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  • Bridgeport Questions

    Greets all! wow I'm finally back in. Have been lurking for quite some time but I couldn't
    remember my password -- managed to get in on attempt 5 out of 5.

    I'm glad to see everyones been up to no good.

    Disappeared for a while -- did the married thing -- then I don't know how it happened -- but
    I ended up with a kid. cutest little girl this side of the BBS.I've tried to get her started
    early on the lathe, but she just keeps teething on the cross slide.

    After a big ugly move (how did I end up with so much junk?!) I'm finally getting my shop back
    up on its feet.

    So to the bridgeport questions.. its an old Jhead.. before I put it in its new home I thought
    I'd clean it up a bit. Would love to do a complete teardown but I know i'll need it as soon
    as I take out the first bearing. So overall cleaning, maybe some paint here and there.

    Its a 3ph machine. I don't have 3ph power anymore. I've got it running off of an inverter
    for now (so no table feed yet).. and am wondering if this could do damage.
    The inverter puts out 230V 3ph.. the motor is the pancake style 2/1 Hp (2800/1428 rpm?) and
    the switch physically has 5 positions.. two speeds in one direction, off position, two speeds
    in the other. So I'm guessing 2 speed motor right?

    My inverter will trip (over voltage) if I switch the mill right to high speed.. but if it take
    it slow.. works like a charm.

    Am I going to do any damage?

    Second, I've never gotten the back gear to engage properly on this thing. The problems always
    been pushed back when it needed fixing and figured now is good time to do it since I'll be taking
    some stuff apart. Gears grinds.. like not properly meshed.. and spindle always stays in high speed.
    The cams seems to move all the way up / all the way down, so I've never opened it up in fear of a
    major overhaul.

    Anyone run into a problem like this? tough fix / easy fix? I know thats not alot of info to go
    on, but I'm sure there are alot of BP owners out there that may have run into it.

    Thanks all.. and now that I have my password again (wrote it down this time!), I look forward to
    participating again.


  • #2
    First, get a notebook or binder and keep a list of all the web sites names and your username and password in it.

    Second, I doubt the "inverter" is powerful enough to run the mill if it kicks out when trying to start in high but it isn't hurting anything.

    Third, download a copy of the parts breakdown manual for your mill and start taking it apart. It may be something simple causing it to not engage. It's been so long since I have had one apart I can't help you there.
    It's only ink and paper


    • #3
      >Second, I doubt the "inverter" is powerful enough to run the mill if it kicks
      >out when trying to start in high but it isn't hurting anything.

      My "test" setup was a hogging cut 1/2" deep using a 4 flute 1/2" endmill
      in a block of aluminum. Don't know what my feedrate was as i just turned
      it through by hand trying for 'reasonable' chip size without clogging the cutter.
      No problems from the inverter.

      I think switching the motor right to high causes some sort of quick spike
      that trips the inverter. Switching from off to "1" and then subsequently to
      "2" (since the motor is up to speed) doesn't seem to cause a problem.
      Tested with same cut as above.

      Have since milled some 5/8" slots in about two dozen 2mm stainless
      steels tubes -- 5 slots per tube -- for a stainless steel railing project, with
      no complaints from the inverter.

      I'm not a big electrical guy and maybe I'm just skeptical of something
      "different" -- my fear is that I could be doing long term damage to the
      motor if I end up using it say, all day, on the inverter at 230V instead of 400
      whatever its looking for.

      Thanks though.


      • #4
        You shouldn't be doing any damage to your motor so long as it is current-limited (which your inverter tripping sounds like it is).

        I'm going to grossly oversimplify things here, I'm sure the more technical sorts will be along to correct me.

        A motor at rest pretty much looks like a short - which turns into 'infinite' current (which really only happens in a theoretical scenario, we'll just call it a bigass spike). Since you're trying to start the motor at max power we'll say it's more infinite than the lower setting. Once the motor is actually turning, the motor no longer looks like a short, so you're not going to pull that infinite current through the motor and trip the inverter when you go to the higher power setting.

        Just remember that your motor's now derated from its nameplate horsepower, and you may want to eyeball the motor temperature to be sure it's not getting any hotter than your old setup.


        • #5
          Originally posted by knucklehead
          > I'm not a big electrical guy and maybe I'm just skeptical of something
          "different" -- my fear is that I could be doing long term damage to the
          motor if I end up using it say, all day, on the inverter at 230V instead of 400
          whatever its looking for.

          Thanks though.
          Hi Knuckle,

          I am guessing that you have re-wired the motor to low voltage 240 volt from the high voltage 480?


          THINK HARDER




          • #6
            Hi Knuckle,
            I am guessing that you have re-wired the motor to low voltage 240 volt from the high voltage 480?



            • #7
              Originally posted by knucklehead
              My inverter will trip (over voltage) if I switch the mill right to high speed.. but if it take
              it slow.. works like a charm.

              Am I going to do any damage?
              I first read this that you were flipping a speed switch that reconfigured the motor winding while the inverter was running. I will keep the answer to that question and then address the other interpretation below.

              Best to avoid switching speed, direction, on/off, or disconnect wires while the inverter is running as this can stress solid state drives.

              Suppose that 10 amps are flowing through a motor winding at the instant you disconnect. A motor winding is an inductor. Inductors like to keep their current constant. Which means that the inductor is going to try to keep 10 amps flowing. Since there is no where for the current to go, it will spike up the voltage until the energy stored in the winding is absorbed by stray capacitance or until the voltage gets high enough for something to break down. Switch contacts are also bouncy so it may be disconnected one instant and connected at another. Thus the motor can get reconnected to a drive by bounce, or the contacts can arc over, during the spike. This can be hard on the transistors and diodes in the drive. Some can take it, some can't and it can depend on the motor and whether there are snubber networks or other loads to absorb the spike energy. The inverter may also have a varistor to absorb spikes; these eventually deteriorate and then short out. If you are lucky you have to replace a fuse and varistor when this happens but the shorted varistor can also take out a transistor. Note that loose wires are also to be avoided for this reason as well as ones related to heat and fire.

              The likelihood of damage depends on your particular combination of motor and drive as well as other elements in the circuit, etc.

              Do you really need to switch speeds using the motor windings? If you aren't stuck with a fixed frequency inverter or other loads hanging off it (such as pumps) that need fixed frequency, use the inverter/VFD to change speeds where possible. There may be some advantage to switching motor windings, on a motor with switchable pole configurations, for best torque at the highest and lowest speeds due to inductance of the windings limiting current at high speed or the physical position of the windings at low speed but when you aren't at the extremes, changing the frequency of the drive has more or less the same effect as reconfiguring the windings.

              Some Bridgeports may have had a "speed" switch for the motor without actually having a two speed motor. Rather, it was a reversing switch because the back gear made the spindle turn backwards unless you also reversed the direction of the motor. Same issues would apply but there would be no electro-mechanical advantage to flipping the switches as opposed to controlling speed/direction entirely from the VFD.

              A good drive will let you control on/off, speed and direction from the drive.

              VERY loosely speaking, switching the load while the motor is running is like switching gears while the motor is running without synchronizers. If you can't just set them and leave them, set your switches to the position you want (including on/off), then turn the inverter on and turn it off before changing. Treat the switches like gears.

              I may have read your post wrong, though. You may have been talking about switching the VFD on in high speed vs starting it in low speed and ramping up. If you were getting an overvoltage alarm, instead of overcurrent, this could be because switching transients were worse under heavy load. Better to ramp up as this gives you a soft start. Less strain on the mechanical structure, gears, belts, and linkages, the motor, the drive, and the power line if you ramp up. Some VFDs will let you program in a automatic ramp up/soft start, if you don't want to mess with the dial.

              A few years back, I designed a soft start system for large industrial motors. While it could also control speed, that was a fringe benefit; customers were looking for such a device for the benefits of eliminating hard starts and willing to pay hefty prices to get it. If you can get those benefits for free as a side effect of single to three phase conversion, why not take advantage of it?

              You should be aware that using a motor that wasn't designed for inverter duty on a inverter can cause stress the motor in some ways it wasn't designed for.
              Your overvoltage warnings may be an indication that you have a problem with voltage spikes which may stress the insulation in the motor and/or the drive. Subtle differences in the design of the drive can have a significant impact on the level of the spikes. Soft starting can reduce the current and the spikes at startup but you may find the problem also occurs when taking heavy cuts. If you have access to an oscilloscope and high voltage probe you might want to look at the level of the spikes.

              If the machine has been stored in a humid environment, having the motor baked or at least running it with a light load and letting it warm up for a good while before trying to take any heavy cuts may reduce the likelihood of insulation failure due to water absorption.


              • #8
                If by inverter you mean VFD, then you are not supposed to be putting any switcting between the VFD and the motor. Most VFD manuals carry a warning about this. You might be getting away with it for now, but don't assume it will always "just trip" without damage.

                Also... a VFD is programmed and/or learns a certain set of motor characteristics. Apart from voltage transients, switching to another set of windings disrupts what the vfd has been tracking, appears like a fault condition and the vfd attempts to protects itself by shutting down.

                My 2c - hardwire the motor to ONE set of windings, set the motor current, braking type/time and other parameters, then use the low voltage speed control and direction functions of the vfd...


                • #9
                  Thanks all for the great info. Yes, by 'inverter' i mean VFD.
                  I'll break out the manual and hardwire past the switch this weekend.
                  It is programmable. The manual is an inch thick. Again, not a particularly
                  electrical kinda guy, but I should be able to make my way through it.

                  I'm not sure if this motor has any explicit wiring info on it -- but maybe
                  the manual has some info on rewiring for 230V.

                  Thanks again.


                  • #10
                    My Long Chang First was fitted with a two speed motor. They're also known a pole changing motors because they do just that, change from a 4 pole motor in low speed and into a 2 pole motor for high. They also swap between star and delta to keep the speed/voltage ratio constant. This means you can't reconfigure it for a lower voltage (unless it has a LOT of terminals and some motors do). Since my motor was set up only for 415v I couldn't run it from a VFD and I used a static phase converter.

                    When the mill made its move to my new machine tools only workshop I wanted to ditch the converter and put it on a VFD (which I already had on the shelf). I sent the original motor up to Sir John and he sent back a new (used/rewound) motor configured for 240v and with a modified shaft and adaptor plate. I've tested it on the VFD and I now have variable speed, jog facility and DC injection braking to bring spindle to a smart stop.

                    The innability to engage backgear might just be swarf between the drive dogs, or it might be that the pulley bearing housing is partially siezed in the top casting. Pulling off the motor and top end of the head isn't too difficult and you can find the manual on-line.
                    Paul Compton


                    • #11
                      EVPaul, Thanks. I've found the manual. Not perfect, but its got some nice exploded views. I'll have a closer look this weekend. Maybe even call an electrician if I find a lot of terminals on that motors.

                      Sir John -- if you're reading this -- please drop a 220V bridgeport in the mail. In return I'll send you a 440V bridgeport and the VFD for your troubles.


                      • #12
                        I suspect your motor is wired for what the VFD is set for otherwise you would have some issues with it running at all. 440v motors don't run very well on 220v supply. They do funny things.
                        It's only ink and paper


                        • #13
                          Hey Tony,welcome back!

                          How many leads on your motor?
                          I just need one more tool,just one!