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    Quote Originally Posted by Paul Cataldo View Post
    Hey I meant to ask you guys about fusing this 20hp RPC. I can honestly say I'm a little overwhelmed with fuse info on the net.
    I have read (on this forum) that I should use a time delay fuse, BUT NOT a "current limiting" fuse.
    As to fuse sizing, is the proper method here just to size the fuses to protect the 4awg wire of my 3 legs?
    4awg THHN is good for 95amps, so what.....100amp time delay fuses that are NOT current limiting???
    What are your thoughts on this?
    What happens when the fuse blows? Does everything shut off or just the idler and now all of your three phase equipment is single phased?

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    Well I suppose that all depends on where you put the fuse block.
    I've read the NEC requires a motor circuit to have three components/layers of protection:
    Fuses (to protect the wire)
    OL relay -and-
    Circuit breaker.
    I've seen MANY RPC's incorporating fuses, so I was just trying to follow these guidelines, and figure this out (ahead of time) as best I can. Hence the sole reason I'm here asking you guys.

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    Quote Originally Posted by Paul Cataldo View Post
    Well I suppose that all depends on where you put the fuse block.
    I've read the NEC requires a motor circuit to have three components/layers of protection:
    Fuses (to protect the wire)
    OL relay -and-
    Circuit breaker.
    I've seen MANY RPC's incorporating fuses, so I was just trying to follow these guidelines, and figure this out (ahead of time) as best I can. Hence the sole reason I'm here asking you guys.
    To be honest the supply side before my RPC will burn up well before any motor I have on it does, and I'm not really experienced enough to reccomend a specific size, but on three phase equipment I've had more issues caused by fuses than they ever protected.

    If you just have one motor you will be using the same size wire all of the way from the circuit breaker to the motor, so I fail to see a need for fuses. I would expect them only to be needed for smaller wires branching out to smaller motors, or for equipment where the maker can't know for sure what upstream protection is in place.

    I fail to see how connecting any three phase load to single phase power is going to be NEC compliant, and NEC was not made with things like phase converters in mind. It may be worth doing what will better protect the equipment moreso than what makes it to code.

    In that case it depends on how you wire your setup as to what protection should or should not be implemented.

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    Quote Originally Posted by Paul Cataldo View Post
    Hey I meant to ask you guys about fusing this 20hp RPC. I can honestly say I'm a little overwhelmed with fuse info on the net.
    I have read (on this forum) that I should use a time delay fuse, BUT NOT a "current limiting" fuse.
    As to fuse sizing, is the proper method here just to size the fuses to protect the 4awg wire of my 3 legs?
    4awg THHN is good for 95amps, so what.....100amp time delay fuses that are NOT current limiting???
    What are your thoughts on this?
    Simplest and safest and the way to avoid all the "fuse" issues AND the side-channel noise about "single phasing" is to simply not USE fuses at all.

    Use a "common-trip" 2-pole breaker on the single-phase 240 VAC input line side of it all as the intentionally weakest or "most sensitive" device. Square-D QO clip-on that gets you to 100 A and a bit if even you need that much.

    Any MAJOR fault, you WANT this bugger to be the one to trip rather than downline ones, as it kills the whole shebang in one go.

    "Pony" start can still see the odd mis-matched phase thump, but in general, it is a "soft enough" start that slo-blow and time-delay should not be major needs atall. "QO" breakers DO seem to know the difference between ordinary starting inrush and an actual fault, Not their first rodeo, 60-odd years into the game.

    On the load side of the RPC, use individual "common trip" 3-pole, 3-phase circuit breakers for your each and several load devices. 15A, 20A, 30A, 40A, 50A..wotever THEY are supposed to have to not be so over-supplied as to just slow-bake to death.

    House those breakers in a 3-Phase load center dedicated to that RPC's output side ONLY. It needs to be rated for corner-grounded Delta. Those from Square-D and other major makers already are, and say so right on the box's label.

    I stressed "common trip" because it means ANY leg trips, the breaker opens ALL legs.

    "Ordinary" fuses are not good at that!



    With the common-trip breakers, OTOH, no "single phasing" need apply.

    Now.. where was it you were going to put those inherently independent trip fuses, that CAN leave you with a dropped-phase and a still "live" leg or two as might bite your surprised ass?

    Back on the damned shelf at the store is close enough to "the dustbin of ancient history" to work for me.


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    Thanks thermite. I think you have me convinced. NO fuses.
    In my research, I've read a lot about fuses to protect the wiring, which is the primary cause of most electrical fires.
    Guys on this forum have stated that a fire from melting wires can and will start (and I quote) "long before any OL relay or breaker will ever see the problem"
    This was the cause for my original line of thinking to use fuses.
    So many options...

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    Quote Originally Posted by Paul Cataldo View Post
    I would be interested to see that actually. Always interested in seeing RPC info.
    Don't make me post up that picture again....

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    Quote Originally Posted by Paul Cataldo View Post
    Thanks thermite. I think you have me convinced. NO fuses.
    In my research, I've read a lot about fuses to protect the wiring, which is the primary cause of most electrical fires.
    Guys on this forum have stated that a fire from melting wires can and will start (and I quote) "long before any OL relay or breaker will ever see the problem"
    This was the cause for my original line of thinking to use fuses.
    So many options...
    The issue with slow-cooked wires is techncially not even a circuit FAULT is where the problem arises. it is an installer FUBAR wherein current WELL within the design capacity of the circuit has been forced to flow through a poorly conducting area, thereby locally overheating it.

    Quite common where Joe Homewonder or builder's alleged Electrician J. Greedyfast used back-push spring-grip wall outlets that didn't have enough metal in contact to support the loads on the downline daisy-chain.

    Found one when I moved into THIS house by the discoloration on the damned wallpaper adjacent the outlet. No fuse or CB would detect that. 15A circuit, pair of ignorant 100 W lamps the only draw on it? Smoke alarm's job, they get THAT bad!

    All-new, and long since. Found four ALUMINUM wire runs as had been given COPPER ends so they weren't detectable, too.

    Busy guy for several years, here, I were!.

    Fuses, OTOH, CAN BE magical WHEN a weird situation exists because they can be so easily "tailored" in the field to suit a situation by swapping.

    You'd not want to know, OTOH, how many varieties exists.

    Or can be made. Were it not fifty years out of print, I'd say "read that chapter in my book".

    Even had to DO it under duress. CAT Thailand, International Gateway switch of a high-stress Sunday wee-hours. wherein three Thai technicians and I jury-rigged a coupla dozen pencil-eraser sized fuses on PCB's. Some careless folks in Europe had swapped the PSU lead polarity on the new kit we were installing. Blew the brains out of the whole rig.

    We made them out of fine strands of Copper lamp cord wire. Monday morning, Bangkok woke up and went global, business traffic never knew.

    All that said, modern-day circuit-breakers very well WILL do a proper job of protection. Some of those I have used are small as a pencil-eraser as well. Big ones need MOTORS to move their arse.

    Are the Square-D "QO" the best in the world? Probably not. But I honestly do not know.

    They have always been good enough I did not have to care! And they are stocked, just about everywhere. Might guess I ain't the only guy as likes them?


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    Quote Originally Posted by jim rozen View Post
    Don't make me post up that picture again....
    No need, Jim. Got a factory in China ready to print that tired old dog onto toilet paper rolls just on the off chance there's ANYBODY left on-planet as has managed to MISS it!

    Just sand the splinters off the wood, wilyah?


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    Quote Originally Posted by Paul Cataldo View Post
    Thanks thermite. I think you have me convinced. NO fuses.
    In my research, I've read a lot about fuses to protect the wiring, which is the primary cause of most electrical fires.
    Guys on this forum have stated that a fire from melting wires can and will start (and I quote) "long before any OL relay or breaker will ever see the problem"
    This was the cause for my original line of thinking to use fuses.
    So many options...
    Bad advise. What's new about that...

    Fuses protect short circuits and over-load relays are for long term heating. Two different things that are used together.
    Last edited by rons; 10-13-2018 at 03:59 AM.

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    Quote Originally Posted by Strostkovy View Post
    To be honest the supply side before my RPC will burn up well before any motor I have on it does, and I'm not really experienced enough to reccomend a specific size, but on three phase equipment I've had more issues caused by fuses than they ever protected.
    Hold that thought. It has a sound historical basis, but we need not re-create the environment where it was one of the most common places it happened actually and often.
    If you just have one motor you will be using the same size wire all of the way from the circuit breaker to the motor, so I fail to see a need for fuses. I would expect them only to be needed for smaller wires branching out to smaller motors, or for equipment where the maker can't know for sure what upstream protection is in place.
    Fuses haven't vanished. This need still finds them soldered to PCB's (LG Fridge-freezers) or in small bulkhead mounts, (Pee Cees) etc. They still have traction in sub-feeds for HVAC,, but.. for the most part it makes more sense to mount a circuit breaker as part of the system. They come in all sizes. Many of those in residential HVAC are commonly self-reset capable thermals, for example. Not new. 1956 DeSoto automobile used self-resetting circuit breakers on critical need circuits, and they were not new THEN, either. These just are not in a panel or loadcenter "in your face".

    I fail to see how connecting any three phase load to single phase power is going to be NEC compliant, and NEC was not made with things like phase converters in mind. It may be worth doing what will better protect the equipment moreso than what makes it to code.

    In that case it depends on how you wire your setup as to what protection should or should not be implemented.
    Back to that "thought". As the manager of manufacturing and the only Engineer as to utility-ish thingies amongst 3,000 merchants, HVAC, safes & security systems, and energy management for an 18-store + warehouses chain was dropped onto my "additional duties" list. All done with contractors, of course, so it wasn't hard.

    But here's where "dropped phase" entered. It was a CHRONIC condition for many years, 1974-1984, that slow-cook overloads of long hot summers AND/OR passing thunderstorms would blow one fuse of three on rooftop 3-Phase HVAC systems. This often happened while they were running, and it could be weeks or even months after the event before a motor running with a dropped-phase finally damaged itself and went hard-down. Moving off fuses to common-trip circuit breakers was a blessing.

    However.. you do not GET a dropped-phase condition for your load motors nor even worklights off an RPC if/as/when one of only TWO upstream fuses on its only ONE "phases" blows!

    What you get instead is all loads down. Worklights included.

    AND .. a shock-hazard.

    One of the only TWO "legs" is still above ground potential.

    A common-trip circuit breaker, 2-pole for that singe-phase side, they'd both have gone dark.

    But there were never THREE, as we had on all those rooftops running off utility-mains 3-Phase.

    The load motors downstream of your RPC idler would not run even if they were single-phase. One phase is all you had, it no longer has a compete path. Just ONE "hot" wire.

    The other two a "native" 3-Phase still had were never there unless the idler was doing what idlers do. And it no longer can. It coasted-down. There is no return path for your load motors on any other leg, either.

    Put your wet-meat hands in the wrong place, Earth Ground may light YOU up off the lone remaining "hot", but the motors and even the work lights should be less carelessly wired!



    To keep in mind is that modern circuit breaker families have accessories to mount them as "onesies". One standalone breaker in a rated NEMA box where you need that and might otherwise have used a legacy set of three side-by-side fuses.

    If that is one placed onto the 3-Phase "output" side of your RPC, and the RPC input has NOT gone down, then once again a "common trip" circuit breaker is your friend.

    One leg goes pear-shaped, the breaker takes them all away at one go.

    BTW FWIW-actual-value, Do NOT used breakers "made into" common-trip with clips (Allen-Bradley) or cross-pins and spacers (even Square-D f**ked this dog) that mechanically connect the external operating handle toggles. They only work in a UL test.

    Out in the real world they fail with DISMAL regularity because the one pole that tripped could not impart enough force nor MOVE FAR ENOUGH to do anything more useful than tilt the silly bar or pin at a slight angle.

    Just do not USE that shit. It is a hazardous "fiddle". If you find it "legacy" installed? Just make it a priority to replace it with PROPER common-trip breakers.

    The common-trip breaker you want only HAS one external toggle for three poles (or two). Connection common-up is made down inside where it actually does what it was meant to do reliably.

    3 poles worth. Plus belt-and-braces. Nice 27 KVA Delta-Wye EGS/Hevi-Duty FULL ISOLATION transformer on the output side of my own RPC.

    You want to "single phase" that puppy off a gone-full-dark RPC feed, you'll have to go get rons to bring a special ration of his magical electrobullshit out of storage.

    Resourceful Californio, rons is. He WILL find a way to mess up a steel ball with a rubber hammer.

    But it will only be fairytale theory for sake of argument, so no harm done unless you try to wire it his way in the real world!

    You don't really want to do that... unless maybe you are in Californikyah and just bored spitless for lack of better Tee Vee?


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    Quote Originally Posted by thermite View Post
    Hold that thought. It has a sound historical basis, but we need not re-create the environment where it was one of the most common places it happened actually and often.


    Fuses haven't vanished. This need still finds them soldered to PCB's (LG Fridge-freezers) or in small bulkhead mounts, (Pee Cees) etc. They still have traction in sub-feeds for HVAC,, but.. for the most part it makes more sense to mount a circuit breaker as part of the system. They come in all sizes. Many of those in residential HVAC are commonly self-reset capable thermals, for example. Not new. 1956 DeSoto automobile used self-resetting circuit breakers on critical need circuits, and they were not new THEN, either. These just are not in a panel or loadcenter "in your face".


    Back to that "thought". As the manager of manufacturing and the only Engineer as to utility-ish thingies amongst 3,000 merchants, HVAC, safes & security systems, and energy management for an 18-store + warehouses chain was dropped onto my "additional duties" list. All done with contractors, of course, so it wasn't hard.

    But here's where "dropped phase" entered. It was a CHRONIC condition for many years, 1974-1984, that slow-cook overloads of long hot summers AND/OR passing thunderstorms would blow one fuse of three on rooftop 3-Phase HVAC systems. This often happened while they were running, and it could be weeks or even months after the event before a motor running with a dropped-phase finally damaged itself and went hard-down. Moving off fuses to common-trip circuit breakers was a blessing.

    However.. you do not GET a dropped-phase condition for your load motors nor even worklights off an RPC if/as/when one of only TWO upstream fuses on its only ONE "phases" blows!

    What you get instead is all loads down. Worklights included.

    AND .. a shock-hazard.

    One of the only TWO "legs" is still above ground potential.

    A common-trip circuit breaker, 2-pole for that singe-phase side, they'd both have gone dark.

    But there were never THREE, as we had on all those rooftops running off utility-mains 3-Phase.

    The load motors downstream of your RPC idler would not run even if they were single-phase. One phase is all you had, it no longer has a compete path. Just ONE "hot" wire.

    The other two a "native" 3-Phase still had were never there unless the idler was doing what idlers do. And it no longer can. It coasted-down. There is no return path for your load motors on any other leg, either.

    Put your wet-meat hands in the wrong place, Earth Ground may light YOU up off the lone remaining "hot", but the motors and even the work lights should be less carelessly wired!



    To keep in mind is that modern circuit breaker families have accessories to mount them as "onesies". One standalone breaker in a rated NEMA box where you need that and might otherwise have used a legacy set of three side-by-side fuses.

    If that is one placed onto the 3-Phase "output" side of your RPC, and the RPC input has NOT gone down, then once again a "common trip" circuit breaker is your friend.

    One leg goes pear-shaped, the breaker takes them all away at one go.

    BTW FWIW-actual-value, Do NOT used breakers "made into" common-trip with clips (Allen-Bradley) or cross-pins and spacers (even Square-D f**ked this dog) that mechanically connect the external operating handle toggles. They only work in a UL test.

    Out in the real world they fail with DISMAL regularity because the one pole that tripped could not impart enough force nor MOVE FAR ENOUGH to do anything more useful than tilt the silly bar or pin at a slight angle.

    Just do not USE that shit. It is a hazardous "fiddle". If you find it "legacy" installed? Just make it a priority to replace it with PROPER common-trip breakers.

    The common-trip breaker you want only HAS one external toggle for three poles (or two). Connection common-up is made down inside where it actually does what it was meant to do reliably.

    3 poles worth. Plus belt-and-braces. Nice 27 KVA Delta-Wye EGS/Hevi-Duty FULL ISOLATION transformer on the output side of my own RPC.

    You want to "single phase" that puppy off a gone-full-dark RPC feed, you'll have to go get rons to bring a special ration of his magical electrobullshit out of storage.

    Resourceful Californio, rons is. He WILL find a way to mess up a steel ball with a rubber hammer.

    But it will only be fairytale theory for sake of argument, so no harm done unless you try to wire it his way in the real world!

    You don't really want to do that... unless maybe you are in Californikyah and just bored spitless for lack of better Tee Vee?

    I'm not saying fuses don't have their place, just that putting them here is fairly useless. If you have a 50 amp breaker feeding through a run of 6 gauge wire and into a motor, what good would a 50 amp fuse in series with that do, unless you need some fairly specialized trip curve, which you really don't for an idler. For the record I put fuses on all small equipment I make, and a fuse any time a wire that can carry high currents branches into a wire that can't, unless it is a very short run that stays inside the device or panel for driving panel meters or whatever. They should probably be fused too, but that's a lot of fuses.

    That depends on how you wire in the motors and protection. There are a multitude of configurations that are possible. One way protects both motors, but requires fuses large enough to run the idler and load motor, which may have to be rated high enough to not provide adequate protection for just one of the motors. The other way protects just the idler, but allows the RPC load to single phase, unless you have an output contactor.

    And I'm being a bit pedantic here but in a situation with a split phase power source disconnected by a single fuse, every wire and connection will be hot. If you touch the load side of the blown fuse it would hurt just as bad as touching the load side of the intact fuse, because one is just the other but through a very low resistance (relative to you) motor winding.

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    Quote Originally Posted by Strostkovy View Post
    The other way protects just the idler, but allows the RPC load to single phase, unless you have an output contactor.
    Not unless you have put fuses on the OUTPUT side, it does not. But we are making the same argument at the end of the wire in any case. So it ISN'T really an "argument".

    And I'm being a bit pedantic here but in a situation with a split phase power source disconnected by a single fuse, every wire and connection will be hot. If you touch the load side of the blown fuse it would hurt just as bad as touching the load side of the intact fuse, because one is just the other but through a very low resistance (relative to you) motor winding.
    Yah. What I meant when I said the loads would not have operating power, but there was still a shock hazard. And that it was of the sort as could "surprise" you.

    Common-trip! Or stick with water power!

    Now.. about the hazards of water power...


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    Quote Originally Posted by thermite View Post
    Not unless you have put fuses on the OUTPUT side, it does not. But we are making the same argument at the end of the wire in any case. So it ISN'T really an "argument".


    Yah. What I meant when I said the loads would not have operating power, but there was still a shock hazard. And that it was of the sort as could "surprise" you.

    Common-trip! Or stick with water power!

    Now.. about the hazards of water power...

    There are two configurations that should be considered here and I'm not adequately explaining it. I'll draw a picture in the morning.

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    Quote Originally Posted by Strostkovy View Post
    There are two configurations that should be considered here and I'm not adequately explaining it. I'll draw a picture in the morning.
    Yeah but.. are we not in agreement those are the very configurations to JF not utilize?

    For a public venue, it might be wiser to only post examples of safe(er) practice, yah?

    Seems there's ALWAYS some hurried soul as grabs the first diagram Go Ogle finds and starts wiring it up figuring "the experts" have solved all problems, and he has nary a care!


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    Quote Originally Posted by thermite View Post
    Yeah but.. are we not in agreement those are the very configurations to JF not utilize?

    For a public venue, it might be wiser to only post examples of safe(er) practice, yah?

    Seems there's ALWAYS some hurried soul as grabs the first diagram Go Ogle finds and starts wiring it up figuring "the experts" have solved all problems, and he has nary a care!

    Hold on...Always new ideas are welcome...

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    Quote Originally Posted by Matt Matt View Post
    Hold on...Always new ideas are welcome...
    Wiremen who did one of the IWCS sites, Northrop-Page for US Army Signal Corps must have thought so too.

    Technical Documentation, Georgetown, DC we keep exchanging one telex after another with a site in Viet Nam trying to figure out where the error was in the as-built drawings that traced a wire through SEVENTEEN telco equipment racks - and could no fine way be correct.

    Drawings were in fact correct. Circuit originated on a neg-48 VDC alarmed "grasshopper" fuse, traversed 17 racks.

    Terminated to a ground lug.

    Staff worked around it. Absent our badgering them, they had no time to do the detective work. Some bullshit excuse about there being "a war going on" or such..

    They just placarded it as "fuse tester circuit" and left it empty.

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    I have a 15hp idler started by a 1/2hp pony.

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    Quote Originally Posted by metalmagpie View Post
    I have a 15hp idler started by a 1/2hp pony.
    If I NEEDED one, I'd use a motor-vehicle / agricultural / grounds-keeping 12 VDC starter motor "turbo'ed" off 24 VDC. Only because I have "more than enough" of a battery-bank already in-place and on-float for other purposes to power it, though.

    Hardly any "engineering" required, as it is what they were built to do. Just some pretty cheap parts-bin cobbling, used goods being more than good enough for such light work, "standard" parts just as easily replaced if/as/whenever.

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    Quote Originally Posted by thermite View Post
    If I NEEDED one, I'd use a motor-vehicle / agricultural / grounds-keeping 12 VDC starter motor "turbo'ed" off 24 VDC. Only because I have "more than enough" of a battery-bank already in-place and on-float for other purposes to power it, though.

    Hardly any "engineering" required, as it is what they were built to do. Just some pretty cheap parts-bin cobbling, used goods being more than good enough for such light work, "standard" parts just as easily replaced if/as/whenever.
    Ah, there we go, I was wondering when you would bring DC into this.

    Nothing keeps it simpler than adding two batteries, a charger, and having to fab up some way to attach a nonstandard shaft to the idler.

    If that's the best solution you can come up with with what you have on hand your stockpile is not nearly as great as you claim it to be.

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    Quote Originally Posted by Strostkovy View Post
    Ah, there we go, I was wondering when you would bring DC into this.

    Nothing keeps it simpler than adding two batteries, a charger, and having to fab up some way to attach a nonstandard shaft to the idler.

    If that's the best solution you can come up with with what you have on hand your stockpile is not nearly as great as you claim it to be.
    Sech iruffleiant boolshite!



    The battery plant is simply to run the fridge-freezers so I don't have to run the MEP-803A all night in a power-outage, and is 24 V so it can also START the ignorant gen set if the onboard pair of 12V batteries are flat.

    My Dee Cee motors are nearly all 180-volters, all Reliance RPM III Type T, and I don't THINK I have any under 2 HP and well over 100 lbs. One five-hoss is IIRC 387 lbs. About an order of magnitude more mass than that of the new-but-cheap-arse 10 HP Weg ODP idler.

    I would only want to "pony-start" an RPC idler. Not launch its ass into low-earth orbit!

    A lawn-tractor or Japanese motorcar-shaped-obect starter? You can hold those in one hand and bolt 'em up to a simple plate of HRS, roughly six holes to be drilled. Only one big one, and it CAN be a notch, instead.

    Use of a "starter" ain't even about "DEE CEE" anyway. Just what they are already..

    Its the free "Bendix" drive built-into a starter that is already a.. "starter". It kicks the bugger back off-duty once the load RPM of the idler picks-up.

    The other "plus"? There'd not only be the lower-load on the utility mains from the pony-assisted idler start. The pony itself would place ZERO load on the mains. Still, as said, only because I already have it for another purpose. At 10 HP idler, I don't actually pony it anyway. One of Jim's Phase-Craft boxes cap starts it.



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