Understanding and Mitigating Electromagnetic Interference

Understanding EMI

Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI) is electromagnetic signals that interfere with the normal operation of electrical equipment. EMI problems often result in poor equipment performance (contributing to a loss of serial communication, nuisance VSD trips, and disturbance of control signals) as well as increased maintenance costs. Each type of interference problem includes a source, a receptor/victim, and a transmission path as illustrated below:

The two main types of high and low-frequency interferences that must be considered when troubleshooting potential sources of EMI are as follows:

• Conducted EMI is interference that uses conductors as a path from a source to a receptor. Example: a motor encoder grounded to a noisy connection would conduct noise to the VSD encoder interface. The conducted noise could cause the VSD encoder interface to receive inaccurate voltage signals preventing the VSD from interpreting the motor speed correctly or cause VSD faults.
• Radiated EMI is interference that uses a wireless path from a source to a receptor. This is commonly seen in control panels with AC motor wires laid in parallel next to low-voltage control wiring. The result is coupling between the wires causing disturbances on the data transmission line. Example: motor wires laid in close proximity of a serial link between a PLC and a VSD may corrupt the data packets being transferred between the controller and VSD.

VSD Impact on EMI

EMI problems caused by VSD’s can be attributed to the non-linear components of the VSD drive and the high-frequency PWM switching of the output transistors.

• EMI is created by a VSD’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.
• VSD’s use pulse-width modulation (PWM) to provide a voltage to the AC motor. The VSD output transistors modulate the DC bus (DC Link) voltage by turning the transistors on and off at a very high frequency 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 VSD’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 VSD result in radiated electric fields, and voltage spikes that are conducted along with the motor cabling. It should be noted that the output of a VSD 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.

EMI Mitigation

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.

Cable Spacing

To prevent high-frequency coupling, wires (AC supply power, motor cable, high power DC voltage cables, and control and data lines) must be spatially separated from each other (a minimum of 20cm when laid parallel to each other). This precaution will help prevent signals from capacitive coupling with one another and generating excessive noise. Extra caution should be taken with the VSD output which is especially rich in EMI due to the high-frequency PWM switching.

Earth Grounding

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 suggested and providing an increased surface area for connections – 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).

Grounding the VSD achieves two important goals: limiting the possibility of electric shock by providing a conductor to earth ground, and mitigating EMI issues that could affect equipment across the connected power network. Also ensure the VSD panel is grounded appropriately. 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 that allow EMI to circulate.

Shielded Control Cables

Shielded cable on the load and line side of the VSD can also be used to reduce the threat of EMI. Shielding sensitive control signals can be used to mitigate radiated EMI (always advised to provide an encoder cable with shielded signals). Use high-quality encoder cables with double shields and twisted pair wires for noise immunity. These cables offer fast, error-free, and noise-immune connections for almost any application. Shielded cables must always be grounded correctly.

Ferrite Rings

A relatively inexpensive option to reduce common-mode noise is to install correctly sized ferrite rings at the output of the VSD. 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 VSD output and cable, thus filtering high-frequency currents.

EMI Filters

High switching frequencies of the IGBTs at the VSD output interact with stray capacitances of the electric system producing parasitic currents. These parasitic currents generate excessive heat in the inverter and can be transmitted to the supply power through the VSD, potentially disturbing sensitive equipment connected to the supply. An EMI filter does two things.

• Protects the VSD from high-frequency noise that is being generated by other electrical loads on the power supply – minimizes the impact of high-frequency electrical noise.
• Drains parasitic currents to the ground instead of conducting them back onto the supply cable.