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Author Topic: When a servo motor smokes ...  (Read 4856 times)

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Offline rcaffin

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When a servo motor smokes ...
« on: October 24, 2011, 05:27:51 AM »
Came back from lunch to a really bad smell of an electrical fault, on a 'stationary' machine.
Noticed Y axis vibrating, or humming.
Tapped jog to try to stop vibrations - whee, off to one end and thump! (and eStop automatically).
Motor stinking hot and leaking very hot ???grease???

Testing:
Disconnect motor from servo amp and from ball-screw.
Motor turned by hand OK, smoothly.
Motor still spins when low voltage applied.
Optical encoder has power but signal lines are static (not changing) when motor turns. (Vampire tap inserted in line.)
Signals very obvious on other encoders.

Diagnosis:
Faulty optical encoder on the motor giving conflicting signals to servo amp, which was oscillating trying to hold position.

What amazes me:
That the servo amp survived the experience.
That the thermal and electrical current limits did not trip.
That the motor survived (maybe I caught it in time). (ElectroCraft Servo S240 series)
That the guys who assembled this apparently put the trim panels on around the axis with small button screws and an airgun! (Biggest hassle was getting these screws out to get access! Tiny hex keys ...)

Hopefully, a new optical encoder (HEDS-5540A13) wil restore sunshine and joy.

Cheers


Re: When a servo motor smokes ...
« Reply #1 on: October 24, 2011, 11:39:27 AM »
Current limit set too high by the sound of it!!.
If the servo drive is capable of 30 amps continuous say, the drive is OK supplying that current but if the motor is stalled it could be well out of the continuous torque rating of the motor. the motor is not capable of producing BEMF to reduce the current.
Nosmo.

Offline BR549

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Re: When a servo motor smokes ...
« Reply #2 on: October 24, 2011, 12:02:32 PM »
IF the servo was smoking hot and leaking grease I would replace the motor.

(;-) TP

Offline rcaffin

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Re: When a servo motor smokes ...
« Reply #3 on: October 24, 2011, 04:48:39 PM »
Current limit set too high by the sound of it!!.
If the servo drive is capable of 30 amps continuous say, the drive is OK supplying that current but if the motor is stalled it could be well out of the continuous torque rating of the motor. the motor is not capable of producing BEMF to reduce the current.
Fortunately not relevant *in this case*. The motor was not stalled, it was oscillating. The servo amp was pumping some serious AC power into the motor, which was getting hot due to high frequency eddy currents in the steel.  I doubt the current levels were all that high in fact.

Quote
IF the servo was smoking hot and leaking grease I would replace the motor.
Yeah, serious thought was given to that. Perhaps 'smoking' is a bit of an exaggeration - I would say the motor had reached 50 - 60 C when i shut it down. Just a shade too hot to touch. I think the smell was due to some grease being near the brushes and getting vaporised.
Yes, this does bring into question the state of the brushes and the commutator. Unknown at present, but spinning freely.

I think (he says very hopefully), that the fault had only just happened, so that no great amount of damage had been done to the motor. For the present I will see how it goes with the existing motor.

All very educational. Shows the value of human supervision at all times! Hit eStop when one leaves for any significant length of time is the obvious answer, except that one always loses precise registration that way. Hum ...

Cheers


Re: When a servo motor smokes ...
« Reply #4 on: October 24, 2011, 05:34:07 PM »
With a Brushed  DC motor the current and hence heat is produced in the armature, so that by the time the heat has radiated to the stator the temperature of the armature is at a very much higher level.
If it is the ElectroCraft 240 DC brushed model, the spec shows max armature temperature to be 155°C!!
Nosmo.


 

Offline rcaffin

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Re: When a servo motor smokes ...
« Reply #5 on: October 24, 2011, 05:48:44 PM »
With a Brushed  DC motor the current and hence heat is produced in the armature, so that by the time the heat has radiated to the stator the temperature of the armature is at a very much higher level.
If it is the ElectroCraft 240 DC brushed model, the spec shows max armature temperature to be 155°C!!

Interesting, and my thanks for that info.
Would you have a data sheet for the motor with that sort of info maybe? I haven't been able to find much data on the web.
And can you suggest any suppliers as well?

Cheers
Re: When a servo motor smokes ...
« Reply #6 on: October 24, 2011, 07:30:25 PM »
Data sheet.
N.

Offline rcaffin

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Re: When a servo motor smokes ...
« Reply #7 on: October 24, 2011, 07:41:10 PM »
Data sheet.

Thank you indeed.
Now, just to stretch the friendship just a little, would you by any chnace have the data for the 0240-04-022 part number? That's the actual motor I have here.

Cheers
Re: When a servo motor smokes ...
« Reply #8 on: October 24, 2011, 08:51:42 PM »
Sorry, that all I have.
1993 and 1997 catalogue do not show it.
N.

Offline rcaffin

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Re: When a servo motor smokes ...
« Reply #9 on: October 24, 2011, 09:58:14 PM »
No worries - just a small variant anyhow, with 8 mm shafts for us in metric land. The data you supplied will be enough - thanks!

cheers
Roger