Page 274 - Industrial Power Engineering and Applications Handbook
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Philosophy of quality systems and testing of electrical machines 11/253
The bridge method to allow for free circulation of air. In normal conditions,
a distance of 2 m is sufficient. The duration of the
Here, the unknown resistance is compared with a known temperature rise test is dependent on the type and rating
resistance using a suitable bridge. Resistance above 1 R of the motor. For motors with continuous rating, the test
can be measured by Wheatstone bridge. Resistance less should be continued until thermal equilibrium has reached.
than 1 52 can be measured by a Kelvin double bridge, Whenever possible, the temperature should be measured
where the lead resistance must also be compensated. both while running and after the shutdown.
All precautions must be taken to obtain the temperatures
of the windings when measuring cold resistance. These 1 While conducting the test the negative sequence voltage
temperatures, when measuring the cold resistance, may should be less than 0.5% of the positive sequence
be obtained by a thermometer placed in contact with the component. In terms of current, the negative sequence
windings or by resistance temperature detectors, if they current should not exceed 2.5% of the positive sequence
are provided in the windings. The temperature of the component of the system current. Measurement of
surrounding air will not be regarded as the temperature any one of the components will be adequate.
of the windings, unless the motor has been idle for a 2 For continuously rated machines, readings should be
long period under similar atmospheric temperature taken at intervals of one hour or less. For non-
condition. If the resistance of a copper conductor is known continuously rated machines, readings should be taken
at one temperature, it may be calculated for any other at intervals consistent with the time rating of the
temperature by using the following formula:
machine. The temperature rise test should continue
(235 + t2) until there is a variation of 1°C or less between the
R,' = .R1 two consecutive measurements of temperature.
(235 + tl )
For motors with a short-time rating, the duration of
where the test should correspond to the specified short-time
R1 = resistance measured at tl"C, and rating. At the end of the test the specified temperature
R,' = resistance at t20C rise limits should not exceed. At the beginning of the
If R1, R,' and tl are known, t2 can be calculated. test, the temperature of the motor should be within
In slip-ring motors the rotor winding resistance will 5°C of that of the cooling air.
be measured at the point of connection of the rotor winding In motors for periodic duty, the test should be
to the slip-rings, so that the slip-ring resistance is continued until thermal equilibrium has been reached.
eliminated from the measurement of the true rotor winding Unless otherwise agreed, the duration of one cycle
should be 10 minutes for the purpose of this test.
resistance.
Temperature measurements should be made at the
end of a cycle to establish thermal equilibrium.
11.3.2 Temperature rise
This test is intended to determine the temperature rise in Measurement of temperature
different parts of the motor windings while running at When thermal equilibrium is reached, the motor must be
rated conditions and the permissible temperature rise stopped as quickly as possible. Measurements must be
limits are specified in Table 11.1. While preparing for taken both while the motor was running and after shutdown
the temperature rise test the motor should be shielded (wherever possible). No corrections for observed
from currents of air coming from adjacent pulleys, belts temperatures are necessary if the stopping period does
and other components to avoid inaccurate results. not exceed the values given in Table 11.2.
Sufficient floor space should be left between the machines Where successive measurements show increasing
Table 11.1 Permissible temperature rise for naturally (indirectly) cooled motors
Method of measurement Insulation class
A E B F H
a b U b a b U b a b
Resistance "C 60 60 75 - 80 80 105 100 125 125
ETD "C 65 65 - - 90 85 110 105 130 130
~~
On the basis of IEC 60034-1
a - < 50OO kW
b - 2 5000 kW
Notes
1 The temperature rise limits are based above an ambient temperature of 40% For temperatures other than 40°C see Section 1.6.2(C).
2 For exact details refer to IEC 60034-1.
3 For other types of cooling methods refer to IEC 60034-1.