Electrical question or better HELP !

[Mr Fixit]

Hello Group,
I should know the answer to this question but I am drawing a blank. I have a 240v 3* heating element that has a resistence of 15.5 ohms for each coil. I am wanting to use this element for a powder coating oven and will be building controls to be able to step the elements on and off at 240v 1*.
The long drawn out question here is what is the current draw if all 3 elements are on at the same time?
I know 240v / 15.5 = 15.48 A , so do I add the 3 element ohms to get 46.5 ohms then divide by 240v to get 5.16A or do I multiply the 15.48 x 3 to get 46.44A or do I do something else? I think this is where l'm missing it. What is option #3?

I will watch for the answers to fly in, I know this is a simple question but l'm just stumped. Looking forward to all your HELP !


Tx Mr fixit for the family
Chris

[lakeside53]

If you put them in parallel that answer is X3 or 46.44 amp.

If you want to use a contactor to turn them on/off, get a 3 pole and run one element per pole - easier on the contacts.

[Jaakko Fagerlund]

I might suggest a PID controller and 3 SSR (solid state relay). No need for stepping the power manually or javing to listen to a contactor banging open and closed all the time. But this requires a contactor too, as SSRs tend to leak current a little and thus they prevent a shock hazard. I wired a contactor to my own HT oven to cut the power when the door is opened.

[lakeside53]

If you want a recommendation for PID controller, the OMEGA CN 7823, a K type thermocouple and 3 25 amp SSRS will do a fine job. Sure, you can buy cheaper PID's on Ebay - all Chinese rip offs of the 7823, zero support. Omega will support you for hours on the phone

http://www.omega.com/pptst/CN7800.html

The 7823 is a bit of an overkill for a powder coat oven, but you can also use it for your heat treating needs!

[Paul Alciatore]

Lakeside answered the question and Jaakko's suggestion is a good one.

But, I would question where you got the 15.5 Ohm value for each coil. If you measured it with an Ohm meter, it is probably misleading because the coil would have been cold (room temperature). The resistance will change when it is hot. It will probably go up and the current will probably go down. For accurate figures you either need the specs for the coils or you need to measure current while they are hot and figure the resistance from Ohm's law (R = E / I).

Of course, when power is first applied, there will be a larger, initial current until the coil heats up. Probably for a second or so.

[J Tiers]

Better yet, rather than measuring, find out the kW rating of the heater. It will probably be findable, if the thing is at all recent, by a company in business.

The rating may also be on a tag attached to the unit. That is usual, unless it is a part of some larger item.

Then take the kW rating, and divide by the rated voltage and you have the current. The hot resistance can be found also, if that is of interest.

The contactor, if rated for the current, will have an easy time of it anyhow, unless the resistance goes up a lot when hot. A resistive load is the easiest possible, and the contactor will have the highest current rating for a resistive load.

Heaters often do change a bit in resistance as they heat up, since there is no good reason not to, and stable resistor material is more expensive. It's not as nice a heater if there is a wide variation, so usually it isn't anything like a light bulb.

[EVguru]

If you want a recommendation for PID controller, the OMEGA CN 7823, a K type thermocouple and 3 25 amp SSRS will do a fine job. Sure, you can buy cheaper PID's on Ebay - all Chinese rip offs of the 7823, zero support. Omega will support you for hours on the phone

http://www.omega.com/pptst/CN7800.html

The 7823 is a bit of an overkill for a powder coat oven, but you can also use it for your heat treating needs!

The 7823 is indeed a good controller. They have a little label on the side which says 'Made in China'.

I've got three on the bench right now going into soil sample digestors with quite a complicated ramp and soak profile. I've been replacing Eurothem controllers in old units and building complete new ones adding an RS485/Modbus interface in the process. The manual is terrible, but so is the Eurotherm one and whilst the Modbus software is also very poor, it's free unlike the expensive Eurotherm package.

My heat treatment oven has one of the generic Chinese controllers in it. It works perfectly and the manual was quite adequate for a plain single setpoint unit.

I'd forget all the theoretical nonsense about your heating elements. Hook one up to a supply and just measure the current!

[J Tiers]

I'd forget all the theoretical nonsense about your heating elements. Hook one up to a supply and just measure the current!

Hardly "nonsense".....

Makes a lot of sense to find out what the thing is. Especially if it is printed on a tag attached... why would you not? And why not look it up if the information is available?

Helps a lot to find out what you need to have as far as supply and switching to handle the load. Even if brand new, I'll usually just check the resistance and check to earth/frame.

And, the information can be extremely important. We do NOT KNOW what this heater came out of. Or is intended for.

As an example..... A heater coil for HVAC may have an element rating of 5kW per section. BUT.... That kW rating only applies if the thing is in an air stream.... which cools it (heating the air). If you were to try to use it in an oven type application, it would be likely to literally melt.

[Mr Fixit]

Thanks for the responces group,
J Tiers you are correct it does matter, it is a replacement element for a commercial convection oven with no other information then 240v on it but because it has 6 terminals I assumed it is 3* but maybe not. There is no manufacturer or tag data info with it. So. does it need air flow for my application being from a convection oven? Being that it will cycle and is in a larger space i've built then original cabinet (again assuming this) does it matter?

Tx Mr fixit for the family
Chris

[lakeside53]

Hmmm... it might need airflow (fan) if it's not designed to be stand alone. Post a picture.

[lakeside53]

The 7823 is indeed a good controller. They have a little label on the side which says 'Made in China'.


My heat treatment oven has one of the generic Chinese controllers in it. It works perfectly and the manual was quite adequate for a plain single setpoint unit.

Well of course Omega is made in China, like just everything else. Buying the generic might get you and exact copy, or you might end up with a pos. My comment about support is mostly about the connections and PID setup. If you are first time user of PID, and electrically challenged, it can be a little daunting decoding even omega's "manual". Even just understanding the results of Autotune can be mystifying

[J Tiers]

Pleeeze.... the picture to post, yes?

[Mr Fixit]

Hello Group,
Got some pictures to share,





the element, PID controller, temp readout and SSR's l'm looking at using that will be controlling and switching the loads of the heater.Not sure that this element is from a convection oven now that we've talked about it as it is a surplus item I've had for a while and don't know its story.
Glad for all the input and look forward to more input.

Tx Mr fixit for the family
Chris

[macona]

I wouldn't say it is three phase, per say. It is just three elements. You could switch in different elements for more power if you wanted. Or just tie each side together and run at full power.

About he only way you are going to find out if the SSR is going to be big enough is to hook the heater up to 240 and see what kind of current it draws. As mentioned it will draw much more at turn on so you need to size the SSR to that. And you will almost certainly need a hear sink for the SSR. They are not intended to be ran at full power without one. You can find dedicated SSR heat sinks on ebay.

[J Tiers]

Well, I have no idea if it needs forced airflow or not. I suspect it does not, given how similar it is to electric stovetop elements.... and the fact that it is not made in a way that is set up for ducted airflow. At about 3600W per element, it appears credible. I've seen the same sort of thing in gas stoves that have electric convection ovens. But I do not know if those use a fan.

I believe it WILL at least need *open access* to air, since it is intended as an air heater (convection oven.... convection implies airflow due to heating). So it probably should not be used to heat the outside of an enclosure, or embedded in the lining as with a pottery kiln.

I don't know much about electric stoves other than that I hate them for cooking. I associate them with a neat spiral of burnt food on the bottom of the pot.....from the old days, and thin pots.