Temperature Limits in Large Machines

The present Institute standards allow a maximum limit of 105 deg. for fibrous insulation and 125 deg. for mica insulation with an allowance of 150 deg. subject to special guarantees of the manufacturer. In high-voltage machines of large size, the effect of actual copper temperatures higher than 105 deg. may cause softening and disappearance of binding materials, bulging of insulation, and consequent powdering of insulation under periodic pounding of copper on softened material, and ionization at voids so created. Operating experisence of four large size machines demonstrated the above effects to repeatedly take place in machines operating at copper temperatures of 150 deg. cent. None of these effects were noted in machines operating at copper temperatures of 105 deg. cent. or under. Two machines have safely operated for over three years at maximum copper temperatures of 130 deg. cent. Incorrect conclusions may be made as regards the safe temperature limits by judging the performance of machines unless actual copper temperatures are known. Lower ambient temperatures and fractional loads may reduce the operating temperatures 25 to 35 deg. below the assumed limits. Machines designed for high temperatures are less efficient than machines designed for cool temperatures, in one instance the difference being as several hundred kilowatts at all loads. The calculation of ventilation of large machines is relatively uncertain; it is of importance to aim at a conservative limit rather than set it too near the danger point.