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Unread 27-03-2011, 23:43
Dale(294engr]'s Avatar
Dale(294engr] Dale(294engr] is online now
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FRC #0294 (Beach Cities Robotics)
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Re: Dead cim

Been there done that ..have photo's!!

Worst case scenario for a CIM is 125A locked rotor which cant be avoided

Every (CIM) rotor starts at zero RPM therefore for some finite amount of time
( couple to several hundred milliseconds depending) until the motor comes up to speed (increasing rpm draws less current than stall at increasing torque until max eff point reached due to armature inductive reactance increasing impedance i.e. AC resistance kicking in (ref: back emf)

1: more gearing =less stress (lower ~7fps highest torque) less time at 125A
2: less gearing = highest stress Especially start from stopped w/max throttle
2a: better use shift on fly Xmsn always start in low then shift after initial inertia
3: throttle demand at each start ea full throttle FWD or REV draws 125A / CIM

Analysis:
125A heats carbon/copper brushes & commutator.. enough heat followed by 5k rpm eventually spins some of the pole wiring loose.. putting more stress on those remaining (if geared poorly i.e. aggressively hi 15fps with lots of full throttle starts or pushing another bot.. can sustain 125A for close to a second limited by the 40A breaker which opens at 50A eventually but will pass 125A initial current for a long time!! ref: see breaker curves time Vs current

next start causes a lot more current to be drawn at declining torque as poles are lost

when enough pole connections fall off the motor burns up.
To do: then disassemble CIM to verify this visually, it is usually pretty obvious

The CIM is a VERY robust motor, high heat absorbing mass

However this is because it is designed for a relatively low RPM (5K)
at low duty cycle (mobile home load leveler)
so has no convective heat removal "fan" & holes typ of those of low mass high RPM (20K) which have potential to fail quickly if unable to remove heat quickly

CIM's high mass large area commutator heat slowly and recover well between matches with only conductive heating through the mounting to chassis.. design mounts with this in mind.. massy mounting=desirable

.. CIMs = not at all delicate!! in fact quite robust if not abused too much

Still for a 2 min 15sec match they hold up wonderfully well ! best we've got!
 


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