Dead cim

[548swimmer]

Team 548 fried a cim! Has anyone else fried a cim?

We checked it with a 12v power supply, and it actually is a fried motor, not just an electrical problem. The head ref at a^2 was quite impressed, especially since it was a drive cim, paired with another in a 12:1 p-80 dual from banebot.

Do any of you have ideas as to what may have caused the cim to die?

Thanks.

[sanddrag]

I've encountered plenty of burned out CIMs. Not uncommon at all, if it's geared for speed, and used for lengthy periods of time, or under heavy load. I've even had burned up CIMs become shorted internally to the point where they'll blow Victors.

[548swimmer]

Originally Posted by sanddrag

I've encountered plenty of burned out CIMs. Not uncommon at all, if it's geared for speed, and used for lengthy periods of time, or under heavy load.

That would make sense. This was a new CIM, used only on the competition robot, and paired with another to drive one side of the robot. It was never "abused," which is why it doesn't make sense.

[M.Wong]

From what I have heard from electrical and motor engineers, CIMS are actually fairly frail.

However, from a FIRST standpoint, the only time I have seen CIMS fail were due to mechanical accidents.

#1: Small fragments of steel fell into the CIM through the mounting holes. Fragments of a bolt that was removed (ie haphazardly cut off) fell down into the CIM.

#2: During the 2010 build season, one of our members drove our robot up the ramp at an odd angle and cracked the CIM at the connection between the casing and the metal face plate. This was due to a design error with the CIMs facing outward (though the design called for them to be internal)

[Dale(294engr]]

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!

[Al Skierkiewicz]

Dale et al,
The CIM stall current is 133 amps but more importantly to our use the CIM is not ventilated. Prolonged high currents cause internal heating that damages the windings insulation and eventually leads to electrical failure of one or more windings. The bearings on a CIM are not designed for significant side loading and for many teams, this is an issue. If you are not using the CIM in a transmission for which it was designed, you need tor provide additional bearing support to the shaft end or friction will raise your load currents and eventual bearing damage will result. Remember that the CIM was designed for a special use in which the loads were intermittent and the speed relatively constant with no side loads on the bearing. Although CIMs have failed in our use, most were due to rough handling as described above. Long term heavy use, i.e. demo, can cause excessive heat. Keep an eye on motor heating and give the motors a chance to cool off before using again.

[Racer26]

1075 was experiencing some of our first CIM issues at the end of Waterloo regional this weekend.

I think this must have been a progressive failure, as they were still *working*, just not working *well*. We're using them in a 2+2 Swerve configuration, I'm unsure of the gearing, but its reasonable.

They were used all weekend at FLR, and then all weekend at WAT, plus whatever testing we did back in build. Everything seems free, so we dont think anything is binding. Its the first time we've seen the CIMs failing in 8 years of FRC.

[Grim Tuesday]

We burnt out all four CIMS in our 2010 robot during driver practice. One the alumni was acting the aggresor with another one of our robots, and, though I wasn't there, they somehow managed to start a small fire in the robot.

[joek]

we have been re-using our cims for 5 years now, and have not had a single one burn out. we have more of a problem with burning out controllers, as we have 3 or 4 bad victors. that said, we almost always have them run through a 12.75:1 gearbox. the only exception iknow of was 2009, where we used 2 cims to run a pair of stacked 6" 07'-08' KOP wheels; they were actually geared up slightly.

in any case, you want to be careful when working around motors or electronics, neither like metal shavings in them. we always tape off our RS and fisher-price motors when working around them, and vacuum out the area when done.