SCR Power control for heating

[condern1]

Condern1
I am looking for some tips in troubleshooting a system I am working on consisting of a 3 phase 480 volt line conntected to a 3 phase fired scr power controller with command signals of 4 to 20 ma from a temp controller. The output is sent to the primary of a 110 kva transformer where voltage is dropped from 480 to 107volts. This 107 volts is fed to a delta heating unit where each element is 36.7 kw at 107 volts. The total FLA is around 132 amps. The SCR controller has a RC snubber amd MOV protection and is fuse protected. The unit also has RMS current regulation and RMS current Limit as well as Overcurrent Trip Auto Reset. The problem I am seeing is that the unit heats up and for some unknown reason the SCR power controller stops running and trips out on overcurrent and never resets itself automatically. I have monitored the voltage and current of the lines and see nothing to cause a trip. The RMS current limit is set at 100% of the units rating and the overcurrent trip is set close to 200% of the RMS current limit. When system shuts down I am still seeing a command signal. I have 3 units and I switched the units and the problem did not follow the hardware. I would like to hear if anyone has some ideas,
Thank you
Condern1

 

[itsmoked]

Why have you not contacted the manufacturer? They likely have seen exactly this before.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[condern1]

I have reached out to manufacturer and they stumped with it also.

 

[itsmoked]

Hmmmmm. Well OK. A couple of things.

1) I would never expect any heater controller that tripped out on overcurrent to "automatically reset". I find that confounding! Are you sure about that?

2) Seems like you've done some reasonable checks and aren't seeing the overcurrent that the unit seems to be detecting. It's very possible there is a failing component. I've seen several phase controllers have component failures that took them down with peculiar happenings. It's likely something in the current sensing logic that's causing the measured value to be wrong. Have you got a schematic?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

I like Keith's responses, but if the problem did not follow the hardware, by which I take it to mean you put in a different controller and the same problem took place, it's unlikely to be a failing component.

First off it is also important to understand the TYPE of SCR controller you are using. For heating controls there are two basic types, Phase Angle controlled, and Variable Time Base (aka Zero Cross) controlled. If you are using a transformer on the output side, it can ONLY be done with a Phase Angle controlled SCR unit. A Zero Cross / Variable Time Base controller will cause transformer saturation, which can lead to ... wait for it... Over Current tripping of the controller!

So before you go off chasing phantoms, make sure the entire operation was not misapplied from the start. If you are unsure, post the make and model of the SCR controller, we can tell.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[condern1]

As I explained, we have three units that are identical. We took the SCR controller out of the system and put it in a different unit and it worked. WE then took the unit taken out of the working unit and installed in the line that was the original problem and the samething occurred with a different SCR controller that was working in a different line. The Scr controller is a phase angle controller for sure. We are fairly sure the problem is being caused by the load, but what it is I cant figure out

 

[davidbeach]

It would seem that the least you could do would be to answer the question as to the make and model of the SCR controller.

Why is it that people so often ask about some problem they have and then respond to every response with "no, that isn't it" without ever providing enough information for anybody else to ever come to a conclusive solution?

 

[waross]

You may have a failing transformer. The next step may be to change out transformers for an identical transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[OperaHouse]

Any new machinery installed around it lately? Had a customer that had a heated bag sealer at General Mills going crazy. I suggested installing a three phase RC network on the line and that fixed it. Just a case where the problem was external.

 

[condern1]

Thank you operahouse, but the controller has a built in snubber and MOV in it. Please explain if you think that would not be enough

 

[jraef]

The snubber circuit that is in the SCR controller is there for the effects the SRC turn on would have for other SCRs in the same circuit, so if one of the snubbers has failed, it can cause a spike from dV/dt on the line that will cause one or more of the other SCRs to self-commutate (turn themselves on when not commanded), which is going to have a similar effect on transformer saturation. I've seen it happen from other sources of spikes as well, for example when an air compressor starter, which used a 480V coil, would come on and cause multiple SCRs to fire in nearby controllers and soft starters, even though they were not running, so they were getting false Shorted SCR trips. Yours would be more likely something more continuous though, hence the thought that one of you snubbers is bad or something similar nearby. A bad snubber generally results in the loss of an SCR in short order, which might be eminent here as well, it just hasn't happened yet.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Compositepro]

You may have a problem with moisture in your heater insulation.

http://www.plasticstoday.com/files/Watlow_0902.pdf

Another possibility is that during heat-up, something is expanding and causing movement that results in a short.

 

[Skogsgurra]

Is "Thank you, but I don't think so" your only possible response?

A few questions have been asked - it may be that you answering them could help...

My tip: Use a fast recorder to see what happens. That is usually the only sure way to solve this kind of problems. Three voltage channels and three current channels are usually adequate. If you don't have one, hire one.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[condern1]

jraef,
The explanation of the snubber circuit being a result of the problem is a good one , but how do you explain that the unit has no problem in another line . I would also like your explanation of why the same symptom effects whatever controller I put in this one line and are not effected in the other 2

 

[condern1]

composite pro
This line had gone thru the drying process during startup and was in operation 24/7 for afew months and this problem started showing up. I would hope that moisture would have been takenn care by this point, but maybe you could enlighten me about it more.

 

[condern1]

itssmoked,
I apologize for not getting back to you sooner. I am sure that the scr unit has auto trip reset, When the fault clears it automatically resets and keeps running. Many ScR controllers have this option. I will send you the manufacturer name and model number on monday as I dont have the info with me and I want to be precise about the information.

 

[condern1]

itsmoked I do have a schematic from the manufacturer and I will post on monday also

 

[condern1]

Waross, I also think that problem is the transformer due to the back that the SCR controller works in the other lines when installed. I am wondering if you have any ideas on how to prove that without replacing the transformer for a test. I dont think that management would be to keen on spending the dough for a new transformer without some type of evidence if it is all possible

 

[itsmoked]

condern1; Thanks. I missed the whole "doesn't follow the controller". I now feel it's not the controller. You can probably skip the schematic.

First thing to check is ALL the down stream connections.
I'd take a temp gun to the party. As soon as it trips out I'd gun all the connections as fast as I could. If you see a warm one investigate. A loose connection could cause lots of strange readings to the controller.

It can also be the heating elements or the connections to them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Remove the controller and replace it with a known good controller. If the good controller develops problems you have cause to suggest changing the transformer.
You can also do some testing on the transformer. A megger test is a good place to start. Others can suggest other tests.

Bill
--------------------
"Why not the best?"
Jimmy Carter