AC Paper Shedder Motor Failure

[SETEC_Astronomy]

I was using my paper shedder today when it suddenly started humming. My first thought was that it was jammed. I switched the shedder to reverse and still the same noise. To fix the jam I opened up the shedder housing and couldn't find anything wrong. To further investigate I switched it on and noticed the motor throwing sparks from one brush. I pulled the brushes and other than a few very minor scoring marks they're fine. The brushes have a good 3/4 of an inch left. Next step was to measure the windings. Out of five pairs (defining a pair as opposing pads that connect to the brushes) two measure 7 ohms with the rest measuring 16. I have no experience troubleshooting AC motors and any tests to perform or diagnoses would be much appreciated.

Warranty repair is out the window because it's been opened and was previously opened to fix a what appeared to be a faulty on/off switch. Turned out a piece of CD polycarbonate made it's way from the CD cutting slot to the switch mounted on the front of the unit. How it happened I'll never know, the switch is "far" from the CD cutting slot, under a PCB, mounted in a plastic cavity with enclosed ends and a dust slide. Baffling.....

[Ed446]

Could the switch be bad and the motor is trying to run in both directions at the same time?
Edwin

[Externet]

Hi Setec.
I like Edwin's post; not just running in 'both' directions, but any another of the internal switch terminals shorting a winding to the wrong place.
Measure resistance to chassis (and motor shaft), should be open.

Or, just give it back to the China importer for warranty. You already tried to avoid their headache twice in good will.

Miguel

[SETEC_Astronomy]

Good points, thanks.

After some more checking I found that the motor still spins in the proper direction as set by the switch. The switch is in good working order. There isn't continuity between the motor frame and anything else I could probe. The only other parts on the control board other than the on/off switch is two diodes and an LED with resistor. The diodes test fine as does the LED. There's continuity between all windings on the motor. I have to admit AC motors are still a bit of a mystery to me but I'm going to assume that's not a good sign. The working theory is the motor overheated, the enamel melted and shorted the windings. A thermal fuse is ziptied to one coil but it apparently didn't open fast enough.

[Dean Huster]

Shredders are so underpowered, at least the ones you buy at the office supply store are. One that I was given had a lot of power but it ended up stripping the gears. "If it isn't one thing, it's a mother" as another of the board denizens would say.

The school where I taught in Oklahoma City had a big shredder designed to shread quickly. It had a huge input hopper and a massive garbage bag for catching the shreads. Put in a piece of paper and the high-speed cutter had it shreaded in less than half a second. Feed it four sheets at a time it jammed. Two sheets at a time slowed it down considerably. Pointless. The SR-71 is the fastest plane in the world, but it only carries two people and a limited amount of luggage and on the ground leaks fuel faster than an H2 Hummer can consume it.

[Bob Scott]

Brush sparking means excessive current. If the brush sparks excessively with the motor idling (not loaded down physically by shredding stuff) something is definitely wrong. It probably means that an armature winding is shorted, probably to the rotor shaft. I would test the resistance of all the commutator segments to the shaft.

My Dad had an old drill like that. It would blow fuses. I removed the ground connection from the cord's plug. Then it would work, but I'd get a ticklish electrical buzz in my hand ever time I turned it on and touched ground at the same time. It was a grounded metal case type, the cheapest drill my Dad could find in 1965.

[SETEC_Astronomy]

Shredders are so underpowered, at least the ones you buy at the office supply store are.

More accurate you could not be, the motor is about the size of what you find in electric pencil sharpeners.

Interesting about the school shredder. I got a commercial grade shredder from a mortgage company and assumed it could handle anything. I was wrong. Like you I found it could only handle one or two sheets before getting jammed. I was really bummed to find that a single page of newspaper could jam the thing. I imagine this shredder would do nicely http://www.youtube.com/watch?v=9JUBBEhrvfc

Brush sparking means excessive current. If the brush sparks excessively with the motor idling (not loaded down physically by shredding stuff) something is definitely wrong. It probably means that an armature winding is shorted, probably to the rotor shaft. I would test the resistance of all the commutator segments to the shaft

I'll be sure to check that the next chance I get. I think that's the case.