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Safe sequencing of power

Discussion in 'General Electronics Discussion' started by hevans1944, Jan 22, 2016.

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  1. hevans1944

    hevans1944 Hop - AC8NS

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    A recent newby poster, @haqq, asked:

    I have a product testing line with 30 products (1KW each) plugged in at the same time and after completion of the test, all these items are plugged off at the same time.
    I need to automate this process i.e:
    Turn on all the 30 items at the same time
    turn off after completion of required testing time
    Reset timer automatically to start the cycle again

    Kindly guide me how to achieve this objective.


    The thread was rightly closed by a moderator because the OP wanted to switch 30 kW of power on and off simultaneously to thirty devices, each drawing one kilowatt. Clearly, the OP has no idea what he is asking!

    When a power failure occurs in my neighborhood, after the fault is fixed an electrical utility worker closes a circuit-breaker (a large fuse) mounted high on a wooden power pole, using an insulated stick with a hook on the end to snag the hinged, dangling, cartridge fuse-holder and push it back in place into a matching receptacle. Presumably the fuse has been replaced if necessary. I watched this happen once a few years ago after a car accident took out a utility pole, downing live wires on the road. The power company temporarily disconnected the fuse to de-energize, and allow workers to repair, the downed wires. From the size of the arc (this was a 7 kV distribution line) when the fuse was re-inserted, there must have been a load of several hundred kilowatts on that line... something on the order of at least thirty, perhaps as much as one hundred, amperes of current. But, Voila! All the lights (and motors and space heaters and whatever) in the surrounding neighborhood came on simultaneously! The moral of this story: if you wanna switch a lot of power all at once, you damn sure better know what you are doing.

    The key to possibly answering the OP's question lies in the fact that the products being tested are plugged in, perhaps into outlet strips. He wants to power up and test thirty of them at the same time, power the thirty units down, presumably replace them with thirty more products to test, and then power those thirty products up. Wash, rinse, and repeat until the shift is over.

    So we can assume that all of these products have power cords with plugs that can be inserted into an electrical outlet of some sort. And they probably all each have a power switch. Current practice is probably to plug them in, one at a time, and then run down the line flipping the power switches on. After the test, someone runs down the line again flipping the power switches off. And after that a new set of thirty products is brought in for testing.

    The key to an automation solution may be this: It is probably NOT NECESSARY that all thirty products be powered up simultaneously! They can, in all probability, be powered up SEQUENTIALLY, one at a time, just as if a minimum-wage intern or apprentice was running up and down the line flipping switches and, later, swapping out products to test. If this is true, all we need is a solution that sequences power to thirty contactors or heavy-duty relays or solid-state switches, each capable of switching one kilowatt, which is about the power consumed by a toaster or microwave oven. POC! Any one of those three devices, informally known as a do-dad or (sometimes) a do-hickey will work.

    So, @haqq, get a licensed electrician to select and install thirty do-dads suitable for the power drawn by each product being tested. Have the electrician run the do-dad control lines back to a Black Box that will enable each do-dad in sequence, say, at two second intervals. Provide the Black Box with START and STOP buttons.

    When START is pushed (momentarily) the sequence begins and ends one minute later with all thirty do-dads energized. When STOP is pressed (momentarily) the do-dads are de-energized in the same sequence: First One ON is also the First One OFF (FOOFOO operation). That way every product runs for the same length of time, and there are no excessive power surges on the plant wiring.

    Here at Electronics Point we can help with the Black Box design. Your licensed electrician can purchase the do-dads off-the-shelf at any electrical supply house and professionally connect them to the Black Box and to outlets for connection of your products.

    You will still need that intern to change out the products, but we can leave that aspect of automation to another time.

    Hop
     
    Harald Kapp and davenn like this.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    I fully agree. Power sequencing is a good solution to this task. Any PLC can control the sequence at reasonable cost.
     
  3. shrtrnd

    shrtrnd

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    Maybe he was referring to Chinese kilowatts.
    (That was intended to be an (insensitive, maybe) joke
     
  4. Minder

    Minder

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    A PLC would be good, also on a cheaper scale a SmartRelay could do it.
    M.
     
    hevans1944 likes this.
  5. hevans1944

    hevans1944 Hop - AC8NS

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    I am thinking a PIC driving a 32-bit shift register, each bit available for output to drive each of the 32 power switches. When it is first turned on, all the bits are cleared. Then when the START button is pressed the PIC presents a logic 1 to the input of the shift register, issues a clock pulse, and then two seconds later issues a second clock pulse. This continues until all 32 bits have been set by the PIC clock pulses. The PIC then just waits for the STOP button to be pressed.

    When the STOP button is pressed, the PIC replaces the logic 1 on the input of the shift register a logic 0 and issues a clock pulse. Two seconds later it issues another clock pulse and this continues until all 32 bits have been cleared by the PIC clock pulses. The PIC then just waits until the START button is pressed.

    A SmartRelay would also work, but because of the number of outputs required it could be a lot more expensive. Shift registers are cheap, as are PICs, but you might want to place optically-coupled isolators between the shift register outputs and whatever is controlling on/off power application to the product under test. I consider solid-state switches to be part of the "real world" so, even though they normally already have optical isolators built in, I will add another optical isolator at the end of a long cable run. This is cheap insurance against someone on the switch end accidentaly connecting mains power to the low-voltage control terminals. Also provides a simple interface from a logic family to the real world.

    So, @haqq, do you have an electrical contractor lined up to purchase and install your 32 remotely switched outlets? Can we help you with the design of the Black Box control unit? Perhaps the electrical contractor could do that too.

    Hop
     
  6. Tha fios agaibh

    Tha fios agaibh

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    If one has to ask a forum to acquire the knowledge to control power to these devices, they are probably not qualified for the product testing.
     
    Harald Kapp likes this.
  7. davenn

    davenn Moderator

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    which is why I told them to employ a registered electrician and then I closed the thread
     
  8. Tha fios agaibh

    Tha fios agaibh

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    ..And what makes you a good moderator.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Which is why I suggested the person plugging in the products, running up and down the line to turn them on and off, and swapping them out with new ones to test, was probably an intern or apprentice. Not directly applying that to @haqq, but if the shoe fits... And if it does fit, maybe he or she can take our "solution" to management and win some brownie points... but maybe also get replaced by some muscular lunk who doesn't mind lifting microwave ovens or whatever off a cart to be tested on a bench. Can't leave 'em on the cart to be tested, BTW; that cart is needed to go back and get more product while the current products are being tested. The whole idea of moving manufacturing to another off-shore location is to minimize cost, maximize profit, and maybe avoid taxes. So, @haqq may be either on the way up the corporate ladder or out the door.
     
    Tha fios agaibh likes this.
  10. Bluejets

    Bluejets

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    Did a sequence switcher like this about 10 years ago which also involved priority switching.
    Controlled 6 /30 metre poles with 3 x metal halide 400w fittings on each spread over 2 football fields.
    Used a small commercial PLC with aux relays at a central location controlling contactors at each pole base.
    Took quite a lot of thought and experience to get it all working just right.
    Supply authority were happy too.:)
     
    Last edited: Jan 23, 2016
  11. Tha fios agaibh

    Tha fios agaibh

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    I agree, but not because of simultaneously switching 30kw.
    Rather, thinking it can be answered merely by knowing a kw rating.
     
  12. hevans1944

    hevans1944 Hop - AC8NS

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    Huh? Go look at the reply again. @haqq clearly stated he wanted to switch a 30 kW load on and off and asked how to do that. The moderator just as clearly stated this was a job for a licensed electrician, not suitable for a hobbyist forum, and closed the thread. I wholeheartedly agree with that.

    This forum is plagued with posters who describe what they think is a solution: Switch a 30 kW load. What they should post is the problem they are trying to solve: How do I automatically power up 32 devices that require 1 kW each, wait an interval while the devices are tested, then power them down again? That sort of problem statement, and its solution, is within the realm of this hobbyist forum, with the caveat that the heavy lifting (switching mains power) be performed by a licensed (or at least qualified) electrician.

    The number of devices to be switched is not part of the problem, except that the total power switched is an increasing function of the number of devices. The solution to sequence the application of power has nothing to do with the number of devices if the mains circuit (or circuits) can provide the required power when all devices are on. Hence the need for a qualified electrician to evaluate the situation.

    This "problem" and its proposed "solution" is not limited to 32 appliance products. It could involve switching a hundred motors or a thousand lamps, or any number of loads consuming any number of watts. The "problem" is how to turn those loads on and off, not necessarily how to do that all at once... unless you are the qualified electrician responsible for restoring power to the neighborhood, as I mentioned in my post #1 on this topic.

    It might be necessary to consult a qualified electrical engineer if it's a really big plant with other equipment that could be affected by switching a "largeish" load on and off, whether all at once or in increments of power. And that is outside the purvey of this forum.

    For the example case proposed by @haqq, it is obvious that the existing plant can provide the power required when all the products are turned on, and no deleterious surges occur when the power is turned off after the testing is completed. So a professional electrical engineer is probably not required.

    Power companies are well aware of the problem of what happens to the grid when a significant load is suddenly applied or removed. Sags and surges occur. The same thing would occur in a manufacturing plant, but large plants are equipped to deal with it.

    Utilities providing power to the grid would love it if every service-entrance drop was equipped with a remotely controlled switch to allow them to "more gently" restore power after a power outage. In some areas of the United States such switches are already installed on major loads such as air conditioners and water heaters to allow those loads to be shed during "brown-out" conditions when the grid is becoming overloaded. After the grid recovers, those loads that were shed can be selectively restored. IIRC, the utility will pay for the remote switching device and/or offer a reduced electrical rate to the customer agreeing to have it installed.

    So, sequential load switching is not new technology. It's not even rocket science, although NASA uses it extensively. It is frequently used in electronics where power must be provided in a particular sequence to avoid circuit damage. How to safely do it should be a topic for discussion in this forum because it does have hobbyist applications. But unless @haqq posts a response to what has already been discussed here, this thread should probably be closed (again).

    Or maybe someone will hi-jack it to talk about how BJTs work. Oh, wait! There is already an open thread for that.:cool:

    Hop
     
    davenn likes this.
  13. davenn

    davenn Moderator

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    NO, that was the MAIN reason for closing it
    That is a serious electrical systems undertaking

    Thanks Hop :)
     
  14. Tha fios agaibh

    Tha fios agaibh

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    Yes, I'm aware of that and have no problem with the inquiry provided it's done safely by someone qualified.
    Perhaps, but it's also possible his solution is ideal for his application.
    You don't know enough about the application to say.
    Don't get me wrong. This certainly raises an eyebrow with me, but not because of simultaneously switching 30kw, but the absence of detail as to why its warranted.

    I never said it did.
    30kw is not a "largesh" load, certainly not in an industrial setting anyway. Turning on and off 30kw loads is not a "problem" either, especially at higher voltages.

    I didn't read "I'm doing this in my house" So, The idea of a brown out and having to restore power to the neighborhood is a bit over the top. Heck, even the residential panel in my house has a capacity of 48kw.

    But yes, I agree having a qualified electrician perform the work is of top importance.

    BTW, Just because someone is licensed, that doesn't necessarily make them qualified.

    John
     
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