Building an RFC, should I size my contactor based on expected in-rush current?
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    Default Building an RFC, should I size my contactor based on expected in-rush current?

    I've been wondering, how should I select the contactor size for my RFC? I'm building it with a 30hp motor so I know there will be a substantial in-rush current to get it started. Do I need to account for that in-rush current by getting a bigger contactor or are they made to handle short spikes in current without damage?

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    Quote Originally Posted by setlab View Post
    I've been wondering, how should I select the contactor size for my RFC? I'm building it with a 30hp motor so I know there will be a substantial in-rush current to get it started. Do I need to account for that in-rush current by getting a bigger contactor or are they made to handle short spikes in current without damage?
    Your idler motor dataplate figures are all THREE PHASE ones. Single-phase service to it for use as an idler will involve higher current.

    Allow for that and the contactor should be OK with the associated inrush too.

    The RPC starter/controller "manages" that startup, but that is not the same as magic. It will need serious power to it.

    How much?

    Simplest to use what the maker of an RPC starter/controller for a 30 HP idler says to use in their specs.

    Don't HAVE such a manual?

    No fear.

    Just use the specs from some other commercial 30 HP RPC maker.
    Or the many in PM's "stickies"!



    And/or check several of the major maker's specs and make your choice.

    You might not expect it, because their nature at start-up is very different, but @ 10 HP, my Phase-Perfect actually wants almost identical single-phase service amperage as my factory-bought @ 10 HP idler RPC starter/controller specifies.

    Manual being handy for the 30 HP Phase-Perfect, those figures are:

    136 AMPS of single-phase to deliver 80 Amps sustained "steady state" to the 3-Phase load. That also supports a peak of 96 Amps for a shorter while, and a surge of 400 Amps.

    So if I HAD a 30 HP P-P? I'd use a 150A 2-pole breaker, #2 copper wire, and a contactor comparably rated on the input side.

    No harm to use an oversized contactor. But the wire and breaker have to meet code.

    So much for one set of specs. You can find more to compare in the PM RPC stickies.

    Hopefully other PM'ers will have other numbers to share "shortly".

    Hopefully you have 400A service, too!


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    Quote Originally Posted by thermite View Post
    ...
    Simplest to use what the maker of an RPC starter/controller for a 30 HP idler says to use in their specs.
    ...
    Subtext here is, he's using capacitor starting? If so, he may need to upsize the contactor for that, a tad larger. I personally
    would go with 1 inch diameter contacts for that....

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    I have a 35hp RPC. It uses a 150a contactor on the motor, 30a contactor to turn on start capacitors and it has an additional 80a contactor on the output. Its fed with 100a breaker single phase. Been running this way for about 6 years with no issues.

    Not 100% sure why they use the extra contactor on output? What would be the purpose of this?

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    Quote Originally Posted by mmurray70 View Post
    Not 100% sure why they use the extra contactor on output? What would be the purpose of this?
    Wise move. Very!

    A predictable and controlled "single point" to both insure the idler always starts unloaded by whatever it serves .. and reduce risk the power off the single-phase feed - common to two of the wires, nature of the beast - could reach - and lock-up or over-heat - 3-Phase loads if the generated leg is "not there yet" OR has failed!

    There's ONE contactor yah might want to power FROM the "generated" leg as well.

    Reason should be obvious?

    You wouldn't want to trust a fallible "human" to ever-and-always have all loads switched OFF at start-up, let alone react rapidly to a dropped-phase, wuddja?

    I do not!

    The Old Fart just ain't that fast OR reliable!


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    You wouldn't want to trust a fallible "human" to ever-and-always have all loads switched OFF at start-up, let alone react rapidly to a dropped-phase, wuddja?
    And this is why we use contactors that drop out on power loss and need to be manually restarted, not just a switch

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    Quote Originally Posted by SomeoneSomewhere View Post
    And this is why we use contactors that drop out on power loss and need to be manually restarted, not just a switch
    Well, yeah, but

    .. whom do I get to blame when I am the one to "drop out on power loss", am not MEANT to "switch" and there's nobody as hasn't already fallen asleep to push the button?

    "They say" that youth is wasted on the young?

    I still remember THAT part well enough to hide back of the the Fifth Amendment protection against self-incineration over the blame for it!

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    Quote Originally Posted by thermite View Post
    Wise move. Very!

    A predictable and controlled "single point" to both insure the idler always starts unloaded by whatever it serves .. and reduce risk the power off the single-phase feed - common to two of the wires, nature of the beast - could reach - and lock-up or over-heat - 3-Phase loads if the generated leg is "not there yet" OR has failed!
    This makes sense. So the output contactor should be delayed until after its started? Would this be done with the timer relay used for the start capacitors? Starting caps cut in, and when they drop out the output contactor comes on? Or are you saying the generated phase is what energizes the output contactor? Wouldnt the start caps mess with this? Possibly energizing it before motor is up to speed?

    Kinda seems like the output contactor comes on as soon as you hit the start button, which doesnt make as much sense. All happens really quick but im 99% sure the output contactor comes on before start caps drop out. Maybe ill do a video of start up and slow it down to get closer look.

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    General use contactors in the USA are sized by load power, and that takes all the inrush into account, as they are sized to handle closing into, or opening, locked rotor current.

    If you use IEC contactors, you are assumed to have the systems designed by a registered engineer, who calculates to find your particular individual inrush, and specifies a contactor that will just carry that current.

    So with US types, the correct class of contactor will need no consideration of the inrush.

    Quote Originally Posted by thermite View Post
    Your idler motor dataplate figures are all THREE PHASE ones. Single-phase service to it for use as an idler will involve higher current.

    ................

    Why would you say that?

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    I have a contactor on the output side of my rpc, it’s hooked to a timer that delays the output by 8 seconds. Before this I inadvertently flipped my lathes drum switch on some time and when I started my RPC the start cap voltage also started my lathe, I could swear I seen that SB16 hop!

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    Quote Originally Posted by mmurray70 View Post
    This makes sense. So the output contactor should be delayed until after its started? Would this be done with the timer relay used for the start capacitors? Starting caps cut in, and when they drop out the output contactor comes on? Or are you saying the generated phase is what energizes the output contactor? Wouldnt the start caps mess with this? Possibly energizing it before motor is up to speed?

    Kinda seems like the output contactor comes on as soon as you hit the start button, which doesnt make as much sense. All happens really quick but im 99% sure the output contactor comes on before start caps drop out. Maybe ill do a video of start up and slow it down to get closer look.
    Someone else posted use of a delay timer, on the basis that if you had intentionally powered-up the RPC at all, having it go ahead and deliver power to such downstream distribution, switchgear, protective devices, outlets, loads - however it is arranged - is a "given".

    That works.

    Manually operated as a second step under deliberate choice is what I use.

    I have a four-idler rig, have to choose which combination I want that hour/day for starting, then what to drop back to for running.

    It still depends on the generated leg being furnished by "somebody" to hold-in.

    Transformer & 24 V contactor coil it powers are not "zero", but are rather insignificant as a load compared to even my lightest of motor loads.

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    Quote Originally Posted by thermite View Post
    Manually operated as a second step under deliberate choice is what I use.

    I have a four-idler rig, have to choose which combination I want that hour/day for starting, then what to drop back to for running.

    It still depends on the generated leg being furnished by "somebody" to hold-in.

    Transformer & 24 V contactor coil it powers are not "zero", but are rather insignificant as a load compared to even my lightest of motor loads.
    Yeah of course. Manually operated makes even more sense but for some reason I didnt think of that.

    Can you post more details of your 4 idler setup? Guessing you only need start caps for first one? Additional idlers start themselves? I may be looking at adding another idler in the future to increase capacity. What would be max reasonable size to add to my 35hp setup without getting into start current issues?


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