Sensor choice is driven by the frequency range of the discharge, the noise environment and whether the machine can be opened.
HFCT (High-Frequency Current Transformer): A split-core clamp around the neutral-to-ground bond or the phase earthing lead, typically 3–30 MHz bandwidth. It is the fastest sensor to retrofit and needs no outage beyond access to the terminal box. It must enclose a single conductor only, and it will also capture discharge from the fed cable and switchgear, so it is best used with time-domain separation.
Stator Slot Couplers: Broadband travelling-wave antennas bonded into the slot under the wedge, on the winding surface. They give the highest signal-to-noise ratio of any motor sensor and can discriminate discharge inside the slot from discharge in the endwinding, because the propagation path differs. Their limitation is installation: they must be fitted during a rewind or a major outage, and the number fitted determines location accuracy.
Capacitive couplers: 80 pF to 1 nF epoxy-mica or ceramic couplers on the phase terminals, used with a 50 Ω measuring impedance. This is the standard configuration for calibrated offline measurement and for the phase-resolved reference.
Complementary methods: Acoustic (piezoelectric) sensors and ozone sniffing confirm that an electrical signal is a genuine internal discharge rather than conducted interference