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    #16
    Originally posted by myspark View Post




    Lagging power factor is inherent in industrial locations where huge AC motors are common. In some commercial and residential locations where resistive loads are predominant. . .there is no discernible economic benefit that can be derived from it.
    ......

    I wonder. What [remaining] loads in a residence are resistive? Well, electric dryer. Ooops, just heard of a heatpump based dryer. Lamps, err. Ovens? I assume they may be PWM controlled. Tesla charging?

    Have there been any recent studies of residential power factor and non-linearity?

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      #17
      Maybe they simply change the $/Kwh rate to compensate.
      Master Electrician
      Electrical Contractor
      Richmond, VA

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        #18
        Originally posted by ka4koe View Post
        Thank you for the replies. I hardly ever deal with PF in my day to day duties. The problem is somehow these vendors get access to those near the top of the command chain and then we have to deal with it. Another issue as I see it is the devices in question are not switched out after the motor loads stop. Then the power factor goes the other way----now we are capacitive.
        Since (as you stated) that this is vendor-supplied equipment, I would assume they would deal with “TURN KEY “ contracts.

        This is the usual procedure in dealing with government projects. The government would invite contractors to bid on projects from scratch. . . meaning the contractors would only be given the criteria as to how and what purpose the project is going to be.

        In essence, turn-key contracts are: you build us something and you give us the key to turn the thing on when its done.. . . that is all the contractor needs to do.
        This is why we "paeans" (engineers) are not privy to powers-that-be decisions.
        Here are the plans . . . you do it.

        And people would wonder why it cost so much. . . to build a toilet to operate in gravity-free environment. Water doesn't flush up there in space you know. ]

        Is that off topic?

        Lockheed for example is invited to create a supersonic fighter jet. This is all what the engineers have to go by. This also applies to projects like building structures.

        Now, back to power factor correction using PFC capacitor.

        Your idea of shutting down the caps when the motor loads stop is good. Anything you don't need when things are supposed to remain idle is a plus. Anything that emits heat when not in use is just wasted energy. Resistors known as vampire loads do consume energy those--that are used for controls and sensors.

        But there is an issue when you connect/disconnect a capacitive load. Capacitors store energy that needs to be drained (discharged) before the next command to turn on. Undesirable results would happen if not heeded.

        There is this inherent current surge every time you engage a capacitor in the circuit.

        Your application (HVAC) loads are meant to cycle on and off. For huge capacitors in industrial application, this “draining “ process takes time. Normally (theoretically) it takes 60 seconds.

        If your project can live with one minute waiting period after you shut down, then, by all means more power to you.

        NEC requires this draining time. OK NEC hounds, I'm ready for the lynching.

        Having said this, the equipment supplier must have included this feature in their design.



        Last edited by myspark; 11-09-19, 02:40 PM.

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          #19
          I just examined the bill for reactive power here. It's about .01% of the total. I'm going to try and slam-dunk this and put it away for good.

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            #20
            Originally posted by myspark View Post

            Efficiency encompasses everything especially in a situation where OP is in.
            We are talking about efficiency at the utilization end. . .not at the power generation end. (generator or PV)
            Efficiency is the state or quality of being efficient or being able to perform or accomplish something with the least amount of wasted time and effort and also minimum expense.
            Improved power factor as I mentioned identifies with minimum of waste.

            A motor for instance that is running at low efficiency (for the most part) generate heat. Heat is wasted energy (Joule's Law) that would otherwise would have been used productively.
            Efficiency of a motor isn't necessarily directly related to the power factor.

            Efficiency of a motror is simply the ratio of power in to power out. Low power factor indirectly leads to possible increased losses in the supply lines but doesn't change overall efficiency of the motor itself.
            I live for today, I'm just a day behind.

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              #21
              Originally posted by kwired View Post

              Efficiency of a motor isn't necessarily directly related to the power factor.

              Efficiency of a motror is simply the ratio of power in to power out. Low power factor indirectly leads to possible increased losses in the supply lines but doesn't change overall efficiency of the motor itself.
              Well, that is a rudimentary knowledge on how inductive loads provide useful work to our benefit.

              Low power factor and its effects are more noticeable on the energy distribution sector.

              This low power factor increases the non-useable energy that is turned to heat. Heat in distribution systems is the consequence of low power factor.

              This heat build up that occurs in the distribution lines increases resistance of conductors and thereby cause voltage drop. This voltage drop propagate to the system being served.

              Motors running on LPF also run in elevated temperature. . .a consequence of LPF. ( some modern motor design can compensate for this only to a certain degree. )

              This doesn’t save you however from the pitfall of LPF because it (load) draws higher internal current.

              Power supply running outside unity PF reduces efficiency of motors connected as mentioned in my previous posts.

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                #22
                Originally posted by myspark View Post


                Power supply running outside unity PF reduces efficiency of motors connected as mentioned in my previous posts.
                That's simply incorrect.

                Comment


                  #23
                  Originally posted by Besoeker3 View Post
                  That's simply incorrect.
                  Agreed.

                  Also:
                  Motors running on LPF also run in elevated temperature. . .a consequence of LPF. ( some modern motor design can compensate for this only to a certain degree. )
                  Motors are DESIGNED to operate safely and at rated efficiency at the PF they are tested to have. So the motor temperature is not "elevated" in the sense that it will be higher than "normal" because of "low" power factor, the motor efficiency and temperature rating is DEFINED based on the rated PF. Then if the PF is lower than rated, it means the motor is unloaded, ergo the unloaded state by itself lowers the motor temperature, so the lower PF is irrelevant from a thermal standpoint.

                  You (myspark) are absolutely correct though that the BURDEN of low PF is felt by the distribution system as a whole, which is why the utilities charge penalties for having low PF to large commercial and industrial users. It's just not significant from the end user standpoint, which is why they use penalties; they are incentivizing those large contributors to correct their PF in order to save the UTILITY from the losses in THEIR equipment, such as having to over size transformers.
                  __________________________________________________ ____________________________
                  Many people are shocked when they discover I am not a good electrician...

                  I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

                  Comment


                    #24
                    I agree with Besoeker and Jraef.

                    power factor,high or low, is going to cause losses in the distribution, require larger transformers, conductors, etc.

                    But you don't correct the PF of a motor, it was designed to operate the way it is. Even if you connect PF capacitors right inside the motor terminal box, all you have corrected is from that point to the source, the motor windings themselves still operate at or very near the PF they were designed for, and they need to for the magnetic properties to be within design parameters or it will effect output torque. All that happens with PF correction is the reactive current of the motor windings gets exchanged between the winding and the capacitor instead of being exchanged between motor winding and the source, if sized for adjustment to 1.0 PF then there is no reactive current between the point of correction and the source. Even VFD's have approximately same power factor between the drive and motor as would be with a motor connected directly to a true three phase source, but any power factor on the drive input is distortion power factor from the solid state switching and not displacement power factor from running an inductive load.
                    I live for today, I'm just a day behind.

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                      #25
                      Originally posted by kwired View Post
                      But you don't correct the PF of a motor, it was designed to operate the way it is. Even if you connect PF capacitors right inside the motor terminal box, all you have corrected is from that point to the source, the motor windings themselves still operate at or very near the PF they were designed for, and they need to for the magnetic properties to be within design parameters or it will effect output torque. All that happens with PF correction is the reactive current of the motor windings gets exchanged between the winding and the capacitor instead of being exchanged between motor winding and the source, if sized for adjustment to 1.0 PF then there is no reactive current between the point of correction and the source.
                      But the PF of an induction motor depends on its load. On light load, the motor PF is poor.

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                        #26
                        Originally posted by Sahib View Post

                        But the PF of an induction motor depends on its load. On light load, the motor PF is poor.
                        That is correct, but the PF is worse at light loads because the real power decreases by a significantly larger factor than the reactive power does under these conditions. So the smaller PF at light loads will not result in higher losses from reactive power, but instead these losses will actually decrease somewhat.

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                          #27
                          Originally posted by synchro View Post

                          That is correct, but the PF is worse at light loads because the real power decreases by a significantly larger factor than the reactive power does under these conditions. So the smaller PF at light loads will not result in higher losses from reactive power, but instead these losses will actually decrease somewhat.
                          But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor. So motor PF and efficiency are correlated for standard induction motors.

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                            #28
                            Originally posted by Sahib View Post

                            But look at the efficiency of the motor. It also decreases with decrease in full load unless it is an energy efficient motor. So motor PF and efficiency are correlated for standard induction motors.
                            They are correlated with the load and by coincidence therefore, each other.

                            The point is that applying external PF correction will in no way measurably affect the efficiency of the motor.

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                              #29
                              Originally posted by Sahib View Post

                              But the PF of an induction motor depends on its load. On light load, the motor PF is poor.
                              And that PF at the motor still remains the same. Capacitor only effects the PF measured on the supply side of wherever it is connected.

                              Even if you put the capacitors inside the motor housing, the motor coils still operate at same PF as they would if there were no capacitors in the circuit. The reactive current either flows between the motor coils and the source, or the motor coils and the PF capacitors - at the rating of the capacitor, any reactive current above the capacitor rating still goes to the source.This reactive current is necessary for the motor to do what it does.

                              Yes light loads have lower PF but the load is lower also, so net kvar can still be lower than when motor is loaded, which means resulting losses from having power factor are still lower than they are at full load without any correction.
                              I live for today, I'm just a day behind.

                              Comment


                                #30
                                Originally posted by ka4koe View Post
                                Thank you for the replies. I hardly ever deal with PF in my day to day duties. The problem is somehow these vendors get access to those near the top of the command chain and then we have to deal with it. Another issue as I see it is the devices in question are not switched out after the motor loads stop. Then the power factor goes the other way----now we are capacitive.
                                You may have switching transient overvoltages, when you are capacitive. Consider providing surge arresters.

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