Page 40 - Offshore Electrical Engineering Manual

Temperature Rise 27

machines but is very common in larger machine stators. In small machines the only
means available in practice may be to determine the temperature either by the change
in resistance of the winding, or by measuring the surface temperature by thermom-
eter. The surface temperature will obviously be less than that of the hot spot, and to
a certain extent the difference will depend on the depth of winding. From previous
research and experience, values of surface temperature corresponding to specified
hot-spot temperatures have been determined and are now universally accepted. If
the resistance method is used, it is evident that as there is a temperature gradient,
the resistance must lie somewhere between what it would be if the whole coil was at
hot-spot temperature, and what it would be at surface temperature. The temperature
determined by resistance is therefore higher than the surface temperature, and the
accepted difference will be noted in the standard tables.
For Classes A and B, hot-spot temperatures of 105 and 130°C respectively have
been universally accepted for very many years. These figures correspond roughly to
about 20 years’ working life under average industrial conditions. It must be remem-
bered, however, that in industry there are usually peak periods of loading interspersed
with off-load or reduced load periods for meal breaks, etc., and these periods of rest
have considerable influence on the life of machines. Under marine conditions some
machines may run for days at a constant load, and experience has shown that under
these conditions the life may be reduced to about 15 years. No hard and fast rule can
be made because conditions vary, but taking into account the necessity for the utmost
reliability in marine installations the need for a more conservative approach to this
problem is indicated. It is generally accepted that insulation life is approximately
halved for each 10°C increase above the accepted hot-spot temperature limits for
Class A and Class B materials.

TEMPERATURE RISE
A continuously rated machine will eventually reach a steady temperature at which the
heat in the windings and magnetised cores and the heat arising from frictional losses
will be dissipated at the same rate as they are generated. The difference between
this steady temperature and that of the incoming cooling air is the temperature rise.
For all practical purposes, other parameters being equal, this rise is always the same
regardless of the temperature of the cooling air. For example, if a machine is tested
in an ambient or cooling temperature of 20°C and a machine temperature of 55°C is
recorded, the rise is 35°C; when the same machine is in the tropics and the cooling
air is at 45°C the rise will still be 35°C, giving a total machine temperature of 80°C.
When the commutators’ sliprings or bearings of machines provided with water
coolers are not in the enclosed air circuit cooled by the water cooler but are cooled by
the ambient cooling air the permissible temperature-rise above the ambient cooling
air should be the same as for ventilated machines.
The appropriate hot-spot temperature for a given class of insulation is determined,
and from that the surface temperature. Then the permissible temperature rise is arrived