Page 276 - Industrial Power Engineering and Applications Handbook

Philosophy of quality systems and testing of electrical machines 11/255
possible. The thermometer method is also recognized in 0 By 5°C if the temperature of the cooling air exceeds
the following cases: 40°C by 5°C or less
By 10°C if the temperature of the cooling air exceeds
When it is not practical to determine the temperature 40°C by more than 5°C but not more than 10°C and
rise by the resistance method, as in the case of low By agreement if the temperature of the cooling air is
resistance windings, especially when the resistance of more than 10°C above 40°C.
joints and connections form a considerable percentage
of the total resistance. Temperature rise of bearings
Single-layer windings, revolving or stationary and The temperature rise of bearings may be measured as
When, for reasons of mass production, the thermometer follows:
method alone is used. although the resistance method
would be possible.
For sliding contact bearings (sleeve or thrust bearings)
the temperature readings must be taken from as near
In thi\ method. the temperature is determined by to the bearing surface as possible.
thcrniometers applied to the accessible surfaces of the For ball and roller type bearings the temperature
windings of the motor (see also Figure 10.10). The term readings must be taken at the stationary race.
'thermometer' may include mercury or alcohol bulb For oil lubricants it is customary to measure the
thermometers as well as embedded thermocouples and temperature in the oil reservoir.
rcsistance thermometers, provided the latter are applied 0 In forced lubricating systems, incoming and outgoing
to points accessible to the usual bulb thermometer also. temperature readings may be taken.
When bulb thermometers are employed where there is
a likelihood of a magnetic field, alcohol thermometers
should be preferred to mercury thermometers. as the latter Notr
I
The permissible temperature riw of the beat-ings will depend
are unreliable under such conditions. upon the type of lubricant used and the recommendations made
by the lubricant and hearing manufacturers.
2 It may not be possible to measure the temperature riw of thc
Measurement of cooling air or gas temperature hearings at the races in all case\. In such a case. it may be
during test permissible to measure the temperature Ihr ball and roller type
bearing< at the stationary race or at the bearing cover.
The cooling air temperature should be measured by several
thermometers placed at different points around and half- Temperature correction
way up the motor, at a distance of 1-2 m and protected
from heat radiation and draughts. The value to be adopted I For motors specified for operation at an altitude higher
lor the temperature of the cooling air or gas during a test than 1000 m, but not in excess of 4000 m. the correction
.;hould be the mean of the readings of the thermometers may be made as shown in Table 1.8.
placed as mentioned above, taken at equal time intervals 2 A temperature rise test may be carried out at any
during the last quarter of the duration of the test. convenient cooling air temperature. When the
In order to avoid error, due to time lag in the temperature temperature of the cooling air during the test is lower
measurement of large motors and variation in the cooling than the stated site cooling air temperature by less
air or gas temperature all reasonable precautions must than 3O"C, no correction should be made on account
be taken to reduce these variations and errors as much as of such a difference. When the temperature of the
possible. cooling air is lower by more than 30"C, the permissible
In cooling by means of forced ventilation, or where temperature rise on test should be the permissible
the machine has water-cooled air or gas coolers, the temperature rise under specified site conditions,
temperature of the air or gas, where it enters the motor reduced by a percentage numerically equal to one-
must be considered as the cooling air or gas temperature. third of the difference between the specified
For motors fitted with heat exchangers the temperature temperature of the cooling air at site and the
reading of incoming air or gas or water should also be temperature of cooling air on test, where both
taken. temperatures are expressed in degrees Celsius.

Example 11.1
Cooling air temperature If the specified temperature of the cooling air at site is 48°C
and the temperature of the cooling air during test is 17°C (a
The motor may be tested at any convenient value of difference of more than 30"C), the reduction in temperature
cooling medium temperature less than 40°C. But whatever rise to take account of this difference will be:
the value of this coolingmedium temperature, the
permissible rise of temperature during test should not -== 48-17 10.3
3
exceed those shown in Table 1 1.1.
In motors intended to operate under conditions in which The permissible temperature rise on test will therefore be
thc maximum cooling air tcmperature may exceed 4U°C, 100 - 10.3% = 89.7% of the recommended temperature rise.
the temperature rise as given in the relevant specification These reductions apply to all the classes of insulation as
indicated in Table 11 .I and the test is to be carried out at the
must be reduced as follows: manufacturer's works.