In deference to Eric et al…
Please do not use the 12v unlimited current from a tab to case to clear (burn open) the RS-555 armature short!!
Justification is The theory of a metal flake causing the short is flawed.
I have examined lots of 550 shorted case failures at multi Regionals & championships, none had a metal flake. All had armature winding short to the metal armature at the turn around (sharpest point). None had evidence brush holder to case debris. Post test cleared the brush holder every time. Every short traced to contact to case via rotor, thru end bushings bearings.
A metal flake is highly unlikely to cause a short to CASE! wire is enamel insulated
so such a short would need to be in the brush holder tab to case in which case it would easily shake out!
The HARD short of the armature winding is another story.
It is caused by mfr relying on epoxy insulation + enamel on wire, rather than customary brown fiber covering sharp iron ends so tight wraps have a buffer.
Done to maximize copper window in small armature winding areas + save $.
Check out CIM armature brown fiber insulation use to effectively eliminate this failure mechanism.
As motors become smaller mfrs seek alternate methods like omitting fiber barrier.
Enamel wire tightly wound over sharp cornered steel laminated in 550 armatures has extensive history of high short to case fail rate. high as 50-70% new out of boxes at Regionals where I was RI in 2011 going back to 2010
Burn open short clearing is not only DANGEROUS but also BAD fix. :ahh: :mad:
12v ~800A: blows apart offending single wire short armature to case ~debris?
This disables one armature pole reducing motor power plus asymmetrical excitation. Not something you ever want to put back into a competition robot!
Also it may set up a condition of intermittent dynamic to case continuity!
…cause now you have two burnt loose copper ends free to flip around.
In improperly geared CIM’s I’ve observed a similar gradual degradation where too high a gear ratio combined with heavy throttle by driver pushing opponents etc keeps CIM at High current, low torque part of curve too long.
Much self heating results weakening copper wire bond to commutator.
Subsequent High RPM force causes wire to detach, fly off, or high current burnout from concentrated current density in a short work weakened copper (bending back & forth), opening a pole on the armature.
The degraded motor still starts up & spins with remaining poles but at greatly decreased performance and not so smooth anymore! (& lower start-up & running torque due to lost pole)
Hint Perform Destructive Failure Analysis of failed motors as a class exercise to gain better understanding of what happened to apply forward. (destructive = observe closely then unwind the rotor carefully inspection as you go for problem areas (wire may break as you unwind). Take close up photos when you find a ‘smoking gun’, document in daily log pics & text description + hypothesize why/how it happened, then publish results.
Good luck… all teams going with RS-550’s again this year!!
Stock up on pre-tested open spares if you need capability it serves!
The Spare Parts will run out of their limited replacements EARLY!
Teams NOT using their RS-550s please Graciously bring them to regionals
to turn-in to Spare Parts to help your fellow competitors!!
