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Which relay should I use?

Discussion in 'General Electronics Discussion' started by roig12, Oct 29, 2016.

  1. roig12

    roig12

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    Oct 29, 2016
    Hey Guys,

    I'm looking for a relay to switch a mode inside my ebike's hub motor.

    The hub motor receives 88v DC and 45A from the ebike controller and I need to have a relay for each of the three phase wires of the hub motor.
    * In a motor that has 3 phases, how much voltage and current are passing through each phase wire? the whole 88v 45A or is it divided by three?

    It has to be a 5 pin relay.

    It seems impossible to find a DC relay with such high voltage.

    Many people who do this, used 12v and 24v DC relays with about the same voltage as me and fried their relays so that's why I'm here...

    Would appreciate some help with this :)


    Have an awesome week,
    Roy
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    Jan 21, 2010
    Why not use mosfets to switch the current?

    What are you actually trying to do? (I.e. What function will this relay perform?)

    What do you mean "it must be a 5 pin relay"? SPDT?

    If this is three phase, don't you need 3 relays?
     
  3. roig12

    roig12

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    Oct 29, 2016
    Yes i will need 3 of those.
    It will change the phase configuration from wye to delta.
     
  4. duke37

    duke37

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    718
    Jan 9, 2011
    Three times 24V = 72V, connected in series, probably near enough to stop the relays overheating.
    Wire them up so that they are not energised for most of the time..
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I've done some research on delta/wye switching.

    You're switching fairly significant inductive loads. If this is done under load, I'm not surprised that the relays don't last long.

    Are you switching under load? Can you remove power briefly while you reconfigure the motor?

    Doing this electronically requires 6 switching circuits, each with it's own floating drive -- not as trivial as it might sound.
     
  6. roig12

    roig12

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    Oct 29, 2016
    Yes it's always under load, it's like shifting gears.

    I heard people that do this use two relays for each phase but I didn't know how it's done, can you really spread the load onto 2 or even 3 relays for each phase? say 3pcs 30v DC relays in parallel somehow for each phase wire?
    How exactly do I connect it so the load will spread over a few relays?
    and what about AC relays? they are high voltage buy low current...
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    Jan 21, 2010
    Well, if you're shifting gears, you take the load off the gearbox while changing gears (even in an automatic gearbox).

    The problem with picking a random moment to switch from wye to delta (or vice versa) is that you'll get huge voltage spikes with enough energy stored in the inductors to destroy the contacts in fairly short order. It would be far better to remove power for maybe 100ms before switching.
     
  8. roig12

    roig12

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    Oct 29, 2016
    I can behave myself and remove the load for a second but shouldn't the relays handle the load if I use them within specs?

    so which ones should I get? I prefer that the voltage that switches the relay wouldn't be above 12v.
    What about my question about the ac relay and it's low amperage? also, can I connect relays in parallel to spread the load?
     
  9. Maxwell

    Maxwell

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    3
    Oct 1, 2016
    Yes but you're not using them within spec, see what steve said about inductive discharge, you'll be putting a lot more than 88v into them when switching if you don't power down first.

    Using AC relays on DC power is problematic, you'll have to use overdimentioned relays (24v instead of 12v for ex.) or you'll fry it, might also be slow to close because of that, or not close at all, or stick closed because the coils iron cores become magnetized permanently by lack of reverse polarity. I think you're better off using DC relays on DC power, put them in parallel to divide the load across them (same deal as using multiple thin wires to power a load to large for just one).

    Typically, 3p 45A means 3x45 amps. So you'll have to use 3 16a relays per phase as a bare minimum. Also, this would mean the E-bike consumes upto 7Kw, that seems like a lot?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
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    This relay may be suitable.

    DPDT, 30A contacts. Place them in parallel to get 60A. Rated for 125VDC with a 12V coil.
     
  11. roig12

    roig12

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    Oct 29, 2016
    It's a 4.2kw setup.

    Can you give me example of a suitable relay from ebay that has the highest dc voltage?
    When I parallel relays, does the voltage spread over the paralleled relays like the current does or do I have to find 100v dc relays?
    I need them to be normal size, unlike what steve showed as they would be inside the hub motor, so about a 2cm cube.

    Can I use a DC input AC output solid state relay to avoid the AC relay not sticking or permanently sticking problem? This one can be triggered with 3-32v dc.
    http://www.ebay.com/itm/1-2-5-10PCS-Solid-State-Relay-SSR-25DA-25A-250V-3-32V-DC-Input-Output-24-380VAC-/191937191357?var=&hash=item2cb0591dbd:m:mOSoAEPm4X5UJIGg8lC1HcQ

    EDIT:

    Even found this:
    http://www.ebay.com/itm/JGX-33-80A-...210454?hash=item3603379d96:g:WLAAAOSwImRYE~T9

    BUT, right when it seems that SSR's are the perfect solution for low voltage DC input and high voltage DC output, it seems that none of them are SPDT as I need to switch from one wire to another and not just an ON/OFF switch.
     
    Last edited: Oct 30, 2016
  12. Minder

    Minder

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    Apr 24, 2015
    Relays made to switch very large DC inductive loads have special qualities built in in the form of arc quenching such as magnetic arc-blowout etc, when a contact carrying this type of load opens that does not have protection, a plasma arc can be produced that continues to conduct Well after the contact has opened which destroys the contact completely.
    M.
     
  13. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016


    H
    i,

    I was referring to the coil of the switch not the actual switch, which is just a contact that can/will handle anything up to it's rated spec. Using an AC coil on DC can cause said issues. This has nothing to do with the actual switch which is just a contact and will conduct whatever you put through it, up to it's rated spec.

    Using relays over rated spec causes all kinds of issues like arching, contacts sticking, etc. If you're going to use mechanical relays, prepare to use either large ones or have them constantly cause issues.

    The 80A 3p SSR seems suitable for what you're trying to do, but you'll have to do some checking to see if it can handle the inductive load when switching... or you could power down before hand like steve said.

    Also, the SSR may run hot when you're pounding the E-bike, it's a FET so it will produce heat.
     
  14. roig12

    roig12

    21
    0
    Oct 29, 2016
    It's supposed to be inside a hot hub motor...I guess I can expect more problems from heat or is that an issue also for relays?
    But again, the SSR isn't suitable for me since it doesn't have SPDT, only ON/OFF.

    So relays again...what do you recommend (you guys probably know the common relay ratings more than I do) that is not much bigger than a 2x3cm cube. so parallel 3 pcs of 32v DC? why those things on ebay don't say the switching voltage (input) and just the output? is that the same? 32v input to switch it on?
     
  15. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016
    Have a look at this, you can make your own ;-)

    High current mechanical relays will be bulky, there's no way around that as the materials needed to withstand the current are bulky, SSR with SPDT are hard to find, and will likely also be bulkier than SPST ones. I guess then your best option is to make the circuit for every one of your phases, and then you can use simple (and small) FET to handle the load.
     
  16. roig12

    roig12

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    0
    Oct 29, 2016
    I don't have the technical knowledge to understand and build such a circuit even though it isn't complex.
    Do you know if there is a website or any way that I can pay someone to do this for me?

    About the relays, is the voltage being spread in half if I parallel two relays?
     
  17. duke37

    duke37

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    Jan 9, 2011
    I have been trying to follow this thread without success. It seems that there is a mix up between the relay coil supply and its switching capability.

    To switch motors between wye to delta, contactors rather than relays are used since they are made to stand the energy generated when the contacts open. If the motor is three phase there will be no DC present.

    The contactor coil will need to have suppresion but not too much or the contactor will be slow. Chose a coil voltage to suit you.

    Why do you need to switch between wye and delta? Won't the controller do the variation required?

    Why do you want anything in the motor case? Just bring out the wires to the external control.
     
    (*steve*) likes this.
  18. Maxwell

    Maxwell

    47
    3
    Oct 1, 2016
    Good point, 3 phase motors don't take DC...

    Maybe it would help if you supplied the motor type?
     
    Last edited: Oct 30, 2016
  19. roig12

    roig12

    21
    0
    Oct 29, 2016
    These motors are called brushless dc motors that look like that:
    https://www.aliexpress.com/item/Hig...2695068434.html?spm=2114.40010208.4.40.nitm3o
    It's not the exact one but that is the idea.

    If they take DC then how come it's not DC inside the motor?

    Why switch, well in wye setup (standard) the max speed is about 68kmh for 88v 45A and in delta it's about 110kmh (yeah.) BUT in delta you loose much of the torque so you can't have delta from 0 speed, just like in a car.

    Why not pull wires out of the hub and have the switch done externally, well some people do this but in order to pull that off you have to drill the motor shaft to enlarge the whole in order to have 3 extra phase wires through it and it weakens the shaft.
    Also, It's a huge mess that I'm trying to avoid.

    I got more confused than I was before with all of the new info...
    So, if it's not DC, is it AC?
    What are contactors?
    Are we still talking about paralleling DC relays?
     
  20. roig12

    roig12

    21
    0
    Oct 29, 2016
    I think I found the best relay for me!
    http://www.ebay.com/itm/2pcs-5pins-...039495?hash=item4656dd1287:g:ddIAAOSwpLNX~o4t
    DC 48v 30A, buy 6pcs and parallel two for each phase wire.
    What do you guys think?

    The 48v is max voltage right? (not THE voltage) because I may use lower voltage than that (meaning lower than 96v since I'm paralleling them), I'm asking because the lithium battery that provides my 88v reduces it's voltage when it discharges and can go under 70v even.

    Two things I don't get about the above relay are one, what is the voltage that is needed to trigger the relay and two, why does it say 48v in the model name but in the specs it says 30v DC (all info from the actual photo of what's written on the relay).
     
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