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5 hp 3 phase - VFD - what amp breaker

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  • 5 hp 3 phase - VFD - what amp breaker

    Looking at new (to me) lathes. Had a 2 hp single phase Chinese machine, recently moved, movers tipped lathe over and smashed the carriage gearbox - they paid me out for it.
    Looking at a Colchester 15 x 30, 5hp 3 phase machine. I have installed a VFD before and I think I understand the benefits. 5hp is probably 2X what I would ever use. Available circuit is 230V single phase, 8ga. wire, 40A breaker. I'm thinking 5 hp may be too big for the available power ?

  • #2
    What does the VFD manual say? usually they use higher than you may think,, but the fuses are only to protect the wiring if the VFD goes dead short, not the vfd itself.

    40A is should be fine for 5hp on a vfd. You get soft start via the vfd so traditional startup currents are not in play.

    Remember, you will either needs SINGLE PHASE INPUT 5hp vfd, or a 3 phase unit at 7.5 to 10hp (programmed for 5hp motor currents), and feed that with single phase.

    Here's an example of single phase 5hp VFD. Max input current is listed 36 amps at 230v. Rules of thumb for a typical domestic breaker (and some code wiring issues) is 80% of the rating for continuous so a 40 amp breaker would be 32 amps, but you will not be drawing that "continuously". https://www.driveswarehouse.com/pc1-50
    Last edited by lakeside53; 06-28-2020, 12:53 PM.

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    • #3
      Originally posted by lakeside53 View Post
      What does the VFD manual say? usually they use higher than you may think,, but the fuses are only to protect the wiring if the VFD goes dead short, not the vfd itself.

      ......
      Not so fast..... The UL rating on many VFDs is CONTINGENT ON the breaker being what is called for in the manual. Some that call for a fuse may have breaker sized for the wiring only.

      In those cases, the "fault test" will not be passed without the right breaker, and if you oversize it, the VFD may actually blast out flames and arcs in case of a fault inside it

      So for the ones with no fuse called out, you need to use the breaker size called for, and size the wire to suit the breaker.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Originally posted by cuslog View Post
        Looking at new (to me) lathes. Had a 2 hp single phase Chinese machine, recently moved, movers tipped lathe over and smashed the carriage gearbox - they paid me out for it.
        Looking at a Colchester 15 x 30, 5hp 3 phase machine. I have installed a VFD before and I think I understand the benefits. 5hp is probably 2X what I would ever use. Available circuit is 230V single phase, 8ga. wire, 40A breaker. I'm thinking 5 hp may be too big for the available power ?
        First off. Look for a VFD that is rated for 5HP on single phase. Polyspede makes one.

        Amps X Volts = Watts
        750 Watts = 1HP apx

        A little over 16 amps at an actual 5hp load. That much load is not likely except as described below.

        Add a little safety margin for loss and peak loads.

        If you let the VFD spin the motor up slowly peak loads will only be bad when you accidentally break a half inch tool in a cut (not likely) or stomp on the foot brake for an instant stop (back EMF). You will lose a little in motor efficiency and in the VFD, but not a huge amount. Gnerally jus tlet the VFD spin up and spin down the motor over several seconds. Add a braking resister if you want fast stops and still let the VFD stop the motor.

        A 40 amp circuit should handle it just fine as long as the VFD is up to the task.

        *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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        • #5
          I should add it is not necessary to run the machine on a 5HP VFD. I run my Hurco KMB1 on a 3.7Kw rated VFD. Yes 3.7Kw is 5HP, but its rated for that when fed 3 phase. I am only running it on single phase , and the manual say derated 30% for single phase. I just calculate all my cuts at 3HP or less. Only time I ever had any issue was when I made cuts hard enough to brake a 1/2 inch or larger end mill. Even then sometimes the mill gave before the VFD tripped an error. On the rare occasions when I overloaded the spindle the VFD just goes into error. I have to power cycle the machine (electrical interlock on cabinet doors) to reset the VFD. If I had the cabinet doors open I could just push the reset button.

          Mitsubishi, Delta, Hitachi, and Dan Foss all exhibit similar behavior in my shop. I can't speak for Huan Yang. I have a few, but do not currently have any of them in service.

          The only one I have a braking resister on is the Mitsubishi on the Hurco. I can stop the spindle from full speed in a 1/2 second or less, but I don't TC tap currently with that machine so its not necessary. I currently have it set for 3 seconds spin up and spin down. The machine with a 5HP spindle and 3 huge servo motors is run on a 35 amp breaker.

          The Polyspede I mentioned earlier is rated for full power in single phase. Drives Warehouse stocks them. I do not know who else.
          *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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          • #6
            40 Amps is plenty.
            21" Royersford Excelsior CamelBack Drillpress Restoration
            1943 Sidney 16x54 Lathe Restoration

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            • #7
              Thanks for your responses guys. Now to see if I might be able to fit this thing in my shop.😏

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              • #8
                My first guess was a 30 amp breaker is all you need. Your 40 amp should be no problem- the wire is heavy enough for that, and lighter wire will not protect anything in the VFD if there's a bad fault.

                Just for reference, our RPC at work is 20 hp, and it's fed through a 100 amp breaker.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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