oven temp control with two controllers in series

We have a small plastic parts oven (SP at 210F). It used to have an old bimetal temperature regulator. We are upgrading it using two temperature controllers. Watlow EZ-Zone with mechanical relay outputs. One for regular SP control, the other for the high limit. The output contacts of these two controllers will be wired in series. Here is my question.... For the high limit controller, should I configure it with the heating mode and use the normally open contact? or the cooling mode and use the normally closed contact?

I don't have my references available, but I thought the EZ Zone controller could be configured as a Limit controller to do exactly as you wish. In that case, you would configure the first unit as heating mode (P, PI or PID) and the second unit as limit mode. You then set the alarm level and the output. It has been a while, so I apologize in advance if my thoughts are not correct.

I'm not an oven control expert, but I've worked around a few over the years. In my experience, the high limit controls do more than just block the addition of heat. They also drop and disable the automatic cycle of the system. It sounds like your system is a lot simpler than all of that.



That being said, I would set the output of the limit controller such that it fails safe. In other words, it needs to be powered on and healthy as well as the temperature being safe to allow the system to run.I would design the system such that it takes operator intervention to resume heating if the high limit control ever detects a high temperature condition.

It appears there are EZ-Zone controllers and EZ-Zone limit controller. What we have are PM6C1EJ. Based on an online manual I found about EZ-Zone limit controllers, the limit controllers are PM#L (or M).

https://www.watlow.com/-/media/documents/user-manuals/pm-pid-1.ashx

https://www.watlow.com/-/media/documents/user-manuals/pm-limit-user-guide.ashx

Thank you for your thoughts

Hi Joseph,
I am a plant electrician. We don't have an electrical engineer so I am tasked to make a simple design for this small oven.

A newbie question for you. I can set up one of my EZ-Zone controllers with an alarm function that de-energize the output and latches it off. Why do I need a high limit controller? Is that for safety redundancy?

Thanks for the clarification on the temp controllers vs limit controllers. I don't have my references handy. Sorry about misleading you.
Joseph - good call on the fail-safe wiring. That is important.
Regarding the use of limit controllers, it is a good idea to have the limit controllers for safety redundancy. If the main controller fails, the limit controller will back it up.

unsaint33 said:

We have a small plastic parts oven (SP at 210F). It used to have an old bimetal temperature regulator. We are upgrading it using two temperature controllers. Watlow EZ-Zone with mechanical relay outputs. One for regular SP control, the other for the high limit. The output contacts of these two controllers will be wired in series. Here is my question.... For the high limit controller, should I configure it with the heating mode and use the normally open contact? or the cooling mode and use the normally closed contact?

Click to expand...

You want to make sure that POWER LOSS to the High Limit ALSO drops out power to elements. This should answer your question about what contact to use.
Also a high limit requires reset button to reset the high limit condition (code requirement). That is why usually it is an instrument that IS a high limit instrument, not just a controller with an alarm relay.

"A high temperature limit. It is required to ensure that the maximum temperature specified by the oven or furnace manufacturer cannot be exceeded. It must require manual reset, must be of the indicating type and must be used solely for high temperature limit. (It cannot be an auxiliary contact of the temperature controller.) It must drive upscale upon thermocouple break."

The other kicker to remember...the High Limit MUST BE "FM Approved" (Factory Mutual)...... if you want insurance to cover ANY damages caused by failure of instrument..