Originally Posted by
Thrust Augmentation
Does it really seem feasible that there wouldn't be a thermal (or otherwise) protection device? Without thermal overload protection, one of the possible outcomes would be thermal overload & all that goes with it.
Electric motors are not really my area of expertise, but I’ll throw out a few things and maybe someone who does work with this type of equipment can help.
The stab motor is reported to be a 3-phase DC brushless electric motor. The available circuit diagrams shows circuit protection on the AC side of the AC/DC power converter, but none between the power converter and the motor itself (doesn’t mean there isn’t one, but none are shown). Previous Boeing aircraft had op manual references to a thermal relief circuit. The NG and MAX have none. The non-normal procedures for an inoperative stab trim motor make no reference to any type of circuit breaker or cooling period.
I’ve given some more thought about what would actually happen if the stabilizer was physically prevented from moving. Just speculating here, but there is a clutch system that connects the stab trim motor drive to the jackscrew mechanism. This clutch system favors the manual trim system, which is why you can grab the trim wheel in the cockpit and keep the stab from moving any further. You can demonstrate this on the ground by activating the main electric trim and then stopping the trim wheel with your hand or foot as desired. When you do this, you can hear and feel the clutch slipping/disengaging. The important point here is that the trim motor will continue spinning against the clutch even though the stab is not moving. What I am suggesting is that the motor overload protection may be less
electrical and more
mechanical. When faced with a jackscrew load that exceeds the motor limit, rather than the motor stalling, perhaps the clutch will slip. If that were to happen, there would be sounds of the clutch slipping on the CVR and the FDR traces would show no stab movement while the yoke switch was engaged (none of which were actually reported). I really don’t know for sure because there is nothing in the available literature that discusses the system in this level of detail.
However, worth repeating, even though none of the participants here know for sure what happens in these circumstances, someone on one of the accident investigation teams certainly does. Again, there’s nothing in the official commentary regarding the stab motor having insufficient power to move the stab, no calls to redesign the motor or anything else in the Main Electric Trim system. No regulators are asking Boeing to do anything different with the Main Electric Trim system before allowing the MAX to fly again. All the concerns that have been raised so far about this aspect of the trim system have been reasonable, but none of them have been sustained after close examination.
Regards the motor being disabled completely, in both directions / not just one, that's also not necessarily true & would depend on how smart the motor logic / protection is & how it's been designed.
Again, not my area of expertise. I only know that on previous Boeing’s, the thermal relief circuit would shut down the motor entirely until it cooled off.