What Are Electromagnetic Signatures?

Arcing is a low-voltage electrical discharge involving lower frequency current that is several orders of magnitude greater than that produced by Partial Discharge (PD). Arcing is rich in low-frequency harmonics, extending through a wide range of the radio frequency spectrum. Arcing is characterized by discharges with measurable rise and fall times when displayed in real time on an oscilloscope.

Arcing results from the loss of continuity in conductors, loose-bolted or crimp joints, broken conductors in stator coils, or broken bars in rotors. Frequent sources of arcing are sliding contacts like shaft grounding brushes, exciter commutator brushes, or slip ring brushes.

The detected audio component is usually erratic due to the intrinsic instability of the source. Depending on the source, the discharges may or may not be synchronized with the power frequency.

Corona is an electric current discharging into a gas or liquid that appears during both positive and negative half cycles. In air, corona can be found on clean conductors operating above 2,000 volts to ground and at frequencies below 10 MHz. If the media is oil or pressurized hydrogen, this voltage can be higher.

Low-frequency corona is a common signature for asphalt mica-flake insulated machines, which is the same pattern detected by PDA techniques as internal insulation voids. Machines with modern VPI synthetic resin insulation systems often have corona activity only above 2 MHz. Dirty and contaminated windings can produce corona even with 2.3 kV machines.

The detected audio component has a sound similar to bacon frying.

A gap discharge (partial discharge) is produced when two surfaces separated by a gap are at different potentials sufficient to spark over the gap and generate a current pulse. A gap discharge can occur one or more times during each half of the power frequency cycle because the rise and fall time of each discharge is extremely fast. Usually the pulse repetition frequency (PRF) is fixed at 1 to 15 events per half cycle at any location and is synchronized with machine phase voltage.

The PRF and amplitude of gap discharge vary as a function of gap dimensions, impressed voltage, insulation quality of the surrounding gas, and the RF impedance of adjacent conductors. EMI generated from gap discharge is rich in broadband harmonics characterized by high amplitude throughout the RF spectrum.

The detected audio component has a popping or rasping sound depending on the source PRF.

Micro-sparking is similar to a gap discharge except the gap is extremely thin, usually in the order of 0.4 mm. The discharge PRF is 15 to 30 pulses per each power frequency half cycle compared to gap discharges which occur at PRF of 1 to 15. Each micro-spark has a very short duration.

The EMI produced is usually found to be above 20 MHz. Often, transmission-line hardware micro-sparking is measured at the generator neutral. Micro-sparking is often intermittent due to the very thin gap involved that can be contaminated with dirt, chemical residue, oil, or rust.

The detected audio component is a buzzing sound that is a function of the PRF.

Most EMI signatures are a combination of the five basic types, and they are always combined with a variety of man-made noise sources. Gross problems such as broken or shorting conductor strands are obvious and easy to detect. Other problems such as loose phase rings, contamination, and internal corona are more subtle and harder to isolate particularly when deterioration is in the early stages.