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Installation
Tests Before Operation
Insulation Resistance
Check insulation resistance prior to connecting motor
to power supply.
A hand cranked or solid state
electronic insulation resistance tester, at least 500 volts
d.c., but not over rated voltage, is usually used (see
Maintenance).
See
IEEE
Recommended
Insulation Resistance Rotating Machinery IEEE Std 43.
Dielectric (Hi-Pot) Tests
All motors receive a factory dielectric test in
accordance with ANSI and IEEE Standards.
Dielectric testing may result in
personal injury or death.
Operate dielectric test equipment only
with qualified personnel, in accordance
with manufacturer's instructions.
If a dielectric test is made on an old or repaired
winding, to evaluate service reliability, the test voltage
applied may vary from the rated terminal voltage to
some higher value. The factory should be consulted
when establishing the tes
old or repaired equipment.
CAUTION
Excessive dielectric testing may cause damage to
insulation.
Test only in accordance with IEEE Std. 43.
Winding Resistance (Temperature)
The change in resistance of a winding provides an
accurate measure of the average temperature of a
winding, and is generally used to determine the
temperature of the stator windings. The
measurements must be made carefully with
instruments known to be accurate, and preferably with
the same instruments for both hot and cold
measurements.
Practice
for
Testing
DANGER
t voltage and procedure for testing
NOTE
If normal vibration or noise will be objectionable (as in
Instruments that operate on the principle of the Kelvin
Bridge are preferred. (See "Temperature Measuring
Devices" - IEEE Publication No. 119.)
The cold resistance, or the resistance at normal room
temperature, must be measured after the machine has
been idle for some time, usually before starting, or cold
resistance value may be obtained from the factory.
The cold temperature of the coils should be measured
because coil temperature may not be the same as the
surrounding air.
The average temperature of the winding is obtained by
taking resistance measurements, at the motor load
terminals and using the following equation:
R
=
T
r
Where
T = hot temperature in degrees C
R = hot resistance
r = cold resistance at temperature t
t = cold temperature of winding in deg C (amb)
As an example, assume the cold resistance of 0.50
ohms at 25°C, and the hot resistance (taken
immediately after motor is de-energized, and has
stopped rotating) is 0.61 ohms, then:
. 0
61
=
T
. 0
50
=
T
82
The temperature measured by imbedded detectors or
by the change in resistance is generally higher than
thermometer measurements and is closer to the true
hottest spot temperature in the machine. For this
reason, the Standards permit higher observable
temperatures when measurements are taken in this
manner.
- 9 -
+
−
(
234
5 .
t
)
234
5 .
+
−
(
234
5 .
25
)
234
5 .
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