over temperature cutoff - Page 1
Forum section: electronics / projects

author: joetorelli
author_rank: Contributor
timestamp: June 29, 2014, 04:04:55 pm
content: Hi,I build mirror position controllers for flight simulators. I have been doing this for about 7 years. It is not enough to live on but a nice 2nd job. My company (just me) make about 6 to 10 units a year out of my workshop. I use an ultra sonic sensor to measure the position of the of the mirror (it is a sheet of mylar about 2m by 5m) and the micro controller generates a pwm to control a vacuum pump that moves the mirror. I have a 12v power supply that runs the sensor, micro and lcd display. The pump runs off the mains.They now want a thermal cutoff is case of fire in the unit. They want the input mains disconnected when there is a fault. I thought of using disc thermostats or some kind of PEPI device. The problem that I have is there can be different types of inputs. 220v/Neutral/Ground or 110v/110v/Ground or 110/Neutral/Ground. So I want to cut off both sides of the mains. I am concerned that if I use 2 thermal cutoffs they may not switch a the same time. Does any body have any ideas what would be a good way to do this?Thanks for any advice.Joe Torelli

author: Kremmen
author_rank: Super Contributor
timestamp: June 29, 2014, 04:18:07 pm
content: Could you make the thermal cutoff operate a relay through which the main power is fed to the system? That way the thermal sense part can be a SELV circuit and simultaneous disconnect of all supply lines is guranteed.

author: SeanB
author_rank: Super Contributor
timestamp: June 29, 2014, 04:38:49 pm
content: You can place a thermal one time switch in the pump electric box if it does not already have one, and use a 135C thermal fuse to disconnect the pump power, using only one lead. Use some fibreglass sleeving to insulate it electrically and crimp the one side to the existing wiring common of the motor and the other side to the terminal that it used for common. That way the motor failing hot will stop it running, but will not protect if the wiring inside goes to ground.Mains input you can use a 2 pole mains rated breaker on the input side to protect against overcurrent, to disconnect power to the unit. Place a mains switch after the breaker, as they are not designed to be a power switch, only being rated for a few cycles. Make sure the motor frame, and all metalwork, are attached by a green/yellow earthing cable to mains ground, and these cables must have crimp terminals both ends, and must be at least the same size as the mains conductors. Use shakeproof washers on each screw connection, and have a dedicated screw per item for this cable.

author: joetorelli
author_rank: Contributor
timestamp: June 29, 2014, 08:30:29 pm
content: First I have to say that this is an amazing site! I found this around three months ago and I have found the people here are great! I can't believe how fast there was a response to my question. I hope I can contribute. Thanks.Kremmen: The relay idea was my first thought. I was going to have a NC thermal switch on the coil. But I didn't want a relay that was energized all the time. These things will run 24/7. The sims are on motion and beat this system to death 20hrs a day. Although this is what I might wind up doing.SeanB: A few years ago I had to go thru the CE certification on the box. That was an expensive learning experience. I had to do a lot of rewiring and already have done what you said about the grounding. I had 2 pole mains breaker and it was replaced with a dual fused power module. The motor is thermally protected and was tested and certified during the CE testing process. They literally stuck a screw driver in the pump until it shut off. Let it cool down and did 3 more times! But they still want a fire shut off killing the main coming into the box. They will allow the cutoff after the power module but from there no more power into the box.So currently I have 2 disc thermostats right after the power module. They are rated at 80C. I was surprised that I could not find a double pole assembly. I'm still looking into it.Thanks for the replies.

author: Kremmen
author_rank: Super Contributor
timestamp: June 30, 2014, 04:36:44 am
content: Quote from: joetorelli on June 29, 2014, 08:30:29 pmAll static (i.e. no external power) cutoff switches with thermal protection that i am aware of, are of the normal circuit breaker style. Meaning the thermal protection applies to the current passing thru the switch itself. Those are "easy" since a bimetal contact will work there.The alternative would be some kind of latching bistable movement that you can trip with an external pulse. But i am not aware that such a beast exists. A regular latching relay is not really suitable for this.Of course you could get creative and employ say a multiphase RCD device. All you need is to artificially create a minor ground fault through the cutoff device to trip the RCD and bob's your uncle. But i suspect that is not really up to code although i have not seen it expressly forbidden anywhere. Hmm... in fact, isn't that exactly what the test button in a RCD does...?

author: joetorelli
author_rank: Contributor
timestamp: June 30, 2014, 06:49:06 pm
content: Here are some pics of the mirror. This is what the pump sucks back.emptyshell is before the mylar is applieddrawback is the first time it is sucked backfinished is the end productpoped is when there is an OH Shit!ThanksJoeT

author: SeanB
author_rank: Super Contributor
timestamp: June 30, 2014, 07:25:17 pm
content: I have not seen a themal cutout in a 2 pole configuration, unless you go to a custom unit. I figure the 2 clixon devices will work, just use the non self resetting ones with the red press to reset button on it. Will need a service call for resetting but then if they trip you have lost a fan anyhow.Fire protection the old method was to put a halon bomb inside the case, but now you cannot do that and need external fire suppression, using a panel and cylinders of fire suppression gas.I was at a fire chief recently, and aside from the fire engine ( his own) in the yard, and the collection of fire fighting equipment and fire axes he also has a whole collection of carbon tetrachloride bombs, the original fire extinguishers.

author: joetorelli
author_rank: Contributor
timestamp: July 01, 2014, 05:38:38 pm
content: I missed the non resetting type. I have found them after you mentioned it. It looks like a solution. I was concerned that one might set/reset and the other didn't.If there is a fire it will be more than just a service call!The sims have "VESDA" systems. They sniff for smoke and shut down power at the wall. They run pipes all over the sim to sample the air. The old sim bays used halon and the new ones use WATER. This is the first request for fire shut off and it is only from 1 customer.Here are some pics of my baby. RSI is the company that make the visual systems and they sub-contract me for the mirror position controller. The amount of curvature need for the mylar it very critical (<1mm) and because of it's size is very sensitive to changes in building air pressure.OutsideBox: is the front with pump at the top and connections at the bottomInsideBox: shows the pump and microOutsideSim: the bowl in the front is for the mylar mirror that the outside world is projected on.American Airlines is my night job and building boxes is my "day" job. Our oldest sim is a 40yr old 727 and newest is a 1month old 787.The 727 runs off a TI mini computer with toggle switches and leds. It was originally using 8" floppies and we have since converted them with a windows PC and Hard drive. I know how Dave likes antique electronics and someday I would like to send him the mini computer for the mail bag.JoeT

author: SeanB
author_rank: Super Contributor
timestamp: July 01, 2014, 06:14:02 pm
content: You have one of those in South Africa, at SAA in Johannesburg? 747 simulator. Went for a flight on it years ago ( part of a tour to the technical side) and of course crashed on take off. when asked for a destination airport I remembered my father and his worst airport to land in, and chose Hong Kong Kai Tak, as he remembered the approach including the right turn on approach, and flying next to the skyscrapers on approach. I watched others crash there, having already had my turn.Was interesting going through the clean room facility there as well, and seeing the only airworthy Junkers being restored.

author: joetorelli
author_rank: Contributor
timestamp: July 01, 2014, 06:36:19 pm
content: I used to teach maintenance and modeling classes to the airlines that would by the visual systems. I taught SAA many years ago at the Texas facility. They were one of those memorable classes, a good group of guys.I have flown every thing from helicopters to fighter jets to jumbo jets. I have done in air refueling, landed/launched on carriers, oil derricks and many airports including Hong Kong. And never left the ground!JoeT

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2014, 04:26:50 am
content: Yes, SAA lost a lot of pilots after the training and the payback period to other airlines. There is a good chance that if you fly on any large non US airline the pilot is ex SAA or ex military.