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    #31
    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.
    But that is not why supply PFC is generally used.

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      #32
      Originally posted by Besoeker3 View Post
      But that is not why supply PFC is generally used.
      But motor load is generally predominant load in a power system.

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        #33
        Originally posted by Sahib View Post

        But motor load is generally predominant load in a power system.
        Your point?

        Some motors can be cage machines, others would rotor, some subsychronous converter cascades, some sychronous, Unity Power Factor DC systems, some active frond VFDs and some like large anodising plants where the load is predominantly high current rectifiers.

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          #34
          Originally posted by Besoeker3 View Post
          Your point?
          Most use over sized motors operating at poor PF. So supply PFC is generally aimed to ameliorate it.

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            #35
            Originally posted by Besoeker3
            Not in my experience - but what do I know??
            We did several UPF drive systems around the several MW range for paper mills for example.
            It has relevance to OP: he may check his predominant load is motor load. A field survey reveals motors rarely operate at full load and so in OP case also at lagging PF most of time. So PF improvement may be beneficial to him.
            Last edited by Jraef; 12-02-19, 02:27 PM. Reason: Edited for context of other edits.

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              #36
              Originally posted by Sahib View Post

              It has relevance to OP: he may check his predominant load is motor load. A field survey reveals motors rarely operate at full load and so in OP case also at lagging PF most of time. So PF improvement may be beneficial to him.
              Being beneficial, to the extent of being cost effective, is only likely if the customer pays a hefty PF penalty on their utility bill, and not necessarily even then.

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                #37
                Originally posted by GoldDigger View Post

                Being beneficial, to the extent of being cost effective, is only likely if the customer pays a hefty PF penalty on their utility bill, and not necessarily even then.
                The OP is not proposing for PF correction; it is rather imposed on him by his superiors. There may be small saving due to reduction in current in cables if he puts the PF correction box at the load center rather at the main disconnect.

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                  #38
                  Originally posted by Sahib View Post

                  The OP is not proposing for PF correction; it is rather imposed on him by his superiors. There may be small saving due to reduction in current in cables if he puts the PF correction box at the load center rather at the main disconnect.
                  The longer the circuit run the more might be gained as it will lower watt losses in the circuit run. You still have watt losses in the run, just a little less when the reactive power component has been (mostly) eliminated from that run.

                  Most the power factor correction that is done (probably in most of the US) is done to avoid penalties from the power supplier, which usually are much more severe financially than the watt loss would be if that were the only cost to not correcting the PF.

                  And yes it is true that at low load level most the time the PF will be lower, but remember at low load level you also have lower kW and lower net kVA even though the PF is low to contribute to lower overall line losses than you would have at full load.
                  I live for today, I'm just a day behind.

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                    #39
                    Originally posted by RD35 View Post
                    I, too, work at a gov't installation. Our installation owns the MV distribution which consists of two very large subs. We have switched Cap banks at each sub. An entity recently approached us wanting to do PF correction "at the load". Their angle was that by correcting at the load, the caps would switch on and off with the load itself. Not a bad idea....however, in our case we are charged a penalty based on PF lower than .9 at the sub and the automatically switched cap banks take care of that. So, there is really no economic benefit. I am totally sold on the concept that PF correction "at the load" is by far the best approach scientifically, but economically it just doesn't work for us here.
                    At the installation stage of a piece of electrical kit, an economic benefit is that it may reduce the size the size, and thus cost, of the cable feeding it. And the labour to install that cable.
                    70mm2 XPLE is cheaper than 95mm2 XPLE for example. Without PFC, the 70mm2 XPLE might be just too small so you'd have to use the next standard size up, the 95mm2 .

                    With the PFC you may well get by with the smaller cable. It's just an example. You'd have to factor in the cost and installation of the PFC. Each case needs to be taken on its own merits.

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                      #40
                      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.
                      My limited experience has shown that an "energy efficient motor" is optimized for (near) full load and the efficiency at say 25% or 50% load is worse than that of a "standard" design. Check with your motor supplier.

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                        #41
                        Originally posted by Besoeker3 View Post

                        At the installation stage of a piece of electrical kit, an economic benefit is that it may reduce the size the size, and thus cost, of the cable feeding it. And the labour to install that cable.
                        70mm2 XPLE is cheaper than 95mm2 XPLE for example. Without PFC, the 70mm2 XPLE might be just too small so you'd have to use the next standard size up, the 95mm2 .

                        With the PFC you may well get by with the smaller cable. It's just an example. You'd have to factor in the cost and installation of the PFC. Each case needs to be taken on its own merits.
                        It should not be against the spirit of NEC recommending spare capacity.

                        Comment


                          #42
                          Originally posted by GeorgeB View Post

                          My limited experience has shown that an "energy efficient motor" is optimized for (near) full load and the efficiency at say 25% or 50% load is worse than that of a "standard" design.
                          No. The Efficiency of Energy Efficient motor is higher than that of standard motor at all operational points.

                          Comment


                            #43
                            Originally posted by Sahib View Post

                            It should not be against the spirit of NEC recommending spare capacity.
                            It's quite simply a commercial matter to minimise the cost of installation by using a smaller cable.
                            And, in my case, not relevant to the NEC regulations. I thought the conductor sizes I gave might have clued you in.

                            Comment


                              #44
                              Originally posted by Besoeker3

                              Is it against the spirit of NEC recommending spare capacity?
                              On what sound basis can you make that assertion?
                              No it is not. 90.8 even mentions it, but also does not require nor does it prohibit allowing for spare capacity.

                              90.8Wiring Planning.

                              (A) Future Expansion and Convenience.

                              Plans and specifications that provide ample space in raceways, spare raceways, and additional spaces allow for future increases in electric power and communications circuits. Distribution centers located in readily accessible locations provide convenience and safety of operation.
                              I live for today, I'm just a day behind.

                              Comment


                                #45
                                Originally posted by kwired View Post

                                No it is not. 90.8 even mentions it, but also does not require nor does it prohibit allowing for spare capacity.
                                And, even if it did, if my 70mm2 was marginal on, say a 0.7pf motor, using PFC to get it up 0.95, typically what gets specified here, the current would drop to about 74% of its previous value. So, plenty of margin.

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