4.2.1 WINDING CONDITION ASSESSMENT
Be sure to disconnect the AVR and ground the leads of the Resistance Temperature Detector (RTD)
before conducting this test. To determine the condition of the windings, the insulation resistance is
measured from each phase-to-phase and from each phase to neutral. This measurement should be
conducted as part of regular maintenance after extended shutdown periods, or if the insulation resistance
value is suspected to be low, e.g., wet or dirty windings.
Caution is necessary when it is possible that alternator windings may be excessively damp or dirty. A low
voltage, typically 500V, Mugger (mega-ohm meter) or similar instrument should be used to obtain the
initial measurement of the insulation resistance. Test voltages should be applied gradually and only for a
short period, only long enough to make a determination about the status.
In no event should high voltage tests be conducted until the alternator windings have been dried and
cleaned as required.
Insulation Test Procedure
Disconnect all electronic components including Avers, electronic protection equipment, etc. If present,
ground all resistance temperature detection devices. Short out all diodes on the rotating rectifier. Examine
the system under test and look for any connected components that might cause false readings or that
could be damaged by the test voltage.
Conduct the insulation test in accordance with the operating instructions for the test instrument.
Test the insulation resistance from each phase-to-phase and from earth to each phase. New or unused
generators should have insulation values between 10 M to 25 M or greater. Used generators should
be above 1 M. If the winding resistance is low it must be dried (see below) or repaired.
4.2.2 DRYING METHODS
Dry Run
In some cases, a generator with a low IR value can simply be run without excitation. The natural flow of
ambient air through the generator will tend to dry the windings. This may raise the IR above 1.0 Mȍ and
allow the unit to be put into service. To do so, run the generator for 10 minutes with AVR terminals K1
and K2 open, and inspect the windings to see if they look dry. Re-measure the insulation resistances. If
all the IR values are above 1.0 Mȍ, the windings are good. If not, use one of the other drying methods.
Forced Air Drying
Remove all covers from the generator to allow the moisture to escape. Be sure air can flow freely into
one side of the generator and out another. At a minimum, there should be two openings.
Using two electric fan heaters rated 1 to 3 kW; direct hot air into one of the openings. Make sure the heat
source is at least 300 mm (12 inches) away from the windings to prevent overheating and insulation
damage. Heat at the entry point should not exceed 80°C (180°F). A period of 3 hours is typical to restore
the IR of a generator.
If the generator will not be run immediately, insure that anti-condensation heaters, if installed, are in
operation, and retest windings before running.
