Coil Heating
The rise in coil temperature as the current through the winding turns into heat across the resistance. Because copper resistance increases with temperature, current drops in a coil driven with constant voltage and the same command produces less flow as the day goes on; this is the classic cause of machines behaving differently in the morning and in the afternoon.
Copper resistance rises about four percent for every 10 °C increase. The difference between a cold coil and one heated to 80 °C is not to be underestimated; moreover the coil is heated not only by its own losses but also by the hot oil passing through it and by the heat of the engine block.
- Symptom: a machine that works normally in the morning slowing down in the afternoon at the same lever angle.
- Permanent solution: current control drive. Because the drive holds the current constant, the temperature drift disappears.
- Permanent damage: at excessive temperature the winding insulation breaks down, a short between turns forms and the coil fails irreversibly.
In applications where the coil stays energized continuously, using drives that switch to a low holding current noticeably reduces both heat and energy consumption. A coil being too hot to touch is not a fault on its own, but if it is above expectations the current draw should be measured.
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