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Discussion Starter · #1 ·
Hi,

I'm converting an older, 3-phase deck oven to single phase, and am having a problem getting the contactor, tripped by the thermostat, to engage automatically.

A little background, I contacted the manufacturer and have all of the wiring diagrams, etc.... The oven has 4 decks that each operate independently of one another, and have 3 heating elements per deck. The elements were natively wired in delta (208v), but can support 240 volts. There are no motors in the units. The original layout for each deck was basically,
L1->Switch->Element->3-pole contactor(connected to thermostat)->L2.
L1->Switch->Element->3-pole contactor(connected to thermostat)->L3.
L2->Switch->Element->3-pole contactor(connected to thermostat)->L3.

I re-wired by rearranging the wires to the elements so that all three elements are connected to the 240v L1 and L2, single phase.
(3 X) L1->Switch->Element->3-pole contactor(connected to thermostat)->L2

The ground is connected to the cabinet itself, and neutral is not used. All components use 240v.

The contactor is a C25DNF340 by Eaton/Cutler Hammer.

The problem I have is that the contactor is not engaging, unless I push the contactor button in manually. When doing so the ovens heat up fine. Initially I kept the contactor wired as 3-phase, meaning, L1 goes in the top and out the bottom of pole 1, L2 goes in the top and out the bottom of pole 2, etc... I've researched online that I could use a 3 pole for single phase, and to cluster the wires into pole 1, put them in series through pole 2 and 3, and out pole 3. The circuit seems the same to me in this configuration, but regardless, the result is the same that the contactor only engages manually, and not automatically.

As mentioned, there are 4 decks to this oven that all operate independently, and they all have exactly the same issue so I'm confident that the contactor is OK. I have confirmation that the thermostat and switch are working.

Any thoughts on why the contactor is not engaging? It seems as though, perhaps, the contactor is not getting the power it needs to close the circuit. I'll replace it with a single phase model, but from what I've seen online 3 pole contactors should work...

Thanks in advance for any help you can give me.
 

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How did you wire the contactor coil into the circuit? I see in your description the elements but you need to engage the contactor somehow.
An actual wiring diagram would be easier for to interpret.
 

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the problem is in the control wiring between thermostat and contactor, the thermostat, or the contactor coils.
 

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How did you wire the contactor coil into the circuit? I see in your description the elements but you need to engage the contactor somehow.
An actual wiring diagram would be easier for to interpret.
yeppers.

while the power connections have been described, the control circuit has not and that appears to the problem.


The problem I have is that the contactor is not engaging, unless I push the contactor button in manually. When doing so the ovens heat up fine. Initially I kept the contactor wired as 3-phase, meaning, L1 goes in the top and out the bottom of pole 1, L2 goes in the top and out the bottom of pole 2, etc... I've researched online that I could use a 3 pole for single phase, and to cluster the wires into pole 1, put them in series through pole 2 and 3, and out pole 3. The circuit seems the same to me in this configuration, but regardless, the result is the same that the contactor only engages manually, and not automatically.
you don't need to do that with the contactor. You can run L1 through one pole and L2 through another and leave T3 open but that doesn't make any difference to your issue.


where in circuit is the thermostat and relay coil? What it sounds like is you to not have power tapped before the relay to power your control circuit so what is happening is when you manually engage the contactor you then have power to hold the contactor in.

A word of caution: as it sounds there is no disconnect though which would mean your control circuit will always be powered.

also since this operated on 208 before, using 240 will change the heat capacity of the appliance. Was this appliance rated for 208-240 voltage?
 

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What's the make/model of deck oven. Even if it was originally 3phase seeing a wiring diagram would help a lot.

What's the coil voltage of the contactor? Where is power coming from for the contactor/thermostat?
 

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Discussion Starter · #10 ·
Answers to questions and diagram

Thanks for all of the quick replies.

Here are some of the answers for various posts:
From the metal plate attached to the outside of the unit:
The unit is an Elektro - Dahlen AB (now Sveba Dahlen, from Sweden)
Type is US - 43C, No. 54411
3X208 - 220 V 60 Hz
31.8 kW (all four decks)
Element, 214 V (I was told the element is spec'ed at 214 volts, but can handle 240 Volts, but will run hotter).

The contactor and thermostat are both rated at 240 volts.

I've attached a couple of wiring diagrams, and a couple of pictures of what I have, before (wired for 3 phase) and after (wired single phase). I did check the voltage just above the fuses, but not at the contactor itself; it was 240 volts. I will do so tomorrow at the contactor and post back.

One other piece of information that I didn't post, when manually holding the contactor in the oven heats after the rewiring. If I set the thermostat to say 300, once the temperature of the oven reaches that level you can feel the contactor button pushing back as I was holding in. Just saying that it appears that the thermostat is talking to the contactor.

I assume the coil is part of the contactor, is that correct? My naive impression was that the contactor was by default open, and was not getting the power to close the circuit. But it does appear to be getting the signal to open the circuit from the thermostat when temperature is reached. I've rewired 3 out of 4 of the ovens and they all have the same issue, so I assume that it is something incompatible that I've done, and not a hardware failure.

Thanks again for all of the help.
 

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I assume the coil is part of the contactor, is that correct?
the coil of the contactor is what you energize to activate the contactor.

I cannot see the drawings clearly enough to track the power through it.


One other piece of information that I didn't post, when manually holding the contactor in the oven heats after the rewiring. If I set the thermostat to say 300, once the temperature of the oven reaches that level you can feel the contactor button pushing back as I was holding in. Just saying that it appears that the thermostat is talking to the contactor.
do you have power to the thermostat at all times with the lead leaving the thermostat connected to the coil on the contactor with the other leg of power to one of the leads on the contactor coil hot at all times?
 

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When you connect the 3 phase oven to a single phase power supply, one out of every three heating elements must be dead unless you unhook it and reconnect it to different terminals.

Each element is connected to one of: the A and B phases, the B and C phases, or the A and C phases.

When you connect up the single phase supply, you connect the A phase oven terminal to the A power leg and connect the B phase oven terminal to the B power leg. Finally you connect the C phase oven terminal to the A power leg so the B-C elements get powered. The element(s) connected to the A-C phase combination will get zero volts.

You cannot connect the C phase oven terminal to both the A power leg and B power leg or else you will get an immediate short circuit. Instead you must do some internal rewiring, unhooking the A phase to C phase elements and connecting them to either the A-B terminal combination or the B and C terminal combination.
 

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Need a couple things to determine if this is all correct.

Those contactor's coils can be swapped out for a different voltage. I suspect that since the unit was converted from 208 three to 240 single that the current contactor coil voltage is 208.

Where do the red wires go exactly? Is there a safety thernostat in as well? Whats the resistance across the terminals of both when closed?

Does the thermostat itself use or require power? Some bakery equipment I've worked on in the past use a thermostat that isn't a simple kx style but uses power to switch a set of contacts as well. Could be that.
 

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Discussion Starter · #14 ·
Thank you for the help! Your hints and questions pointed me to the solution. I traced the path of the red wires through the switches, thermostat, etc..., and when I moved the wires to support single phase from 3-phase it resulted in both sides of the conductor coil being on the same side of line... I just had to flip the 'red' wires, and the conductor works as expected and the oven heats up, lights come on, etc...

But, AllanJ, I'm concerned with your comment,
"You cannot connect the C phase oven terminal to both the A power leg and B power leg or else you will get an immediate short circuit. Instead you must do some internal rewiring, unhooking the A phase to C phase elements and connecting them to either the A-B terminal combination or the B and C terminal combination."
I've not yet had all three sets of elements on simultaneously... Two on at the same time; can you elaborate further on this?

Thanks!!
 

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Do you need to have all three sets of elements on at the same time?

You have just two feed wires, A and B.

To power up the A-B elements, of course you connect the two oven terminals (A and B), lets say to the two feed wires (A and B) respectively.

To power up the B-C elements you connect the two oven terminals (B and C) to the two feed wires (A and B) respectively but in any order. The B oven terminal was already connected to the B feed in the preceding step so you connect the other oven terminal (C) to the other feed terminal (A).

To power up the A-C elements you need to connect the respective oven terminals (A and C) to the two feed wires (A and B) respectively but in either order. The oven terminals are already connected via the preceding two steps but both A and C are connected to the A feed terminal. You get zero volts between A and C so the A-C elements remain cold. You cannot re-arrange any of the oven terminals without causing something else to stop working.

If you connected an oven terminal to the neutral of the feed, you might get all three sets of elements to heat up but some of the elements will be getting 120 volts instead of 240. The total amount of heat produced will probably be less compared with having just two sets of elements working on 240 volts.
 

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Discussion Starter · #16 ·
Thanks, Allan.

I'm sort of following, but not completely. From looking at the circuits it appears that the elements are now independent of each other. So, it is not element (E) A-B, and E B-C and E A-C, as was wired in delta in 3 phase, but is now E A-B, E C-D and E E-F . So, points A, C and E of Elements 1, 2 and 3 are connected to Leg 1 of the single phase service, and points B, D and F are connected to Leg 2 of the single phase service.

Each deck of the oven operates with 3 sets of elements. The 'top' set has six banks of four elements each, the 'bottom' set has four banks of four elements each, and the 'front' has two banks of four elements each. Thanks for the tip. I know that they work individually and in pairs, but I won't turn on all sets until I can confirm that it's possible. If it is not possible, I will just disconnect the front set to avoid any possible damage.
 

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What he is saying is you'll need to rewire the elements as well if you want all the elements on at the same time. It doesn't damage the elements at all, but they will be energized when you are heating.

Its like having a live outlet. There is voltage there, but no work being done.
 

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It sounds like you have a functional solution. However, I wonder about the current capacities of some of your parts. Your total current at 240V will be 167 amps. That is an awful lot of current. Depending on how you re-wired the contactor, you may have over 3X the current on each pole as the original wiring arrangement. In converting to single phase and raising the voltage, you really probably should put all poles of the contactor in parallel, and you may need to replace internal wiring with larger conductors since you're increasing the total current, and condensing it from three current-sharing conductors down to one circuit. Without a more readable diagram, it's impossible to tell exactly how you have this set up.
 
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