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  1. bentov

    MCSF limits for VFD centrifugal application?

    It does often feel like I'm battered by the need for some attempt at dynamic analysis despite not fully understanding the regular kind, oh well! Regarding use of VFDs for flow control based on suction pressure, I do have a local example. A canal company captures drainage water in large ditches...
  2. bentov

    MCSF limits for VFD centrifugal application?

    Well this got busy, been away, sorry . . . this thread was kind of inactive, just noticed while checking for replies to my other one (hoping for a rule of thumb on downthrust k factor, no luck there yet) that I left it hanging, felt guilty so followed up (while sleepy, short on time). For the...
  3. bentov

    MCSF limits for VFD centrifugal application?

    Thanks again for the replies, sorry for responding late here. I do like playing with my VFD toys. This part still seems magical to me: "1750rpm selection would be 7", 200gpm @ 48', NPSHr = 25', 3.6hp . . . using the 9" with a VFD, we can get 200gpm at 40+ ' at 1225rpm, NPSHr = 5.8, 2.6hp"...
  4. bentov

    Hydraulic thrust K factor rule of thumb?

    We repair vertical motors, sometimes see premature thrust bearing failure that needs to be explained. Assessing the load requires K factor data on the pump that is often unknown. A recent example: 800hp 1800rpm motor rated 10,300lbs downthrust - on a site visit we recorded production of...
  5. bentov

    MCSF limits for VFD centrifugal application?

    Hello, thanks for the replies. I have a sketch I'll try to scan and upload to clarify. We are going to try this with the full diameter impeller, will see what happens at the low speed where MCSF is indicated, maybe set a minimum to tune out any problems. There's a typo in my post, "7.63 inch...
  6. bentov

    MCSF limits for VFD centrifugal application?

    A customer's new pump (Berkeley B3ZPM) will be supplied by a 900ft 4" PVC underground pipe fed by a water district turnout with a "normal operating range" of 17.5-20psi. The pump supplies water trucks (12ft high) and a fire water tank (16ft high) through a back flow preventer (10psi pressure...
  7. bentov

    Multiple sources, voltages, & transfer switches - solid neutral?

    Hey Bill, The genset is 480 only, has a 250a CB. The pump guys are tapping bottom of that, taking it the 125ft to the 60hp panel, installing a transfer switch there. The 200a 240V greenhouse branch feeder circuits are bigger than needed at the moment, not my call though to reduce that...
  8. bentov

    Multiple sources, voltages, & transfer switches - solid neutral?

    A customer has a 60hp pump supplied by a remote (300ft away) 480/277V service, so the neutral conductor is not present at the pump (ground only). 125ft from that is a 600a 240/120V 3ph 4W delta service supplying (2) greenhouses on separate 200a branch feeder breakers. Next to that service...
  9. bentov

    (yet another!) 277V lighting question

    How this turned out: guys I quoted own the greenhouses,(hemp)guys with the lighting are renters. So the renters did their own thing -(2) strands of 6-4 SO cord per house, stuck into un-glued PVC conduit clamped to trusses, spliced in PVC conduit bodies to pigtails; owner said the generator...
  10. bentov

    (yet another!) 277V lighting question

    Hey Bill, your suggestion is perfect, proposed exactly that configuration the next day after making the list and pricing it out (but haven't heard back - I bet they weren't expecting the cost, 1.5 miles cable, 480V panelboard, big pile of breakers, plugs, connectors, strain reliefs, contactors...
  11. bentov

    (yet another!) 277V lighting question

    The fixtures they've ordered come with a whip and plug, plus they've ordered the pigtails to go with them (cord with a female connector body). I guess I'll presume anyone working on a single fixture will be smart enough to unplug it? These will be staggered over long open houses, so a tripped...
  12. bentov

    (yet another!) 277V lighting question

    Thanks for catching that OCPD thing, I'll probably put 1p breakers or fuses in the J-boxes. About the neutral, the 8/5 is good for 28a, a segment will actually draw that much . . . will the #8 neutral in the bundle see more than 28?
  13. bentov

    (yet another!) 277V lighting question

    A greenhouse customer has ordered 277V lighting fixtures (46 x 660W for each of 3 houses), needs to run on a generator for short grows, then roll everything up, take to other sites on a rotating annual basis (so everything is temporary/portable). My task is to provide wiring, etc. One of my...
  14. bentov

    Soft start into a spinning load?

    Transitions can definitely be interesting! We're probably overthinking the back EMF thing, should ask EASA but you guys are more fun. I should specify we're talking exclusively standard squirrel cage induction motors, which I understand generate current if driven above synchronous with voltage...
  15. bentov

    Soft start into a spinning load?

    What we know is we can run any of them at 60V and 120V without tripping the breaker. Occasionally we trip at 240V (on a 500hp), for sure we trip above 250hp at 480V - but all those voltage steps are open transition with the current setup (just energizing the transformers dims the lights). So...
  16. bentov

    Soft start into a spinning load?

    Thanks for thinking about this. I have never used a sync check relay, only read about them for paralleling generators. Will one work in this case (with an unpowered spinning motor)? Will it detect (to sync with) back EMF arising from residual magnetism? I may be barking at the wrong squirrel...
  17. bentov

    Soft start into a spinning load?

    Thanks for the replies. My plan is to use a standard soft starter with internal bypass, point is to get to xline (across-the-line) no load running condition for extended test at rated voltage. Two concerns then - what if the soft start (correctly) senses that acceleration is complete, just...
  18. bentov

    Soft start into a spinning load?

    At our motor repair shop we have 200a 240V 3 phase service equipment, but because our actual operating loads are low the utility supplies us with a whopping 45kva. We routinely rewind motors up to 500hp 480V. The problem is getting motors up to full speed/full voltage for no-load testing. We...
  19. bentov

    Low voltage VFD closed transition synchronous transfer to mains available yet?

    But they (Robicon, Eaton, Toshiba) do it with large HP MV motors right? Synch the VFD & mains, then close the mains to the motor AND the load side of the VFD, then disconnect the VFD? Maybe there's just not enough perceived market for same on LV. As you mentioned in that old thread, around...
  20. bentov

    Low voltage VFD closed transition synchronous transfer to mains available yet?

    Good Morning Jeff, From the RSYC-01 Operation description: BUFOUT signal reaches 5 volts indicating “synchronic” status for the drive application program.  RSYC unit gives a control pulse for the contactor control circuitry.  Drive control logic switches the inverter modulation (IGBT...

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