Power for Oven

Little Bill said:
I may have missed some of this as the wires were hard to follow.
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As you say, you may have something wrong, but your description is consistent with my guess.

IMHO you have 12 elements, each rated for 240V and 2400W.

You could reconnect all the elements to parallel, and get the same performance. This would require 120A at 240V, and the control circuit would need to handle the 120A.

You mentioned that the elements come on for 12 seconds and then shut off. How long are they off for? You might be able to trade current for longer on time, if the application will permit.
 
From that description, you have 2 elements in series on each L-L leg.
To connect the elements in parallel as Jon mentioned, you could do the following:
1. Of the jumpers you mentioned, remove the three that connect two elements together (i.e, element #1 to #2, #3 to #4, #5 to #6).
2. Leave the connections to L1, L2, and L3 in place
3. Wire the now open side of each element to the corresponding line shown below:
#1 to L2
#2 to L1
#3 to L3
#4 to L2
#5 to L1
#6 to L3
Wire these directly to the line inputs so that the current carried does not have to flow through the jumpers that remain.
Make sure that there is adequate ampacity in place for the 2x line current in this configuration.
 
Many of European ranges are setup so it can work on single or three phase.

Before further discussion, I think a photo of the schematic would be useful.
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3 Phase Power Systems And How To Know That You Have Wired Them Correctly

The 3-phase systems are called 3-phase delta and 3-phase wye and this document will explain how to connect your single phase heaters to such a system, and how to check that it’s done correctly.
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winnie said:
As you say, you may have something wrong, but your description is consistent with my guess.

IMHO you have 12 elements, each rated for 240V and 2400W.

You could reconnect all the elements to parallel, and get the same performance. This would require 120A at 240V, and the control circuit would need to handle the 120A.

You mentioned that the elements come on for 12 seconds and then shut off. How long are they off for? You might be able to trade current for longer on time, if the application will permit.
Click to expand...
I believe they are on 12 and off 12.
 
synchro said:
From that description, you have 2 elements in series on each L-L leg.
To connect the elements in parallel as Jon mentioned, you could do the following:
1. Of the jumpers you mentioned, remove the three that connect two elements together (i.e, element #1 to #2, #3 to #4, #5 to #6).
2. Leave the connections to L1, L2, and L3 in place
3. Wire the now open side of each element to the corresponding line shown below:
#1 to L2
#2 to L1
#3 to L3
#4 to L2
#5 to L1
#6 to L3
Wire these directly to the line inputs so that the current carried does not have to flow through the jumpers that remain.
Make sure that there is adequate ampacity in place for the 2x line current in this configuration.
Click to expand...
Unless I use a RPC, I won't have but two lines/legs. I'm trying to determine if I need the RPC to feed the transformer, or wire the oven in single phase with lower voltage. Or single phase and step up the voltage. New building voltage will be 240V.
 
Little Bill said:
Unless I use a RPC, I won't have but two lines/legs. I'm trying to determine if I need the RPC to feed the transformer, or wire the oven in single phase with lower voltage. Or single phase and step up the voltage. New building voltage will be 240V.
Click to expand...

The elements are each are "2-wire" and from your descriptions are made for 240V. Whether the 240V comes from a 240V 3-phase delta connection or 240V single phase will not affect their operation. And so from an electrical standpoint, you should be able to remove all jumpers and connect all the elements in parallel across a 240V single phase supply. The ampacity to handle the current is the main concern. Also, having wiring that is appropriate for the temperatures involved is required. Of course, there may be other practical considerations without knowing other details.

Since the elements themselves are single-phase 2-wire devices, the cost and electrical losses for providing 480V 3-phase to supply the present series/delta configuration by using a transformer and RPC doesn't seem very attractive.
 
synchro said:
The elements are each are "2-wire" and from your descriptions are made for 240V. Whether the 240V comes from a 240V 3-phase delta connection or 240V single phase will not affect their operation. And so from an electrical standpoint, you should be able to remove all jumpers and connect all the elements in parallel across a 240V single phase supply. The ampacity to handle the current is the main concern. Also, having wiring that is appropriate for the temperatures involved is required. Of course, there may be other practical considerations without knowing other details.

Since the elements themselves are single-phase 2-wire devices, the cost and electrical losses for providing 480V 3-phase to supply the present series/delta configuration by using a transformer and RPC doesn't seem very attractive.
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Thank you!
 
Little Bill said:
I'm not sure what the wattage is on the elements so I don't know what the current would be when wired in parallel.
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For any configuration where the elements are getting 240V each, the total power usage will be the same. So if your OP means that in the present configuration for 480V 3-phase, the 3 line currents are 34A, then the total power usage is 480V*34A*sqrt(3) = 28.3 kVA. (In the OP you say 16 kVA, but I think you forgot the sqrt(3) factor.)

That means if you rewire the elements to all be in parallel and connected to 240V single phase, the total current will be 28.3 kVA / 240V = 118A.

Cheers, Wayne
 
wwhitney said:
For any configuration where the elements are getting 240V each, the total power usage will be the same. So if your OP means that in the present configuration for 480V 3-phase, the 3 line currents are 34A, then the total power usage is 480V*34A*sqrt(3) = 28.3 kVA. (In the OP you say 16 kVA, but I think you forgot the sqrt(3) factor.)

That means if you rewire the elements to all be in parallel and connected to 240V single phase, the total current will be 28.3 kVA / 240V = 118A.

Cheers, Wayne
Click to expand...
I probably did leave off the sqrt(3).
I understand the total power will be the same. However, the way it's wired now, there would be less power through each element vs wired in parallel where more power would be used per element.
I guess I'm asking if the elements would need to be changed to a different/higher wattage?
 
ptonsparky said:
Am I missing something? I’ve always used the resistance of an element to determine amperage and wattage.
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Changing from 480V to 240V and changing from a Delta/series connection to a single phase parallel.
Since power through each element was lower with the Delta, my concern was the power through the parallel single phase would be higher through each element. Which led me to think the wattage on the element might should be higher than what's there now.
 
Little Bill said:
I understand the total power will be the same. However, the way it's wired now, there would be less power through each element vs wired in parallel where more power would be used per element.
Click to expand...
Based on the numbers so far, if an element has 240V across it, it will have about 10A going through it (so presumably the warm resistance is about 24 ohms, even though you measured a cold resistance of 19.3 ohms). That's true whether it's one element across 240V single phase, or one of two in series across 480V line to line. The power across each element is about 2400W in either configuration, no difference.

The question is whether you have provide ~120A @ 240V, and whether you can replace the internal conductors for the increased ampacity as necessary.

Cheers, Wayne
 
Little Bill said:
This relates to my previous thread(s) concerning 3ph equipment. I have more info to work with.
The oven currently runs on 480V (from a transformer). It is fed with 3ph but the elements are just single ph.

There was some extra/spare elements there, so I measured the resistance on them. They measured19.4 ohms on the elements.
The current draw that I measured on the working elements was 34A. There are 12 elements in the oven.

The oven is being moved to a new building that will only have 120V/240V. They will not have 3ph.
What I want to do is just use the 240V single ph and rewire the elements.

I came up with around 16Kva for the power as it runs now. But that doesn't exactly match the resistance I measured on the spare elements.
480Vsq/19.4ohms = 11.9Kva
I was only able to measure the current but not the voltage as the guy was leaving, so the voltage may not be exactly 480V since its from a 240V to 480V transformer.

I suppose my questions are:
Do I have to take the 12 elements and consider them parallel circuits, or is the 19ohms total R?
If this requires nodal analyzes, it's been too long for me to remember how to do that.

Or to jump ahead and just ask what my current draw would be at the new building using 240V instead of 480V?
The elements currently only come on for 12 secs, then off. So if the voltage is reduced to 240V, would that give enough heat if the time were increased?

Keep in mind, I would only be using 2 legs instead of 3 legs since the new building is single ph.

Ask me any info that I may have left out.
Click to expand...

Resistance same, not depend on voltage. Just measure resistance You want know hot resistance value to use, switch on for some time, then switch off, measure Resistance immediate, that about hot Resistance use
 
Little Bill said:
However, the way it's wired now, there would be less power through each element vs wired in parallel where more power would be used per element.
Click to expand...

The above is true if you keep the supply voltage the same. But you will change both the connection and the supply voltage.

When you change from 2 in series at 480V to 2 in parallel at 240V, the voltage across each element stays the same and the power stays the same.
synchro said:
The elements are each are "2-wire" and from your descriptions are made for 240V. Whether the 240V comes from a 240V 3-phase delta connection or 240V single phase will not affect their operation.
Click to expand...
This is an important point to emphasize. The load isn't a 3 phase load, it is a bunch of single phase loads arranged in a three phase combination.
synchro said:
Also, having wiring that is appropriate for the temperatures involved is required. Of course, there may be other practical considerations without knowing other details.
Click to expand...
Changing the wiring might be enough trouble that a transformer to get 480V is worth it. Current handling limits of the control circuitry might be another reason for 480V. But in general I agree it is better to adjust this load to match the 240V supply then to adjust the supply to match the current load configuration.
 
Since I/we are still in the planning stages. I have requested a quote for an RPC that is large enough to handle the transformer and also run a 10 HP air compressor. They already have a transformer. However it's wired as a step up and looks to be a step down. It's been running that way for a few years.
 
Always good to look at options when you are making plans.

You might also look at the cost of bringing in 480V three phase service, and then using the transformer as a step down.

IMHO you will find that it makes much more sense to convert the oven to single phase rather than try to synthesize 3 phase for it. It _might_ make sense to keep the oven wires for 480V single phase and use a transformer to get the 480V.

A step down transformer reverse driven as step up is yet another kettle of fish to deal with.

-Jonathan
 
winnie said:
Always good to look at options when you are making plans.

You might also look at the cost of bringing in 480V three phase service, and then using the transformer as a step down.

IMHO you will find that it makes much more sense to convert the oven to single phase rather than try to synthesize 3 phase for it. It _might_ make sense to keep the oven wires for 480V single phase and use a transformer to get the 480V.

A step down transformer reverse driven as step up is yet another kettle of fish to deal with.

-Jonathan
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I got the quote back from the RPC dealer. He says I would need two RPCs, one for the air compressor, and one for the 480V transformer. The quote was north of $12K and that didn't include tax and S/H.

I like the idea of leaving the oven wiring and step up the voltage from 240V to 480V. I would think buying a new transformer would be cheaper than a 2nd RPC.
 
Little Bill said:
I like the idea of leaving the oven wiring and step up the voltage from 240V to 480V. I would think buying a new transformer would be cheaper than a 2nd RPC.
Click to expand...

You would still need to change some of the oven wiring, just less.

If you step up to 480V single phase you would need to reconnect the elements from delta to single phase. But you could do this by moving one internal jumper and adding one external jumper.

IMHO the decision between complete rewiring to 240V single phase vs getting a transformer and partial rewiring to 480V single phase would be the specifics of the control system.

My suggested decision tree:
1) If the existing controls would work with 240V single phase, I'd go that route.
2) If the existing controls would work with 480V single phase but not 240V then I'd use the single phase step up.
3) If I had to replace the existing controls in any case I'd go the 240V route, unless... 4) If replacing the controls was going to be more expensive than the RPC (giving consideration to the additional electricity costs of the RPC) only then would I consider the RPC route.

Jonathan
 
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