480v 3 phase to 480/277v 3 phase

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SFMaintenance

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I'm working on a project where i'm trying to figure out the best way to heaters on a plastic injection press. The press I'm working was wired for 480v 4 wire, the power supply at my facility is 480 3 wire. The press has heaters wired for 480v and 277v. When trying to size the transformer to switch the 480v from 3 wire to 4 wire do I just figure in the amps for just the 277v heaters or do I need to figure in the 480v heaters too. The 277v heaters I need 600 amps and if I add in the 480v heaters it would be 1300amps.
 
SFMaintenance said:
... The press has heaters wired for 480v and 277v. When trying to size the transformer to switch the 480v from 3 wire to 4 wire do I just figure in the amps for just the 277v heaters or do I need to figure in the 480v heaters too. ...
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Can you wire for two separate feeds? One 480D for the 480 heaters and 480/277Y for the 277V heaters. That would be uncommon but possible.

SFMaintenance said:
... When trying to size the transformer to switch the 480v from 3 wire to 4 wire do I just figure in the amps for just the 277v heaters or do I need to figure in the 480v heaters too. The 277v heaters I need 600 amps ....
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Tell us some more about the 277V heaters. It is highly likely they will be connected as 480/277Y. Is this 480/277Y at 200A? Or 480/277Y at 600A. Don't be adding the three phase current to come up with a number.

SFMaintenance said:
... if I add in the 480v heaters it would be 1300amps.
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The 480 heaters will be connected 480D. What exactly is the 3phase load?
 
This:

jumper said:
kVA is kVA at any voltage.
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Are you saying that your incoming power is delta-supplied, and there is no grounded conductor?

And that the machine requires a neutral no matter how you wire it?

If your 277 loads are perfectly (well, close to) equal, could you wire them as if there was a neutral, and leave that point floating?



Added: I agree that, if the loads that must have a neutral are or can be wired separately, then you only need size the transformer for that portion of the load.
 
LarryFine said:
This:




Are you saying that your incoming power is delta-supplied, and there is no grounded conductor?

And that the machine requires a neutral no matter how you wire it?

If your 277 loads are perfectly (well, close to) equal, could you wire them as if there was a neutral, and leave that point floating?
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I am envisioning something that has say six elements, 3 480 volt, and 3 277 volt, and that these elements are not all on at one time. Even if they were, how would you get 277 volts across a single 277v element with a 480-volt Delta power supply?

From the OP's description, I took it to mean he does not have a neutral, thus no ability to provide 277 volts.

SFmaintenance, posting a picture of the name plate will help us to help you answer your questions. A few pictures of the press itself would be neat as well, it is not everyday that I see something that (apparently) requires 1300 amps of 480 volt power.
 
LarryFine said:
By connecting the three in a wye configuration.
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:thumbsup:
As long as:
1. The neutral point is left floating, since there is nothing to reference it to directly, and
2. The loads on the three legs must draw very close to identical current and each matched set of heaters must be switched on and off at the same time. Otherwise the voltage on some elements will be higher or lower than 277.
 
GoldDigger said:
:thumbsup:
As long as:
1. The neutral point is left floating, since there is nothing to reference it to directly, and
2. The loads on the three legs must draw very close to identical current and each matched set of heaters must be switched on and off at the same time. Otherwise the voltage on some elements will be higher or lower than 277.
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If you have a situation where only two of the three are in the circuit - those two are in series and each will see 240 volts if they are equal resistance.

SFMaintenance said:
All the 277v heaters added up will draw 600amps
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But how are they connected and controlled? If always in groups of three balanced loads you can do as mentioned above and connect in wye. If there are times when there isn't balance you need a real neutral from a separately derived system or an autotransformer derived neutral (though it may have more complications to use such).
 
LarryFine said:
By connecting the three in a wye configuration.
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perhaps I am failing to understand a basic tenant of electricity here, but if the heating elements are in a wye configuration, connected to a 480 volt Delta supply, with no neutral connected at the center point, how does one get 277 volts across any one element? As Kwired wrote, from what I am seeing, the best you could do is operate two in series and have 240 volts across two elements as they would be supplied by what amounts to 480 volt single phase. Or am I way off?

Discounting all of that for a moment, if the OP needs 480y/277 volts 1300A secondary, and has a 480 volt Delta primary, what size Transformer would he need?
I came up with 1 MVA, though this seems to me to be excessively large just to have a neutral point and 277 volts on the secondary side.
 
kwired said:
If you have a situation where only two of the three are in the circuit - those two are in series and each will see 240 volts if they are equal resistance.
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True. I was thinking of the more complex and less likely scenario where you have groups of heaters with unequal wattages and you turn on one from the high current group and one on a different phase from a low current group. The voltage divider from 480 down is no longer a 50-50 split and one heater could see more than 277.
 
JFletcher,

I'm not the person who should be answering. In short it has to do with phase angles and that magical square root of three number. I'll provide a link that explains it much better but it does require some trig knowledge. And as Larry made clear this solution only works if all three heaters are always on-line at the same time.

https://www.electricalpereview.com/square-root-three-3-electrical/
 
JFletcher said:
perhaps I am failing to understand a basic tenant of electricity here, but if the heating elements are in a wye configuration, connected to a 480 volt Delta supply, with no neutral connected at the center point, how does one get 277 volts across any one element?
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If you supply three identical 277v loads from a 480/277v supply, with the center point of the loads connected to the supply neutral, there would be zero neutral current. Opening the neutral would have no effect, and you would read 0v between the supply neutral and the load center point.

It's actually the same as a perfectly-balanced 120/240v 1ph system. If you have a pair of perfectly balanced 120v loads wired to a 120/240v supply, there is zero neutral current, and you could disconnect the neutral and still measure 0v between the supply neutral and the load center point.

Opening the neutral would make no change in either case. Remember, now, we're talking theoretically, but if the loads are truly equal, the theory works in reality. If the loads are imbalanced, the center-point voltage will vary from zero. Most balanced loads like this are close enough to work in reality.
 
The heaters are hooked up with one line going to power the other line going to a neutral. The press is set up for 480v 4 wire my building has 480 3 wire. I dont have the room to change the heaters over to 240, that would requaire double the relays and fuse(no room in the panel boxes for that. I know i can use a transformer to turn the 480 delta to 480/277y i'm just trying to figure out what size of transformer I need. Is it better to power the 480 heaters off the main power and use the transformer to power just the 277v heater? or do I run all the heaters (480 and 277) off the same transformer how do I size for that?
 
SFMaintenance said:
The heaters are hooked up with one line going to power the other line going to a neutral. The press is set up for 480v 4 wire my building has 480 3 wire. I dont have the room to change the heaters over to 240, that would requaire double the relays and fuse(no room in the panel boxes for that. I know i can use a transformer to turn the 480 delta to 480/277y i'm just trying to figure out what size of transformer I need. Is it better to power the 480 heaters off the main power and use the transformer to power just the 277v heater? or do I run all the heaters (480 and 277) off the same transformer how do I size for that?
Click to expand...

I would run the 480V heaters off the 480V supply and just size the transformer for the 277V loads.
 
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