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9/22/2017 12:11:25 AM
Posted: 11/13/2001 9:01:02 AM EDT
Hey All, Thanks for looking! Well, I have a 3 phase power box I am preparing to tap and run a (used and just purchased) dry transformer off of. I am wondering what type of 3 phase it is, and if it is compatible with the transformer I have. I am reading the bars from left to right: L1 L2 L3 I I I I I I I I I Here are the values of the lines: L1 to L2 = 492v L2 to L3 = 504v L1 to L3 = 490v L1 to GR = 361v L2 to GR = 124v L3 to GR = 428v The main box says it's 480v 400A ************** Now, my transformer says it's "Delta" The Hi v says 480 with the Lo v's at 240 and 120. (on the ID label) It is a 75kva box. It allows for primary taps at: 432v 444v 456v 468v 480v 492v 504v ******** So my questions are... Is this standard 3 phase "480" or one of these other exotic (or antiquated) "High leg" deals? Is this a suitable transformer for this power source? The transformer is currently tapped equally on all three legs at the 480v position. Should it be left there or should I tap each leg at the closest value to each leg's actual voltage... *OR*... should I tap all 3 coils to the *same* value that is closest to the average of all three legs (492v)? Some of my machines are 480v. Can I run them directly from the input legs? I don't see any provision to pull 480v from the transformer if I have the taps at 492 although it may be there. Yes, I am going to enlist a professional electrician, but I've been collecting all the parts, boxes, and wires, and I'd like to see if I'm on the right track. I will be doing all the conduit and mounting of the breakers and transformer myself to save some dough and just have the electrician do the final hookups. How am I doing? Many Thanks!!!
Link Posted: 11/13/2001 9:08:57 AM EDT
Can't help you. I was hoping it was a simple question like: Brown Orange Yellow or Black Red Blue. Sorry!
Link Posted: 11/13/2001 9:27:12 AM EDT
The leg-to-leg voltages are dependent on what loads are present on each one, and will probably vary at different times of the day, as different loads are switched on and off throughout the neighborhood. I woudn't attempt to adjust each leg tap to compensate for it, but would probably choose the 492 volt tap. In a system with all 3 legs sharing equal loads, the leg-to-neutral voltages will also be the same. Obviously, yours isn't. From time to time, the power company comes through the neighborhood and adjusts this, by moving some single-phase loads over to other less-loaded phases, so again, you can expect these voltages to change, too. It certainly sounds like this is an appropriate transformer. I trust that the electrician will check the transformer's output voltages before connecting any shop equipment or other loads, just to make sure.
Link Posted: 11/13/2001 9:36:36 AM EDT
If it is a delta wound secondary, you will have a 'high leg' on the 120/240 side of the transformer. One leg of the secondary will be 277V to ground. On the primary, I also would go with the 492V tap. If you want 120V to ground on all legs of the secondary side, you will need to buy a Wye type transformer, then you will have 120V to ground on any leg, and leg to leg will be 208V. this is definitely a job for a good electrician, experienced in wiring transformers.
Link Posted: 11/13/2001 10:20:16 AM EDT
Pour water on the ground under the box, and insert screwdriver. Sorry I can't help. If it was 3 phase 220 I could tell you anything you want to know.
Link Posted: 11/13/2001 10:50:17 AM EDT
[Last Edit: 11/13/2001 10:44:18 AM EDT by sr15]
Originally Posted By SameShot: L1 L2 L3 I I I I I I I I I Here are the values of the lines: L1 to L2 = 492v L2 to L3 = 504v L1 to L3 = 490v
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These values seem a little high for 480 V service, but I guess it's possible.
L1 to GR = 361v L2 to GR = 124v L3 to GR = 428v
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These values seem way off. There is no reason there should be that much voltage unbalance.
The main box says it's 480v 400A ************** Now, my transformer says it's "Delta" The Hi v says 480 with the Lo v's at 240 and 120. (on the ID label) It is a 75kva box. It allows for primary taps at: 432v 444v 456v 468v 480v 492v 504v ******** So my questions are... Is this standard 3 phase "480" or one of these other exotic (or antiquated) "High leg" deals?
View Quote
Seems standard to me. It only becomes "exotic" if you connect it in an unconventional way. You could connect it in a way to give you a high leg by using the delta winding as the secondary and grounding one of the legs.
Is this a suitable transformer for this power source?
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Seems like it, but to fully answer that question I'd need to know about the load you intend to hang off of the secondary side.
The transformer is currently tapped equally on all three legs at the 480v position. Should it be left there or should I tap each leg at the closest value to each leg's actual voltage... *OR*... should I tap all 3 coils to the *same* value that is closest to the average of all three legs (492v)?
View Quote
Leave it on 480V. Once everything is hooked up and running a full load, check the voltages at your load(s). If the voltage at the load is too low, move the feeders to a lower voltage tap. If the voltage at the load is too high, move the feeders to a higher voltage tap.
Some of my machines are 480v. Can I run them directly from the input legs? I don't see any provision to pull 480v from the transformer if I have the taps at 492 although it may be there.
View Quote
Other loads should be run directly from the breaker pannel, but yes you could connect the 480V loads to the same terminals as the 480V feeders coming from the panel.
Link Posted: 11/13/2001 4:03:28 PM EDT
[Last Edit: 11/13/2001 4:10:47 PM EDT by Halfcocked]
Piece of corn. What your incoming power source is, is a 480 volt delta. It is not grounded therefore the values to ground are meaningless. There is no reference to ground. Draw an equal lateral triangle (all three sides the same length) with each side 480 inches (cm, or mm whatever)long. The corners represent the phase (phase to phase voltages). The output of your transformer will be the same thing with the lengths of the triangle sides only 240 inches long. Now half way across one of the legs of your triangle place a dot. The distance from the phase (pointy part of the triangle) opposite the dot, to the dot, will measure 208 inches. This is called the wild leg. That dot point is usually grounded. This gives you 2 phases that will be 120 volts to ground, (half way across a 240 leg is 120, right) and one that is 208, (the point opposite the ground) and 3 phases that are all 240 apart from each other (the points of the triangle). Piece of corn right? I would stick with the taps at 480. The input voltages are within 5% of nominal and so should your outputs. Some times however there is a little voltage drop compensation built into these transformers and they may add a little bit of voltage. Check it after it's wired to see. If your rotation is backwards you can just swap any 2 legs to reverse it. Edited to add that yes you can add three phase 480 volt equipment directly to the main panel provided the total draw does not exceed 80% of the 400 amps. There may be other limitations depending on the size and characteristics of any large motors you may be starting. Motors usually take up to 5 times there full load running amps (FL amps) to start (Locked Rotor LR amps). E-mail me if you need more help.
Link Posted: 11/13/2001 7:25:20 PM EDT
I'm an industrial electrician and deal with 480V 3P most every day. I'm having a little difficulty visualizing what you are intending to do. First, whether your 480V 3P incoming power is industrial or residential, it should be a delta or triangle shaped circuit. Each leg of the triangle represents some kind of load, and each point L1 L2 L3 is a voltage source. Your L1-L2, L2-L3,L3-L1 voltages look pretty much normal for a completely unloaded circuit. There should be no ground connections on L1 L2 L3, BUT - EVERYTHING wired up as a load should have 4 wires - 3 phase leads and an equally heavy green ground lead connected between every machine frame and the box's earth ground terminal. It's suicidal to neglect the frame grounds at these voltage and amperage levels. I don't understand the transformer. Are you trying to step down 480V to 240V for some things? If you have 480V coming in and most of your machines are 480V you don't need the transformer. Where I work we occasionally build and repair industrial gear that runs on 240V, 380V, 575V instead of the usual 480V. For these jobs we hook up various transformers mounted on carts for mobility. Otherwise, all the rest of the dozens of machine tools and welders are hard-wired or plugged into the many 480V circuits in the building. On any given machine, check the rotation direction immediately on all drive motors, pumps, fans, etc. If any newly-connected motor runs backwards, disconnect the power source to the machine, and reverse any 2 of the motor leads. DO NOT reverse your 480V distribution leads in the box or elsewhere upstream of other motors. :-) You adjust the phase of each motor to the source. Don't neglect frame grounds and proper fuses or breakers. Occasionally at work someone will hose down a 480V outlet box. The explosion and fireball are very impressive, and a new outlet and a handfull of new 100-amp fuses is expensive. :-) A clarification on the transformer's role might help.....
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