546 Electrical testing would also show that the deformed coil was indeed the coil that  failed, rendering the motor  useless . The motor vendor, which was gracious to participate in these efforts, agreed to these conclusions.  

         Figure 4. Motor Coil Images post Destructive investigation . Note waviness to coil in 3rd image, top coil  

Though the symptom of the failure was obviously  coil overtemp, the investigation team was unsure why  these motors would fail at 25° C in a minute when motors of identical design were used on the EDU and  lasted for up to an hour at 60° C without issue. Unfortunately, due to scheduling constraints, the program  required the rebuild the AMT before the fundamental cause could be determined . The team chose to protect  the motors from future failure  by creating a “duty cycle” for the motors at 25° C and above. This duty cycle   would be determined by the rebuilt hardware in the TVAC facility at vacuum . A sequence of increasingly  long run times followed by coil impedance measurements would determine when the coils would reach a  maxiumum allowable temperature. The impedance would then be monitored until it returned to a near - ambi ent temperature, allowing for the motor to be cycled again. Each mo tor coil was monitored, and the  performance ranges between coils and motors were larger than expected. The first lot of duty cycle  discovery gave a maximum of 40 seconds on, 25 seconds off . This duty cycle would keep the motors from  reaching a “red line” test limit of 110° C maximum coil temperature. The team chose to use the duty cycle  operation at the 25° C and 45° C functional tests.  

Lessons Learned from AMT TVAC Test Failure 1   1. Pressure, tem perature and current data should be collected at a much faster rate than we  previously thought during TVAC testing involving DC motors.  These motors weigh less than  4 grams, and the coils may be a few tenths of a gram each. Their overheating still produced a  measurable pressure spike of nearly 200% above the ambient pressure at 0.5E-5 Torr.  One second  data rate was chosen for further testing.   2. Thermocouples should be place d on available motor surfaces and a nearby heat sink to estimate  thermal conductance. Close correlation of the two temperatures are a great sanity check that the  system’s heat is conducting as expected.   3. This motor coils’ performance has a wide range of vari ability . You cannot base performance of a  system on a previous motor set . (The team would later discover that within a single motor, one coil  would require a hefty duty cycle of 50% while the other didn’t need one at all . The team b elieved  this to be l argely because the motors were not or iginally designed for a vacuum application. ) 

AMT TVAC Test 2, Failure at - 35°C 

The Flight AMT w as rebuilt with new flight motors and returned to the TVAC facility in June of 2019. Using  the newly employed duty cycles at ambient and hot temperature ranges, the team felt confident that the  issue was resolved using the operational change. The ambient and hot functional tests went without  issue .  The hardware then was tested at the cold extreme of -35°C. After a 20 -minute continuous operation, the  motor current spiked— indicative of a short in the windings due to overtemperature— and the test was  aborted. The failure at the second protoflight TVAC test was completely unexpected and gave more insight