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Thread: RPC Question

  1. #1
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    RPC Question

    Hi,
    I purchased a 10 HP rotary phase converter from a local industrial shop and they make the RPCs themselves. The motor is a Siemens and it is rated 27 amps at 230 volts. So I had it wired by an electrician using a 30 amp breaker and 10 awg wire.
    Problem is when I am using my Centauro 700 CL bandsaw (which has a 4.6 hp 3-phase motor), the breaker trips after 5 minutes of use. When I called the guy who sold me the RPC unit, he said I should try a 40 amp breaker. I felt I needed to consult with fellow creekers before I install the new 40 amp breaker as this seems to me a problem with the RPC rather than the breaker.
    Thanks in advance for your uinput,
    Sal.

  2. #2
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    You haven't told us how you wired from the rpc to the motor. You are supposed to use an input wire rated 250% so it sounds like the 10 gauge is light. You can't increase the breaker without increasing the wire. From the rpc outlet are you going back to a cutoff or panel with the manufactured leg and feeding L1 and L2 from the main panel? Dave

  3. #3
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    I think you need to add the PRC amps and the saw motor amps for the breaker size. Don't forget, 10 awg wire is a bit light for a 40 amp circuit.

    John

  4. #4
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    Quote Originally Posted by David Kumm View Post
    You haven't told us how you wired from the rpc to the motor. You are supposed to use an input wire rated 250% so it sounds like the 10 gauge is light. You can't increase the breaker without increasing the wire. From the rpc outlet are you going back to a cutoff or panel with the manufactured leg and feeding L1 and L2 from the main panel? Dave
    The wiring is as follows: 12 awg line with 30 amp breaker to a 3-phase 30 amp disconnect. While the two lines feed to the disconnect input, the third leg is provided by the rpc motor. All the wire between the disconnect, the rpc motor and the outlet are 4 line 12 awg cords.
    Thanks for the feedback,
    Sal.

  5. #5
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    Quote Originally Posted by Sal Kurban View Post
    The wiring is as follows: 12 awg line with 30 amp breaker to a 3-phase 30 amp disconnect. While the two lines feed to the disconnect input, the third leg is provided by the rpc motor. All the wire between the disconnect, the rpc motor and the outlet are 4 line 12 awg cords.
    Thanks for the feedback,
    Sal.
    So I get this straight. 12 awg from the panel to the disconnect and then from the disconnect to the rpc. I though you were running 10 awg? Dave

  6. #6
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    Quote Originally Posted by David Kumm View Post
    So I get this straight. 12 awg from the panel to the disconnect and then from the disconnect to the rpc. I though you were running 10 awg? Dave
    True, the 10 awg is only between the panel and the disconnect. Others are just cords and 12 awg size.
    Sal.

  7. #7
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    If I were wiring it I would run 8 ga to the disconnect with the 30 amp breaker, then 10 ga to the rpc and back. You should not running 12 on a 30 amp breaker unless the disconnect is fused and you have a 20 amp time delay fuse. Even then you will suffer some voltage drop with the the long cords. First run 10 from the disconnect and see if that helps. If not, put the 8 to the panel. Don't increase the breaker to 40 unless you also run 8 ga from the disconnect to the rpc unless you put a 25 or 30 amp time delay fuse in the disconnect. A fused disconnect allows for heavy wire and big breaker to the line side and the proper sized fuse for the rpc. Dave

  8. #8
    As a point of reference, I run my phase converter, which has a 15 hp idler motor, off of a 30 amp breaker. It runs my felder saw, (7.5 hp + 1 hp), and scmi shaper (5 hp + 1hp) . It has popped the breaker a couple of times under heavy load on the shaper, but it has run without a hitch at all other times. I cobbed it together myself, but with much heavier wire. I think that I have 8 gauge for the wiring from the breaker to the RPC and 8 to the idler motor. The idler also has a thermal overload for protection. To the machines is 10 gauge.

    My impression would be go remove all the 12 gauge wire. I did some testing with 12, and it got hot.

    Also, you may want to measure the running voltages and currents to see how they differ. Rotary phase converters are not very efficient, but it may be possible to use some run capacitance to straiten things out, lowering the overall current draw.

    The Practical Machinist site has a forum dedicated to this sort of things, and plenty of local gurus that would offer to help.

  9. #9
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    Hi Measure the voltage and current at the bandsaw on all three phases while cutting.

    You may have unbalanced voltages which will cause your bandsaw to draw unbalanced currents.

    Once you have these results recorded, measure the input current to your RPC under the same conditions.

    Regards, Rod.

  10. #10
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    Quote Originally Posted by John McClanahan View Post
    I think you need to add the PRC amps and the saw motor amps for the breaker size. Don't forget, 10 awg wire is a bit light for a 40 amp circuit.

    John
    Presuming that your 3-phase equipment is also feed from the disconnect, John is correct. Your 4.5 hp motor is pulling around 15 amps per leg, and your RPC is pulling probably around 20 - 22A to generate the 15A third phase required for the 4.5 hp motor. If your 3-phase equipment and the RPC are both fed by the same breaker, you need at least a 40A breaker and 8 ga wire from the breaker to the disconnect.

    If you're feeding two of the

  11. #11
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    This won't be of direct assistance Sal, but I wired my 3kW 380V 50Hz UK spec 24in Agazzani to run on a 230Vin/380V out 5.5kW RT rotary made by these guys in England: http://www.powercapacitors.co.uk/Con...Literature.pdf (the Agazzani is wired so you can't wire the motor to accept 230V 3 phase - it's delta wound. (?) )

    Installed per their sheet http://www.powercapacitors.co.uk/Con...es/RT%20OI.pdf it's been faultless. It's on the recommended 4mm2 cable and 30A type C (slower acting motor/inductive load type) breaker, but bear in mind it's UK/Irish 50Hz power and there may be design differences as well so treat it as general information only. That said they reckon 40A and 6mm2 cable for their 10hp unit on UK power. They have some stuff to say about which phases to use for controls when hooking up to machines too - the synthetic one on units like this can be a bit iffy it seems so far as holding relay solenoids open is concerned.. They reckon their unit holds the start up current to about x3 the load motor full load amps.

    Your unit is presumably a rotary. I seem to recall reading that the synthetic phase/the power factor is only spot on for some median load the unit is optimised for. Your load is smaller than your unit, so maybe this requires some allowance for the extra current that would result from this too. It's seemingly possible to tune the capacitors to optimise the match to a given load too.

    Big band saws with cast iron wheels are a surprisingly severe test of an RPC because it takes a while to wind up the inertia, so it's probably not a good machine to sail close to the wind with the electrics...

    ian
    Last edited by ian maybury; 03-19-2013 at 7:59 PM.

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