"Engine Rise Temp" switch

[jolmos]

"Engine Rise Temp" switch, when to come RISE or IN?

[JRBarrett]

I believe that normally, it would be set to "rise". This system monitors the oil temperature of the generator drive units on each engine.

The "generators" on a 737 are actually "alternators", which produce 3-phase AC voltage at approximately 118 volts. The problem with an alternator is that the frequency of the AC voltage it produces will vary with the speed that the engine is turning, but the systems on the aircraft depend on the frequency of the AC being precisely 400 Hz.

The generator drive unit sits between the engine accessory gearbox, and the generator. It works somewhat like an automatic transmission on an automobile, and will turn the generator at a constant speed, no matter what speed the engine is rotating.

The drive unit has its own dedicated oil supply, oil pump and oil cooler.

When the switch is set to "IN", the lower scale of the gauge is measuring the temperature of the oil that is entering the drive unit, after passing through the oil cooler. In "RISE" position, the associated gauge is showing the relative increase (rise) in the oil temperature after it has passed through the drive. (Read on the upper scale). If the temperature rise of the oil, after passing through the drive unit, exceeds 20 degrees C, it could indicate the the generator load is too high. If the temperature gauge reads in the yellow zone with the switch in the "IN" position, it would likely mean there is a problem in the oil cooler

If the drive oil temps remain high, and cannot be corrected by reducing generator load, it could indicate an impending failure of the drive unit itself, in which case it may be necessary to activate the guarded "disconnect" switch. This will mechanically disconnect the generator drive input shaft from the engine, and will result in the loss of the generator voltage on that side. Once the disconnect system is activated, it can only be reconnected on the ground, by a maintenance engineer.