SCR Power control for heating
SCR Power control for heating
(OP)
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
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





RE: SCR Power control for heating
Keith Cress
kcress - http://www.flaminsystems.com
RE: SCR Power control for heating
RE: SCR Power control for heating
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
RE: SCR Power control for heating
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
RE: SCR Power control for heating
RE: SCR Power control for heating
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?
RE: SCR Power control for heating
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: SCR Power control for heating
RE: SCR Power control for heating
RE: SCR Power control for heating
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: SCR Power control for heating
Another possibility is that during heat-up, something is expanding and causing movement that results in a short.
RE: SCR Power control for heating
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: SCR Power control for heating
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
RE: SCR Power control for heating
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.
RE: SCR Power control for heating
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.
RE: SCR Power control for heating
RE: SCR Power control for heating
RE: SCR Power control for heating
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
RE: SCR Power control for heating
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
RE: SCR Power control for heating
So that just eliminates the possibility of a failed snubber, but some other source of noise spikes peculiar to that location could be doing the same thing in terms of causing misfires of the SCRs. Skogsgura's suggestion of a much better look at what is going on is the right approach, because by moving the controller to another location and having it work, then also putting a known good controller in to this location and having it fail is proof that it is something local. With no better test equipment, your only other choice is to methodically remove other components of the overall system until the problem goes away. The Advanced Random Scientific Wildly Inaccurate Process of Elimination (ARSWIPE) method.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington