Power for Oven

Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
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.
 
I'm unclear on what the conditions are under which you measured 34A. Was it with 12 elements in a delta configuration (4 in parallel on each pair of legs), and it was 34A on each leg (which is reasonable to assume even if you only measured one leg)?

If so, and assuming all of the demand is from the elements, then the total current on each parallel set of 4 elements was 34A / sqrt(3) = 20A. So each element was drawing 5A at 480V (assumed).

If you instead apply 240V, each element will draw 2.5A; and with 12 of them in parallel the total draw will be 30A. The heating power will be 1/4 of what you had at 480V, as both the current and the voltage will be halved. You could use 4 times as many elements to get the same heating power, or get different elements. Or, if the elements only run for 12 seconds at a time with a duty cycle under 25%, you could run them 4 times as long, if the different heating profile works for the process involved.

Cheers, Wayne

P.S. The 19.4 ohm measurement doesn't really fit anywhere in the above. It is close to what you would get if you measured 4 elements in parallel with each other but out of circuit (not as part of the delta, because in that case you'd also have the other path around the other two legs of the delta).
 
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wwhitney said:
I'm unclear on what the conditions are under which you measured 34A. Was it with 12 elements in a delta configuration (4 in parallel on each pair of legs), and it was 34A on each leg (which is reasonable to assume even if you only measured one leg)?

If so, and assuming all of the demand is from the elements, then the total current on each parallel set of 4 elements was 34A / sqrt(3) = 20A. So each element was drawing 5A at 480V (assumed).

If you instead apply 240V, each element will draw 2.5A; and with 12 of them in parallel the total draw will be 30A. The heating power will be 1/4 of what you had at 480V, as both the current and the voltage will be halved. You could use 4 times as many elements to get the same heating power, or get different elements.

Cheers, Wayne
Click to expand...
I failed to say that I measured the current on all three legs and each was drawing 34A. The oven, as far as I could see, only has the elements, nothing else, so the current was all from the elements.
It would very difficult to add more elements.
 
Little Bill said:
I failed to say that I measured the current on all three legs and each was drawing 34A. The oven, as far as I could see, only has the elements, nothing else, so the current was all from the elements.
Click to expand...
OK, so another way to state the point of my last post was that if each element is getting 480V single phase, and you instead supply each element with 240V single phase, you'll get 1/4 the heating power, as the voltage and the current are both halved. Your 34A measurement says the heating power @ 480V is 34*480*sqrt(3) = 28.3 kVA. Divide that by 4, and by 240V to get a current at 240V of 29A.

Cheers, Wayne
 
Bill, I would try approaching it from a slightly different angle. I would expect either a nameplate on the equipment that tells you it KW or KVA rating. Barring that though, I would think you could find a part number for the elements either by finding a manual on the equipment or a part number etched on the element. That should give you the KW of the individual element which we could then use to provide the correct power to them.
 
Strathead said:
Bill, I would try approaching it from a slightly different angle. I would expect either a nameplate on the equipment that tells you it KW or KVA rating. Barring that though, I would think you could find a part number for the elements either by finding a manual on the equipment or a part number etched on the element. That should give you the KW of the individual element which we could then use to provide the correct power to them.
Click to expand...
No nameplate could be found on the oven. I have an element in hand. It has no markings, or tags on it. I can't swear the elements in the oven are the same as the spares. I can only say they look alike, and the guy says they are the same.
 
wwhitney said:
OK, so another way to state the point of my last post was that if each element is getting 480V single phase, and you instead supply each element with 240V single phase, you'll get 1/4 the heating power, as the voltage and the current are both halved. Your 34A measurement says the heating power @ 480V is 34*480*sqrt(3) = 28.3 kVA. Divide that by 4, and by 240V to get a current at 240V of 29A.

Cheers, Wayne
Click to expand...
So do you think a transformer (single phase) is needed, and if so, would you uses a B&B, or step up, transformer?
or
An RPC & transformer?
 
Little Bill said:
So do you think a transformer (single phase) is needed, and if so, would you uses a B&B, or step up, transformer?
Click to expand...
Outside my area of expertise, but I would think the first question would be what the process requires, does it need to be such intense heat for 12 seconds, or would 1/4 the heating power for longer (say 48 seconds) also work?

Also, do you have 120A @ 240V available? If you do, and if the higher heating power for only 12 seconds is required, then you could either use a single phase transformer or try to find replacement elements rated for 240V @ 10A. Either way you'd rewire the elements from delta to all being in parallel. The wiring to the elements may also need upsizing, as the current would go from 34A to 60A (with a 480V transformer) or 120A (with new elements).

Cheers, Wayne
 
wwhitney said:
Outside my area of expertise, but I would think the first question would be what the process requires, does it need to be such intense heat for 12 seconds, or would 1/4 the heating power for longer (say 48 seconds) also work?
Click to expand...
Something else I just thought of. There is a conveyor, powered separately, that runs the items through the oven. The oven is to dry paint (not powder coated). The conveyor is ran/powered through a VFD, so I think the conveyor could be slowed down to allow extra time for the paint to dry.
The 12 secs that the oven is on is directly related to the speed the conveyor runs the items through the oven. So maybe increasing the timer to more than 12 secs, and slowing the conveyor might work.

I really wish the customer would get an engineer involved, but they just kind of threw it on me to figure all this out.
 
Are you sure that 480V is being applied across each element, and with 4 elements in parallel across each L-L voltage?
It might be that each group of 4 elements consists of either: 1.) a series connection of two pairs, each pair having two elements in parallel, or 2.) a parallel connection of two pairs , each pair having two elements in series. Then each element gets 240V, and your measurement of 19 ohms on an element would be more consistent with your measured current. The resistance will get higher as the element heats up as Dennis mentioned.

If the elements are designed for 240V, then perhaps the oven was made to accommodate a 240V 3-phase delta feed using an alternate internal wiring of the elements at the factory.
 
@synchro has it right. You need to figure out the wiring configuration and the voltage applied across each element before you move further.

You measured the complete oven operating at 480V and 34A. This works out to 28.3kW (34 * 480 * 1.732)

Based on the measured system power, the number of elements (12) and the measured cold resistance, my _guess_ is that the elements are rated for 2400W each at 240V with a hot resistance of 24 ohms.

You cannot simply connect this oven to 240V single phase and expect it to work properly. I am guessing that the elements are in a 2 series 2 parallel delta arrangement; 4 of the elements will get half voltage and only produce 1/4 power, 8 of the elements will get 1/4 voltage and only produce 1/16 rated power. You will get both less output and uneven heating.

You might be able to reconnect the elements to all be in parallel, and then the oven will run at full power directly from 240V single phase. Of course this will draw about 120A at 240V single phase. So the temperature control hardware will also need significant modification.

-Jonathan
 
The equipment that will be used is currently being used by the current owner. He contracts to do the job for my customer. The man is retiring and my customer is buying the equipment and moving it to a new building to do the work in house. I say this to say I haven't had ample opportunity to check everything. So looks like I need to arrange to spend some time there so I don't interfere with his work.

I will schedule a time to check everything that I can think of and report back.
 
Little Bill said:
I will schedule a time to check everything that I can think of and report back.
Click to expand...

Sounds good.

I suggest checking how the heating elements are connected and if you have easy access to reconfigure that connection (could go from bad, cables jump from element to element hidden away to medium, all elements connections are exposed in one location where you can rewire the connectors to great, all elements have terminal blocks in a controls area so you can easily reconfigure the connections).

I'd also check for how the temperature control system operates. Do you have a single 3 phase 480V contactor or do individual elements have their own temperature control, or something in between such as single phase contactors powering banks of elements . Do you have solid state relays switching power to the elements, etc.

Finally I'd check for the voltage and current ratings of any disconnect hardware.


-Jonathan
 
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