cable) and how these are to be routed (proper separation) and grounded. Only after using proper EMC installation guidelines do we recommend providing filters and other interference suppression measures to achieve even more suppression.

Chris Jaszczolt, industrial drive product management at Yaskawa: Most VFDs are used in industrial environments where the drive's electromagnetic interference (EMI) does not effect localized electrical equipment in any significant manner. In sensitive environments the user may find it prudent to implement some form of EMI mitigation. There are two halves to EMI; conducted and radiated noise.

Radiated noise occurs when the drive's output waveform turns the motor leads into a large antenna. This type of noise can effect equipment near to the motor leads. It is always recommended to minimize the motor lead length to limit the radiated noise. Also, reducing the drive's carrier frequency can help to reduce the amount of noise radiated. To eliminate radiated noise altogether, armored/shield cable or grounded metal conduit can be used.

Drives generate conducted noise when the transformer sources current for the drive's high frequency switching needed by the drive to generate its output waveform. These high frequency currents can effect negatively effect the operation of equipment on the same power source. Reductions in the drive's carrier frequency helps, but input filtering would be needed to significantly mitigate these currents. Low cost solutions include the use of ferrite cores.

Margie Rawlins, lead control systems engineer at Thermo Systems, a Control System Integrators Association (CSIA) member: Electromagnetic interference, often referred to as “noise”, on your feedback device can be very detrimental in a motion system. In a motion system, the drive is trying to control its set speed based on the feedback it is receiving. Noise changes the feedback signal that your drive is getting and has the potential to be seen as movement that your device will try to correct for. In most motion systems, there is little room for error. To help minimize the effects of electromagnetic interference, you should be sure to use shielded twisted pair cables that are properly grounded for your feedback devices. Also, keep in mind that the most common cause of electromagnetic interference in a motion system, is the high voltage side of the drive itself, so try not to run high voltage motor cables and feedback cables alongside each other for long distances. If it is possible, they should be run in separate conduits.

Chris Cusick, senior VFD product specialist at Mitsubishi Electric Automation: The best way to avoid EMI is by using good wiring hygiene during the panel or machine design. Issues such as grounding, cable shielding and ‘wire dress’ are all very important. A radio filter will also be useful to limit mains conducted noise coming back on the line, which is the worst manifestation of radio noise. Noise directly radiated from a drive is much less of an issue.

Joel Kahn, product manager, inverters, at Lenze Americas: VFDs can be paired with or may incorporate an EMI/ RFI filter. Ensure you follow the manufacturer’s recommendations for motor cable type (this most often should be a quality shielded cable) and maximum cable length. Also, good wiring practices should always be observed and will ensure the system is properly grounded.

Rick Akey, senior application engineer, low-voltage drives groups, at ABB: Electronic interference from a VFD can cause problems for other equipment in the area in which the drive is installed and at the same time it is possible for other equipment to cause problems for the proper operation of the drive. The drive itself can produce interference through a broad range of frequencies and magnitudes. This can affect radio communications, instrumentation or other equipment or devices.

The chance of issues occurring in an application can be greatly reduced by carefully following best installation practices for wiring and grounding. Manufacturer’s recommendations for cable selection, grounding and installation are critical to good results. Generally, each wiring class should be kept separately from each other where possible. Input and output wiring to the drive should not be placed next to each other and should also be separate from control cables. Shielded cables between the drive and the motor that are properly installed will help to keep the noise within the cables and the noise will then have a low impedance path back to the source. This will greatly reduce the chance of the noise finding its way into other cables and also help to keep it from radiating into the air.

Sometimes noise from a VFD can go back on the AC line and cause interference to other devices on the network or it can even propagate through the air to interfere with other equipment. In that case it may be useful to apply a filter to suppress the noise. Some VFDs may provide this kind of protection either as a standard or optional feature in the form of a filter. It is also possible to source or specify this type of Electromagnetic Interference/ Radio Frequency Interference filter and apply it on the input of the drive to mitigate the noise.

Mark Harshman, director, systems engineering, at Siemens: There are two types of EMI that are addressed by most VFD manufacturers: radiated emissions and conducted emissions. Most VFD manufacturers provide their control inside protected and sealed metal cabinets or cubicles that have been tested for radiating emissions and immunity to outside EMI. Typically, the metal cabinets are designed to limit the EMI and are tested to conform to various IEC, IEEE, or CE standards for the generation or susceptibility of EMI. All VFDs should have a certification indicating their rating for EMI.

For conducted emissions which are high frequency components that are conducted out of the VFD through power conductors, the standards still apply, and the control systems are designed to limit conducted emissions and tested to comply with one of the standards. Generally, some type of filter will be added to the control power connections that eliminate any conducted emissions that are present on the line because of EMI from the control system or semiconductor switching transients.

The key to both types of emissions is to make sure the VFD has been designed to an EMI standard and has the appropriate label indicating its compliance to those standards.

ALSO READ: The basics of variable frequency drive installation

About the author: Mike Bacidore

Mike Bacidore is the editor in chief for Control Design magazine. He is an award-winning columnist, earning a Gold Regional Award and a Silver National Award from the American Society of Business Publication Editors. Email him at [email protected].