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VFD power cord question

Dean Center

Dean Center

Joined
May 4, 2010
Messages
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Location
Bozeman, MT
I would appreciate some help from those of you who know electrical stuff.

I'm replacing the deceased Delta VFD on one of our high school's 3520B lathes. It's a single phase input, 3 phase output situation, as I understand it. The 240v power supply cord has 4 wires, red-black-white-green which I understand are 2 active/live/electricity carrying wires, a neutral wire, and a ground wire (in that order, as far as I know). The Delta VFD had R-S-T connections and I believe the white wire was connected to one of them, like a 3 phase wiring. With the single phase 240 input, my understanding is that only the 2 active/live/electricity carrying wires are needed, and they are supposed to be attached to the L and N terminals on the new VFD.

I might add that the plug required to match the school's system is much larger than what is on my 3520A lathe, with 4 prongs, 2 of which have lips/extensions on them, and we had to have an electrician replace the plug on one of the other 3520B lathes to match the wall outlet. He'll be coming back to do the same with this power cord.

Am I correct in assuming that the white is a neutral wire that doesn't carry any current? What do I do with it?

Thanks for the assistance.
 
White is neutral, Red and Black are Hot, Green is Ground. The way it works (basically) is there's 120v between Red-White and 120v between Black-White. Red-Black gets you 240.

If you're wiring for 240 single-phase (what my 3520B uses) then you use Red and Black (and Green/Ground).
If you're wiring for 240 2-phase (or split phase, whatever - me not an electrician, and have forgot a lot of theory/terminology I learned in school), then you wire up with Red, Black, and White (and Ground).

It sounds like the wall plug is 2-phase (4 prongs with a twist lock - it has a name, but me no remember). So basically for your application the White wire would go unused.

In my shop, I've wired 240 2-phase outlets in the ceiling (4-prong twist-lock), and have a 4-wire cable drop to the lathe. On the back of the lathe, I have wired a 240v single phase plug (3 prongs, Red/Black/Ground) plus a couple standard 120v outlets (using Red-White on one and Black-white on the other). So I only have one wire to plug in but have some convenient outlets for lights, etc. Have the same setup on my bigger 240v bandsaw, too.
 
My factory incoming feed has black, white and green wires. Looking at Doc Green's website wiring diagram is the same as mine. Unless being a school, there is a need for an extra grounding connection, the white wire being not used as a hot would be an extra...Tips and Tutorials section a thread on VFD replacement is a picture of the feed wired into the new drive, that feed has only three wires. Mine and the one in the forum are both 3520Bs.
 
Dave Landers said:
It sounds like the wall plug is 2-phase (4 prongs with a twist lock - it has a name, but me no remember). So basically for your application the White wire would go unused.
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I am also not an electrician but I do recall running into 2 types of 220v outlets, a 3 prong and a 4 prong. I thought the only difference was whether one was grounded or not with the 3 prongs an older non grounded style. I don’t think it is anything about two-phase power.
 
Brian Nowak-Thompson said:
I am also not an electrician but I do recall running into 2 types of 220v outlets, a 3 prong and a 4 prong. I thought the only difference was whether one was grounded or not with the 3 prongs an older non grounded style. I don’t think it is anything about two-phase power.
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You always have to have a ground. The three prong would be two hots and a ground. The four prong would be two hots, a neutral and a ground.
 
I was saying my 3520B, the one John wired his new VFD in the Tips and Tutorial thread and the wiring diagram in Doc Greens web site for a 3520b all have black and white hot wires with a green to ground. If the feed wire coming into the old Delta VFD is the original, it probably does not have a red wire, and if it does, I would be curious as to where on the old delta it was connected to, don't think it is the OEM wiring.
 
Dean Center said:
I would appreciate some help from those of you who know electrical stuff.

I'm replacing the deceased Delta VFD on one of our high school's 3520B lathes. It's a single phase input, 3 phase output situation, as I understand it. The 240v power supply cord has 4 wires, red-black-white-green which I understand are 2 active/live/electricity carrying wires, a neutral wire, and a ground wire (in that order, as far as I know). The Delta VFD had R-S-T connections and I believe the white wire was connected to one of them, like a 3 phase wiring. With the single phase 240 input, my understanding is that only the 2 active/live/electricity carrying wires are needed, and they are supposed to be attached to the L and N terminals on the new VFD.

I might add that the plug required to match the school's system is much larger than what is on my 3520A lathe, with 4 prongs, 2 of which have lips/extensions on them, and we had to have an electrician replace the plug on one of the other 3520B lathes to match the wall outlet. He'll be coming back to do the same with this power cord.

Am I correct in assuming that the white is a neutral wire that doesn't carry any current? What do I do with it?

Thanks for the assistance.
Click to expand...
You do NOT connect the white wire if the shop power is 240 volts single-phase. (it's not really relevant to this discussion, but the white wire is neutral and is a current-carrying conductor in a 120-volt circuit). You will need to consult the operating manual, but the R, S, and T inputs are probably for connecting to three-phase. shop power. The manual will tell you which two of those input terminals to use when the input power is 240 volt single-phase.

As to the plug sizes being different, look at the number that is on the plug. If the two plugs are twist-lock and look alike, but one is slightly larger then I'll bet that one is for a 20 Amp branch circuit and the other is for a 30 Amp branch circuit. BTW, a four-prong twist-lock connector is called a generator connector.

As Rusty said, you always must have a ground. The ground is only connected to the metal frame/case of a machine, not to any live circuit. Its only purpose is fire and shock safety by tripping the breaker if any live circuit contacts the frame.
 
Bill Boehme said:
You do NOT connect the white wire if the shop power is 240 volts single-phase. (it's not really relevant to this discussion, but the white wire is neutral and is a current-carrying conductor in a 120-volt circuit). You will need to consult the operating manual, but the R, S, and T inputs are probably for connecting to three-phase. shop power. The manual will tell you which two of those input terminals to use when the input power is 240 volt single-phase.

As to the plug sizes being different, look at the number that is on the plug. If the two plugs are twist-lock and look alike, but one is slightly larger then I'll bet that one is for a 20 Amp branch circuit and the other is for a 30 Amp branch circuit. BTW, a four-prong twist-lock connector is called a generator connector.

As Rusty said, you always must have a ground. The ground is only connected to the metal frame/case of a machine, not to any live circuit. Its only purpose is fire and shock safety by tripping the breaker if any live circuit contacts the frame.
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Bill, my lathe came with a three wire feed to the VFD, one is black, one is white and one is green
 
Marvin Jones said:
Bill, my lathe came with a three wire feed to the VFD, one is black, one is white and one is green
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So that's ok. Normal cable you'd buy would be colored like that. For example, if you buy 12/2 With Ground cable, you'd get a black, white and uninsulated ground (12 gauge). 12/3 would have Black, White, Green probably.

The electrons don't really know about the colors :) Colors are just a convention. If that's on a 240 outlet, then just know that the white is not really neutral but is another "hot" and you shouldn't lick it. (Disclaimer: you probably shouldn't be licking any of the wires to any of your machines)
 
So, being an old man, I began to doubt what or how I actually connected my 3520B to power 16 years ago. I thought I put a plug on the pigtail that came with the lathe so I dug up the operator's manuel, in case my mind was wondering. It states to connect the white and black to power and the green to ground. I was not trying to confuse anyone but he did say his lathe was a 3520B...John Coppola shows a great picture of his pigtail wired in with the same black, white and green wires as well as the 3520B wiring diagram on the Doc Green web site. Leads me to believe if he has a four wire feed to the VFD, it is not Oem. That is all I am saying...
 
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In my old shop I had a subpanel. When it was inspected, I was told to separate the ground (bare or green) from the neutral (white) until the main box. I just added a new terminal block to the box for the bare or ground and passed.

I understand that this is to prevent stray currents in the ground and neutral. Only one ground for the house, prevents some damage from faults and lightning.

I am not an electrician or an electrical engineer, so this is my understanding. (I associate with lawyers to much hence the disclaimer).

Stu
 
Marvin, if you will re-read my post, I was responding to Dean where he stated that he had a 4-wire cord that was connected to 240-volt single-phase power. My answer to his situation was correct. The point that I was making is that neutral is not a part of a 240-volt single-phase branch circuit. Your situation is different because you have a three-wire cord. It would be a good idea in your situation if you marked the white wire with either red wire-marking tape or red wire-marking paint.
 
I agree with the marking the white to red, I'm sure Powermatic could have purchased red, black and green conductor pigtails. Powermatic also shipped the big 24" swing lathe with the same black, white and green wires. Both Powermatic manuals state a four wire feed is only needed when 3 phase power is at the lathe. Sorry to add unneeded length to a simple question.....
 
Thanks for all the comments. If those of you with 3520B lathes all have 3 wire power cords, then the school must have replaced the wires with the 4 wire cords.

As I thought I mentioned in the first post, the old VFD had places for the red, black and white wires, but the new VFD only has connections for the hot wires. I know to connect the red and black wires there.

The question I have is simply, what do I do with the white wire? Do I wire it to something to keep it out of trouble, or do I cap it with a wire nut, or something else?
 
Dean Center said:
The question I have is simply, what do I do with the white wire? Do I wire it to something to keep it out of trouble, or do I cap it with a wire nut, or something else?
Click to expand...
As I previously stated, if you use the four-wire power cord with black, red, white, and green wires then the white (neutral) wire isn't used because it is only used for 120-volt circuits and the VFD is a 240-volt device. So the best thing to do is to cap and stow the white wire or just snip the white wire flush where it exits the jacket (the jacket is the outer black rubber insulation). You can cap and stow two different ways. The simplest is a small wire nut and then wrap with rubber or stretch plastic electrical tape. The neater way if you are a nerdy engineer is to fold back the wire about 3/4" and then put a heat shrink sleeve over it.
 
Just want to 2nd what Rusty posted. Make sure you know what voltage is across the 2 wires you plan to connect to the new VFD. Referring to the Delta VFD-S Instruction Sheet, the R/L1, S/L2 and T/L3 terminal connections are for 3 Phase input. You could have 2 or 3 different voltages depending on your electrical distribution system. Maybe the white wire, assuming it was connected to T/L3, wasn't connected at the plug or socket side.
 
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