No announcement yet.

Three Phase Question

  • Filter
  • Time
  • Show
Clear All
new posts

  • Three Phase Question

    I have 3 three phase machines. My lathe has a 3.4hp motor. My mill has a 2(?)hp motor in it, and my press has a 7.5hp motor in it. I have a 10hp motor that I use as a rotary phase converter (i.e. no capacitors). It works fine for the lathe and mill, but will trip a 50 amp breaker when I try to use the press. The plate on the press motor says that it draws 22 amps at 220v (or 11 at 440v). I realize that I have to include the 10hp motor for the phase converter in the draw, but with the current wiring (6 gauge 240 to the rpc and 10 gauge to each of the machines), I am stuck if I want to use the press. I had originally considered using a VFD on each machine, but my lathe has a 2 speed motor, which I have been told will burn up if I try to use it with a VFD. I have considered getting a generator to supply electricity for the 3 phase machines, but I am not sure if that would be a good idea. There is a 15kw unit that is 3 phase capable for a reasonable price on CL. My question is this. Since I have only manual machines, could I use a generator to run my 3 phase motors without damaging them? Or would there be better options for me?
    Hi, my name is Wilson and I am a tooloholic.

  • #2
    Hydraulic press? Probably that hardest starting load you can have in a shop. I think your RPC is undersized. Should probably be at least 2-to-1 larger than the press motor. You can start one of the other motors first and see if that give you the extra kick to start the press. Every motor that it running adds to the start capacity. Also you could look for a VFD to drive the press.
    Central Ohio, USA


    • #3
      I had thought about a VFD for the press, but it seems like a waste to have a high dollar VFD for just the press. I guess that I am looking for a single source of 3 phase for all my machines.
      Hi, my name is Wilson and I am a tooloholic.


      • #4
        The breaker tripping could be due to low power factor which may be the result of the highly inductive load of the rotary phase converter and the press motor itself. Thus, the 7.5 HP (5.6 kW) motor would draw 25.4 A at 220V at an ideal power factor of 1.0, but most motors have a PF of more like 0.8 so the actual current may be 20% higher or 32 amps. That still does not explain tripping a 50 amp breaker, which will actually hold 150 amps for a few seconds, and should take as much as 500 amps to trip instantaneously. You may have a defective breaker, or it may be a fast-trip type. I design, build, and service circuit breaker testing equipment which can be used to verify the breaker characteristics. Here is a video of a test of a 60 amp breaker showing the time delay and instantaneous trip:

        It is probably much cheaper to purchase a new breaker than get it tested, unless you know someone with the test equipment or have the ability to make your own test jig. With similar test equipment you can measure the transient current drawn when the press starts up and the breaker trips. A three phase motor typically draws about 3x nominal current on start-up (locked rotor current) but it could be as much as 10x, especially if one of the phases collapses during start-up, or if the voltage drops excessively.

        The three phase generator should provide the power you need, especially if it has enough rotary momentum to ride through the start-up torque. But it may be better to use a VFD which can control the motor start-up and deceleration which will reduce the current surges that are typical of full-voltage start-up. I don't know if the press motor is starting into a high immediate load, or if it is driving a hydraulic pump which may take some time to achieve full pressure. Using a VFD with the generator may be the best option, but it may also be OK with the rotary phase converter. The problem is that a VFD presents a high capacitive load and poor power factor on single phase, but with true three phase it does not need as much capacitance.

        Another thing that may help is power factor correction capacitors on the line near the motor. This will reduce the current through the breaker, possibly enough to keep it from tripping.

        I don't think the two speed motor will burn up with the VFD, but it may depend on which speed you use. Sometimes one speed will be a lot higher power than the other, and usually you can get a more powerful single speed motor in the same frame size. Also, some motors are designed for VFDs, and those that are not may suffer stress on the insulation, causing degradation and breakdown.

        Hope this helps, and is not too overwhelming.
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030


        • #5
          Try turning on your lathe and mill (unloaded) and then see if you can start your press. If not, you need a bigger RPC.



          • #6
            Have you considered a reduced current starter? A couple of stove elements in parallel until the motor spins up then shorted to place the motor load across te line whem it gets up tp speed.


            • #7
              I've wondered more than a few times whether a clutch of sorts would make sense on a motor which has to start against a large load. I'm thinking centrifugal clutch, and it could possibly have a feature whereby the initial start of the motor would inhibit the action. Once the motor came up to say about half speed, this inertial inhibit would be gone and the clutch would begin to engage very strongly.

              I can't see it being very complex or trouble-prone. After all, washing machines used to last for decades with their simple clutch mechanism.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


              • #8
                Why? 99.999999999% (+/- a couple) of motors don't have a clutch and they work fine. Size the motor correctly for the load. Weird stuff like washing machines with wimpy motors are an exception.

                The OP just needs a larger rotary converter. Me? I gave up on all the rpc related stuff and dropped the $$ on a used 10hp Phase Perfect. Never been happier. YMMV.

                As to a vfd burning up a two speed motor - na... configure it correctly and there will be no issue at all.

                OP : check your voltage on T3 (measure t1-t3 and t2-t3). If it's sagging badly when you start up your press, your rpc is undersized. An Analog meter is the easiest way to see it all happening. BTW, your rpc is probably iding at about 20 amps - lousy pf, but current is current and seen by the breaker. I'm betting your T3 sags badly, and the 7.5hp motor does not come up to speed fast enough so it's hogging current. The breaker is doing what is supposed to do - protecting the wiring. You might want to consider motor overload protection for your 7.5 (if not already in place).
                Last edited by lakeside53; 11-18-2013, 03:03 AM.


                • #9
                  You can use a soft starter for the press motor also known as reduced voltage starter. It should solve the problem.
                  Helder Ferreira
                  Setubal, Portugal


                  • #10
                    Thanks for all the replies. The press does not trip the breaker immediately. It will run for 5 seconds or so. It seems to be the load, because if the motor is running backward (no load) it will run just fine.

                    PSTechPaul, the breaker is a 3 month old Square-D QO. I would not think that it has gone bad so soon.

                    Lakeside53 are you running your Phase Perfect on 240? What is the largest size motor that you are running on it?
                    Hi, my name is Wilson and I am a tooloholic.


                    • #11
                      I suspect that your RPC just isn't big enough to start the press motor. The usual rule for motors that start under light load is a factor of 1.5. Your 10hp RPC is undersized for the press motor by 1.25hp, if it started under no load. But the press starts under load and thus a bigger RPC is needed, perhaps by a factor or 2 or 3. In my opinion a VFD for the press would be iffy with only single phase power available.

                      You might try getting the additional starting current from the other two motors by running them when you start the press. That is close to an additional 5hp of RPC. Another option would be to modify the press hydraulics to allow the pump to start with the output directed to the reservoir. Once the motor is up to speed your RPC should carry the motor's running current.


                      • #12
                        You have a hard start load.... And the RPC is undersized.

                        The issue with an RPC is that the 3rd wire always has a higher series impedance than the others, which go straight through. Starting a motor requires a lot more current than running it, there is a large starting surge, which can be between 4 and maybe 7 times the running current.

                        The RPC needs to be able to supply a credible proportion of that, and do it at the proper phase, in order to provide start torque to the motor. You don't always need FULL power line start current, but you usually do need at least 1/3 to 1/2 of it. More for a hard start load.

                        Typically, that means at least a 1.5:1 ratio of RPC to load, for "normal" loads. More for hard start, 2x to 3x is a good number. That ratio gets the series impedance lower, so that the 3rd wire neither drops in voltage so much that it loses current capability, nor gets shifted in phase so much that it is ineffective.

                        You need a bigger RPC to get the series impedance down. Likely that will be cheaper than a bunch of VFDs, or a Phase Perfect. Especially with single phase input to the VFD...although that is why you NEED the VFD, single phase capability costs money.
                        CNC machines only go through the motions.

                        Ideas expressed may be mine, or from anyone else in the universe.
                        Not responsible for clerical errors. Or those made by lay people either.
                        Number formats and units may be chosen at random depending on what day it is.
                        I reserve the right to use a number system with any integer base without prior notice.
                        Generalizations are understood to be "often" true, but not true in every case.


                        • #13
                          Originally posted by WilsonT View Post

                          Lakeside53 are you running your Phase Perfect on 240? What is the largest size motor that you are running on it?

                          Yes, 240. 70 amp breaker on the single phase input; 30 amp (max) fused disconnects to each machine on the output. It is a 10hp unit which mean it will start a fully loaded 10hp motor and put out 30 amps continuous delta three phase. Right now the biggest motor I run a 5hp "really hard starting" motor on it - no issues even when running a 3hp compressor and a 3hp mill. I started 15hp unloaded with ease. The PP has a large overload capacity. It also runs a 240-480 15kva transformer to give me 480 volt 3 phase for some vfd's or odd-ball machines that cannot be easily converted to 240.

                          A new (they are almost never for sale "used") 10hp unit will set you back a bit under $3k. Cheap for a precision 30 amp three phase supply - and it's "real" 3 phase, better than utility in many cases.

                          There are many 10, 20, 30 (and 2x 30 for 60hp) installed around here. Cheaper than paying for 3 phase from the utility for a small shop, if you can even get it in residential or rural areas. They are also UL listed which means you can get an electrical permit/inspection and pass OK, so some home shops will skip this but for a business in this county... that's a different risk.
                          Last edited by lakeside53; 11-18-2013, 12:51 PM.


                          • #14
                            For the lathe, a lot of people just wire it for single speed and use the VFD that way. One thing you could also do is use a Cerus VFD. They have an output to control a contactor to switch between motors and it saves parameters for each. You could use this to drive the two speed switch to switch between windings.


                            • #15
                              Often a 2 speed lathe motor will be Dahlanger wound. In that case you just connect to the high speed (higher power) windings and use the vfd to get the lower speed. The resultant HP at the lower speed will be close to the same you could have got by the original switching so nothing is gained by trying to use both windings.