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  • Phase converter

    My lathe has a 1 hp 3 phase motor. I purchased an all ready made box with capacitors, and everything is labeled. Good for me. The box is labeled as 5 hp. I have a idler motor I can use, but it is 3 hp. If I use my control box as 5 hp, and my 3 hp idler motor will this supply necessary 3 ph for my 1 hp lathe motor? Stan

  • #2
    Is the "ready made box" something that is home built or is it fabricated by a company? If it is company made, can you contact the company to ask about using the smaller motor? Are the capacitors run capacitors (oil filled) or start caps? It might run just fine or the caps may be too big ( uneven voltages on the third leg). Good luck.
    Sarge41

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    • #3
      3HP motor should work fine. Put it together and if you have problems post them here where a few of us who have built RPC's can chime in. Unless you are using the neutral in circuit for motor controllers or the like uneven voltages are usually not that important (just put on nomex suit). I use a 3hp motor on a home-brew RPC with no run caps to power my HVLH without issues for many years. Just don't hook the generated leg to anything connected to neutral. Lots of 3 phase theory involved but my generated leg is about 180V to neutral vs 110 on mains legs.

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      • #4
        What you have sounds like a "static phase converter". That is, essentially, a box with all the required parts to start an idler motor, possibly without the so-called "balance" capacitors that many (not all) rotary phase converters have.

        Most such devices are made to cover a range of motors. Usually, 3 HP and 5 HP are two separate categories, but the difference is generally just the total start capacitance provided, and possibly the size of a contactor. The contactor is just as good for 3 as for 5 HP, and the capacitors can be changed if necessary. So the box you have is either OK as-is, or is easily changed to accommodate a 3 HP motor.

        The 3 HP is very adequate to handle a 1 HP load, so while you may need to make some minor changes (or maybe not), the system should be OK for what you want to do.
        2730

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

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        • #5
          Originally posted by 1937 Chief View Post
          ....................................... If I use my control box as 5 hp, and my 3 hp idler motor will this supply necessary 3 ph for my 1 hp lathe motor? Stan
          Yes

          Rich
          Green Bay, WI

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          • #6
            Show a pic of what is inside the box..

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            • #7
              I wish I could show a picture. Maybe when my granddaughter comes over. I purchased this unit from a company called Phase Craft in Los Angeles Ca. I've had this converter for at least 4 years. I think Covid must have put them out of business. No one to answer my calls. Stan

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              • #8
                Originally posted by 1937 Chief View Post
                I wish I could show a picture. Maybe when my granddaughter comes over. I purchased this unit from a company called Phase Craft in Los Angeles Ca. I've had this converter for at least 4 years. I think Covid must have put them out of business. No one to answer my calls. Stan
                Maybe gone before the Covid? Sounds like it was a one man operation that made quality:

                https://www.practicalmachinist.com/v...t-gone-349875/

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                • #9
                  The box you have and a 3 HP motor will probably work but may not be optimized for your lathe motor. There is very little chance that you would damage anything if you hook it all up and try it. Then you can take some Voltage and/or current measurements and change what is necessary to optimize it. The Voltage between each of the three pairs of legs should be about the same and should equal the nameplate Voltage of your lathe motor. That is what you would be checking when you test run it.

                  Just be sure your motors are rated for at least the line Voltage you will be using to power all this. With 230 VAC power the motors should be rated for at least 208 V, 3 phase. And if your idler motor does not start, do not let it sit there with power applied for too long (over a minute). You can try to pull start it but if that does not work, turn the power off while figuring out what is wrong. This will prevent overheating and the production of smoke.
                  Paul A.
                  SE Texas

                  And if you look REAL close at an analog signal,
                  You will find that it has discrete steps.

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                  • #10
                    Assuming your converter uses a potential relay to engage the start capacitors I’d say there is a 50% chance the 3hp motor is not big enough to trigger the 5hp converter. If you get nothing but loud clicks, shut it down.
                    Last edited by SVS; 03-26-2021, 05:45 AM.

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                    • #11
                      Forget using this setup and get and VFD. Simpler, easier and you get fwd/rev and speed control.
                      Peter
                      Grantham, New Hampshire

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                      • #12
                        I agree with Paul on this. It should run, but it may not ne optimized for your motor.
                        In my experience with balancing caps, RPC's are rather forgiving, some people run with no caps at all, and some run balanced caps, and I have run voltages up to 280'ish.

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                        • #13
                          Originally posted by SVS View Post
                          Assuming your converter uses a potential relay to engage the start capacitors I’d say there is a 50% chance the 3hp motor is not big enough to trigger the 5hp converter. If you get nothing but loud clicks, shut it down.
                          There is no trigger from the load motor.
                          If your rpc wont start without its load, then you got a poor wiring diagram or bad parts or something.
                          A RPC with potential relay starts just fine alone & by itself.

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                          • #14
                            It doesn’t become an RPC until the static converter he has starts the idler motor.

                            Ran my whole shop for a long time on a GWM brand 7.5-10hp static converter with run caps. No specific idler motor. Had 4 machines that ran direct from
                            converter. To run machines smaller than 5hp needed to start one of the 7.5hp loads first to act as an idler. A 1-3hp & even some 5hp motors, started alone, made the phase converter click uselessly.

                            Mfg. literature said this would happen, so it wasn’t a surprise or defective. Why or how is beyond me.
                            Last edited by SVS; 03-26-2021, 12:50 PM.

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                            • #15
                              Originally posted by SVS View Post
                              Assuming your converter uses a potential relay to engage the start capacitors I’d say there is a 50% chance the 3hp motor is not big enough to trigger the 5hp converter. If you get nothing but loud clicks, shut it down.
                              Does not work that way, you manually turn on the idler then the load. And his 3 HP is the idler, the 1 HP is the load. That should work. The box is for a 5 HP idler, and might need some adjustment.
                              2730

                              Keep eye on ball.
                              Hashim Khan

                              Everything not impossible is compulsory

                              Comment

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