Choosing a relay

Discussion in 'General Electronics Chat' started by Konstabel, Feb 17, 2008.

  1. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    How do I choose a relay that will be able to handle the required load? I want to switch a load of 220Vac,2200W (kettle element). The switching side is not a problem to me. I know how to specify that. (In this case 9Vdc switching).

    When looking at the manufacturer sheets, they specify VA ratings, Vac ratings, current and W ratings. So which one do I choose?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    VA = V x A = Volts x Amperes = Power (in Watts) in a DC circuit.
    In an AC circuit, VA also is dealing with loads that have reactance (like coils, inductors, motors, etc.)

    Although your kettle element is mainly resistive, there may be a minor amount of reactance to it, as they are typically wound rather than being straight conductors. If you multiply the VA rating times 0.6, you should see the result in Watts. Conversely, multiplying power in Watts by 1.67, you'll see the VA rating.
     
  3. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    So going from VA to W and vice versa you are basically moving between the RMS and peak values? Right? So the 0,6 you mention is actually 1/sqrt(2)?

    So if looking at the following relay - found at this website -

    http://docs-europe.electrocomponents.com/webdocs/0514/0900766b80514594.pdf

    I'll be able to switch a maximum of 16A continuous current. So from P = IV the max continuous current is going to be 2200W/220V = 10A. So this would be fine.

    And also it is rated as 250Vac so it will be able to handle that.

    From the above I reckon it should be fine for my application - switching the element on and off via a 5V signal with a transistor circuit. But what I do not understand is the load rating.

    In this case it is given as 550W. Will I still be able to use this relay?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The AC15 rating is for inductive loads like motors, solenoids, etc.

    Your load is mainly resistive (heating element) although there may be a small inductive component that would become apparent at higher frequencies, your load doesn't fall into this category.
    This page will help to describe it a bit better:
    http://www.electronicproducts.com/ShowPage.asp?FileName=surr_phoenix_aug2007.html

    Something else to consider is the life expectancy of the contacts; running near the maximum load will result in shorter contact life. Your relay has a life expectancy of 2.7e5 operations at 10A, and your heating element is cycling on and off every 5 minutes, that is 288 make/break cycles per 24 hours. At that rate, your relay would fail on the 902nd day, or roughly 2.5 years.
     
  5. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
    52
    0
    To all, this is the information I managed to gather. Hope that someone finds it useful.

    When choosing a relay the first step is defining your load, ie. resistive, inductive or capacitive. This is important because it defines the power angle of your power triangle (just in the case of AC loads). This is basic power electronics.

    The reaction of the relay contacts to the different kinds of loads is significantly different. Lifetime, make/break operations and all that are influenced. So the designers are making the contacts of different types of metal alloys to cope with the different loads.

    Also the phase angle has an influence on the inrush currents, which also influence the choice of relay. For a purely resistive load, the inrush current (from what I understand) is basically the same as the continuous current rating or your load. If inductance or capacitance are introduced the inrush current starts to increase.

    So why then a volt-ampere(VA) and watt(W) rating then? From all the datasheets I have been viewing, most of them only has a VA rating. So basically in a purely resistive load you have that amount of W the load can handle. If not (phase angle increases), it is up to you to calculate your power triangle to make sure that your reactive and real power components fits the load rating.

    The for the coil side. I believe it is very straight forward but I'll mention it any way.

    Decide whether or not you want to switch your relay with an ac or dc voltage. Then choose the voltage rating in this category that suits your application.

    So that is it. Very basic but I hope it helps. I also found this guide on how a mechanical relay works. Very informative. Check it out.
    http://www.schrackrelays.com/schrack/techn/relbook.asp
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Buy one which is rated 240 VAC(rms) and 20 A(rms) and you will be ok.
     
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