Hi folks, I bought a new chinese plasma cutter for $404. I received it promptly enough. Upon inspecting the 220V twist lock power plug I discovered it's two power pins were reversed from what my garage is wired for. The garage is wired with HomeDepo sockets. Which one is wrong?
I promptly replaced the cutter's power plug to match my garage, hooked up compressed air.
The cutter works like a charm. It cut through eighth inch steel like.....butter! I experiment some more later. Looks like a good buy though.
Neither one. There's no particular order to the hot pins. The ground has to be on the correct pin, but either of the hot wires can go on either of the hot pins. It's AC, there's no polarity to be observed.
Probably neither you nor the manufacturer were "wrong". As long as they had the two hot lines hooked up to the right two pins, it really doesn't matter if you like to put the red wire on the left or right. The electricity on a single-phase 240V device really doesn't care whether the Conga Line is going clockwise or counter, as long as you have the hots on the hot pins on both sides.
It matters a bit on three phase stuff, but you know to check the rotation the first time. And again after you've loaned out a piece of equipment or had it serviced.
And electricity is colorblind, it doesn't care what color the wire is it's going through. Where the screw ups come in is when the next human to work on it doesn't realize the colors aren't necessarily what they seem to be. (This is why you use ALWAYS put colored tape on the wires at both ends if the white wire isn't a neutral or the green a ground - clue in the next guy. Or yourself, 5 years later.)
Where it matters if they put the neutral (if equipped) or safety ground on the wrong pins and create a bolted fault, that goof can cause all heck to break loose when you power it up for the first time. Especially if the breaker feeding the load has any problems, like it's old and sticky and doesn't necessarily trip right away...
Semi-related. I have an old 3-phase "induction" motor on an ancient Radiarc cut-off saw that I would like to be able to use occasionally. The previous owner told me that his electrician had wired it wrong (all 3 wires coming out of the motor are tar-like black coated) and burned up the motor. I suspect that it may have been wired wrong, but the electrician just decided not to waste any more time on it... so my question is: assuming the motor isn't fried internally; can I use an ohm meter (or other tool) to find out which wires are the "hot" ones and which is supposed to be the neutral? There are black, red, and white wires spliced onto the motor and wired into the starter switch... but I'm guessing the splices into the black motor wires are wrong. Before I cut them apart, I would like to know what I should be looking for. Thanks, David
"Gary Coffman" wrote in message news: snipped-for-privacy@4ax.com...
3 phase motors don't have Neutral conections to them.
3 phase conductors, and an equipment ground conductor, are all that are required. The equipment grounding conductor should have no or very low resistance to the motor frame. The phase conductors should appear to be not connected to the frame when testing with a normal ohm meter
What I should have said was that the two non-ground pins, which have unique physical shapes were reversed. This prevented the plug from entering the socket.
There is no neutral with 3 ph. All three conductors are hot, with each shifted in phase by 120 degrees with respect to the adjacent phase. Their order only determines the direction of rotation. If the motor doesn't rotate the direction you want, then just reverse any two of the wires (doesn't matter which ones) and it'll turn in the other direction.
Note, you have to have 3 ph power in your shop. The motor won't run if fed by ordinary residential 1 ph power. It'll just hum and smoke. Sounds like that's the real issue here.
Now it is *possible* to *make* 3 ph power from 1 ph power by using a rotary converter, a Variable Frequency Drive (VFD), or at reduced power by using a static phase converter. You have to choose *one* of those options if you want to run a 3 phase motor from 1 ph residential power.
If someone already tried to run the motor directly from 1 ph power, it may indeed be burned up. But if it isn't, you can buy/build a rotary converter, VFD, or static phase converter to make it run.
Thanks for the info. I have 3-phase power and the machine was here when I bought the place... so hopefully being hooked up to single phase was not an issue. There are about a dozen other 3-phase machines here that all work fine, so somebody must have known what they were doing at some point. Although, I can't figure out why the previous owner said the motor was burned out from being wired wrong if there is no "wrong" way to hook it up. It seemed odd that the "starter" switch box has two lines with some sort of fusible link on the contacts and one line that just connects directly when you press start; so I was afraid it did matter which line went to the unfused one. The motor turns freely by hand and I have 2.2 ohms between any pair of wires; so I'll just have to find a plug and try it for myself... POOF! LOL Thanks again, David
"Gary Coffman" wrote in message news: snipped-for-privacy@4ax.com...
Ok, one of the plugs is a L5-20 and the other is a L6-20. The only physical difference between the two is that the two non-grounded pins are reversed. The electrical standard difference is that the L5-20 is only to be used in
120 VAC circuits while the L6-20 is intended to only be used in 240 VAC circuits. The difference in the pin arrangements is to keep you from plugging a 120 volt machine into a 240 volt circuit, or vice versa.
In NEMA nomenclature, the leading L means the plug is a twist-lock. The next number is the voltage rating, with a 5 being 120 VAC and a 6 being
240 VAC (7 is 277 VAC, 8 is 480 VAC, and 9 is 600 VAC). The number following the dash is the amperage rating, in this case 20 amps.
The large doglegged pin is the safety ground. For the L5-20, the smaller of the other two pins should have a W stamped next to it. That's the neutral, the other pin is the hot. For the L6-20, one pin will be stamped X and the other Y. Both of these are hots, and there is no particular order as to which hot wire goes on which pin.
Well ... there is a wrong way -- connect only two of the leads, and you are trying to run it from single-phase, even though that is two of the three lines needed to make three-phase power.
They were just saving a little money. The current through that one un-fused one has to pass through one or both of the other two lines, which *do* have the fuses, and cooking off any one of those drops power from all three lines. (Actually -- is is passing through heaters, which under normal circumstances don't get hot enough to cook off the links, which can be permanently fusible ones, or ones which can be reset at your convenience, and the current through those links is what maintains power to the coil of the contactor (relay), so any one opening drops power to everything.
I hope that it works for you. Make sure that all three are getting power, or it will just cook of the protectors.
That didn't work. I also bypassed the starter switch and just wired the 3 motor leads to the plug and tried it; but it just makes about 10 - 15 degrees of rotation and stops... then when you unplug it, the motor is "stuck" in that spot until you break it free by hand. Then it rotates freely again. There's power to all three leads going in, but I imagine something is burned out or melted inside? I can pull the motor and take it in to get checked for little or nothing. Unfortunately it's kind of an oddball 1.5hp motor with a 3/4" diameter shaft; which doesn't appear to be locally available. (NEMA 204 frame?) It's not a big deal, I just hate having stuff in the shop that doesn't work when you need it. Thanks to everybody for the info, David
"DoN. Nichols" wrote in message news:c5sq7s$ps8$ snipped-for-privacy@fuego.d-and-d.com...
Hmm ... it sounds as though one phase is not getting power -- or perhaps one phase has a shorted turn or two. Can you use a clamp-on ammeter to see whether all three leads are drawing the same current? A shorted turn or two on one winding would cause it to draw a lot more current than the other two. Your resistance checks eliminate the alternative option -- an open circuit.
You might just set it up with some 220V lamps in series with the three windings, and see whether they all three glow at the same brightness (assuming that all three are the same wattage -- say perhaps
250W per lamp.) This will allow you to run it long enough to take some measurements.
There are two probable wirings inside the motor for a single-voltage motor:
1) Y (wye) -- the three windings join at a point which is not brought the outside.
2) Delta -- the three windings are connected so one end of each winding is connected to the other end of the next winding.
I'm not sure what happens if you reverse one of the three windings in either of those connections. I suspect that it will produce symptoms similar to what you see -- but since the inner end of each winding is joined to the other two inside the motor (for a Wye), or the joined leads are brought out with a single wire (for a Delta), you should not have the option to get that one wrong.
If it is a dual-voltage motor, there are quite a few ways to wire it wrong. For the most common, dual-voltage Wye connection, there are nine leads brought out. Three are the outer end of an initial Wye for the lower voltage. Both ends of the other three windings are brought out, so you can either wire them into a second Wye connection, and hook that in parallel with the first one for low-voltage operation, or hook each winding in series with the corresponding winding of the initial Wye, to get the higher voltage.
For dual-voltage delta motors, I would guess that all six windings are brought out at both ends, for a total of twelve wires.
Both of the dual-voltage options give you plenty of chances to connect things wrong. The wires should all be identified with numbers, and you should have plenty of information on a data plate on the motor, or perhaps inside the connection box.
If the motor is that hard to find, you should be able to get it re-wound by a good motor shop for a lot less than a new motor of a specialized format.
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