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Starting lathe on rotary 3ph

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  • Starting lathe on rotary 3ph

    Now that I have my lathes in place and the rotary phase converter running. It seams like my lathe comes up to speed slower than when I was using a static phase converter. I guess I got spoiled with start caps pushing the lathe motor up to speed. My voltages on the rotary seam fine. I have never ran a lathe on true 3ph power is this normal?

  • #2
    Well, the very first question is how big is the idler, how big is the lathe motor, and does the lathe have a lot of gearbox stuff turning when it is idling?

    Also, how big a motor was the static converter rated for?

    I have a 1 HP capable RPC, a lathe motor of around a half HP, and the starting is quite snappy unless I have a heavy large diameter workpiece in the machine. Then it is slower, but that would be somewhat the case with powerco 3 phase as well.

    Static converters start the machine with a start capacitor. They are made for ranges of motor HP, so if the motor was at the lower end of the range, it might be started much quicker than if it were the maximum size motor the static converter was intended for.
    1601

    Keep eye on ball.
    Hashim Khan

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    • #3
      The idler motor is 25hp
      The lathe motor is 7.5hp
      The lathe is direct drive no electric clutch. So at 600rpm 3-1 reduction.
      The old static was rated for 15hp so it was over kill.


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      • #4
        With the static, it had quite a bit higher than needed start capacitor, which could easily be rather snappy in starting.

        On the other hand, the idler is 3x the load motor, which normally leads to a good start also.

        If you check the manufactured leg voltage during a start, and it stays up without dropping much, then my conclusion is that the static converter was actually giving a boosted start, and you are probably experiencing a "normal" start now.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          Ok I'll give it a look this morning. Do you want me to check to ground or another leg?


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          • #6
            Originally posted by jeremy13 View Post
            Ok I'll give it a look this morning. Do you want me to check to ground or another leg?


            Sent from my iPhone using Tapatalk
            I have almost the same setup, a 7.5hp lathe motor running on a 15HP RPC. If you take the voltage between phase while the lathe is running, it should be balanced and check if your voltage drop too much on the lathe motor.

            Masimec's home shop

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            • #7
              For power voltages, you almost always want the "line-to-line" voltage, as in this case.
              1601

              Keep eye on ball.
              Hashim Khan

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              • #8
                Ok my lathe is smarter than me. I'll have to wait for help to get a starting voltage drop. I cut the breaker off the the lathe turned it on and went to flip the breaker on and get a reading no dice. The lathe did not come on with breaker on I cut lathe off and back on started just fine. Must be some kind of safety.

                L1-L2 are my mains L-3 is made up leg

                With rotary only running
                L1-L2 241V L1-L3 236V L2-L3 240V

                With lathe on
                L1-L2 240V L1-L3 226V L2-L3 229V



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                • #9
                  OK, the generated leg is a bit low.

                  Does the RPC have any "balance" capacitors in it? Those would be metal cased parts that are always connected, not the typically black plastic cased starting capacitors.

                  As for starting, you always have to have the RPC already running in order to start to machine. If you cut the breaker that supplies both, nothing may start. The starting voltage would be what it is at the instant of starting. With a digital meter, it can be hard to obtain, unless the meter has a function that reads the minimum or maximum.

                  You'd get it by reading the voltage from L3 to L1 or L2, with the lathe not running, but the RPC running. Then start the lathe while watching the meter. The lowest reading is the one we want. Since digital meters only read once or twice per second, the reading is not always an accurate measurement of the drop. With a function for min or max, you connect the meter, set for a minimum reading, then start the machine. The meter should "hold" the minimum reading it saw. Do that 3 times or so, and the lowest number is likely what it drops to on starting.

                  If no "minimum hold" on the meter, then just do it several times, and report what the lowest number you see is.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    Starting lathe on rotary 3ph

                    I do have run caps in line. A set L1 to L3 and same amount L2 to L3 top of pic

                    I don't have any true 3ph caps in line

                    I did have the rotary running. The lathe must have a relay that cut it off if power is off while the lathe is on.


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                    Last edited by jeremy13; 08-16-2016, 12:03 PM.

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                    • #11
                      It's about 5% low with the lathe running, not too bad, but the real question is how much more it drops with a heavy starting load on it.
                      1601

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                      Hashim Khan

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                      • #12
                        Ok with rotary on and starting lathe at 2000rpm I get a maximum drop of 146V


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                        • #13
                          Originally posted by jeremy13 View Post
                          Ok with rotary on and starting lathe at 2000rpm I get a maximum drop of 146V
                          That seems like a huge drop for a 240 VAC nominal P-P. That would definitely cause a slow start. Can you measure the current during startup? Better yet, view the waveform of the current and the voltage. A $25 DSO would be adequate for the job.
                          http://pauleschoen.com/pix/PM08_P76_P54.png

                          Paul: www.peschoen.com
                          P S Technology, Inc. www.pstech-inc.com
                          and Muttley www.muttleydog.com

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                          • #14
                            I'll have to tell my buddy to bring his oscilloscope over and we can look at that


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                            • #15
                              I would expect a drop, since the thing will be pulling about 6x full load current. But RPC is 3X oversized, vs normal of 1.5x, so it seems as if it should be better than that. possibly not.

                              I will need to take a look at mine for a reset on expectations. Have not done in years.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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