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Baldor 3-phase motor for RPC

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  • #76
    Originally posted by Jim Stewart View Post


    Well, Paul, perhaps you haven't been dealing with old electronics - by "old" I mean from the twenties and thirties.

    My first radio receiver was an RME (Radio Manufacturing Engineers) superhet, manufactured in '36 or thereabouts. The dial was calibrated in Kilocycles and the manual used MFD throughout. Tubes with grid caps on top. So MFD does have historical precedence.

    -js
    Absolutely correct. And marked on old components as well. Old can be the 50's or possibly 60's, not just 30's.
    1601

    Keep eye on ball.
    Hashim Khan

    Comment


    • #77
      Originally posted by Fear View Post
      I don't know how to type the upside-down lower case "h" for microfarad so I've just been using MFD.

      How come most schematics for homemade RPCs show run caps between L1&L3 and L2&L3? Isn't it going to balance out (as in literally balance, like minimal vibration) if caps are spread more uniformly?
      To get mu on a windows computer hold down alt, and with it held, type 230 on your NUMPAD (it must be using a numpad) and you'll get your ยต.
      21" Royersford Excelsior CamelBack Drillpress Restoration
      1943 Sidney 16x54 Lathe Restoration

      Comment


      • #78
        I just hope we can get Fear's RPC working. I'm really curious about why it has presented so many confusing symptoms and still won't properly generate the third phase. It may be that a two pole motor has to spin a lot faster than a four pole, and it needs to be running at least about 80% of synchronous speed before the third phase is sufficiently generated. Another thought is that perhaps the motor is actually wired for 480 VAC in which case it will bog down on 240 and cause problems as he has experienced.
        http://pauleschoen.com/pix/PM08_P76_P54.png
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030

        Comment


        • #79
          Originally posted by PStechPaul View Post
          I just hope we can get Fear's RPC working. I'm really curious about why it has presented so many confusing symptoms and still won't properly generate the third phase. It may be that a two pole motor has to spin a lot faster than a four pole, and it needs to be running at least about 80% of synchronous speed before the third phase is sufficiently generated. Another thought is that perhaps the motor is actually wired for 480 VAC in which case it will bog down on 240 and cause problems as he has experienced.
          I've checked the wiring to the field coils of the motor so many times. It is definitely wired for 240vac.

          Comment


          • #80
            Can you connect 240 VAC to T1 and T2, and then spin the rotor using an electric drill or other means to get at least 3000 RPM? Once up to near synchronous speed, it should run on single phase. Repeat the test for T2-T3, and T3-T1, to make sure all three windings are OK. Could be a reversed winding or mislabeled connections.
            http://pauleschoen.com/pix/PM08_P76_P54.png
            Paul , P S Technology, Inc. and MrTibbs
            USA Maryland 21030

            Comment


            • #81
              Originally posted by Fear View Post

              I've checked the wiring to the field coils of the motor so many times. It is definitely wired for 240vac.
              Excess current, and bogging down can be from many causes. Among them are the following reasonably common causes in RPCs.

              1) Start caps not disconnecting. Contactor or control system fault. Usually, if run for long, start caps will fail spectacularly or at the least become very hot. I think that it was mentioned that some already failed, probably from the bad timer board. Others may be damaged or open due to overheating even though not "exploded".

              2) Idler motor loaded down with too much "balance" capacitance. Remove ALL balance capacitors, and check operation with ONLY start caps. It should work, with possibly a slightly low generated leg.

              3) Idler started with a load on it. Output contactor (if present) may have failed, or control for it may be defective. Load machine simply left on. Disconnect all loads and try again, then if OK, check load machines or contactor.

              4) Motor wired for correct voltage, but has at least one reversed coil in one of the phases. Triple check the coils both for voltage configuration, AND for correct polarity, correct as needed

              5) Motor actually has a shorted turn in a coil. Check for possible heating in one specific area of the winding. If tested too much or for too long, one portion of winding may become discolored. Shorted turns are difficult to find without specialized equipment. They are not too common unless motor is known to be abused, electrically or physically, both of which leave traces, either discoloration from overheating, or evidence of physical damage due to impacts, etc. No cure for this, get a different motor.

              A useful test is to disconnect everything from the motor, get the motor rolling over, and connect two wires to 240V power. The idler should accelerate and run. If it does on both L1/L2, and L2/L3 pairs, then shorted turns are not the issue, and problem is in "control box".
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #82
                Originally posted by J Tiers View Post

                Excess current, and bogging down can be from many causes. Among them are the following reasonably common causes in RPCs.

                1) Start caps not disconnecting. Contactor or control system fault. Usually, if run for long, start caps will fail spectacularly or at the least become very hot. I think that it was mentioned that some already failed, probably from the bad timer board. Others may be damaged or open due to overheating even though not "exploded".

                2) Idler motor loaded down with too much "balance" capacitance. Remove ALL balance capacitors, and check operation with ONLY start caps. It should work, with possibly a slightly low generated leg.

                3) Idler started with a load on it. Output contactor (if present) may have failed, or control for it may be defective. Load machine simply left on. Disconnect all loads and try again, then if OK, check load machines or contactor.

                4) Motor wired for correct voltage, but has at least one reversed coil in one of the phases. Triple check the coils both for voltage configuration, AND for correct polarity, correct as needed

                5) Motor actually has a shorted turn in a coil. Check for possible heating in one specific area of the winding. If tested too much or for too long, one portion of winding may become discolored. Shorted turns are difficult to find without specialized equipment. They are not too common unless motor is known to be abused, electrically or physically, both of which leave traces, either discoloration from overheating, or evidence of physical damage due to impacts, etc. No cure for this, get a different motor.

                A useful test is to disconnect everything from the motor, get the motor rolling over, and connect two wires to 240V power. The idler should accelerate and run. If it does on both L1/L2, and L2/L3 pairs, then shorted turns are not the issue, and problem is in "control box".
                Work has had me very busy for the past few months. So much so that I haven't had time to mess with the RPC.

                I did, however, disconnect and tape off all wiring to run caps, hooked 2 run caps in parallel to the existing 4 start caps for a total of 6 start caps.
                I then held the start button for the start contactor while I hit the start button for the run contactors. The motor started spinning on it's own and looked promising except for not getting up to full speed. I'd release the start contactor after it became clear it wasn't getting any faster and it would slow back down and start groaning like it's labored. Since this motor needs to spin twice as fast as the motor this panel was designed for, I wonder if it doesn't just need a few more start caps and possibly a second start contactor.

                With it now spinning on it's own is promising, but still not where I need to be.

                Comment


                • #83
                  Voltage readings when performing this test would be helpful. Check the incoming 240 VAC line power before and after trying to start the motor. And also measure the current. If there is too much voltage drop, it may not start or run at full speed. But most 240 VAC motors will run OK on 208 VAC, which may drop as low as 200 VAC under load.

                  Once you have determined that line drop is not excessive, measure the voltage from the manufactured "wild" leg (T3) to each of the incoming lines L1 and L2 (which should be the same as T1 and T2 when power is applied).

                  If you can, take a video of your testing. Seeing and hearing things are valuable clues.
                  http://pauleschoen.com/pix/PM08_P76_P54.png
                  Paul , P S Technology, Inc. and MrTibbs
                  USA Maryland 21030

                  Comment


                  • #84
                    Originally posted by PStechPaul View Post
                    Voltage readings when performing this test would be helpful. Check the incoming 240 VAC line power before and after trying to start the motor. And also measure the current. If there is too much voltage drop, it may not start or run at full speed. But most 240 VAC motors will run OK on 208 VAC, which may drop as low as 200 VAC under load.

                    Once you have determined that line drop is not excessive, measure the voltage from the manufactured "wild" leg (T3) to each of the incoming lines L1 and L2 (which should be the same as T1 and T2 when power is applied).

                    If you can, take a video of your testing. Seeing and hearing things are valuable clues.
                    I'm curious how L1 and L2 could be different than T1 and T2. It would be reading each side of 3 contactors with 4 circuit connections each. Aside from contactor failure, what am I looking for?

                    Comment


                    • #85
                      Originally posted by Fear View Post

                      Work has had me very busy for the past few months. So much so that I haven't had time to mess with the RPC.

                      I did, however, disconnect and tape off all wiring to run caps, hooked 2 run caps in parallel to the existing 4 start caps for a total of 6 start caps.
                      I then held the start button for the start contactor while I hit the start button for the run contactors. The motor started spinning on it's own and looked promising except for not getting up to full speed. I'd release the start contactor after it became clear it wasn't getting any faster and it would slow back down and start groaning like it's labored. Since this motor needs to spin twice as fast as the motor this panel was designed for, I wonder if it doesn't just need a few more start caps and possibly a second start contactor.

                      With it now spinning on it's own is promising, but still not where I need to be.
                      There is a load on the motor, that has to be the problem. Almost any motor will start up and reach full speed even if just rope started with no load. It does NOT need to be held on "start" to full speed. Most single phase centrifugal switches will cut out at around 65% of full speed or thereabout, but that is primarily because they are expected to start under load. The start circuit provides torque-producing phase.

                      But the motor typically will not have much torque without the start circuit until above 50% speed. RPM of 80% or 85% of full speed is typically about the max torque speed. So if there is any load on it, mechanical or electrical, it may not be able to accelerate to full speed without the start circuit, and can sound labored.

                      And, with the regular start circuit, phasing is not necessarily perfect, so the motor may not get to full speed, it may stop accelerating below that unless you release the start.

                      Mechanical loads are easy, stuff attached to the shaft, or, potentially bearings so bad that they should be very obvious. Sometimes an odd thing, like the rotor shifts axially and rubs on something. You can check for that sort of thing, but usually you will know easily about it.

                      Electrical loads can be something connected to the generated leg. There should be no load on that, no connection to it (except the voltage relay, if any) until the idler is running normally.

                      Electrical loads can also be due to a problem with the motor wiring or even the windings. A shorted turn on the generated leg winding is a pretty heavy load, and can cause an issue. So can a coil that is accidentally connected in reverse polarity, especially if it is wired in parallel with another coil, as may be normal for the low voltage setting (reversed like that is almost the same as a shorted turn).

                      Check everything carefully. Maybe change which wires are connected up, if motor leads 1 and 2 are connected to L1 and L2 (input wires) respectively with motor lead 3 as the generated leg, try changing to motor lead 2 and 3 for L1 and L2 so motor lead 1 is the generated leg..

                      That may make the problem more evident. For instance, if the winding for the generated leg had a shorted turn, the motor might not start. If switched to put that problem winding on an input wire, the motor will likely run, but would draw excess current and overheat pretty fast. That would tell you there was a problem with that winding, and you can re-check it for polarity. If polarity is OK, then the winding may be shorted. A motor shop can probably test it if you suspect that.



                      1601

                      Keep eye on ball.
                      Hashim Khan

                      Comment


                      • #86
                        Originally posted by Fear View Post

                        I'm curious how L1 and L2 could be different than T1 and T2. It would be reading each side of 3 contactors with 4 circuit connections each. Aside from contactor failure, what am I looking for?
                        That is true, but I am trying to understand what was causing these voltage readings you showed some time ago:

                        Click image for larger version

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                        That was supposed to be with a nominal 240 VAC source. You may want to measure L1 and L2 at some point closer to the distribution box, and then again on the terminals of the contactor. It is possible, although unlikely, that the contactor is not working properly, in which case you would see voltage between terminals L1-T1, L2-T2, and L3-T3. Something seems "hinky", and without eyes and ears on the problem, it is difficult to suss out.
                        http://pauleschoen.com/pix/PM08_P76_P54.png
                        Paul , P S Technology, Inc. and MrTibbs
                        USA Maryland 21030

                        Comment


                        • #87
                          Originally posted by J Tiers View Post

                          But the motor typically will not have much torque without the start circuit until above 50% speed. RPM of 80% or 85% of full speed is typically about the max torque speed. So if there is any load on it, mechanical or electrical, it may not be able to accelerate to full speed without the start circuit, and can sound labored.

                          And, with the regular start circuit, phasing is not necessarily perfect, so the motor may not get to full speed, it may stop accelerating below that unless you release the start.

                          Check everything carefully. Maybe change which wires are connected up, if motor leads 1 and 2 are connected to L1 and L2 (input wires) respectively with motor lead 3 as the generated leg, try changing to motor lead 2 and 3 for L1 and L2 so motor lead 1 is the generated leg..

                          That may make the problem more evident. For instance, if the winding for the generated leg had a shorted turn, the motor might not start. If switched to put that problem winding on an input wire, the motor will likely run, but would draw excess current and overheat pretty fast. That would tell you there was a problem with that winding, and you can re-check it for polarity. If polarity is OK, then the winding may be shorted. A motor shop can probably test it if you suspect that.


                          Am I just oversimplifying my understanding? The start caps get a quick charge delayed enough to surge the "fake-leg" just long enough to get up to speed enough for the two the legs to carry it at speed. Apparently the run caps serve no purpose beyond fine-tuning the balance of the output. So if the start caps were sized to get a motor up to 1800rpm, then they might need twice the capacity to get to over 3000rpm.
                          And adding run caps to at least T1 won't help it get up to speed?

                          I've been scouring to find a slower motor with the same specs but I doubt I'll run across it for under a grand.

                          Comment


                          • #88
                            The size just affects the phase angle with that particular motor, which sets the start torque. You do need "enough" of that.

                            But, as I said, once it is going, the motor has some torque without the caps, and it ought to speed up on its own.

                            See, the motor, with no load on it, will run above rated speed, and close to the nominal synchronous speed. The faster it runs, the less torque it makes, so it actually has to slow down to the rated speed, 1750 or 3450 etc RPM in order to make torque for full power. So it SHOULD speed up OK until the speed and the torque balance out.

                            Once you have it running reasonably fast, it should take off on its own without the start caps. If it does not, that means that something is not right.

                            Electrical loads can be the things I mentioned, and another one is too much "run", or "balance" capacitance. It does not need the balance capacitors in order to run, they are for correcting the voltage.

                            Thing of it is they can provide a load, as well as a small amount of start type capacitor action. You can disconnect them for the moment. You need the START capacitors unless you use a pony motor or other means, but you do not need the balance ones at the moment, and they can cause a problem if not correct..

                            First thing then, is to disconnect the metal case balance caps, just to get them out of the way, and cut down on the number of variables. The start capacitors are probably in black plastic cases, and you do need "enough" of them. Since it does start, you have enough for the moment.

                            If you are CERTAIN that the start capacitors are getting disconnected, then that is OK. But they DO NEED to get disconnected FOR CERTAIN, once the motor is started and running, likely a second or so time-wise..

                            Then, if you do not have any loads connected to the generated leg, and you have disconnected the "balance" capacitors for the moment, you are down to something about the motor itself. Connections, defects of some sort, we do not know yet, except you said it spins free, so that should eliminate the bearings as a possible problem.

                            Go over those connections and be certain you have them right. If you need to, make a schematic drawing of how it IS connected, and compare that with what it SHOULD be connected as. Sometimes that really helps..... DAMHIKT, over the years I have made plenty of bonehead mistakes, enough to not look down on anyone else.

                            If you are sure there are no mistakes, try it again. If it works, then you can start by checking voltages, and seeing if you need to be adding back any balance caps.

                            If it does not work, then try swapping motor leads as I mentioned. If it works then, either you fixed a connection, or there may be a motor fault on the winding that used to be the generated leg. Check temperature, and watch out for the smell of charring varnish.

                            Let us know.
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #89
                              When starting an induction motor using full voltage, the current will be at practically that for maximum locked rotor, until it reaches about 90% of full speed. At that point, the slip causes the back-EMF to rise to approach that of the applied stator voltage, and the current will decrease. It will approach zero current as it gets to synchronous speed. The starting capacitors will provide maximum current to the manufactured leg up to the 90% RPM point. As long as there is no speed related load on the rotor (such as a fan), the torque required will be dependent on the rotational inertia of the rotor, and that will remain constant until it gets closer to rated speed.

                              Normally, a motor should reach operational speed in several seconds. The rate of increase of RPM (acceleration) may not be linear, as energy is related to the square of velocity. So it may take 1 second to reach half speed (1800 RPM), but another 1 second to reach 2700 RPM, and then after another second it might reach 3150 RPM. At that point it is approaching synchronous speed and will soon "lock in" to line frequency.


                              https://electronics.stackexchange.co...or-an-ac-motor

                              Click image for larger version

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                              http://pauleschoen.com/pix/PM08_P76_P54.png
                              Paul , P S Technology, Inc. and MrTibbs
                              USA Maryland 21030

                              Comment


                              • #90
                                That curve appears to vastly under-rate the starting torque for many motors, but is otherwise OK. The peak torque is shown as at quite a high rpm, which is true of some motors. Others have a broader peak, it depends on the design slip. Current is also shown as higher for most of the acceleration range than some other motor types.

                                Also, an induction motor will NEVER "lock-in" at synchronous speed. It will always have slip, because without slip, there is zero torque. Slip depends on the load. With more load, the motor slows, creating more slip, and increasing the "field" until the torque balances the load.

                                Regardless of details, in the graph, the torque is shown as non-zero when the rotor is not turning, which is correct for a three phase motor supplied with three phase power, and as zero at synchronous speed, also correct.

                                When running at single phase, as an idler does, the actual performance will be quite different.

                                Run with single phase, the zero speed torque of a three phase motor is zero. There is nothing creating a rotating field. That is the job of the start circuit, which creates another phase using the start capacitors.

                                But, even with a single phase motor, the motor can and does accelerate after the start circuit disconnects, and the three phase motor will also run on single phase, but at a lower power, once turning..

                                Bottom line is:

                                The motor in question, if wired right, not under excessive load, and having no defects, should happily speed up to it's full speed (just short of synchronous) once it is substantially turning.

                                This one does not, which means it has some sort of excessive mechanical or electrical load, or a condition that prevents it operating correctly (miswired coil, shorted turns, etc).
                                1601

                                Keep eye on ball.
                                Hashim Khan

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