Weirdness in the workshop

December 20, 2013 1:17 AM

Is it possible for a 3 phase motor to switch polarity without changing the input wires anywhere?

One of three machines on a 3 phase input was running in reverse when we turned it on today, after a few weeks of idleness. The other two on the input, and all other 3 phase equipment in the workshop is working fine. Is there any thing, other than a human switching wires, that could cause the switch in polarity?

posted by Kerasia to Grab Bag (14 answers total)

It has been a while, but as I understand it three phase motors often have a capacitor permanently connected to a second winding. If this capacitor becomes faulty or the winding or associated wiring is open circuit, then the motor might go backwards?
posted by Just this guy, y'know at 2:36 AM on December 20, 2013

Permanent split motors (where a capacitor is permanently wired in) are only used on single phase motors to generate enough phase shift to cause rotation. Because the phase motors windings are wired 120 electrical degrees apart, they always have a rotating magnetic field and don't need the assistance of a cap.

Because the rotation of the motor is directly tied to the winding spacing and the rotation of the phases of the service it's hooked up to, it can't switch direction on its own. You need a foward/reversing starter or to manually switch the wires. Maybe the phasing was switched somewhere upstream?

Of course, if it wasn't being powered, it is possible it can rotate backwards - fan wheels can do this if exposed to a backdraft, and when they start it places a huge amount of stress on the shaft.
posted by Naib at 2:49 AM on December 20, 2013 [1 favorite]

Response by poster: Of course, if it wasn't being powered, it is possible it can rotate backwards - fan wheels can do this if exposed to a backdraft, and when they start it places a huge amount of stress on the shaft.

Thanks for your info, Naib. Would/could this apply to our 3 phase motor? It started, turned, and responded to the wood in a nasty way until turned off, assessed to show it was turning in reverse, and checked in all relevant ways. It was only returned to normal by reversing the wires ourselves.
posted by Kerasia at 3:11 AM on December 20, 2013

Response by poster: If anyone reading this is certain that "no, reversing the wires is the only way to reverse polarity" then please speak up now so I assess what to do next. Thanks.
posted by Kerasia at 3:17 AM on December 20, 2013

Best answer: If it really is a 3-phase motor, as opposed to a single-phase motor wired between two phases or between one phase and ground, then the direction it's going to run is fully determined by the order in which the phases energise its windings.

So if you've got a 3-phase motor that one day decides to run backwards, then either (a) two of the phases appearing at the windings were swapped (is there a mysterious hard-to-access reversing switch somewhere on the machine that nobody's noticed before?) or (b) it's not operating as a 3-phase motor any more because the windings are connected in a Y rather than a delta and one of them has failed open.

If (b) I would expect to see reduced power output and increased vibration as well.
posted by flabdablet at 3:51 AM on December 20, 2013 [1 favorite]

Here's a YouTube video that shows what happens when a 3-phase motor is deprived of one phase: Single-phasing a 3-phase induction motor.

Note especially that a motor in this condition has no preferred direction of rotation - it will run equally badly either way. And unless it's given a mechanical start in one direction or the other (though it doesn't take much!) it will just sit there and buzz.
posted by flabdablet at 4:09 AM on December 20, 2013 [1 favorite]

Best answer: flabdablet has it right that changing two leads is the only way to change the rotation of a non reversing starter (short of some sort of solid state controller running a DC motor off of 3 phase). If the unit has some sort of feeder like say a planer or moulding cutter it may have a manual jog switch to allow a jam to be backed out in a non-obvious place. Depending on the control structure a short in that switch could cause your symptom.

I'd bet though if there are more than a couple people working in this space this is human caused. Maybe the cord end was damaged and changed if the machine plugs in. Or someone was vacuuming out the starter and knocked a couple wires out. Has there been any changes or maintenance on the electrical system of the shop at all in the last couple months? Someone changing light bulbs or something? Often what seems like a minor change during maintenance can have these serious effects later. Maybe someone added a plug circuit and moved the breaker serving the effected machine. Did you test out a new piece of equipment lately or have something on loan? Maybe someone borrowed the power from the idle machine and then hooked it up backwards afterwards. I've seen this a few times with stuff like floor polishers where the maintenance contractor didn't have the right plug so they just borrowed one from another machine.
posted by Mitheral at 7:38 AM on December 20, 2013

Best answer: My metal lathe motor has a reverse on it that, if you hadn't been the one to take the lathe apart to clean it and move it bit by bit to its new home, you'd probably not be aware of and it looks more like a piece of decorative trim than a switch.
posted by Kid Charlemagne at 8:01 AM on December 20, 2013

Response by poster: This is our personal workshop in the middle of whoop-whoop.

So there are two possible causes: 1) a stranger has, for some reason, come into the workshop, undone the switch box and swapped the wires; or, 2) there is a sekret swytch that was bumped. I vote #2.

Thanks all!
posted by Kerasia at 12:32 PM on December 20, 2013

Kid Charlemagne, would you care to share a photo of your decorative reversing switch?
posted by flabdablet at 5:43 AM on December 21, 2013

Response by poster: Kid Charlemagne, would you care to share a photo of your decorative reversing switch?

Yes please! It would be handy to know what some of them look like.
posted by Kerasia at 9:01 PM on December 21, 2013

OK, it's a little less subtle than I remember but it is just a little tab of metal with no clear indication as to what it's does (it rotates some contacts forward about an eighth of a rotation). If you took those two little machine screws off you could just pull the switch part right off, leaving no indication besides a tiny slot where a little metal tab goes into the motor to rotate the collar with the contacts.
posted by Kid Charlemagne at 11:47 AM on December 25, 2013

Looks to me as if it wouldn't take much more than a careless bump or a dropped tool to flip that thing from up to down, if the little machine screw was loose.
posted by flabdablet at 1:12 PM on December 25, 2013

Yep, though it's kind of in the back and hidden, which is why I thought of it. I can also imagine incorporating such a switch into part of the motor housing - twist it one way and tighten the screw and it goes clockwise, twist it the other way and tighten the screw and it's counter clock wise. Leave it a little loose and be surprised!
posted by Kid Charlemagne at 10:47 PM on December 26, 2013