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    #16
    Originally posted by synchro View Post
    Yes, VDC = 3sqrt(2)/pi x VLine-Line RMS = ~1.35 VLine-Line RMS

    About 1.4VDC should also be subtracted from the output to account for voltage drops in a pair of diodes inside the bridge.




    I guess it depends on the requirements of the load that is served by the DC supply.
    RMS or Root Mean Square is the square root of the average (aka "mean") of one (or the sum of more than one) quantities that are squared, usually voltages or currents in electrical applications. RMS is more meaningful in AC power applications than the peak voltage of a waveform because power can than be directly calculated as P = V x I =V2/R =I2R for resistive loads when V and I are RMS quantities. Also, heating of conductors is proportional to I2R and therefore to the "mean square" of the AC current waveform even if harmonics or other frequencies are present.

    It's more likely that the average value of the DC supply voltage is a more useful quantity than RMS for loads that use DC, unless you wanted to accurately quantify the total power into a resistive load when there's a significant amount of ripple (which would not be a common siituation).
    Otherwise, the average DC voltage and peak-peak ripple are more useful quantities. For example, if you had a linear voltage regulator IC fed by a rectifier then the minimum voltage on the waveform due to DC ripple is very important. The voltage drop across the regulator cannot fall below a minimum value or it falls out of regulation. An RMS spec doesn't give you enough information in this case, because very short dips in the DC voltage would have minimal effect on the RMS ripple value.
    Thank you for the thorough explanation!
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

    Comment


      #17
      Originally posted by paulengr View Post
      RMS has to do with the AC that we don't care about...we want only the DC component.

      Sent from my SM-T350 using Tapatalk
      I'm not trying to be snarky, but that is not a good response. I want my money back That was basically my question: why not apply the RMS principle to DC like we do with AC. Nothing says RMS is for AC only.
      Ethan Brush - East West Electric. NY, WA. MA

      "You can't generalize"

      Comment


        #18
        If you apply a voltage source (AC, DC, mixed) to a resistor, current will flow and the resistor will dissipate heat. The average power dissipated will depend on the RMS applied voltage, and this is true for AC, DC, mixed AC and DC.

        -Jon

        Comment


          #19
          Originally posted by electrofelon View Post
          I'm not trying to be snarky, but that is not a good response. I want my money back That was basically my question: why not apply the RMS principle to DC like we do with AC. Nothing says RMS is for AC only.
          In DC servo motors torque is proportional to voltage (less counter EMF which is hopefully a small value) so average torque is proportional to average voltage. If the voltage has ripple the average torque is proportional to average voltage. On pure AC if the frequency was slow enough it would just oscillate back and forth. On an AC motor RMS voltage is what matters, to a point. On an AC power meter on resistive loads V x I = P with RMS V and I just as with DC power.

          Usually when taught in school we talk about the fact that in the analog era making a cheap mostly RMS AC meter is simple with just a single diode rectifier circuit but true RMS let alone doing square roots in analog is nontrivial but that's teaching first year students, not second years in the motor class.

          Sent from my SM-T350 using Tapatalk

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            #20
            190711-0846 EDT

            paulengr:

            In DC servo motors torque is proportional to voltage (less counter EMF which is hopefully a small value) so average torque is proportional to average voltage.
            That is an incorrect statement.

            Torque is proportional to current for a constant magnetic field in which that current is flowing. It is true that if a servo spindle is at zero RPM that current will be approximately proportional to applied voltage. But it is not true that voltage produces torque, it is current.

            Counter EMF is approximately proportional to RPM. Why is that hopefully small? If you are talking about a servo there are many times, and possibly mostly when the servo is rotating.

            On a CNC machine mostly doing drilling the X and Y servos are mostly stopped. However, both Z and spindle are moving. But if you are contour milling, then X, Y, and Z are mostly moving, and spindle is at a constant high speed. On most CNC lathes the servos are mostly moving, and the spindle is usually at a high speed.

            .



            Comment


              #21
              Originally posted by Besoeker3 View Post
              It is. With a lot of smoothing you might get get closer to 1.42Vac
              Shouldn't the average DC voltage be higher (assuming appropriate filtering) with three phase rectification?
              The value of:

              VDC = (VAC * 1.65)

              makes sense intuitively, as there is inherently less ripple in rectified three phase, as opposed to rectified single phase AC. Is my intuition wrong here?



              Thanks,

              SceneryDriver

              Comment


                #22
                Originally posted by steve66 View Post
                Can you determine the peak diode current? That's one very important parameter for diode rectifier design. Both at stead state, and also when the power is first applied and the caps. are completely discharged.

                Although, I still don't know how we are getting from a 3 phase supply to a single phase output, so maybe you don't know what the complete circuit is either.
                I have dealt with a lot of rectifiers, mostly at high/relative high currents. Up to 70kA but more usually in the 20-40kA range.

                Typically, these were 12 or 24-pulse arrangements made up of two or four units operating in parallel. The DC usually had a choke (inductor) in series with the output of each. This made the current quite smooth. Peaks were not an issue for such applications.

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                  #23
                  Originally posted by junkhound View Post
                  Scenery - how much is the load?

                  You can buy off the shelf 5 kW Vienna bridge 3 phase PFC rectifiers a lot cheaper than designing a straight rectifier with big caps.
                  About the size of 2 packs of playing cards, that should fit, eh.


                  Post your amperage needs and can probably give you a part number even. Lots of sources, Vicor, Murata, Kepco, Aveox, etc....
                  I had not heard of Vienna bridge rectifiers until I googled them. Very interesting. Active rectification crossed with SMPS tech. Very cool. Looking at needing 60A @ 70VDC peak. Much lower currents most of the time.


                  SceneryDriver

                  Comment


                    #24
                    Originally posted by SceneryDriver View Post
                    Shouldn't the average DC voltage be higher (assuming appropriate filtering) with three phase rectification?
                    The value of:

                    VDC = (VAC * 1.65)

                    makes sense intuitively, as there is inherently less ripple in rectified three phase, as opposed to rectified single phase AC. Is my intuition wrong here?



                    Thanks,

                    SceneryDriver
                    For a 3-phase, full wave rectifier Vdc = 1.35Vac.
                    Others here have shown the mathematical derivation.

                    Comment


                      #25
                      Originally posted by SceneryDriver View Post
                      I had not heard of Vienna bridge rectifiers until I googled them. Very interesting. Active rectification crossed with SMPS tech. Very cool. Looking at needing 60A @ 70VDC peak. Much lower currents most of the time.


                      SceneryDriver
                      You may also want to consider a switched mode power supply. They are usually more complex, and normally include an inductor, but they are typically more efficient and they reduce the input current spikes.

                      With a rectifier circuit, input current only flows when the input voltage is higher than the output voltage across the capacitor. In the case of a simple half wave rectifier, the input current may only flow for a few milliseconds, but it must charge the capacitor enough to sustain the load current for the rest of the cycle. That means the peak input current may be several times the output current. As the output capacitor is increased in size, the output voltage doesn't droop as much, and the capacitor charging time actually becomes shorter, resulting in higher input current spikes.

                      Of course, using a bridge rectifier, or a three phase input, or almost any of the more complex circuits adds more diodes to conduct, and more conduction time. So these all lower the input current spikes. But if you don't calculate the spike peak current, there is a good chance you are going to kill the diodes or something else in the circuit.

                      Switched mode power supplies can also reduce the input current spikes.

                      Comment


                        #26
                        Originally posted by electrofelon View Post
                        I'm not trying to be snarky, but that is not a good response. I want my money back That was basically my question: why not apply the RMS principle to DC like we do with AC. Nothing says RMS is for AC only.

                        As long as the ripple percentage is low compared to the DC component, the difference between average and RMS will be small, and may not justify the extra expense of a true RMS meter or the additional complication to the math.

                        Comment


                          #27
                          Originally posted by Besoeker3 View Post
                          It is. With a lot of smoothing you might get get closer to 1.42Vac
                          How would you ever get above the peak of 1.414?
                          Ethan Brush - East West Electric. NY, WA. MA

                          "You can't generalize"

                          Comment


                            #28
                            Originally posted by electrofelon View Post
                            How would you ever get above the peak of 1.414?
                            If the voltage was above nominal by less than half a percent.
                            That work for you?

                            Comment


                              #29
                              Originally posted by electrofelon View Post
                              How would you ever get above the peak of 1.414?
                              PFC boost circuit, typical output for 3 phase 120V l-n is 350 to 400 Vdc.

                              Comment


                                #30
                                Agreed that current is torque, no question about it. As long as voltage is unlimited this is true. A DC drive in current regulation mode is a torque regulator until we hit maximum voktage. Similarly if we need a speed controller we regulate armature voltage (to a point). Physics says it's all current and this is great when we are trying to understand DC motors and drjves.

                                So what happens if we are voltage limited using the current? No way to tell because we are affecting voltage, not current. What if the bus voltage fluctuates faster than the drive response time? Torque varies. How much? Hard to say except if we apply Ohms Law.

                                I think we can agree torque is proportional to current minus counter EMF that we can hopefully ignore in practice.

                                https://www.motioncontroltips.com/torque-equation/

                                If I have a drive involved that adjusts current faster than supply voltage fluctuations below maximum torque, torque is approximately constant. But if the drive is not involved or too slow, torque will vary as described. If this was not a concern we would not control DC bus ripple at all.


                                Sent from my SM-T350 using Tapatalk

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