Measurement In The Rotor Winding, Exciter And Accessories; Minimum Insulation Resistance; Windings Evaluation And Preservation; Conversion Of The Measured Values - WEG AG10 Series Installation, Operation And Maintenance Manual

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4.5.4 Measurement in the rotor winding, exciter
and accessories
Measurement of the rotor winding

Disconnect rotor cables of the set of diodes;

Connect the Insulation resistance meter
(megohmmeter) between the rotor winding and the
shaft of the alternator. The measurement current cannot
pass through the bearings.
Measurement of the stator winding of main exciter.

Disconnect the power cables from the exciter;

Connect the Insulation resistance meter
(megohmmeter) between the stator winding of the
exciter (terminals I and K) and the frame of the
alternator.
Measurement of the rotor winding of main exciter.

Disconnect exciter rotor cables of the set of diodes;

Connect the Insulation resistance meter
(megohmmeter) between the rotor winding and the
shaft of the alternator. The measurement current cannot
pass through the bearings.
Measurement of the stator winding of the auxiliary exciter
(PMG), if any:

Disconnect cables that connect the auxiliary exciter to
the voltage regulator;

Connect the Insulation resistance meter
(megohmmeter) between the stator winding of the
auxiliary exciter and the frame of the alternator.
ATTENTION
The test voltage for the rotor, main exciter,
auxiliary exciter and space heater must be 500
VDC and other accessories 100 VDC.
It is not recommended the measuring of
insulation resistance of thermal protectors.
On machines that are already in operation, higher values
of insulation resistance, can be obtained compared to the
initial values for commissioning.
The comparison with values obtained in previous tests on
the same machine, in similar conditions of load,
temperature and humidity serves as a better indication of
the conditions of isolation than the value obtained in a
single test, and any sudden reduction is considered
suspicious.
4.5.5

Minimum insulation resistance

Table 4.2: Minimum insulation resistance
Stator voltage ≤ 1000 V
Stator voltage > 1000 V
Rotor and exciter
24 l Installation, operation and maintenance manual – Synchronous Alternator – AG10 line - Horizontal
Minimum R.I.
(referred to 40ºC)
5 MΩ
100 MΩ
5 MΩ
4.5.6

Windings evaluation and preservation

Table 4.3: Evaluation of the insulation resistance "I.R."
Insulation
Un R.I.
< 5 MΩ Critical*
≤1000V 5 a 100 MΩ Acceptable
> 100 MΩ Normal
< 100 MΩ Critical*
>1000V 100 a 500 MΩ Acceptable
> 500 MΩ Normal
* The alternator must not operate in this condition.
NOTE
The data in Table 4.3 serve as a reference. It is
recommended to record all insulation
resistance measurements performed on the
alternator and maintain a history of these
measurements.
Any sharp reduction in recorded amounts
should be investigated.
The insulation resistance is influenced by the presence of
moisture and dirt in the insulation material.
If the measured insulation resistance is lower than the
values reported in Table 4.2, before the alternator come
into operation, the windings must be carefully inspected,
cleaned and, if necessary, dried according to the following
procedure:

Disassemble the alternator by removing the rotor and
bearings;

Place the winding components, which have low
insulation resistance in an oven and heat them at a
temperature of 130°C, keeping this temperature for at
least 08 hours.

Check the insulation resistance is achieved within
acceptable values, as Table 4.3, otherwise refer to
WEG.
DANGER
Immediately after measuring the insulation
resistance, ground the winding to prevent
accidents.
4.5.7

Conversion of the measured values

The insulation resistance should be referred to 40°C.
If the measurement is made at a different temperature, the
reading must be corrected to 40°C, using the approximate
correction provided by the curve of Figure 4.6, according
to the IEEE43 standard.
The correction of the insulation resistance reading to 40ºC
is done by the relation:
R = Kt . Rt
40
Where:
Rt = insulation resistance at temperature "t".
Kt = Correction factor of the insulation resistance as a
function of the winding temperature, as shown in Figure
4.6.
Procedure
Status
Cleaning and drying
Periodic monitoring
Normal operation
Cleaning and drying
Periodic monitoring
Normal operation
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