Page 45 - Offshore Electrical Engineering Manual

32 CHAPTER 1 Insulation and Temperature Ratings of Equipment

DO NOT READJUST AVR DURING HEAT RUN

Portable thermometers are attached to the air intake and outlet; in the latter case it is
advisable to run generator for a short while and determine the hottest spot by hand before
attaching the thermometer. Air thermometers must be no more than 2 m from generator.
When thermal stability is reached, take final hot load readings and running tem-
peratures and then shut down as quickly as possible. No specific time for shutdown
and taking hot temperatures is given in marine specifications.
With digital thermometer check spot temperatures and record as follows:

Stator core (usually accessible through the terminal box)
Stator coils (end windings)
Rotor coils (At hottest spot, usually near coil supports)
Exciter armature (Windings)
Exciter field (Windings)
Bearings (D. E. and N. D. E. temperatures to be recorded)

Take hot resistances of stator and exciter field windings.
On large generators, take rotor winding resistances as soon as possible after gen-
erator has stopped.
As mentioned earlier in this chapter, temperature rise by resistance is given by 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 hot
1
2
(°C) (Fig. 2.1.1).
RATINGS FOR VARIABLE SPEED MOTORS
Where motors are being supplied from converters, phase-angle controlled thyristor
drives or similar systems where the machine is subjected to prolonged running at sig-
nificant load but reduced speeds, a larger de-rated machine may be required because
the cooling effects of shaft-driven fans and windage will be reduced. Alternatively,
an independently driven cooling fan will be required. Because of this, it is preferable
to purchase the drive and motor as a unit with the appropriate ATEX Declaration of
Conformity from the manufacturer.

THERMAL OVERLOADS AND MOTOR THERMAL PROTECTION

During starting, both in the winding of the rotor and in the winding of the stator,
currents may be present that are well above regular on-load currents. A fast trip
must be initiated when a fault occurs during startup, for example, in the event