Electromagnetic Interference

Transcription

1 Electromagnetic Interference

2 What Is EMI/RFI? EMI/RFI is defined as any conducted, radiated or magnetically induced voltage which degrades, obstructs, or repeatedly interrupts the desired performance of electronic equipment. The terms EMI and RFI are often used interchangeably. EMI is actually any frequency of electrical noise, whereas RFI is a specific subset of electrical noise on the EMI spectrum. It consists of any unwanted conducted or radiated signals of electric origin that can cause degradation in system or equipment performance. Conducted RFI/EMI is most often found in the low frequency range of several 3 khz to 30MHz. Radiated RFI/EMI is most often found in the frequency range from 30MHz to 1GHz. For the lower part of the RF range, noise is expected to travel along the lines or cables, rather than radiated from the equipment. The frequency range for harmonics starts from 50 Hz and ends at 2 khz or 2.5 khz. Sources EMI/RFI Switching Mode Power Supplies. Relays, motors and AC/DC drives. Equipment such as business computing devices, work processors, electronic printers. Industrial controls, personal computer. Electronic games.

3 Reduction of EMI/RFI 1. EMI could be reduced by reducing the switching frequency of power electronics converter as they make use of fast switching power semiconductor switches, such as MOSFET and IGBTs. 2. The reduction in EMI can also be achieved by reducing the length of the motor used with the drive as the longer cable length has large surface area as a result radiated EMI will be enhanced. However there are some other techniques for reduction of EMI such as: a. Shielding b. Filtering Shielding of the power cables Shielding is produced by insertion of a metallic barrier in the path of electromagnetic waves between the source of the radiation and the device which is supposed to be protected. Metals are more attractive for shielding due their high conductivity. Thus metal fibers of small diameter are most desirable.

4 The drawback of using metal as a shielding material is: Weight of the heavy metal can be an issue in the case of full metal shielding and plastic matrices with high metal filler content, especially in applications where mass should be as low as possible. Electromagnetic shielding reduces, or rather prevents coupling of undesired radiated electromagnetic energy in electrical equipment. Shielding also reduces the effective cable length of the motors. Tips for Improving the Performance of AC Drives The following three tips for cabling can help reduce or eliminate several performance issues in a VFD system: Shielded cables to address EMI issues in other electronic systems and reduce bearing currents. Cables with adequate insulation thickness to address standing waves. Cables with a symmetrical design to address induced currents related to bearing failure. Reducing Carrier Frequency of ac drives also lessens the impact of EMI Effective motor cable shields The general rule for cable shield effectiveness is: the better and tighter the cable s shield, the lower the radiated emission level. Recommended shielded cables Analog cables: Recommendations for analog signal runs: Use double shielded, twisted pair cable. Use one individually shielded pair for each signal. Do not use a common return for different analog signals.

5 Digital cables: Recommendation for digital signal runs: A double shielded cable is the best alternative, but single-shielded, twisted, multi-pair cable is also usable. Filtering EMI filters are commercially available to eliminate high frequency interference in power lines. A combination of inductors and capacitors make up the EMI filters. The impedance of the node that requires an EMI filter determines this configuration of capacitors and inductors.