Over the years, there has been significant progress from a reactive maintenance philosophy, through preventive maintenance to one based on condition monitoring. There are many motor and generator health monitoring programmes available that carry out periodic measurements to assess the general condition of the machine and its components.

WorldWind Life Expectancy Analysis Program (LEAP) adds the vital dimension that enables an operator to know, with a high level of confidence, how long their motor or generator can remain in service before a failure. This systematic approach to maintenance management, when used in conjunction with the known time to repair or replace a component, then allows service to be performed during planned downtime, rather than during a costly emergency.

Stator winding health the crucial factor

Surveys have shown that for larger motors and generators, above 2000 kW, the most common cause of failure was in the stator (static) windings at nearly 61%. Since these larger machines are in the most widespread used in energy generation industries, Thermal and Hydro Electric projects, HT AC/DC motor and Examples and WorldWind LEAP analysis is focussed on assessing the condition of stator windings.

Furthermore, the need for a new approach to monitoring the condition of stator winding insulation was clearly indicated - since these surveys identified that very few winding failures (just over 8 percent) were being detected under maintenance using existing, conventional techniques.

Predicting insulation ageing.

Stator windings comprise a relatively complex multi-layer structure that, during operation, is exposed to thermal, electrical, mechanical and environmental stresses. On startup a large motor will draw an increased current typically 5 x its normal operating current this current peak causes differential expansion of the copper wire and the various layers of insulation. Over time this creates air gaps that result in weak points.

In service, the insulation material ages as shown by the light blue curve in Fig 1. The stresses due to both normal operation and abnormal events (such as transient currents) is represented by the dark blue curve. When the two curves cross an insulation failure will occur, resulting in expensive unplanned downtime. LEAP provides the operator with detailed information on the condition of the stator insulation so they can plan effective action to prevent the failure and increase the overall life of the machine as shown by the dotted line.

How is a LEAP analysis carried out?

To carry out a LEAP analysis, WorldWind takes a range of sophisticated instrumentation to site that enables a battery of tests to be performed on the stator winding insulation. This instrumentation is available throughout India and certain abroad locations.

Our Health-check and Life Expectancy Analysis program includes:

1) Test objective: defining the test objective

2) Test condition

3) Analysis & observation

4) Recommendations

5) Testing and measurements:
Visual inspection report .
Visual inspection (as per oem manual)
Visual inspection endoscopy (stator)
Visual inspection endoscopy (rotor)
Visual inspection special observations
Stator winding resistance analysis
Insulation profiling by charge-discharge method
Insulation resistance/polarization index before overhauling.
Insulation resistance/polarization index after cleaning, varnishing and dryout.
Tan delta and capacitance analysis
Phase resolved partial discharge analysis.
Wedge tightness check (knocking method).
Coupling resistance.
Digital el-cid test on stator core.
Test on rtd.

6) Main rotor.
IR/PI test on rotor before overhauling.
IR/PI test on rotor after overhauling.
Insulation profiling by charge-discharge method.
Ac impedance test.

7) Main exciter stator.
IR test.
Winding resistance test.
Diode checks.
AC impedance.
Main exciter rotor.
IR test on exciter rotor.
Winding resistance:

8) Capacitance of RC block