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Power switching relay

Discussion in 'Datasheets, Manuals and Component Identification' started by flippineck, Feb 22, 2021 at 2:31 PM.

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  1. flippineck

    flippineck

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    Sep 8, 2013
    Trying to find a DPDT non-latching relay with contacts rated 250 V AC / 40A and ideally a coil able to operate at either 5V DC, or else around 240 V AC

    Closest I have found so far is the 20844-85 at:

    https://deltrol-controls.com/sites/default/files/pdf/data-sheet/270-275 Series Data Sheet.pdf

    I've ordered one to be going on with but the contact current rating is not quite as high as I'd like though.

    Hoping someone might be able to suggest places to look (I'm ordering from the UK). Thanks.
     
  2. WHONOES

    WHONOES

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    May 20, 2017
    Sounds like you need a contactor rather than a relay. They are just bigger relays but designed to handle a lot of power.

    Perhaps something like below?
    2224331.pdf (farnell.com)
     
  3. flippineck

    flippineck

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    Sep 8, 2013
    I used to come across contactors a lot many years ago, working in a panel builders shop.. more substantial yes, but they always seemed to come in 2-terminal NC or NO types, never dual throw, & that's something I need for my current project. I'm using it to choose between two different supplies, a small 2kW 12V - UK mains inverter and a larger diesel generator rated at 10kVA / 8kW.

    Are dual throw contactors commonly available?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Look up automotive relays. Coil voltage is usually 12 V, not 5 V, but high currents can be switched,
     
  5. flippineck

    flippineck

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    Sep 8, 2013
    I've been discounting automotive relays because I was worried they would mostly be designed with 12 or 24 V DC in mind, and putting mains voltage on the contacts would be using the device out of spec.

    I notice the phrase "N.C. mirror contacts" mentioned in Whonoes' link - sounds like it's a specifiable dual throw feature option.. unfortunately not available in the combination of specs I need

    Farnell is one supplier I've not looked at so I'm having a good root around on there, thanks. Potter & Brumfield PRD-11AF0-240 http://www.farnell.com/datasheets/1993351.pdf would seem ideal but it doesn't seem to be possible to actually order that particular spec combination from them.
     
    Last edited: Feb 22, 2021 at 9:14 PM
  6. WHONOES

    WHONOES

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    Are you looking for a change over function?
    Can you draw a diagram of how you want your system?
     
  7. Bluejets

    Bluejets

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    Oct 5, 2014
    You will not find a 240v AC changeover type contactor at that 40a rating.

    There are contactor style relays which changeover but I'm pretty certain they will bow out at around the 20A max.

    Usually in a 4 pole arrangement.

    I for one, would not like to be switching a 40A load "on" through contacts under "unloading" of a contactor, just not designed to do that.

    Come to think of it , it probably violates many electrical standards having a particular load energised under no signal condition as there is no way to isolate in a control fault condition in a fail safe manner.
     
    flippineck likes this.
  8. WHONOES

    WHONOES

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    I was thinking that 2 devices could be used to perform a changeover function if that is what is required.
     
  9. flippineck

    flippineck

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    Sep 8, 2013
    Rough idea attached. As shown, the diesel generator 'hoists itself' onto the main ring by supplying the coil with it's own 240V when it comes on line. I'd also considered controlling the coil fully from the Arduino. The smaller inverter power supply is left running to handle small loads and act as a sentinel for intermittent larger power demands.

    circuit.png

    I've been worried about high currents because of the more powerful generator's 10 kVA rating. Using 10 kVA and taking the mains voltage as 230V gives me just over 43A. If I use it's 8 kW rating and 240V as the upper limit it seems maybe I'll be ok with the 35A relay I did manage to get, and can stop searching for a 40A / 50A relay, or multiple contactors?

    Guess I could also cap the maximum current in the software on the Arduino.

    The generator itself is stamped with a power factor rating of 0.8
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

    10,999
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    Nov 17, 2011
    The 10 kVA rating is what the generator is able to deliver at max. It is not what the generator forces into the load. The actual current is determined by the load. Since the inverter is rated at 2 kW only, the load cannot draw more power than this 2 kW, otherwise the inverter will be fried or, preferentially, shut down due to overload. Consequently if the load cannot draw more than 2 kW from the inverter, it will also not draw more then 2 kW from the generator.
    2 kW / 230 V = 8.7 A. Round that off to 10 A and we're talking sensible values now.
    The relay used should be a safety relay to avoid a short circuit between generator and inverter in case of a failure.
     
  11. flippineck

    flippineck

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    Sep 8, 2013
    The load is a small workshop power supply incorporating a few lighting circuits and power ring with a few 13A UK switched mains power outlets fed by a 2-breaker consumer unit, so the load would vary according to what people plugged in & switched on at any given time. I could limit the current draw of the load to say no more than 35A, with a 32A mcb for the power ring and a 3A mcb for lighting.

    Ideally the Arduino would be programmed to switch over to the larger generator at quite low current levels. One big problem I can think of is what would happen if somebody suddenly for instance, switched a kettle on, or 'blipped' a high current by starting a small motor. The Arduino would have to detect the rise in current before the inverter shut down (it does detect overcurrent and shut down gracefully but I'm not sure how quickly). I'm wondering if I could introduce some form of soft start into the supply to give the Arduino extra time to react under such circumstances.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    How are you going to power 35 A of load from a 2 kW inverter? That's not going to work.
     
  13. flippineck

    flippineck

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    Sep 8, 2013
    The 2kW inverter should be ably to supply up to, just over 8A whilst there are only a few small appliances connected. For instance a couple of LED light fittings switched on and a mobile phone charger or two, maybe a refrigerator, dare I mention a TV. That would generally be the state the system spends most of it's time in. It's only when people decide to come in & use the brew room & decide to switch on things that draw a more substantial current, like toaster, kettle, microwave etc, that the supply would change over to the diesel generator (and disconnect the inverter) if necessary. The exact changeover point would depend on the Arduino software.
     
  14. Bluejets

    Bluejets

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    Oct 5, 2014
    Anything which contains a motor, such as the refrigerator, has to be considered as a high current device.
    Reason being, the startup current of approx. 4 to 6 times the full load rating.
    This will drag a generator's voltage down to a point where the motor cannot start and will bog the whole system, possibly cook the frig motor into the bargain.
    Changeover from one power source to another requires a certain amount of delay to ensure the one supply is really completely off and the second supply is up to speed so-to-speak.
    Then there is the complexity of how the switching is arranged for power from unit one coming back on again i.e. required delays or how to lock it out whatever......
    It is normally a requirement to include some form of mechanical as well as electrical interlock.
     
    Last edited: Feb 25, 2021 at 12:24 AM
    Harald Kapp likes this.
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