BinaryMike
Pelagic EE
There have been many threads on this topic, so when I found an authoritative treatment in the Radio Designer's Handbook (Classic Edition), I decided it was worth quoting here verbatim:
Temperature Rise
This is dependent on the cooling area, the total loss and the ratio of iron to copper loss. To avoid deterioration of the insulating materials, it is necessary to limit the working temperature to 105°C for Class A insulation which includes paper, cotton, silk, varnish and wire enamel. The temperature rise in the winding, as measured by the change in resistance method, will be about 10°C lower than the maximum (hot spot) temperature. Thus with an ambient temperature of 40°C (104°F) plus a margin of 10°C for the difference between measured and hot spot temperature, it will be seen that the maximum permissible rise is 55°C, as measured by the change of resistance.
It is common practice to allow 10°C margin for change in line voltage, frequency, or operation in situations with restricted ventilation. Thus 45°C is generally accepted as the maximum permitted rise above ambient when measured by resistance change.
The temperature difference between winding and core varies between 10° and 20°C according to the distribution of losses. This means that even with an ambient temperature of 25°C (77°F), the core temperature may be 60°C (140°F). This will feel quite hot to the touch, although the internal temperature may well be under the permitted maximum. Measurement of the core temperature may be made with a spirit thermometer if good contact is maintained between the core and the thermometer bulb.
Langford-Smith goes on to discuss the winding resistance measurement method in detail, but his example assumes high enough winding resistance that standard two-wire ohmmeters are adequate. Be aware that low resistance windings such as heater windings generally require the use of four-wire ohmmeters for sufficient accuracy and repeatability. Modern IR thermometers are convenient for core temperature measurements.
Temperature Rise
This is dependent on the cooling area, the total loss and the ratio of iron to copper loss. To avoid deterioration of the insulating materials, it is necessary to limit the working temperature to 105°C for Class A insulation which includes paper, cotton, silk, varnish and wire enamel. The temperature rise in the winding, as measured by the change in resistance method, will be about 10°C lower than the maximum (hot spot) temperature. Thus with an ambient temperature of 40°C (104°F) plus a margin of 10°C for the difference between measured and hot spot temperature, it will be seen that the maximum permissible rise is 55°C, as measured by the change of resistance.
It is common practice to allow 10°C margin for change in line voltage, frequency, or operation in situations with restricted ventilation. Thus 45°C is generally accepted as the maximum permitted rise above ambient when measured by resistance change.
The temperature difference between winding and core varies between 10° and 20°C according to the distribution of losses. This means that even with an ambient temperature of 25°C (77°F), the core temperature may be 60°C (140°F). This will feel quite hot to the touch, although the internal temperature may well be under the permitted maximum. Measurement of the core temperature may be made with a spirit thermometer if good contact is maintained between the core and the thermometer bulb.
Langford-Smith goes on to discuss the winding resistance measurement method in detail, but his example assumes high enough winding resistance that standard two-wire ohmmeters are adequate. Be aware that low resistance windings such as heater windings generally require the use of four-wire ohmmeters for sufficient accuracy and repeatability. Modern IR thermometers are convenient for core temperature measurements.