Partial Discharge Test
Partial Discharge (PD) is a phenomenon that has been around for decades and probably was observed in rotating equipment as higher voltages came into use. In recent decades PD was mainly a concern for operators of high-voltage rotating equipment. But, with the introduction of inverter drives used to power electric motors instead of 50/60Hz sinusoidal systems, Partial Discharge started to be observed in low voltage motors as well.
Today, inverter drives, also called Variable Frequency Drives or Variable Speed Drives, Adjustable Speed Drives etc. are commonplace and their use is growing. When VFDs/ASDs are misapplied, Partial Discharge is often generated in motor windings with serious or catastrophic results.
When using inverter drives, good system design is an important aspect of ensuring long motor life. However, many cases arise when there are limitations, historical or otherwise, to the system design and re-design options available.
Systems often degrade in an unknown manner. In most cases, early changes in motor condition will be the first indication that there is a problem. These changes are best detected by conducting a Surge and Partial Discharge test on the motor with a motor analyzer. If the surge test fails, the motor condition is critical. If the surge test passes, a PD test will provide valuable information about the condition of the motor and the motor’s power system, and provide early warning of insulation breakdown.
Modern motor analyzers such as the Electrom iTIG II Winding Analyzer can include PD measurements, an important and cost-effective tool to detect insulation weaknesses early and diagnose motor systems that use inverter drives. With the iTIG II, Partial Discharge measurements can be part of a fully automatic sequence of tests.
On the following pages we will address more details about the Partial Discharge test, how Electrom Instruments measure PD, and how PD is used as an analysis tool:
- What is Partial Discharge and why is it bad? Including when and where PD takes place.
- PD generated by Inverter Drives (VFD/VSD/ASD)
- PD Measurement methods
- PD measurements by Electrom Instruments; how they are used as motor analysis tools