temporary pilot brushes. Field winding temperature
can be found from the following formula:
can be found from the following formula:
T
2
=
(234.5 + T
1
) - 234.5
R
R
2
1
where:
T
2
= temperature (
O
C) corresponding to final
resistance
T
T
1
= temperature (
O
C) corresponding to
Initial resistance R
1
R
2
=
field volts
field amperes
Field resistance R
1
at temperature T
1
is usually
given on the generator manufacturer's test report.
If these data are not available, resistance should
be measured after the machine has been shut
down for at least 12 hours with constant ambient
temperature and using ambient temperature for T
If these data are not available, resistance should
be measured after the machine has been shut
down for at least 12 hours with constant ambient
temperature and using ambient temperature for T
1
.
Data obtained from field temperature tests can be
used for making up curves as shown in
used for making up curves as shown in
for future use in determining field temperature
from readings of field volts and amperes. These
curves will be particularly valuable if the field
temperature is the limiting factor in overloading
the generator. While making the field temperature
test, readings of the accurate voltmeter and
ammeter used should be compared with the
switchboard voltmeter and ammeter and the latter
should be corrected if necessary. It is important
that the field voltages be measured at collector
rings. If pilot brushes are not provided, an
allowance for brush drop can be made as found
from comparison of voltage measured on the main
brushes and temporary pilot brushes, The
temperature-resistance curve,
from readings of field volts and amperes. These
curves will be particularly valuable if the field
temperature is the limiting factor in overloading
the generator. While making the field temperature
test, readings of the accurate voltmeter and
ammeter used should be compared with the
switchboard voltmeter and ammeter and the latter
should be corrected if necessary. It is important
that the field voltages be measured at collector
rings. If pilot brushes are not provided, an
allowance for brush drop can be made as found
from comparison of voltage measured on the main
brushes and temporary pilot brushes, The
temperature-resistance curve,
, is a
straight line and can be plotted through any two
points of resistance and corresponding
temperature, such as R
points of resistance and corresponding
temperature, such as R
1
, T
1
, and R
2
, T
2
used in the
should be
plotted in the range of about 80 to 130 percent of
full load field amperes and corresponding voltage,
so as to obtain a broad spread in the temperature
scale. Field temperature readings are not
full load field amperes and corresponding voltage,
so as to obtain a broad spread in the temperature
scale. Field temperature readings are not
considered important unless they are near the
limiting temperature. Each curve in
limiting temperature. Each curve in
straight line plotted between two values of voltage
and its corresponding current giving the same
value of resistance. That is,
and its corresponding current giving the same
value of resistance. That is,
R =
=
E
I
1
1
E
I
2
2
each value of temperature for which a curve is
to be made.
to be made.
20. COMPUTING STATOR WINDING
TEMPERATURE RISE
TEMPERATURE RISE
At times it is difficult or inconvenient to measure
stator winding temperature rise by tests at all
desired loadings; therefore, a method of
computing temperature rise, T
stator winding temperature rise by tests at all
desired loadings; therefore, a method of
computing temperature rise, T
R
1
from available
temperature test data is outlined below:
The basic equation to be used is:
T
R
= C
1
C
2
(kVA)
2
This equation has two unknowns, C
1
and C
2
, and
test data are required from two heat runs to find
these unknown values. By substituting T
these unknown values. By substituting T
R1
and
kVA
1
in equation for heat run No. T
R
2
and kVA
2
for
heat run No. 2 and subtracting one equation from
the other, C
the other, C
2
can be found. Substituting C
2
back
into either equation gives a value for C
1
. The
constant C
1
represents stator winding temperature
rise for no-load losses. If it is more convenient, or
if data are already available, stator winding
temperature rise attained during a no-load, rated
voltage heat run may be used for C
if data are already available, stator winding
temperature rise attained during a no-load, rated
voltage heat run may be used for C
1.
After C
1
and C
2
are known, the stator winding
temperature rise, T
R
, for other desired loading
conditions may be calculated.
21. GENERATOR CAPABILITY CURVES
Capability curves for large rotating machines are
required to establish safe operational limits. These
curves are required for inclusion involved.
required to establish safe operational limits. These
curves are required for inclusion involved.
11 (FIST 1-4 3/91)