Page 41 - Offshore Electrical Engineering Manual

28 CHAPTER 1 Insulation and Temperature Ratings of Equipment

at by deducting the maximum ambient air temperature under which the machine will
be called on to operate. When carrying out temperature tests on machines it is impor-
tant to remember that the surface temperature of windings is affected by windage,
and the temperatures recorded while the machine is rotating must not be taken as
the maximum for determining temperature rise. After the machine has come to rest,
a further rise will occur and the thermometers must be observed for several seconds
after stopping, until the temperature reaches its maximum. (Note that this is not to be
confused with the effects of reduced windage in variable speed machines, which is
considered at the end of this chapter).
Temperature rise limits vary according to the application, e.g., rotating machin-
ery, transformers, contactor coils. Reference should be made to the appropriate stan-
dard and/or regulation for a particular application. Table 2.1.1 is an example of limits
for rotating machines on vessels for unrestricted worldwide service. It is based on the
ambient air temperature of 45°C and cooling water at 30°C. The limits for Classes F
and H insulation are 20 and 40°C, respectively, higher than Class B.

AMBIENT AIR TEMPERATURES
For ocean-going vessels and mobile oil installations it is required that temperature
rises should be based on tropical conditions. Whatever the intentions of the first
owner, such vessels sometimes change hands and must therefore be suitable for ser-
vice in any part of the world.
Temperatures recorded in various positions in many types of ship have been tabulated
and studied. It has been shown for instance that certain locations are almost invariably
warmer than other parts of the same machinery space, and it is logical therefore to accept
lower temperatures for switchboards, for instance, than for generators and motors.

BASIS OF MACHINE RATINGS
[Reference should be made to BS EN 60034-1:2004: Rotating electrical machines –
PART 1: Rating and performance]
Insulation and temperature ratings of equipment: Momentary overloads (for
which, for test purposes, a duration of 15 s is recognised) of 50% in current for gen-
erators and of varying amounts in torque for motors according to type, size and duty
are recognised. Continuous Maximum Rating (CMR) does not mean that machines
are incapable of carrying moderate overloads of reasonable duration, but that the
makers are not required to state either magnitude or duration or to submit machines
to an overload test. In actual fact, electric motors, generators and particularly trans-
formers have an inherent capacity sufficient for average requirements.
For exceptional applications where overloads are anticipated in normal service,
the purchaser should seek the advice of the manufacturer or select a standard motor
of higher rating. Such cases might arise with motors coupled to oil pumps, where the
load may be increased for short periods while pumping cold oil.