Page 43 - Offshore Electrical Engineering Manual

30 CHAPTER 1 Insulation and Temperature Ratings of Equipment

Class E insulation was introduced to enable full advantage to be taken of the then
new synthetic resin enamels, and similar materials now in use, which are suitable for
higher working temperatures than the oleo-resin enamels which were available when
the present limits for Class A were established. This has now largely been displaced
by Class B and F materials.

METHOD OF MEASUREMENT OF TEMPERATURE RISE BY
RESISTANCE

If it is desired to measure temperature rise by resistance, it is important to obtain a
reliable reading of the cold resistance and corresponding temperature. The machine
should have been standing at atmospheric temperature for some time so that it is at a
uniform temperature. The initial resistance and the temperature, at which this is read,
as measured by a thermometer on the winding, should be recorded simultaneously.
The hot temperature is determined from the following formula:

t 2 = [R 2 /R 1 (t 1 + 235)] − 235,

where R is the resistance of the winding cold, R the resistance of the winding hot,
2
1
t the temperature of the winding cold (°C), and t the temperature of the winding
1
2
hot (°C).
Since the increase in resistance of copper per degree Celsius is about 0.4%, spe-
cial care must be taken to use calibrated instruments, accurate recording and rapid
measurement after shutting down.
A quick estimation of temperature rise can be made on the basis of 0.4% increase
in resistance per degree Celsius. For instance, if a 20% increase in resistance is
recorded the temperature rise is approximately 20/0.4 = 50°C.
When it is intended to use embedded temperature detectors (ETDs) they must be
built into the machine during construction. They may take the form of either thermo-
couples or resistance thermometers. This method is generally used for large alterna-
tors to record the temperature of stator windings, in which case at least six detectors
are suitably distributed around the stator and placed where it is expected that the
highest temperatures are likely to occur. They are usually placed between upper and
lower coil sides, within the slots and midway between radial ducts, if any.

MEASUREMENT OF AMBIENT AIR TEMPERATURE

The cooling air temperature should be measured at different points around and half-
way up the machine, and at distances 1–2 m away from it. The thermometers should
indicate the temperature of the air flowing towards the machine, and should be pro-
tected from heat radiation and stray draughts.
The value to be adopted to determine temperature rise is the average of these
temperatures from readings taken at the beginning and end of the last half hour