Temperature control of 3 phase heaters

Thread Starter

JonathanL

Joined Jun 12, 2020
16
Hi guys

I am looking for some help on a system which I have here. There is a temperature probe in a tank which is monitoring the temp of a liquid and the controller output is switching a 3 phase contactor on and off when the temp of the liquid drops below the setpoint and then switches the contactor back off when the setpoint is reached again. The problem is now that over the weekend the contactor welded closed and kept heating the liquid which had to be dumped. There is an over temp alarm relay output on the controller which was not connected to the circuit therefore no failsafe. I need to configure this now so that the contactor coil switches on and off as normal through the relay output of the controller for when it drops below setpoint and then switch off when setpoint is reached. But i also must now get the over temp alarm into the circuit to switch the contactor off as well. I am a little confused as to how to go about this as in should i install a safety contactor so that if contacts weld shut no power will flow, should I put in a safety relay, or change the type of temp controller with different outputs instead of relay outputs. Or maybe a logic module of some sort? The temp controller in question is an Omron E5CSV
 

drc_567

Joined Dec 29, 2008
965
... There is wiring information on one of the sides of the OMRON controller. If there is some way to copy that diagram and post it here on the forum, then it may be possible to find a solution to the problem.
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
The controller only offers 2 outputs the temperature output and the alarm output. I need to get this so that the contactor will fail safe should temperature be exceeded
 

drc_567

Joined Dec 29, 2008
965
The model number of the controller indicates that it is made to be used with a relay of the single pole/single throw, normally open type.
It appears that one alarm output is available.
A basic relay wiring diagram is shown on page 14 of the datasheet in the link above.
(Use the relay version shown in the right side column.)
The power supply is shown as 24 volts AC/DC, so avoid using full mains voltage.
The T designation implies thermocouple or platinum resistance temperature sensors.
 

drc_567

Joined Dec 29, 2008
965
...Trying to decode the model number using page 13 of the datasheet link above.
What type of temperature sensor is being used?
 

drc_567

Joined Dec 29, 2008
965
There should be a dial switch that is used to set the platinum resistance temperature sensor range.
What range of temperatures (Celsius) will need to be measured?
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
The temperature is measuring fine that is not the issue. The issue is gettng this to fail safe should the contactor weld itself shut like it did today
 

drc_567

Joined Dec 29, 2008
965
There is also an alarm dial setting, next to the temperature dial.
A setting of 2 on the alarm dial should turn on the alarm when the temperature exceeds the upper limit.
 

drc_567

Joined Dec 29, 2008
965
There is a group of control mode switches. Switch no. 5 needs to be on in order for the Pt temperature sensor to be active.
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
Again this is not the issue with the temperature reading it is an issue with control and shutting off in a known safe state
 

strantor

Joined Oct 3, 2010
5,382
Yes, I think you need to add a contactor "upstream" of the contactor which is switching the element on/off. This contactor should be energized at all times by the controller's alarm output. This means configuring the temperature controller's alarm as a high-alarm-only, and configuring the temperature controller's alarm output action to be reverse-acting (ON when there is no alarm and OFF where there is an alarm).

It would be simpler to use a NC contactor and normal alarm output action, but that would not ensure that power is disconnected from the element. Power could be sent to the element if contacts are welded and the controller is powered off.
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
Yes, I think you need to add a contactor "upstream" of the contactor which is switching the element on/off. This contactor should be energized at all times by the controller's alarm output. This means configuring the temperature controller's alarm as a high-alarm-only, and configuring the temperature controller's alarm output action to be reverse-acting (ON when there is no alarm and OFF where there is an alarm).

It would be simpler to use a NC contactor and normal alarm output action, but that would not ensure that power is disconnected from the element. Power could be sent to the element if contacts are welded and the controller is powered off.

With the temp controller I have I don't think I can invert the outputs to behave like this
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
So in theory i should the first contactor being controlled by the alarm output. So then that contactor is powering the heating contactor with 3 phases and is always on unless there is an alarm. And then the second contactor is being controlled by the temperature controller and switching the coil on to reach setpoint and then back off when setpoint is reached?
 

strantor

Joined Oct 3, 2010
5,382
So in theory i should the first contactor being controlled by the alarm output. So then that contactor is powering the heating contactor with 3 phases and is always on unless there is an alarm. And then the second contactor is being controlled by the temperature controller and switching the coil on to reach setpoint and then back off when setpoint is reached?
Yes
With the temp controller I have I don't think I can invert the outputs to behave like this
You should at least be able to make it a COLD alarm (ON any time the temp is <xx degrees), which would achieve the same end.
 

Thread Starter

JonathanL

Joined Jun 12, 2020
16
Ok thanks. But I guess I still need to figure out why this happened too. the third phase was only giving me 230v between the first two instead of 400. I've checked the circuit and wiring and it looks ok.
 

drc_567

Joined Dec 29, 2008
965
If the control mode switch #1 is placed in the on position, the output relay is then in PID mode, which means 10 sec. on and 10 sec off. Maybe this mode would not cause as much heating stress on the relay contacts. A different setting allows for more rapid PID pulsing. ... Something to try out maybe, as opposed to the relay being on full time.
 

strantor

Joined Oct 3, 2010
5,382
Ok thanks. But I guess I still need to figure out why this happened too. the third phase was only giving me 230v between the first two instead of 400. I've checked the circuit and wiring and it looks ok.
You most likely have a blown supply fuse. The 230V was probably coming back from the wye-connected heater element. Disconnect the heater and retake your measurements.
 
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