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Thread: Another VFD question

  1. #21
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    Aug 2007
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    “I am looking for a sensorless vector VFD, single phase in three phase out to run a 220V three phase 5 hp motor that requires 13 amps at start.”

    That’s awfully low for starting current; likelier that’s the FLA (full load amps).

    If that’s the case, you’d get about 3 ˝ HP out of a 3 HP VFD rated at 10.5 A; you can get a Teco 3 HP VFD for $220.

    “The PC1-50 VFD did not state if it was sensorless vector drive and I do not know if it has been derated for single phase use. Can you all help there?”

    It says it is:

    3.1
    Input Voltage 200 - 230VAC ±10%
    3.2
    Phase 1
    I also asked about the vector vs V/Hz issue, and was told it’s more a matter of precise speed control at low speeds, not torque.

  2. #22
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    Your last point about precise speed control at low speeds is not really correct. It cannot be about precise speed control without also being about torque. High maximum torque is what is necessary to maintain speed when a heavy load is applied.

  3. #23
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    Depends how it manifests.

    If speed can't be maintained, then you're right.

    But if the speed has ripple in it, or it's a few % lower than the set point, which might be unacceptable in some applications, it would be fine by me.

  4. #24
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    Let me add a little. In a volts/hertz drive the output voltage is constant for a given frequency. As the load increases, the motor slips more and the rpm drops. In a vector drive an increased load will cause the output voltage (and consequently the current) to increase to compensate for the extra load. Of course this is not perfect - but the decrease in rpm for a given load is much less. The extra voltage (and current) that the vector drive provides gives rise to the extra torque we are discussing.

  5. #25
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    Jun 2008
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    Quote Originally Posted by Bruce Griffing
    The DC voltage sag that occurs between conduction peaks (2 for single phase, 6 for three phase per cycle) depends on the capacitor that follows.
    Thanks for the correction, Bruce - I must not have been fully awake when I typed 3 for three phase. I corrected my original post.
    TexasTurnado

  6. #26
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    Feb 2007
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    So, basically the PC1-50 VFD that driveswarehouse is offering is not sensorless vector, but would work. They have another unit, SJ200-055LFU, that is, but it is three phase in and three phase out. It is made for 7.5 hp though so it could be derated? Am I getting close? Jay
    "Just build it and be done"

  7. #27
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    Quote Originally Posted by tiptop
    So, basically the PC1-50 VFD that driveswarehouse is offering is not sensorless vector, but would work. They have another unit, SJ200-055LFU, that is, but it is three phase in and three phase out. It is made for 7.5 hp though so it could be derated? Am I getting close? Jay
    That would be my choice
    TexasTurnado

  8. #28
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    I would ask someone at Hitachi America (not a sales person) if it is possible to add a capacitor to the 5hp 3phase version and run it on single phase. The terminals necessary to do this are already on the drive.

  9. #29
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    Quote Originally Posted by Bruce Griffing
    I would ask someone at Hitachi America (not a sales person) if it is possible to add a capacitor to the 5hp 3phase version and run it on single phase. The terminals necessary to do this are already on the drive.
    The current running through each of the 4 legs of a single-phase rectifier is 33% more than the current running through each of the 6 legs of a 3-phase rectifier.

    So a 3-phase VFD will have rectifier diodes that are rated for 1/3 less current than the equivalent single-phase VFD.

  10. #30
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    Robert-
    That is true, but rectifiers come in specific values. If the spec is 8 amps, but the only device available is 15 amps - then the scheme will work. The only potential snag is the one you point out.

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