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Discussion Starter · #1 ·
I have machine that has both a 3ph 240v plug and a standard 1ph 120v plug that need to be simultaneously plugged in for powering it. I would like to tap into the 3ph connection that goes to the motor to get the 120v so that there is a single plug for the machine if possible. I am wondering whether or not it is ok to directly tap into one of the 3ph legs to get the 120v (and have both the 3ph motor running and the 120v drawing at the same time - 120v line would be pulling less than 3 amps)? Would it be ok to have a current imbalance between the 3 hot legs of the 3ph supply while the motor is running? (due to drawing some current for the 120v from one of legs) If not, do you guys know of a small 3ph to 1ph converter that I can jump from the 3ph supply, so that we can take an equal amount of current out of all three legs for the 120v without disturbing the 3ph connection to the motor? Any help or advise would be much appreciated!
 

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Not all 240V 3 phase services are 4 wire, meaning you might not have 120V from A or C to neutral. If you don't know for sure, the safe bet is to install a 240-120V control power transformer on the machine and power it from 2 legs of the 240V. 3A at 120V would make it a 500VA CPT (because they typically jump from 350VA to 500VA), fairly inexpensive.
 

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Discussion Starter · #4 ·
Re: 3ph to 1ph

Thanks for the reply tesla. I completely agree but unfortunately I am not able to send the machine back to the manufacturer for this issue, and need to find a way to combine the power sources. Do you know of any converters that can simply take a 3ph input and output 1ph? Obviously, this seems counter intuitive but is really the only solution I can think of where I can get both supplies from the 3ph while maintaining a properly breakered connection.
 

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Discussion Starter · #5 ·
Re: 3ph to 1ph

JRaef, thanks for the reply. This is a 4 wire 240v. If I run two of the hot legs to a converter for the 120v, do I have to worry about a current imbalance to the motor since I will be simultaneously drawing power from the original 3ph supply for the motor PLUS the new 1ph 120v supply? Basically I am wondering if tapping into two of the leads to the motor and leaving the third untouched (to get my new 120v source) while it is running will create an issue?
 

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If it is a 4 wire 240/120 volt system it has a wild leg....one leg has 208 to the neutral and the other 2 have 120 to the neutral.

I don't think the imbalance is a real issue.
 

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How much the 120 volt load it needs ??

Typpically you can use the A-N or C-N connection but verify the voltage and make sure you don't use the wild phase ( 208 volts line to nettrual )
 

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There is no "converter" needed, because as you have been told, in a 4 wire 240V 3 phase system, phases A and B to Neutral are already 120V. That's what that configuration is for.

Typically there is a limit of 5% of the loads being single phase. That limit applies to the entire service however, not your one machine. Don't sweat over it.
 

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If you don't have a neutral at the equipment (just the 3 phase conductors), then Telsa gave a good solution in installing a control transformer to get your 120V circuit the machine requires. If I had to go that route, I'd want to do this outside the machine instead of rewiring anything inside the machine.

If you don't have a neutral at the equipment, it seems it might be better if you could fish a neutral through the existing conduit system. I gather you looked at that and it's not feasible so that's why your asking here.

I suggest you don't make any equipment modifications w/out written manufacturer approval.
 
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Personally, I wouldn't hesitate to add the control transformer to the machine and get rid of the 120 V cord. Any competent electrician familiar with commercial and industrial equipment could do it with his eyes closed, tied up, under water, with the lights off.
 

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Personally, I wouldn't hesitate to add the control transformer to the machine and get rid of the 120 V cord. Any competent electrician familiar with commercial and industrial equipment could do it with his eyes closed, tied up, under water, with the lights off.
I agree with your statement about the competent electrician, but ..... the competent attorney trumps the electrician.

Once you modify equipment, the manufacturer can void any warranty, you lose any listing by a testing company. If anything happened involving that machine, whether the modifications made by the electrician are responsible or not, the company that hired him will be involved and expected to settle out of court.

I have to wonder if this machine has been tested by a listing company (being that it has 2 cords seems very odd to me).
 

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I agree with your statement about the competent electrician, but ..... the competent attorney trumps the electrician.

Once you modify equipment, the manufacturer can void any warranty, you lose any listing by a testing company. If anything happened involving that machine, whether the modifications made by the electrician are responsible or not, the company that hired him will be involved and expected to settle out of court.

I have to wonder if this machine has been tested by a listing company (being that it has 2 cords seems very odd to me).
PERSONALLY, I wouldn't hesitate to add the control transformer... I don't run and hide over every potential issue that could lead to a lawyer being involved. Hell, literally everything I do everyday, from driving to work, to installing cans in a residence, to adding a transformer to a machine, leaves open the possibility of potential legal action. Brother gots to eat.
 

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PERSONALLY, I wouldn't hesitate to add the control transformer... I don't run and hide over every potential issue that could lead to a lawyer being involved. Hell, literally everything I do everyday, from driving to work, to installing cans in a residence, to adding a transformer to a machine, leaves open the possibility of potential legal action. Brother gots to eat.
Have you ever been the target of a frivolous lawsuit?

I know you've read about them. They exists, and the targets are usually people that did something they considered no big deal, but ..... it was enough against the rules that the bus backed up towards them.

I used to be just like you, but then ...... I had an up front and personal experience with this type of lawsuit.
 

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Two cords is SO odd for a 240 VAC machine.

A control power transformer ought to have been the factory's original enginerring,

No ?

One instantly suspects that the load kicks back counter EMF that screws with the controls logic.

Or that this device NEVER was properly engineered.

&&&&

Shipping it back for re-manufacture is scarcely necessary.

However, one DOES want the factory to assent to the use of a plain vanilla controls transformer.

&&&&&&

Why was it not installed from the start?

Don't your suspicions run towards the NEMA 240 cord being THREE wire -- no neutral conductor ?

It's the DETAILS not spelled out in the OP that spell out mine field. :eek:
 

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I've never seen a four-wire NEMA cord cap.

I suspect that the design was -- and is -- for 208 VAC -- but will tolerate 240 -- but ONLY for the primary load -- not the controls package.
 

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Have you ever been the target of a frivolous lawsuit?

I know you've read about them. They exists, and the targets are usually people that did something they considered no big deal, but ..... it was enough against the rules that the bus backed up towards them.

I used to be just like you, but then ...... I had an up front and personal experience with this type of lawsuit.
Well you've sold me. I'm out of the electrical game. I'm going to be a dirt farmer.
 

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