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  • Motor Control Question

    I just purchased a 1/2hp 90VDC permanent magnet motor that I'm hoping to install on a small lathe. I'm looking for a KB DC Drive to match up with it - they make and support a good product made for industrial use, I'm not interested in playing around with treadmill controllers and such - I want something that I can pull up a programming guide and wiring diagram for and that I have confidence will be rated correctly. Looking for some advice on which model to get, there is quite a variety of models to choose from. I would like to have 115VAC input, 90VDC output, motor reversing, and speed control. Thinking chassis mount so that I can make up a box and mount the controls in a handy location. I'm not counting on using the speed control for large speed changes, from what I understand I will loose torque as as the voltage drops for speed control - is this correct? Having dual voltage (115/230 & 90/180) and being a bit oversized would also be handy in the event I ever upgrade the motor or use the drive elsewhere. And is there a way I can reverse a motor even if it has a non-reversing drive? Is it kosher to put a DPDT switch between the drive output and the motor for that function?

    So what do you recommend based on your experience?

    KBMG?
    KBMM?
    KBIC?
    KBCC?
    Cayuga, Ontario, Canada

  • #2
    Well with KB you have a few models and technologies to pick from, are you looking for PWM, or the cheaper SCR bridge versions?
    You need reversing, so that cuts the list down.
    Or You can put a relay on the output in order to reverse, If you want to ensure off in between, then you would need two relays.
    The other way is with one relay and operate the two Inhibit (I) lines to ensure drive off when reversing, this avoids reversing whilst running in the opposite direction.
    Torque does not drop with RPM, in fact a DC motor typically is capable of highest torque at zero RPM.
    DPDT switch would require zero RPM when operating.
    Last edited by MaxHeadRoom; 11-19-2021, 05:11 PM.

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    • #3
      I'm thinking probably the SCR bridge versions, but I'm open to both.

      Zero RPM for switching the motor direction wouldn't be an issue, I'm not looking for rapid direction changes and just want to have the capability to change direction if I happen to need it. The lathe this is going on is a Simonet D102 (similar to a Schaublin 102) which does not have a leadscrew, so running in reverse would be a rare operation.

      Explain to me how the power curve for a DC motor with a drive like this works, sounds like I may have the wrong impression. I thought the drive reduced voltage to reduce the motor speed, and figured that as that voltage dropped the amount of torque the motor could generate would also be reduced. Am in incorrect in that understanding?

      Leaning towards a KBIC or KBMM at this point as a good match to this motor size with 115VAC input.
      Cayuga, Ontario, Canada

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      • #4
        I use a KB controller with a 1HP DC motor to drive my vertical bandsaw. I forget which exact unit it is, but it is one of the SCR bridge types and is fairly old. I can check later if you are interested.
        Most of those KB drives can also be set up for tachometer feedback to monitor the speed and turn up the power to the motor if it begins to slow significantly under load. It's actually not very difficult to set up and it works great. Gives a very wide speed control range with minimal torque loss. I did this on my bandsaw and can go from slow metal cutting speeds to decently fast speeds for wood cutting without changing belts.

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        • #5
          Originally posted by Tom S View Post
          ........................

          Explain to me how the power curve for a DC motor with a drive like this works, sounds like I may have the wrong impression. I thought the drive reduced voltage to reduce the motor speed, and figured that as that voltage dropped the amount of torque the motor could generate would also be reduced. Am in incorrect in that understanding?

          ..................................
          The controller has a generally constant current characteristic. Current makes torque, so the torque would be constant down much of the way to zero (it's problematic down at really slow speeds, the KB has a current boost to correct that a bit). Power reduces approximately the same amount as speed. half speed, half power.

          Both the SCR and PWM have this characteristic. The PWM probably works better at very low speeds.
          2730

          Keep eye on ball.
          Hashim Khan

          Everything not impossible is compulsory

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          • #6
            Look for an ebay bargain KBCC-125R. Safe, reliable reversing, a braking resistor plus a heatsink all rolled into one. I have 2 and they work great. Mini suckage story: I got one of them brand new (without the KB cardboard box) for $75.00.
            Milton

            "Accuracy is the sum total of your compensating mistakes."

            "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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            • #7
              You need one that senses back EMF and uses that to increase torque at low rpms.

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              • #8
                Originally posted by metalmagpie View Post
                You need one that senses back EMF and uses that to increase torque at low rpms.
                Many, if not all the KBC drives have an option for a low end boost. The problem is that at high voltage/speed, the resistance of the motor is largely swamped out by back EMF, but at low speeds the resistance produces a constant voltage that tends to swamp out the back EMF that should self-regulate speed. The low end boost is set up to compensate to some degree for that.
                2730

                Keep eye on ball.
                Hashim Khan

                Everything not impossible is compulsory

                Comment


                • #9
                  A DC motor, by itself, does have a torque curve that decreases when the drive Voltage decreases. And that means that if you try to drive a 90 V DC motor with 10 or 5 Volts you will probably have reached a point where the motor just stalls - 0 RPM.

                  The DC motor speed controllers use a chopped waveform.

                  With the SCRs it is fairly easy to just feed the rectified, but NOT FILTERED AC to the SCR. SCRs will turn off every time this rectified Voltage returns to the zero value (axis crossing point). This happens 120 times per second if 60 Hz AC is used as the input. A simple control circuit can then delay the point where the SCR will turn on and supply current to the motor. This can be varied from immediately after that zero axis crossing (minimum torque and speed) all the way through the full half cycle to the next zero axis crossing (maximum torque and speed).

                  On the other hand, the better models (FETs) will not just rectify the AC, but will filter it too. This produces a constant DC Voltage (90 Volts). Then the control circuit produces a square wave with a variable duty cycle. This square wave is used to switch the FULL DC Voltage to the motor for some fraction of it's cycle and zero Volts for the remaining part of that square wave's cycle. The percentage of time ON to time OFF determines the speed of the motor which is roughly proportional to the AVERAGE value of the DC Voltage. But, because the full DC Voltage is fed to the motor for the ON fraction of the cycle, the motor produces it's full rated torque during that time period. And conversely, it produces zero torque for the OFF fraction of the cycle. It is the full rated torque that allows the motor to act, more or less, with that torque while running at a slower speed.

                  The biggest advantage for the simpler, SCR style controls is price. In it's most basic form, only a very few parts are needed so it is very inexpensive to produce. The better models need extra parts to both filter the rectified AC and a more complicated circuit to produce that square waveform that switches the FETs on and off.

                  Due to the sinusoidal waveform of the 60 Hz AC, the SCR style controller will only produce the full rated torque if the duty cycle is set at the 50% point or greater. If the turn-on point is delayed until after the maximum point of that sinusoidal waveform has passed, then the greatest Voltage that will be fed to the motor will be LESS THAN the maximum and therefore the maximum torque that the motor will produce will be reduced. At the 5% point, the Voltage will be quite low and the torque will be close to the point where the motor stalls even with no external load. In short, the torque will start getting lower at the 50% point and will fall to zero as the speed drops to zero.

                  On the other hand, the better models will send the full DC Voltage to the motor at all speed settings and the torque will remain much the same all the way down to the zero speed point. OK, that is not completely true. At some point the inductance of the motor's windings will come into play and the current in the coil will lag behind the applied Voltage. At a low speed setting this lag in the current can be longer than the ON time of the control waveform and the Voltage will be switched OFF before the current and therefore the torque will never reach it's maximum rated value. This does not happen until the speed setting is very low (as compared to the SCR style control) so the full torque is maintained for much slower speeds than the SCR style controllers are capable of. The exact point where this happens will be determined by a combination of the motor's inductance and the frequency that the control circuit uses. That frequency, along with the percentage of the speed that has been set, determines the duration of the square wave during the ON part of the cycle.

                  One thing that this tells you is that, with the better controllers, a motor with lower inductance in it's windings, will be able to maintain it's torque better at the slower speeds. I have seen this in operation with what is known as printed circuit motors where the windings have only one or a very few turns and therefore have a very low inductance.

                  Of course, there are many variations for both of these types of DC controllers. The above is a very simplified explanation.

                  There is no basic reason why a mechanical, reversing switch or relay can not be used between either style of controller and the motor. With larger motors (over 1/10 HP) I would not allow the direction of the drive to the motor to be reversed while the motor is still rotating in the original direction. I would stop it first and then apply the reverse drive. But that may be just me. Check with the manufacturer of your controller first.



                  Originally posted by Tom S View Post

                  ...<snip>...

                  Explain to me how the power curve for a DC motor with a drive like this works, sounds like I may have the wrong impression. I thought the drive reduced voltage to reduce the motor speed, and figured that as that voltage dropped the amount of torque the motor could generate would also be reduced. Am in incorrect in that understanding?

                  ...<snip>...
                  Last edited by Paul Alciatore; 11-20-2021, 02:46 AM.
                  Paul A.
                  SE Texas

                  And if you look REAL close at an analog signal,
                  You will find that it has discrete steps.

                  Comment


                  • #10
                    Originally posted by Paul Alciatore View Post
                    A DC motor, by itself, does have a torque curve that decreases when the drive Voltage decreases. And that means that if you try to drive a 90 V DC motor with 10 or 5 Volts you will probably have reached a point where the motor just stalls - 0 RPM.
                    ........................
                    That's what I said..... the resistance (and to some degree inductance) becomes more and more significant as voltage decreases, until at very low voltage, the resistance voltage will be larger than the back EMF, wiping out self-controlling action, which the motor inherently has.

                    This effect is proportional to current, so good drives have a current-dependent boost that can "adjust out" the effect of resistance and leave only the back EMF, giving a net effect more similar to what happens at full voltage. Both SCR and PWM types can have that. It essentially makes "zero" the voltage due to the resistance at the current which is flowing at any moment.
                    2730

                    Keep eye on ball.
                    Hashim Khan

                    Everything not impossible is compulsory

                    Comment


                    • #11
                      On my 9x20 lathe, the previous owner retrofitted it with a 1-1/4hp 90vdc motor. Originally he used a 90vdc SCR control, it worked fine, but there was no reverse and I wanted to add a tach etc,etc.

                      So I went shopping on Ebay and found a NOS KBCC-125 for $100 delivered and built the new system around it. The control is in a box down by the motor and the control panel is in a fiberglass hinged clamshell box mounted over the headstock. I have the on/off toggle switch, speed pot and tach display on the front and a DPDT toggle on the back of the box for FWD/REV. The tach I used was just one of the little $15 Chinese ones.

                      I put the FWD/REV switch on the back of the box, so direction change is a conscious action and not a brainfart flipping the wrong switch.
                      Last edited by wierdscience; 11-20-2021, 05:18 AM.
                      I just need one more tool,just one!

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                      • #12
                        Thank you for the explanations. If I'm understanding it correctly the SCR drivers use a chopped sinusoidal waveform to produce the DC voltage and because of that at below 50% rating torque can drop off because it's after the peak of the waveform. The PWM drives create a square waveform that works better for lower speed operation, but at very low speeds there can be back EMF that causes trouble. I'm not overly concerned about very low speed operation, so it sounds like the SCR will work fine for my application.

                        alanganes where did you get that tach for feedback? I took a quick look online but it looks like a pricey component, unless I'm looking at the wrong thing. Good to know of the option, I had seen that in the wiring diagram for some of the KB models but though it was for a tachometer readout, not for feedback.

                        A few people have mentioned the KBCC model, looks like a pretty package that can do everything I'm looking for. I'll see what I can scrounge up to that end.

                        Cayuga, Ontario, Canada

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                        • #13
                          Originally posted by Tom S View Post
                          A few people have mentioned the KBCC model, looks like a pretty package that can do everything I'm looking for. I'll see what I can scrounge up to that end.
                          KBCC is a SCR version, I have used many of the KB units in the past, I much prefer the PWM models, especially if you intend doing any low RPM's operation.
                          Where they are much smoother and quieter.
                          When doing reversing, I would make use of the Inhibit input when you reverse, this causes the drive to go through the accel stage when you remove the inhibit..
                          The TM versions I use now have gone from SCR to PWM for some time now.
                          Last edited by MaxHeadRoom; 11-20-2021, 11:51 AM.

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                          • #14
                            One thing to look out for.....

                            Some of the KB drives are NOT ISOLATED, so the control pot is live with the AC volts. You need to use a pot that has a plastic shaft, and keep the case of the pot from being accessible, including the attachment nut.

                            2730

                            Keep eye on ball.
                            Hashim Khan

                            Everything not impossible is compulsory

                            Comment


                            • #15
                              Lots of choices. Something like this.

                              https://www.ebay.com/itm/11500340759...4AAOSwdF5hSkrn
                              John Titor, when are you.

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