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  1. #1
    I designed and made a control panel for a furnace a year ago and it has worked well except for one relay in the 3phase control box. This relay powers up a contactor to keep the fan running for the air supply. The furnace is on 24/7 and that relay has burnt out with a coil overheat twice. This happened for the second time today, while I was visiting the workshop, so fortunately a spare relay could be plugged in without the furnace cooling down too much.
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    The only thing I can think of that caused the problem was that the relays are 24vAC and the power supply is a solid 24vDC. The next relay will be DC rated, and I can only think that the AC relay coil limits current to a 'safe' level due to its' inductance.

    I would welcome any input on this problem - in fact I will relay it on to the furnace owner (GROAN !!!).

    Cheers,

    Rob

    BTW I made the front panel on my CNC router !

  2. #2
    Quote Originally Posted by cropwell View Post


    The only thing I can think of that caused the problem was that the relays are 24vAC and the power supply is a solid 24vDC. The next relay will be DC rated, and I can only think that the AC relay coil limits current to a 'safe' level due to its' inductance.

    Rob
    Rob I am sure it is because as you say A DC relay with have a higher resistance and the AC relay will be lower R because inductance will come in to play and limit the current. It would be interesting for you to measure the current with the AC relay fitted and then check again with the DC relay in circuit. ..Clive
    ..Clive
    The more you know, The better you know, How little you know

  3. #3
    I suppose that if they just put the voltage on it would be fair to assume that AC or DC didn't matter ;-)

  4. #4
    Quote Originally Posted by magicniner View Post
    I suppose that if they just put the voltage on it would be fair to assume that AC or DC didn't matter ;-)
    That's what I thought when I used the relays (which I happened to have). The other relays in the main panel are OK, but they don't operate in a 'pulled in' mode 24/7, but this one has a safety relay function, if the gas fails or the electricity goes and the furnace drops below 800C then the air blower won't come back on and air pressure operates the gas solenoid. Above 800C the furnace will relight itself OK when the electricity supply comes back on, but then it will only operate in a low fire mode (reaching about 1000C) until manually reset to full operation, when it will go to the set temperature, normally 1250C.

  5. #5
    Quote Originally Posted by Clive S View Post
    It would be interesting for you to measure the current with the AC relay fitted and then check again with the DC relay in circuit. ..Clive
    Clive, It is in the 3phase box and I would not go in there with power on

  6. #6
    Quote Originally Posted by magicniner View Post
    I suppose that if they just put the voltage on it would be fair to assume that AC or DC didn't matter ;-)
    The relay states 24V AC on the picture. Thinking about it 24V AC will be RMS so the peak with be higher about 34V !!
    ..Clive
    The more you know, The better you know, How little you know

  7. #7
    Quote Originally Posted by cropwell View Post
    The only thing I can think of that caused the problem was that the relays are 24vAC and the power supply is a solid 24vDC. The next relay will be DC rated, and I can only think that the AC relay coil limits current to a 'safe' level due to its' inductance.
    Eh why would you use a AC relay with DC supply you numpty Rob. .
    Would you put AC servo's on your router and Try to run them on DC supply and expect to work correctly.!! . . . . The AC relay didn't limit to a safe level it tolerated it until couldn't tolerate any more then melt down.!!

  8. #8
    Quote Originally Posted by Clive S View Post
    The relay states 24V AC on the picture. Thinking about it 24V AC will be RMS so the peak with be higher about 34V !!
    Yes, indeed it will and you will then have AC running through an inductance, which should tell you something, if it doesn't then at least it tells you why AC or DC is included in component marking and should be observed ;-)

  9. #9
    Quote Originally Posted by JAZZCNC View Post
    Eh why would you use a AC relay with DC supply you numpty Rob. .
    Would you put AC servo's on your router and Try to run them on DC supply and expect to work correctly.!! . . . . The AC relay didn't limit to a safe level it tolerated it until couldn't tolerate any more then melt down.!!
    .
    As I said Jazz - I had them in stock.

    My workshop gets cold in the winter, do you reckon I could wire some stepper motors up to the mains and use them as heaters, I have a box of high inductance Astrosyn jobbies ?

    I wouldn't expect an AC servo to work on DC, cos I know how servos work and I know how a relay works, it is just a solenoid. The first burn-out was caused by excess current and I will put my hand up to not thinking about it. The second burn-out was because the furnace owner replaced the relay with like.

    We put in another AC relay to keep the furnace on over the weekend, but it will be replaced by a DC block on Monday.

    If you want to see the control panel it is monitored by an IP camera
    http://www.akroydglass.supracam.net/
    login: guest
    password: guest

  10. #10
    Quote Originally Posted by cropwell View Post
    .
    As I said Jazz - I had them in stock.

    My workshop gets cold in the winter, do you reckon I could wire some stepper motors up to the mains and use them as heaters, I have a box of high inductance Astrosyn jobbies ?

    I wouldn't expect an AC servo to work on DC, cos I know how servos work and I know how a relay works, it is just a solenoid. The first burn-out was caused by excess current and I will put my hand up to not thinking about it. The second burn-out was because the furnace owner replaced the relay with like.

    We put in another AC relay to keep the furnace on over the weekend, but it will be replaced by a DC block on Monday.

    If you want to see the control panel it is monitored by an IP camera
    http://www.akroydglass.supracam.net/
    login: guest
    password: guest

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