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Adjustable controller

Discussion in 'General Electronics Discussion' started by nob4uask, Feb 16, 2014.

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  1. nob4uask

    nob4uask

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    Feb 16, 2014
    Adjustable power controller

    I'm not an electronics person and not even sure if anyone here can help me but thought I would give it a shot. Here is my situation.

    We have a commercial sandwich press that is 240v 1800w 10a. It is very efficient and once it gets to operating temperature only comes on for a short period to heat back up.

    We are only allowed 1x10a power outlet but wish to add a 240v 700w 10a hot dog grill. I'm not sure on the efficiency of it but it is probably quite good.

    We have a couple of smaller items that require a couple of amps but that is about it. Our problem is that if during the day both of the above mentioned items have to heat back up at the same time it will pop the 10a power board/ strip. We could use a splitter before the power board/ strip and probably get away with the additional power draw but could get in trouble if it is noticed.

    My question is is there anything out there that I can plug the sandwich press and the hot dog grill into to limit the power draw? It might take a little longer for it to heat back up but would solve the limited power problem

    Thank you in advance.
     
    Last edited: Feb 16, 2014
  2. davenn

    davenn Moderator

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    hi there
    welcome to the forums :)

    OK just initially to get something correct

    240V and 1800W does not equal 10A, its only 7.5 A

    and again 240V 700W does not equal 10A, its only 2.9 A

    how does that sound ? :)

    Dave
     
  3. nob4uask

    nob4uask

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    Feb 16, 2014
    Thanx for the reply Dave,

    I meant 10a plug not 15a which some commercial appliances require. I had the same math as you did and it is a 10a power outlet so the numbers still add up to over that plus the little extra that will cause the power board/ strip to overload or worse case scenario trip the bigger circuit although from what I hear they can usually handle a little overload.

    By the way, name is Dave to... ;-)
     
  4. donkey

    donkey

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    hey buddy welcome.
    now I am a noob and I would not recommend ever overloading a circuit board due to fire risks HOWEVER
    the amp rating for both while the numbers don't add up lets assume you do need a 10AMP outlet. now the reason for this is the time while it heats up on the start will draw a lot of current to get to the desired temperatures. THEN it goes into soft power mode that uses less amps to maintain the temperature. at this point I would TRY to turn on the second appliance. now this may blow your circuit but if it doesn't then they will both go into standby until they need to reheat.
    the way to think about it is you have 2 beakers of ice that you want at 50 degrees celcius, but you only have one flame. if you constantly alternate the 2 it will take forever, however if you get 1 up to 50 and then put it on the edge of the flame and allow the bulk amount of the flame heat up the second one then it goes to the edge of the flame to maintain its temp. If you ever watched Apollo 13 its they had to do something similar so as not to draw too many amps from the battery, its all about the right sequence and making sure you only use what is absolutely desired.
    once again you can TRY this but it may blow your board and you may need to reset it.
    on a secondary note the number you supply don't add up. watts is determined by volts multiplied by current so 10amps time 240 volts is 2400watts. I am thinking what you actually have in total when both are heating up is 2500wats which will definitely blow your board, but if you turn one on as I mentioned then the current draw should drop and hopefully by that magic 100 watts to allow the other to work.
    also you say you will get into trouble for the additional power draw, I will point out no matter what you do here it will show on the meter. they measure kilowatts per hour and you are using 700watts more, that's nearly half of what you were using and it will show.
    last but not least you mention you have other appliances I seriously recommend that you test this out in a non busy time and get ready to reset the RCD/safety switch. there are certain limits to what can be done.
     
  5. donkey

    donkey

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    also this takes into consideration that all appliances are new and working efficiently as older items can draw a little more current to make up for the wear and tear
     
  6. nob4uask

    nob4uask

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    Feb 16, 2014
    Thanx donkey,

    I should probably edit my OP because I was specify the plug type not the amps that the device draws. The excessive watts issue was the same math I got and it blows the circuit. While driving today I remember as a kid there was a controllers that controlled the speed of little electric motors by reducing the power. I was wondering if there was anything out there that would allow me to plug the devices in and adjust the power that is given to each device by a little to get under the 10a/ 2400w issue.

    Thanx for your time.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, here's the thing, at 240V, 10A is 2400W, so your 1800W device only draws 7.5A.

    Added to this, the 700W device draws a little under 3A Together they're only marginally over 10A

    Have you actually tried using these on the same powerboard?

    Assuming that the other devices can operate correctly if the power is turned on and off (i.e. they're simple thermostat controlled devices) then the approach I would use is as follows:

    I would sense the current by placing a couple of diodes in inverse parallel in series with the power to the highest power device. The voltage across this would go to a bipolar optocoupler. The output of the optocoupler would be used to control a relay that would switch the other devices on and off.

    This requires mains wiring (and in the simplest case also a low voltage power supply). Since you have no experience with this sort of thing, it's probably both too dangerous and not simple for you to assemble yourself.
     
  8. donkey

    donkey

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    Feb 26, 2011
    Steve the OP stated there were other "smaller items" also
    am I wrong in assuming that the watts stated is only for the initial warm up? after that they would use a lot less right? or even use it for shorter periods which if you are lucky will prevent the 10amp total being reached
     
  9. nob4uask

    nob4uask

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    Feb 16, 2014
    Thanx for that steve,

    You pretty well summed it up. Once initial power up is complete all devices will run just fine until that time comes when both the highest power consumers have to reheat at the same time. This is at a market stall so we are outdoors and with the use of the equipment the odds are pretty good that it will happen. I would just use a splitter on the main lead before the power strip and go about it that way because being slightly over probably wont trip the mains. But if someone sees it I might have to shut some stuff down. Don't thay have something that I can limit the power to each device by an amp or so so that if they do come on at the same time it will still be under the 10a max?

    Sorry but I don't understand the diodes thing and all, like a post earlier I kind of referenced a controller that controls the speed of an electric motor or come to think of it a light dimming switch.

    Dave
     
    Last edited: Feb 16, 2014
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    From the description it sounds like it cycles on and off, so it either draws 7.5A or nothing.

    Of course it might be different but we have no other information.
     
  11. duke37

    duke37

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    Jan 9, 2011
    The devices probably switch between full and off so could be on full together even when warm.

    If you can find a plug in phase angle control for the bigger device, then this would be the safest way to reduce the load. Here in the UK 5A lamp dimmers are common so could be used on the 700W thingy.

    Note that these controls work by chopping the waveform and the peak currents may not be reduced much.

    Have you taken a rickshaw down to you local souk and see what they have there?
     
  12. nob4uask

    nob4uask

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    Feb 16, 2014
    Please look at my previous post responding to yours. Yes they cycle on and off once they get to operating temperature then the thermostat determines when the temp start dropping and turns it back on to get to operating temp. Usually on for a minute or two at the most.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Without experience with electronics and especially with mains wiring my suggestion isn't going to fly for you.

    I'm trying to think of a something by all the options I can think of turn other appliances on, not off.
     
  14. nob4uask

    nob4uask

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    Feb 16, 2014
    Thank you for your time sir, it is much appreciated.
     
  15. shumifan50

    shumifan50

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    WARNING: MAINS WIRING CARRIES ENOUGH POWER TO KILL, SO FOLLOW ALL NORMAL SAFETY PRECAUTIONS.

    1. Take the supply to a normally closed relay with an AC coil.
    2. Connect one load, L1, (lowest priority load) through the 'normally closed' relay
    3. Make a few windings around ONE leg of the wire going to the the other load(L2) creating a sort of transformer that will generate power when the L2 comes on. Use this power to pull the relay, which will turn L1 off while L2 is on.

    This is just a suggestion. I am not an analogue electrics theory person:eek:, so I dont know how many windings would be needed around the power lead or whether the 'transformer' would be able to generate enough power to pull the relay or require an electronic circuit. That would require modifying any of the load devices.

    If this is workable it could be built into a box that plugs into the wall and the 2 loads plugged into the box.:)


    Found this circuit that will detect the electro-magnetic field and amplify it to drive a set of headphones. This should be modifyable to drive a relay.
     
    Last edited: Feb 16, 2014
  16. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I had a similar idea to Steve's. Detect when the heavy load's thermostat turns ON and kill the lower priority loads during that time. But I agree with Steve that it's probably not a suitable project for a beginner.

    One way to make it a bit tidier would be to modify the inside of the heavy load device. You would add an AC-coil relay with its coil connected across the heating element, so it would activate when the thermostat kicks in to heat it up. You would add a mains socket on the unit, and feed it from the mains input via the normally closed contact of the relay. Then you would plug your lower-priority loads into that socket.

    Every time the big load cooled down and the thermostat kicked in to heat it up, the relay would activate and disconnect the mains from the socket, so the lower-priority loads would lose power. Once the heater got back up to temperature, the thermostat would turn OFF, and the relay would drop out, and reconnect the mains supply to the socket and the lower-priority loads.

    This would certainly void any warranties, and would have to be done by someone qualified to work on mains-powered appliances. It would be wise to have it done and/or inspected by a registered electrician. Even then I can't be sure that it would be legal or safe; you would need someone locally to take responsibility for determining that. But it's a possibility I guess.

    Another approach would be to drop the voltage to the heavy load, using a transformer. It would have to be a pretty big transformer, and you would lose some power in it.

    That would work by reducing the current (and power) consumed by the heavy load device. It would mean that it would heat up more slowly, but it sounds like the heater is much more powerful than it needs to be for the application, so it might be workable.

    The heating element has a roughly constant resistance, so you can use the formula P = V^2 / R. That's power equals voltage squared, divided by resistance. If resistance is constant, power is proportional to voltage squared. So to halve the power, you need to reduce the voltage by the square root of 0.5, which is 0.707. So your transformer would have to transform 230 VAC down to about 163 VAC.

    This would halve the power consumption, and therefore the current consumption from the mains supply. Actually not quite, because of power loss in the transformer.

    That method would halve the heating power of the heavy load and would make it take longer to heat up initially, and to come back to temperature during operation, but it would require no modification to any of the appliances. The transformer would be big and heavy, and pretty expensive. You would have to get it custom made, and it would be wise to find the optimum output voltage by trial and error. Also you should check with the manufacturer of the heavy load device that this would be safe and reliable.

    Good luck!
     
  17. shumifan50

    shumifan50

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    The following circuit could be used to detect the presence of an electro-magnetic field around the wire and by some little changes can be used to drive a relay. This will avoid the need to modify the load devices as it can all be done in an external box.

    Electromagnetic field detection circuit

    This is losely based on a Rogowski coil except we are not interested in measuring exact current, just the presence of it.


    If you want to make your own coil it is important to note that the more windings the higher the voltage it outputs. The same also applies: the higher the current the higher the voltage. So for your specific load you might have to experiment with different number of windings.
     
    Last edited: Feb 16, 2014
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I mention this mostly as a curiosity from the past...

    I recall a circuit to run your soldering iron at half power. It consisted of a diode in series with the mains.

    If each of the large appliances had this, with the diode pointing in the opposite direction so each saw opposite half cycles, it might work to share the power by giving each one opposite sides of the waveform. The problem would be that each would effectively run at half power.
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I don't think that would be workable unless (a) the appliances could be grouped into two sets that had roughly the same power consumption, AND (b) the appliances either drew the same amount of current all the time, or switched on and off in unison.

    Since these are heating loads, controlled individually by their own thermostats, I don't think the requirements of (b) could be met.
     
  20. shumifan50

    shumifan50

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    This kept on bothering me. I found an articleon Rogowski coils, but the coils were large and cumbersome (24 inches long flexible coil). So I thought there must be a better solution. I then found an doc on the theory of current measuring and more specifically, current transformers. Then hunted for commercial fairly low current versions, but most are for 100-500 amp.
    However, I found this cheap one that would be perfect for this project to detect when current is flowing to one of the devices so the other can be switched of. The current transformer and switching circuit can be built into a separate box and could even be cascaded to allow more than 2 devices.

    Current Transformer (cheap)
     
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