Overheating problem on Ship
Extremes of temperature will affect the performance and the effective life of any electrical apparatus.
Devices which depend on electromagnetic operation by shunt coils will find that the resistance of the coil increases with temperature and so with less current, the field strength ‘H’ is reduced
( H=NI where N is the number of turns and I is the current in amperes).
Thus contactors and relays may fail to operate correctly if overheated.
Importance of Adequate Ventilation
The total temperature is governed by the ambient air temperature and the heating effect of the electric current in the windings. This heating effect, constituting to a temperature rise, is always about the same for similar loads.
The total temperature, which will affect the life of the insulation and the performance of equipment, will be the highest at the maximum ambient temperature.
For unrestricted service the cooling air temperature is 45°C. For restricted service and vessels intended for northern and southern waters outside of the tropical belt the temperature is 40°C.
Adequate ventilation and avoidance of hot pockets where electrical apparatus operate is important.
When considering suitable operating temperature for a device the ‘hot spot temperature’ is an important factor.
In the field coil of a rotating machine, for example, the hot spot is somewhere in the centre of the winding and there is a temperature gradient from there to the surface.
Previous recorded surface temperature values corresponding to specified hot spot temperatures are acceptable for recording the machine’s performance.
Another method is to record changes in resistance due to temperature in the winding.
Temperature test on Ship Machines
When carrying out temperature tests on machines the maximum surface temperature of the windings is found just after the machine has stopped and it is no longer cooled by windage.
The temperature bulb should be covered by a pad of felt to prevent heat loss when the surface winding readings are taken.
Modem methods of monitoring temperature can also be adopted with Thermistor-based Protection.
