14 | Flameproof motors
On motors still in operation higher
values may be obtained compared to
new motors. On new motors lower
values can be obtained, as solvents
are present in the insulating
varnishes which become volatile in a
later stage during normal operation.
This does not necessarily mean that
the motor is not suitable for operation
once the insulation resistance will
increase after a period of operation.
The comparison with values
obtained from previous tests on the
same motor under identical load,
temperature and humidity conditions
will be a better indication of the
insulation conditions in comparison
to the value obtained from a single
test. Any sudden or high reduction of
the value requires careful attention.
Table 3-2 – Recommended minimum insulation
resistance values at 40ºC (all values in MΩ)
Minimum insulation
resistance
(megohms)
For most ac winding built after
about 1970 (form-wound coils)
For most machines with random-
wound stator coils and form-
wound coils rated below 1kV
MINIMUM INSULATION RESISTANCE
If the insulation resistance measured is smaller than
100 M (for medium or high voltage motors) and 5
M (for low voltage motors), the winding must be
dried according to the procedure below before
starting the motor:
- Disassemble the motor, removing the rotor and
the bearings;
- Take the frame with stator winding to an oven
and warms it to a temperature of 130°C, keeping
this temperature for at least 8 hours. For bigger
machines (above IEC frame 630), it can be
necessary to stay for at least 12 hours).
Double check the insulation resistance to verify if it
has reached the acceptable values, according to
Table 3-2, otherwise refers to WEG for instructions.
POLARIZATION INDEX
The polarization index (P.I.) is traditionally defined
as a ratio of the 10 min. insulation resistance (IR
10
)
to the 1 min. insulation resistance (IR
1
), tested at a
relatively constant temperature.
Through the polarization index, the user can
evaluate the motor insulation conditions, according
to the table bellow:
Table 3-3 - Polarization index
EX ATMOSPHERE INFORMATION
To avoid electric discharge risks,
discharge the terminals right after
performing the measurement.
In order to avoid injuries to
personnel, the winding must be
grounded immedialtly after
measuring the insulation resistance.
PROLONGED STORAGE
3.7.1. INTRODUCTION
The instructions for long term storage described as
follows are valid for motors to be long term stored
and/or long periods of standstill before the
commissioning.
3.7.2. GENERALITIES
The existing tendency, especially during the
construction of the plant, of storing the motors for
several years before commissioning or to install
immediately some units, results that the motors are
exposed to influences that cannot be evaluated in
advance for this time's period.
It is difficult to evaluate the different forms of stress
(atmospheric, chemical, thermal, and mechanical)
imposed to the motor, which might happen during
storage maneuvers, assembly, initial tests and
storage until the commissioning.
Other essential factor is the transportation, for
example, the general contractor may transport the
motor or the complete unit with motor as joint
transportation to the installation location.
The motor internal gaps (air gap, bearings and
interior of connection box) are exposed to the
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