the stator core because of excessive leakage flux
near the winding end turn area. Most machines are
not subject to this trouble, but the condition should
not be overlooked in the few cases where it exists.
near the winding end turn area. Most machines are
not subject to this trouble, but the condition should
not be overlooked in the few cases where it exists.
various components of the machine.
16. CHANGING VOLTAGE TO REDUCE
TEMPERATURE
Some reduction in operating temperature is
sometimes possible, particularly on synchronous
motors, without reducing load, by changing the
operating voltage. Stator iron losses and
temperatures increase with increased applied
voltage, and vice versa. Copper losses and
temperature are proportional to the square of the
stator current. If stator laminations run cool and
coils comparatively hot, an increase in stator
voltage by changing transformer taps will decrease
the copper temperature and increase the iron
temperature, without a change in output. The
minimum stator winding temperature for any kW
load will be obtained at 100 percent power factor.
17. LOAD TEMPERATURE TESTS
For all types of machines, an indication of the
amount of overload which can be carried can be
determined from the mechanical limits and a series
of temperature tests. A prediction can be made
from results of a single temperature run by
computing the temperature rise as outlined in
paragraph 19. More accurate indications are
obtained by making temperature rise test runs at
50, 75, and 100 percent load. However, the best
plan is to make a temperature test at the actual
overload which the machine is desired to carry.
Voltage and power factor should be held constant
for all runs so that the internal voltage and core
loss will be about the same. Switchboard
instruments should be supplemented with
accurately calibrated instruments during
overloading of the machine. Each load
temperature test should be run with constant load,
excitation, cooling water, etc., until the temperature
TEMPERATURE
Some reduction in operating temperature is
sometimes possible, particularly on synchronous
motors, without reducing load, by changing the
operating voltage. Stator iron losses and
temperatures increase with increased applied
voltage, and vice versa. Copper losses and
temperature are proportional to the square of the
stator current. If stator laminations run cool and
coils comparatively hot, an increase in stator
voltage by changing transformer taps will decrease
the copper temperature and increase the iron
temperature, without a change in output. The
minimum stator winding temperature for any kW
load will be obtained at 100 percent power factor.
17. LOAD TEMPERATURE TESTS
For all types of machines, an indication of the
amount of overload which can be carried can be
determined from the mechanical limits and a series
of temperature tests. A prediction can be made
from results of a single temperature run by
computing the temperature rise as outlined in
paragraph 19. More accurate indications are
obtained by making temperature rise test runs at
50, 75, and 100 percent load. However, the best
plan is to make a temperature test at the actual
overload which the machine is desired to carry.
Voltage and power factor should be held constant
for all runs so that the internal voltage and core
loss will be about the same. Switchboard
instruments should be supplemented with
accurately calibrated instruments during
overloading of the machine. Each load
temperature test should be run with constant load,
excitation, cooling water, etc., until the temperature
rise above ambient air temperature reaches a
constant value, and readings should be taken at
30-minute intervals for several hours alter
conditions become stabilized. Specific test
procedures may be obtained from the Denver
Office. A curve of generator amperes versus
stator temperature rise plotted from the test data
may be of value for future reference.
constant value, and readings should be taken at
30-minute intervals for several hours alter
conditions become stabilized. Specific test
procedures may be obtained from the Denver
Office. A curve of generator amperes versus
stator temperature rise plotted from the test data
may be of value for future reference.
18. MEASURING STATOR TEMPERATURE
Temperature of the stator coils should be taken as
the highest reading obtained. Where embedded
RTDs are provided for measuring the stator
temperature and a temperature check should be
made by placing a few thermocouples on the
stator iron (core). Where no embedded RTDs are
installed, it is desirable to use a number of
thermocouples for measuring stator temperature;
thermocouples should be placed on the hottest
parts of the stator steel, well protected from the
airstream, and the highest thermocouple readings
should be used. Thermocouples may be held
against the surface with duct seal. Precautions
should be taken if thermocouples are used to
measure end turn temperatures, end turn coils
could be charged to nearly the turn-to-ground
voltage during machine operation. Specific test
procedures may be obtained from the Denver
Office.
the highest reading obtained. Where embedded
RTDs are provided for measuring the stator
temperature and a temperature check should be
made by placing a few thermocouples on the
stator iron (core). Where no embedded RTDs are
installed, it is desirable to use a number of
thermocouples for measuring stator temperature;
thermocouples should be placed on the hottest
parts of the stator steel, well protected from the
airstream, and the highest thermocouple readings
should be used. Thermocouples may be held
against the surface with duct seal. Precautions
should be taken if thermocouples are used to
measure end turn temperatures, end turn coils
could be charged to nearly the turn-to-ground
voltage during machine operation. Specific test
procedures may be obtained from the Denver
Office.
19. MEASURING FIELD TEMPERATURE
Field temperature for both static or rotating
excitation systems shall be determined by the
resistance method from field voltage and current
readings while the machine is carrying the load for
which data are desired. Specific test procedures
may be obtained from the Denver Office. An
accurate voltmeter and ammeter should be used.
Field voltage should be measured at collector
rings by pilot brushes. If no pilot brushes are
provided, one of the main brushes on each ring
can be disconnected and insulated from the
brush holder by a layer of paper and used as
excitation systems shall be determined by the
resistance method from field voltage and current
readings while the machine is carrying the load for
which data are desired. Specific test procedures
may be obtained from the Denver Office. An
accurate voltmeter and ammeter should be used.
Field voltage should be measured at collector
rings by pilot brushes. If no pilot brushes are
provided, one of the main brushes on each ring
can be disconnected and insulated from the
brush holder by a layer of paper and used as
(FIST 1-4 3/91) 10