Introduction To Electromagnetic Interference With Vfds.docx

Introduction to electromagnetic interference with VFDs Colin Zauner Applications and How-Tos

This is the first of two articles describing what electromagnetic interference (EMI) is, how it affects electrical components in the elevator control panel and common tips that can be used to limit the effects of EMI.

The second article gives 7 actionable steps to reduce the negative effects of EMI. Although the article was written with elevator users in mind, the principles apply to all industrial applications. So let’s get started.

What is EMI? Electromagnetic interference (EMI) or radio frequency interference (RFI) is described as electromagnetic signals that interfere with the normal operation of electrical equipment. EMI can create adverse effects with electrical components in the elevator control panel, contributing to a loss of serial communication, nuisance drive trips and disturbance of control signals. EMI not only degrades the performance of electrical equipment but also decreases the lifetime of components and increases the financial cost to maintain equipment.

Conducted and radiated EMI are two main types of high and low frequency interference that must be considered when troubleshooting potential sources of EMI. As you read, keep in mind each type of interference problem includes a source, a receptor, and a transmission path between the source and victim. This concept is illustrated below:

Source: Wikipedia Conducted EMI is defined as interference that uses conductors as a path from a source to receptor. For example, a motor encoder grounded to a noisy connection would conduct noise to the drive encoder interface. The conducted noise could cause the drive encoder interface to receive inaccurate voltage signals preventing the elevator drive from interpreting the motor speed correctly or drive faults. Initially, it may be assumed that the root cause of the drive operational issues are related to an incorrect parameter setting or possible a faulty drive interface board. Closer inspection reveals the culprit to be poor grounding of the encoder cable. Radiated EMI is defined as interference that uses a wireless path from a source to the victim. This is commonly seen in elevator control panels with AC motor wires are laid in parallel next to low-voltage control wiring. The result is coupling between the wires causing disturbances on the data transmission line. For example, if the motor wires were laid in close proximity of a serial link between the elevator controller and the drive, the coupling of the signals may corrupt the data packets being transferred between the controller and drive.

How is EMI related to Elevator VFD’s? Drive Topology EMI is created by a VFD’s input rectifier stage which consists of a full-wave diode bridge that rectifies the incoming AC power into DC power. The input rectifier draws a non-sinusoidal current from the power supply during each period of AC voltage. This creates current harmonics on the power supply and subsequent voltage distortion which can be conducted to other electrical equipment connected to the utility.

Pulse-Width Modulation VFD’s use pulse-width modulation (PWM) to provide a voltage to the AC motor. The VFD output transistors modulate the DC bus voltage by turning the transistors on and off at very high frequency (in the order of 8kHz) to simulate an AC waveform with a desired frequency and output voltage. A higher switching frequency has the advantage of providing a more sinusoidal current waveform to the motor, however there are several trade offs to consider. During IGBT turn on the voltage at the VFD’s output terminal rises to that of the DC bus, the rate at which the voltage rises is referred to as dV/dt rise time. High dV/dt rates on the motor side of the VFD result in radiated electric fields, and voltage spikes that are conducted along the motor cabling. It should be noted that the output of a VFD is especially rich in EMI noise due to the high frequency transistor switching. This is important when considering how to lay out control and power wiring.

In summary, EMI is described as electromagnetic signals that interfere with the normal operation of electrical equipment. EMI problems often results in poor equipment performance and increased maintenance costs throughout the lifetime of the system. EMI is transmitted to a receptor device from a source via conduction and radiation. EMI problems caused by elevator VFD’s can be attributed to the non-linear components of the VFD drive and the high-frequency PWM switching of the output transistors. The next article in the series will cover 7 tips to help mitigate EMI related issues. If you’d like to learn more about KEB control and automation solutions, you can reach us using the form below.

7 Steps to Reducing EMI with VFDs Colin Zauner Applications and How-Tos, KEB Technology

This is the second of a two-part series reviewing EMI in VFD applications. The purpose of the first article was to get you better acquainted with the basics of EMI as it relates to industrial controls and VFD applications. This article will review 7 tips to mitigate EMI-related problems. When considering EMI, there is always a source, a victim, and a path. Each tip below will use grounding, shielding, or filtering methods to mitigate EMI-related issues.

1. Ensure the VFD and other components are earth grounded appropriately The earth ground is very important and provides a return path to drain high-frequency noise. In general, a low impedance path should be created which will allow the EMI noise to drain. Considering this, the ground connection should be kept as short as possible. A flat braided ground strap is a good choice and provides an increased surface area for connections.

The use of a braided ground strap like this is best practice

Finally, the ground conductor gauge should be sized appropriately for the current – a conductor that is too small will offer high resistance and not drain as effectively.

2. Use a high-quality EMI Filter like the E6 High switching frequencies of the IGBT’s at the VFD output interacts with stray capacitances of the electric system producing parasitic currents. Parasitic currents generate excessive heat in the inverter and can be transmitted to the supply power through the VFD, potentially disturbing sensitive equipment connected to the supply.

For the best performance use a high-quality filter product like KEB’s E6 EMI Filter. An EMI filter does two things. First, it protects the VFD from high-frequency noise that is being generated by other electrical loads on the power supply. Second, it drains parasitic currents to ground instead of conducting them back onto the supply cable.

The EMI filter is placed between the line power and the VFD

It should be noted that not all EMI filters perform equally. An engineer should verify the performance data across the entire frequency spectrum.

Not all EMI filters are created equally. Ask to see the tested performance date into the MHz range.

The use of EMI filters is actually mandated in the EU and is part of the CE mark that machinery gets. Different EMI mitigation levels are defined depending on whether the equipment is used in commercial or residential applications. Make sure the filters are tested to meet performance requirements like those called out in EN 61000-6.

KEB offers back mount and book mount style EMI filters for VFDs

The E6 filters are well suited for packaging, food processing and other machinery applications because they have a low-leakage design. See here, for more information on low leakage EMI filters.

KEB’s E6 filters are suitable for small and large applications

3. Verify the panel is also grounded well to building power More than once, I have had a tech support call from a user who is bench testing a motor. The drive is behaving erratically and it appears that EMI noise could be the culprit. After investigating, the VFD was found to be sitting on a wooden pallet or bench. Without any sort of earth grounding to the building. Poor grounding is particularly problematic when a regenerative drive is used to return excess energy from an overhauling load back to the supply. Regenerative drives measure the main line voltage and frequency in order to deliver excess energy in synchronous with the main line supply. Voltage notching on the main line prevents proper regenerative operation and may cause excessive drive and regen faults.

4.Connect all ground connections to metal common ground block

A common ground block should always be mounted to the cabinet sub panel. All ground connections should be wired to this common block. A common ground block provides a single grounding point to reduce potential differences between multiple ground connections. This will also prevent ground loops which allow EMI to circulate.

The use of a common ground block

5. Use shielded cables for control signals Shielding sensitive control signals can be used to mitigate radiated EMI. For example, it is always necessary to provide an encoder cable with shielded signals. Permanent magnet motors utilizing high-resolution absolute encoders often use analog signals with a 1V peak-to-peak signal to provide position and speed feedback to the drive. Noise on these signals often results in vibration in the motor and poor ride quality. KEB offers high-quality encoder cables with double shields and twisted pair wires for noise immunity. Shielded cables must always be grounded correctly.

KEB cables contain a shield around all conductor pairs

Each twisted pair is also shielded

6. Spatially separate AC supply power, motor cable, high power DC voltage cables, and control and data lines To prevent high-frequency coupling these wires must be spatially separated from each other a minimum of 8 inches when laid parallel to each other.

Extra caution should be taken with the VFD output which is especially rich in EMI due to the high-frequency PWM switching.

7. Install ferrite rings at the inverter output A relatively inexpensive option to reduce common mode noise is to install correctly sized ferrite rings at the output of the VFD. Common mode noise is a result of the interaction of pulse width modulation and parasitic capacitances of the cable and motor. Common mode noise produced by PWM travels throughout the motor to ground.

The result is high voltages and currents which may contribute to nuisance controller faults, premature failure of motor bearings and motor windings. The inductance of the ferrite increases the impedance between the VFD output and cable, thus filtering high-frequency currents.

The use of ferrite rings on the VFD output can dramatically help