Abel Gonzalez and Sameer Kulkarni
The condition of a rotating machine can be evaluated by using a combination of Static and Dynamic testing techniques. Static tests are performed while the motor is disconnected from the system, typically during manufacturing, commissioning, maintenance, or repairs. The goal of Static tests is to provide a proper picture of electrical characteristics of the machine.
As part of Static tests, the status of windings is evaluated by measuring parameters such as resistance, inductance, and impedance. In certain situations, winding impedance is measured at different rotor positions by performing the rotor influence check test. This test is utilized to verify the uniformity of the air gap between the rotor and the stator. Deviations between winding parameters could indicate manufacturing defects, contact resistance issues and, turn to turn faults. The turn-to-turn insulation, which is both the weakest spot and the place where most of the insulation failures begin in a motor, is also evaluated during Static testing using voltage Surge tests. During Surge tests, voltage pulses are injected into the machine windings to simulate the startup process, where a nonlinear voltage distribution exists across the winding. The results of this can be used to diagnose the condition of the machine insulation system and when trended over time be used to plan machine replacement or overhaul.
The status of the ground wall insulation system is evaluated by high voltage DC tests that include tests such as Insulation Resistance, Polarization Index, Dielectric Absorption Ratio, Step Voltage, Ramp, Dielectric Discharge, and Dielectric Voltage Withstand (hipot). Other types of Static tests include the application of high voltage AC to measure the insulation’s power and dissipation factors. In some cases, low frequency AC tests, known as VLF tests, are used to test rotating machine insulation with the advantage of reduced power requirements of test equipment.
This seminar will cover the theory behind each of the different static tests used to evaluate the status of the winding and the insulation of rotating machines. It will discuss best practices, applicable standards as well as the criteria used to analyze the condition of rotating machines.