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  1. DFordPE

    Speed-Torque Curve

    Can anyone tell me where to get a simple description of what is happening during the various parts of the induction motor speed-torque curve? I have found some info using equivalent circuits, but: a. I don't need anything that precise b. It's been forty years since I graduated and longer than...
  2. DFordPE

    Running 3-phase motor on single phase

    I'm curious. What is the application or are you just playing in your "skunk works?" As was already said, VFDs are cheap these days.
  3. DFordPE

    Overload Relay Sizing when using PF capacitors

    It's not about what I "thought was being done;" I never thought about it at all. Adding PF capacitors is just something you do with motors, for overall plant PF improvement. It was not necessary to think about what was going on in the motor, you just did it. I've worked for a couple of motor...
  4. DFordPE

    Overload Relay Sizing when using PF capacitors

    Thanks for all the input. I found the simple answer to be that the nameplate current is "apparent" current not "real" current, so the OLR is simply adjusted to account for that.
  5. DFordPE

    Overload Relay Sizing when using PF capacitors

    I believe that what I've researched says that you subtract capacitor current from motor FLA and set (I prefer IEC OLR) that number. That's where I'm stuck. It appears that I'm de-rating the motor after I improved the system. Next, the chart Skogsgurra drew shows the PF worsening under an...
  6. DFordPE

    3 phase 480 V VFD Contactor and Bypass A-T-L Contactor Pulling in together

    Sorry, I must have read it too fast. mea culpa
  7. DFordPE

    Overload Relay Sizing when using PF capacitors

    Yes, I read that article, but I still am somewhat confused. Using Squeeky's example: I have a motor with FLA of 81.6. I add PF correction and the current is now 69.13. How could going back to 81.6a be considered an overload? It seems to me that by reducing the imaginary current and...
  8. DFordPE

    3 phase 480 V VFD Contactor and Bypass A-T-L Contactor Pulling in together

    To ScottyUK, It's obvious that it's insufficient because it doesn't work! Would you want to try to design a retrofit without having all to the system info? Are you that bold? That's why I referred him to a company the makes bypasses.
  9. DFordPE

    Transfer a motor onload from vfd to mains

    Why not just install a filter ahead of the VFD?
  10. DFordPE

    VFD OVERSIZING

    It's common to see compressors operate into the service factor of the motor for short periods of time. I have also seen motors relabeled to 15% higher HP with a 1.0 SF. There will always be unusual cases or poor operating practices that require special care.
  11. DFordPE

    Overload Relay Sizing when using PF capacitors

    I recently heard that the OLR must be "resized" if a PF capacitor is connected on the load side of the OLR. Ref: NEC 430.22 and 460.9 Has anyone done this? I can't find a way to calculate. However, I have always placed the capacitor ahead of the OLR.
  12. DFordPE

    Transfer a motor onload from vfd to mains

    If all you want to do is bring it up to speed, why not use an SSRV starter. They come with bypass contactors built in.
  13. DFordPE

    200hp motor starting across line every 8 minuets

    Wow, I didn't catch this originally, but 65 years old is pre-U-Frame. Not only that but some refrigeration compressors have specially built motors.
  14. DFordPE

    VFD OVERSIZING

    The affinity laws (cube/variable torque) are a nice approximation of load on fans and pumps, but they only work in a perfect world, not what we have to deal with. However, if they did always work a 10% speed increase would only yield 10% more flow but would require 33% more current. Assuming...
  15. DFordPE

    Location of overload in motor starter circuit

    If it is an IEC motor circuit protector, it will typically be wired ahead of the contactor. These devices replace both the fuses/CB and the OLR.
  16. DFordPE

    3 phase 480 V VFD Contactor and Bypass A-T-L Contactor Pulling in together

    Obviously, your electrical interlock is insufficient as it exists. I wouldn't want to try to design a foolproof circuit without knowing your system and needs. However, a simple stopgap might be to install an on-delay timer in the contactor circuit, coupled with a N.C. aux contact from the...
  17. DFordPE

    VFD OVERSIZING

    Yes, I understand the theory. I just said I've never seen it. The SSRV only prevents water hammer if it has decel ramp - not all do.
  18. DFordPE

    VFD OVERSIZING

    Manufacturers typically have tables showing the HP ratings at different voltages. However, current is the ultimate factor, as was said. Danfoss is a reputable company and they should be able to answer your question. What I do find interesting is using an SSRV for a bypass. I've never seen...
  19. DFordPE

    200hp motor starting across line every 8 minuets

    I haven't seen anything about using a Solid-State Reduced Voltage starter. My guess is that this system is so old that it was originally designed to run the motor constantly and let the compressor "load and unload." The load inertia is only part of the equation because the motor sees inrush...

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