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15 HP RPC Build

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  • 15 HP RPC Build

    I'm starting a shop in the house I live in, moving away from dad's shop slowly. So I need three phase. And since I don't believe in anything small, my first three phase motor I moved in is the 10HP motor in my Daewoo lathe. It's a Y start, Delta run, so it's actually only ~7HP starting load, so the 15 HP idler should be plenty.

    Dad already found me a 15 HP motor he got on a big compressor that had been dropped and he got for a screaming deal. It runs, just needs bearings, so I'm go to go there. All I need to do is build the actual converter. I should mention that the motor is a 200 volt motor, but it's apparently been running fine on 240V for years.

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    Dad went with the Fitch Williams design. It works well. I will too at least on the balancing and power factor correction caps. But I'm thinking of using a pony motor. Partly because I think it would be fun, and partly because I'm sure the start caps would be pretty expensive, and I'm sure the utilities would appreciate the lack of the power spike.

    So what I was thinking was:
    • 1 to 2 horse 3450 RPM single phase motor. A dime a dozen, nobody wants 3450 motors. I've probably got 5. Even one one of those "5" HP harbor freight motors with the 1/2" shaft which is actually like 2.
    • 12V ac clutch from a car. Local junkyard is bound to have plenty, it would just be a matter of finding one with the right shaft size and fighting it off the car.
    • I have a 20:1 transformer, 53A. It's rated for 120V input, but I'm sure it would work for 240V input. It was free, I don't need it. So that gives me plenty of 12V power.
    • Dad has a friend that scrapped out a basically a factory worth of electronics. He has a few semi trailers full. I can get pretty much any circuitry logic for free, but it's mostly 12V.
    • Dad and I have a fair few contactors, but they are mostly 120V logic. I'm not afraid of 120 or 240V logic, I've already wired with it a plenty on the Sidney or Dad's RPC.
    • I don't have a big enough contactor for this motor, but I'll need one regardless.
    • So the idea is to have a completely dumb electronic logic circuit. With one button push the pony motor starts, spins up to full speed (potential relay?, timer?) fires the AC clutch, spins the big motor up to full speed, using a 2:1 pulley reduction and automotive belt. The AC clutch is the small pulley, I'd only need to build the big one. After the big motor is up to speed, the pony motor and clutch are both killed and just the big motor stays on.
    So.... crazy? Stupid? Fun? A waste of motors? All of the above? I dunno, I was just feeling some thing different that would use up some of these junk components I've salvaged and teach me some circuit logic at the same time.

    I'm curious to hear HSM's thoughts. Thanks!
    21" Royersford Excelsior CamelBack Drillpress Restoration
    1943 Sidney 16x54 Lathe Restoration

  • #2
    You don't need to disconnect the smaller motor, so no need for the clutch etc.

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    • #3
      Originally posted by lakeside53 View Post
      You don't need to disconnect the smaller motor, so no need for the clutch etc.
      I don't think the smaller motor could spin the big one up speed without being at speed by itself.
      21" Royersford Excelsior CamelBack Drillpress Restoration
      1943 Sidney 16x54 Lathe Restoration

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      • #4
        Originally posted by J Tiers
        The only thing that jumped out at me was the 120V transformer used at 240V. That ain't gonna happen unless it has a 240V rating.

        The rest of it seems as if it can work. The details will naturally make or break it.

        I'd be inclined to use a standard start, but nothing wrong with a pony. And, yes, the powerco will get fewer complaints if you don't drop 15 HP on the line.

        One thing....... the capacitors. A standard AC motor can generate if it has a little residual magnetism in the core. And with the balance and RPC capacitors, it can actually "build up" and get some significant voltage. I have demonstrated this "generating" action in the lab.

        Then that voltage will draw current, and may bog the pony motor trying to drive the capacitive load on the "generating" idler. I had that happen with the demonstration.... the driving motor bogged, because I had a bit more than enough capacitance to supply exciting current, and it was pulling a lot of current, due to a quite high voltage.

        Most do not bother with the capacitors with a pony start, but if you do, consider connecting them after it is started.
        I was hoping you'd reply, I appreciate your input on electrical matters.

        I honestly think the transformer will work fine for the few seconds it is engaged. It came out of a Raytheon LPS for a 60s laser system. I don't remember the details, but I'm pretty sure they were feeding it 240V anyway. There were certainly transformers in there not being used at their design voltage. They had a triple bank variac, each rated for 120V input, going into another transformer marked 220v in, 300v out. I don't really remember all the details but the voltages were ignored for sure. I'm almost 100% certain it was setup for 240V delta three phase due to the triple bank aspect of everything before it went DC, so there is no way they anything marked 120V was getting 120V, unless they had a stepdown ahead of the input line. Since the DC voltage gauge went up to 750, and I think the AC one went to 300, I'd say it had to be 240V input.

        Ok, so another contactor to connect the caps... got it.

        Thanks JT.
        21" Royersford Excelsior CamelBack Drillpress Restoration
        1943 Sidney 16x54 Lathe Restoration

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        • #5
          Minor point but air conditioner clutches from a car are 12V DC, not AC.

          A friend needed a bigger RPC years ago and I found him a 15HP motor brand new at the junk yard, big corporations junk things you wouldn't imagine. He rigged it up with a pony motor and belt drive but with a foot pedal to engage it. It worked out quite well. The foot pedal allowed gradual engagement.

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          • #6
            Another vote for keep it simple. When the sparks and smoke start to fly you'll be happy you did. First check with your utility on what they require. Y delta and power factor correction, probably not on 15 hp. Check your electric service to make sure it can handle the added load. The motor is not starting under load and should start easily across the line on delta. Voltage balancing caps, definitely. It's a toss up on whether you use start caps and potential relay or pony motor and belt/treadle to spin it up. Use properly sized contactor, overload and wiring. A voltage monitoring control is inexpensive and can be purchased at a HVAC supply house if brownouts are common in your area.

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            • #7
              X2 with deltap. Belt off the pony motor. I'm no RPC guy, but I do know that "keep it simple" is a very good rule to live by.
              25 miles north of Buffalo NY, USA

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              • #8
                I built my 5hp, then upped it to 7.5hp RPC. Yeah I started it with the rope pull thing for fun & giggles.
                I bought a rpc kit from Ebay put it all together, and did the balancing from the Fitch drawing. Works fine.
                Myself, I wouldn't bother with the pony motor, that start cap circuit is just too easy to do.
                All the parts can be had from your local HVAC supplier, the potential relay, the caps, all that are HVAC common stuff and not expensive.
                All the common HVAC stuff is 24v controls and run on 120v or 220v, that stuff is cheap and common.
                If you got a good electrical drawing to begin with, that start circuit is by far easier to build than twiddling around with a pony motor.
                here is a couple online suppliers,, if you need any additional parts
                ​​​​​​http://phaseconverterusa.com/15Hp-Ro...vac-_p_55.html
                ​​​​​​https://usbreaker.com/

                I used 240v control on my contactors, eliminating the need for control transformers
                Last edited by Ringo; 04-19-2021, 09:27 AM.

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                • #9
                  whats better than a one button start? A remote control start! Should be relatively cheap and easy in this day of Arduinos. I've found that I dislike the drone (and bit of power wastage) when I'm not running a machine, but it gets to be a real pita climbing across my crowded shop to turn it on whenever I want to run a machine.
                  in Toronto Ontario - where are you?

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                  • #10
                    Can't help on the rest of it but every a/c clutch I've ever pulled had a tapered hub. Not an insurmountable obstacle but something to consider.

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                    • #11
                      I used a 15HP with 1/2 hp pony motor to start it belt slipped a bit till up to speed.There some good info on a post from a year ago Stone Hammer Simple RPC.

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                      • #12
                        Wondering what happened to this thread? Not displaying properly -- big blank gray area
                        25 miles north of Buffalo NY, USA

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                        • #13
                          Well, I was going to reply to everyone, but Paul's reply has hilariously broken the thread for me. I can't see any other posts or the previous page. Paul Alciatore can you edit your reply if possible to unbreak it?

                          George Bulliss Any way to fix this?
                          21" Royersford Excelsior CamelBack Drillpress Restoration
                          1943 Sidney 16x54 Lathe Restoration

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                          • #14
                            Even more radical idea: no clutch at all. 1hp ought to easily be able to spin over the big motor, especially with a 2:1 reduction on a belt. You could always use a relay setup (HVAC potential relay on the generated leg?) to disconnect the pony motor from the single-phase after the big one starts generating.
                            25 miles north of Buffalo NY, USA

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                            • #15
                              Originally posted by The Metal Butcher View Post

                              I don't think the smaller motor could spin the big one up speed without being at speed by itself.
                              They do easily... sure there is a some start up acceleration lag, but...

                              One I designed years ago runs fine. Schematic has been published on this site several times

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