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Forum Index : Microcontroller and PC projects : Oh no, I am stuck with fixing Abdul's mess!
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| PhenixRising Guru  Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
I was asked to look at the project but it was horrible and so I quoted REALLY high....but they agreed  There is some local "industrial electrician", Abdul, whom I've never met but I seem to keep coming across his "work". He "re-controlled" a machine, nearby and I've seen nothing like it. I'll grab some pics but Mick will be warned to sit down before viewing.  I first saw it many months ago, it involves forward/reverse control of a three phase motor. Abdul's electrical enclosure was a six-pint plastic milk container with the top chopped off. The three-phase (blue wiring which is supposed to be for DC) contactors were just dropped in the container I protested and they brought him back in and now they are at least DIN rail mounted but the wiring is horrid. There's some interlocking logic required (more than just forward/reverse) that is just not achievable with two contactors. Yup, it's getting a PicoMite  |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
Apologies for multiple posts...the first two gave me an error due to the emoji...but they evidently got posted. |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7844 |
Forward-reverse. Mechanical *and* electrical interlocks needed to avoid big bangs and (if you are lucky) blown fuses instead of melted wiring. :) Reversing star-delta starters are fun... Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2434 |
i'd say you need 2 main contactors, one for forward and one for reverse. a forward and a reverse push-button switch. THEN another pair of (small) relays: one whose contacts lock-out the forward switch when the reverse contactor is not open, the second doing the same for the reverse switch / forward contactor. at least, that is how i did it 20 years ago with a 32v DC motor moving a sled up on the lab roof back and forwards (the sled had a couple of GPS receivers on top). there were also reed switches at each end of the sled travel that disengaged the appropriate main contactor when the sled reached the end of travel. the end result is you could press both switches at once and the sled would power to one end, stop, then power back to the other end, stop, and repeat ad infinitum. at each end there would be a satisfying "clunk, ka-clunk" sound as one contactor disengaged followed by the other engaging. with a 3-phase motor i'm picking you'd also need some means of sensing rotation and preventing engaging anything until the motor is stationary. that could take from a few seconds up to a few minutes! cheers, rob :-) |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2601 |
A trick that works on many induction motors is after it is switched off residual magnetism in the rotor generates a weak AC voltage in the stator until the speed gets quite low. Sensing the frequency of that will give you the speed until the voltage gets too low to read. Wait a few more seconds before reversing. Re contactors, an emergency supply transfer kit has 2 contactors on a chassis with mechanical and electrical interlocks already set up. Larger ones usually use motorized circuit-breakers instead of contactors. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7844 |
You can get mechanical interlocks that fit onto the contactors (or "rocker plates" that the contactors screw down onto). Some of these have built-in normally closed auxiliary contacts, one for each contactor. These open when the contactor closes and are wired in series with the coil of the opposite contactor. In theory the mechanical interlock is merely as a safety backup. Unless the control system prevents it in other ways, it's quite common to find anti-plugging timers that are set to allow the motor time to stop before any restart is possible. In actual fact many motors (especially pumps) stop very quickly anyway. The direction of rotation is immaterial, restart is simply prevented for a fixed time after a run. In extreme cases you might find start counters that limit the number of starts per hour. These are generally for very heavily loaded direct on line motors where the inrush current can be very high. I shouldn't be thinking of this stuff. I retired from it almost eight years ago. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2601 |
Prompted me to work it out, more than thirty. Where did it all go? |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
This is what we have: There is a foot-pedal for forward and a foot-pedal for reverse. The output shaft has a range of 0 to 190° There is a limit switch at 0° and another at 190° The ridiculous thing is that; although the contactors drop-out when the limit-switches are tripped, the limit-switches are overshot. The strikers are too short. Common sense would be to fit longer strikers that keep the switches open. But no-can-do for reasons unknown. So if the operator drives the shaft until it stops after the 190° and then has a brain-fart and hits the forward pedal a second time, instead of reverse, the machine crashes and gets all tangled up because it overshot the limit. Similar things happen in the other direction. As stated, doing a proper job with the switch-strikers is not an option.  I plan to use a simple state-machine with NV storage of the active state, in case of e-stop and/or power-loss (shut-down) |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
Oh, BTW, it takes ~5 seconds to rotate from 0 to 190° |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2434 |
i'd be telling them that the limit switch overshoot situation is a health and safety hazard, and that i'm not able to do any further work on the machine unless it is rectified. cheers, rob :-) |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
It's like one of these, though |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7844 |
Agreed. A limit switch is what it says - a limit. If it can be overshot then it can't do its job. It also has to be hard-wired. Of course, you should really have two switches at each end. The first is overridden by selecting the opposite direction only. This one can be handled in software and can be inductive or capacitive if you like. The second switch has contacts to trigger the emergency stop relay (I trust there is one!) by hard wiring and kills the lot. It should never trigger, obviously. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
Semantics. CNC machines and robots throw many axes around at high speeds and they stop based on pulses from a sensor (encoders). In my case, I'm receiving a pulse from a sensor and the controller inhibits any further motion in that direction. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7844 |
Don't you allow for software going wild? Surely you can't depend on pulses to stop it? It was hammered into us that there should always be final limits on anything that was driven under power unless the system was designed to stall if it hit an end stop. I'd have thought that came under basic Health & Safety. Even machines in cages can throw chunks off if they go through an end stop. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
The limit switch will still be dropping-out the contactor, hard-wired, like a typical e-stop circuit. My system will pick-up on the event and prevent the operator from driving it in the same direction. "Final limits". In reality, this is hardly ever the issue. Operators and setup people input a bad value and a machine ploughs into its workpiece. Here it's down to the response time of someone hitting the e-stop. I know of a way to make every manufacturing process 100% safe. Shut the entire factory down. When I was three years old, I grabbed a screwdriver and unscrewed every 240V outlet in the house and pulled the socket away from the wall. How I never stuck my fingers behind, no-one knows. Innocent kids can get into danger in any number of ways at home but when it comes to responsible adults in the workplace, we're almost wrapping them in cotton wool. |
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IanT Senior Member  Joined: 29/11/2016 Location: United KingdomPosts: 108 |
Many years ago I changed the fans on a mini-computer system without switching off the power (the customer wasn't willing to have any downtime, even for "scheduled" periodic maintenance). It was possible to do but not exactly in line with my companys 'safety at work' policy. However, the customer was very insistent and so I reluctantly obliged him. Everything went well, until it came to screwing the back panel mesh back on. On the very last screw, my screwdriver slipped and hit a large capacitor inside the mesh. There was a very loud bang, a bright blue flash and I jumped about six foot in the air. On closer examination, the tip of my screwdriver had melted and it's shaft had welded itself to the mesh. Fortunately, the customer had just left the room (and the computer didn't crash!) so I decided to leave the screwdriver where it was (sticking out the back of the PSU) until the next PM (when I cut it out with the power off). I never agreed to do any maintenance on PSUs with the power on ever again! At least you were only three years old when you 'played' Russian Roulette with the mains PR. I was quite a lot older and really should have known better! Regards, IanT |
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| PhenixRising Guru Joined: 07/11/2023 Location: United KingdomPosts: 1346 |
A few years ago in Brum, I was retrofitting a MicroMite to a CNC machine. It was a Friday and they only worked Monday to Thursday so I had the factory to myself...or so I thought. I went outside to the car for a few minutes and when I came back, there was this guy with his head inches away from exposed three-phase terminals in my control panel and he was messing with my stuff. Turned-out that someone had let the PAT (personal appliance tester) guy in to stick his little sticker on everything . He had no idea. I shouted "FREEZE" like a darned cop. Once I had him clear of the area, he was shaking like a leaf. Had a chat with him and he clearly understood very little about electricity. Bloody stickers all over my plugs Yeah thanks buddy. |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 530 |
The problem with using limit switches on something like you describe is that the motor will not stop immediately when the foot switch is released or when the limit switch is tripped. For something like that you need a brake, most brakes on machines like that are powered to release, so when the power is cut the brake shoes close on a drum and the machine stops. The torque created by the spinning motor will keep it turning after the power is off. Some sort of braking, such as mechanical or plug braking is needed. Plug braking works by reversing the motor but is tricky to get accurate stops like you want. A mechanical brake on the motor pulley or a flywheel would work best as others have pointed out the contactors also need to be interlocked. Mechanically and using auxillary contacts on the contactors pete |
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Bill.b Senior Member Joined: 25/06/2011 Location: AustraliaPosts: 235 |
Way back in time I worked for Phillips Electric lamp manufacturer. On a machine that made fluorescent lights there was a transfer arm to move lams from a conveyor to another machine. The arm moved through 180 degrees then stopped on limit switch. To ensure the arm stopped in the correct position. when the limit switch stopped the motor a short pulse of dc voltage was applied the winding. This immediately locked the rotor. Bill In the interests of the environment, this post has been constructed entirely from recycled electrons. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 7844 |
One of the nicest "Abdul" jobs (very likely a completely different "Abdul"!) I've seen had the DOL starter contactor wedged in. The motor was started and stopped using the door-interlocked isolator. It was pretty badly burned. :) There was obviously no overload protection, only fuses. . Edited 2025-05-30 18:33 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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