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Thread: VFD - V/Hz High Breakaway Load

  1. #11
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    Quote Originally Posted by spraymax6 View Post
    From Jraefs post above the drive always seems to have to start at 0 and ramp up. I would ultimately would like to send 50% voltage and 50% speed command initially to the motor. This will guarantee that the motor will be able to start. But from Jarefs post. It looks like the drive in V/Hz will need to ramp up to get to that point, and I am assuming the issue is that it will fault out by the time it gets to a point that can move the load at starting. The thing about across the line starting it is that you hit it with the 100% voltage and 100% frequency right off the bat. And the motor doesn't have time to overload/fault out. It rips through its locked rotor torque position and keeps moving along its curve.
    I would setting the ramp time to .01 sec and disable faults during ramp. That should get you to your ideal speed relatively quickly and prevent nuisance trips.


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  2. #12
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    Quote Originally Posted by spraymax6 View Post
    From Jraefs post above the drive always seems to have to start at 0 and ramp up. I would ultimately would like to send 50% voltage and 50% speed command initially to the motor. This will guarantee that the motor will be able to start. But from Jarefs post. It looks like the drive in V/Hz will need to ramp up to get to that point, and I am assuming the issue is that it will fault out by the time it gets to a point that can move the load at starting. The thing about across the line starting it is that you hit it with the 100% voltage and 100% frequency right off the bat. And the motor doesn't have time to overload/fault out. It rips through its locked rotor torque position and keeps moving along its curve.
    That is more of a software/firmware question, and Jraef certainly has more experience than I with actual drives that you can purchase. Start at 0Hz and ramp up might be what all available drives actually do.

    In principal there is no reason that a drive could not simply start at a particular frequency and do the equivalent of an 'across the line' start at that frequency. Or start at 0V and XHz and quickly ramp to operating voltage at that frequency. The latter is what we do with the experimental drives we do in our lab, but we are writing the actual PWM software ourselves.

    -Jon

  3. #13
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    Won’t make a difference. The drive is only capable of, at BEST, 150% current for 60seconds and will current limit itself. The only way to get a VFD to start a motor across-the-line (in the sense that is equivalent to a standard starter) is to start with a VFD that is 4 times larger than the motor FLA.

    But there is no point to all of that. Because the VFD is controlling BOTH the voltage and frequency together, it gets equal or better performance out of a motor that you can get with across the line starting. The frequency you start at makes zero difference.

    The REASON your motor doesn’t perform well at low speeds is simply because you are using an old technology (V/Hz mode).l that is inherently sloppy at low speeds as I explained. If you need full starting torque capability from the motor at the very outset of a run command, use vector control of any sort, ie SCV. That’s what that is for.
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    Quote Originally Posted by spraymax6 View Post

    Question:

    It seems like the drive will try to start at 0.001Hz (something arbitrarily close to 0 Hz) and work its way up with a possibility of the load not starting, but if the drive continues to increase the voltage and current without caring if the motor actually started or not eventually it would get to 100%Voltage (460V)and 60Hz at that point the motor would be able to be started at Full Load Torque and the 150% FLT.
    The VFD will limit the current and the voltage. If it could get up to full rated voltage before it started you would six to eight times full load current. Ain't gonna happen.

  5. #15
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    Quote Originally Posted by Jraef View Post
    Won’t make a difference. The drive is only capable of, at BEST, 150% current for 60seconds and will current limit itself. The only way to get a VFD to start a motor across-the-line (in the sense that is equivalent to a standard starter) is to start with a VFD that is 4 times larger than the motor FLA.

    But there is no point to all of that. Because the VFD is controlling BOTH the voltage and frequency together, it gets equal or better performance out of a motor that you can get with across the line starting. The frequency you start at makes zero difference.

    The REASON your motor doesn’t perform well at low speeds is simply because you are using an old technology (V/Hz mode).l that is inherently sloppy at low speeds as I explained. If you need full starting torque capability from the motor at the very outset of a run command, use vector control of any sort, ie SCV. That’s what that is for.

    I spoke with the drive manufacturer. Their model drive in question comes standard in Open Loop control, however, the model drive has such a rudimentary torque control model even in Open loop vector control that it still might work. They recommended a model up where in Open Loop vector there would be no problem at all.

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    Quote Originally Posted by spraymax6 View Post
    I spoke with the drive manufacturer. Their model drive in question comes standard in Open Loop control, however, the model drive has such a rudimentary torque control model even in Open loop vector control that it still might NOT work. They recommended a model up where in Open Loop vector there would be no problem at all.
    .

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    Quote Originally Posted by spraymax6 View Post
    I spoke with the drive manufacturer. Their model drive in question comes standard in Open Loop control, however, the model drive has such a rudimentary torque control model even in Open loop vector control that it still might work. They recommended a model up where in Open Loop vector there would be no problem at all.
    Who's drive is it?
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