VFD application

Discussion in 'General Electronics Chat' started by inwo, Apr 26, 2014.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Are there problems using a vfd on two speed 5hp/3hp 460vac 60hz motor?

    Separate windings. One winding powered, and one winding open, for high.

    To make matters worse. Motor is 60 to100 feet from control panel.

    Vfd near motor is an option. Unprotected outdoor pit in Minnesota!

    There are 4 to change over.

    Seems there are certain designs that have cleaner output for long runs.

    Also seems good practice would mean some type of termination of unused winding.

    In a pinch I'll talk them into changing motors.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it is the motor type that use one winding at a time (not dual connected), you should be able to just run the high speed winding, the other open?
    If the job supports a bit extra I would fit a 3 phase inline choke or inductor at the motor end.
    If running stranded single conductor in conduit or liquid seal, TEW etc, you may want to ensure the conductors are twisted for the full length, earth wire separate, could be a bit of pain for that length a run.
    Max.
     
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  3. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Thank you.
    I hoped you were here.:)

    Yes, one winding at a time.

    Worried about the noise and harmonic effects induced on the unconnected winding.

    Google didn't turn up a warning.

    Are you thinking line and load choke?

    I've used line chokes in the past. Is more better? 5% over 3%

    I don't know if I'm up for re-pulling wires.:eek: How important is it. Or just "good practice"?

    I also need to decide whether to install in "bucket" adjacent to plcs or in the pit outside. May be a third option. External to control cabinet with wires looped from the "base" raceway trough.
     
  4. MaxHeadRoom

    Expert

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    I prefer to use at least a choke or inductor on the load side, if any, it is easier on the motor and the VFD, especially on non-vector rated, or older style motors.
    Baldor have a PDF on it out there, Ideally installed close as possible to the motor.
    http://www.google.ca/url?sa=t&rct=j...m4z0_o1ZYZhkwXt3A&sig2=Ei2RvR5o9SByZX8MlvDoyQ
    If you have already ran the wiring then maybe take a chance over twisting conductors, it is just noise/EMI suppression precaution if using low voltage/sensitive equipment.
    I wouldn't really worry about the open winding, as long as it is terminated off suitably.
    Max.
     
  5. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    By terminated off, you mean insulated/floating?

    Would have never guessed installing the choke near motor.

    Any prefered brands? I've used toshiba, AB, and Hitachi. Teco is available from one of my suppliers.

    What about mounting it a couple "buckets" away from plc in same overall cabinet?

    I remember a job where meter across the line was reading more than double voltage 50' from a vfd.
     
    Last edited: Apr 26, 2014
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    say you were using the two speeds, it would be wired back to the MCC bucket starters anyway, so I don't know what concern you'd have. The VFD supplier would recommend a filter value as well. Having a PLC or remote rack located next to the MCC is a common choice.
     
  7. MaxHeadRoom

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    Yes.

    There are a few sources Hammond or automation direct, and also most of the VFD suppliers will have them.
    If not possible at the motor, the second best is near the drive or in the enclosure.
    Max.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    The much-faster-than-line-frequency rise times cause spikes at the ends of the line where they meet/terminate, and when these spikes coincide/meet on two or more phases they are additive. The star point in a vfd powered motor can experience voltages tens of times higher than what the vfd is putting out. For this reason, inverter duty motors have super thick insulation near the star point. The longer the cable run, the more exaggerated the spikes. I don't 100% understand it but I think of it like a whip. The longer a whip is, as the mechanical wave travels down it, the speed is increased. You only have to put a few M/s into the thick end of a whip to get super sonic speeds at the tip. A shorter whip will have less speed at the tip; a longer one, faster. Same with the cable; the longer the cable, the more painfully you whip the motor (pun intended ;)). Combining non-inverter-duty motors with long cables is a recipe for swift insulation death. If mounting the vfd right next to the motor is an option, that's the best thing you can do to mitigate the problem. Mounting a reactor on the vfd output is a good idea but won't cure the problem by itself and won't be as effective as relocating the vfd. I recommend both. But ideally you would mount the vfds in the main cabinet, with output reactors, and install inverter duty motors, call it a day and walk away confident that you won't be called back for motor or drive failure for at least a few years (as opposed to a few weeks or months).

    Note that installing reactors on vfd outputs can screw with the vfds ability to interpret feedback/regen up the line from the motor which it uses to resolve speed. This can make it difficult or impossible to run the vfds auto tune feature which is needed for tight speed control. If speed control doesn't need to be tight (I assume not since previous installation was just 2 speeds) then put the vfd in V/Hz mode and call it done.
     
  9. strantor

    AAC Fanatic!

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    Another tip: set the switching frequency as low as possible.
     
  10. MaxHeadRoom

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    This is the main reason I fit a 3ph inductor close to the motor, Of all the installations I have done, only one was a vector rated motor and the oldest motors were 3 GE 1950 motors, So far no failures.
    The line reactors are recommended by most manufacturers and in some case it requires one on the input as well as output, but if the choice is limited, pick the motor side first.
    Max.
     
  11. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    This is a very non-critical application as far as speed. They circulate waste products in ponds.

    Fast or slow depending on bacteria count.

    The operators want the ability to slow the process down during warm weather.

    Motors are easy to swap. I think I'll recommend a spare inverter duty motor, rather than switching them all and possibly nixing the project.

    This is the same plant that I posted about power factor problems. I never did get back to check it out.
     
  12. strantor

    AAC Fanatic!

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    Vfds will definately improve power factor. Goid idea about the spare. Replace with inverter duty rated as they fail.
     
  13. strantor

    AAC Fanatic!

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    Interesting. I've never installed them out near the motor, always near the drive and IIRC that's what most vfd app notes suggest but I will investigate. Do you find it more effective when placed near the motor? So when you say inductor are you referring to the same thing I'm referring to when I say reactor? Or are you talking about wound air core inductors?

    http://www.automationdirect.com/adc..._Spare_Parts_-a-_Accessories/AC_Line_Reactors
     
  14. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    I'm still thinking about the best way to do this.

    As there is already interlocked high/low contactors and a high/low selector.

    Just thinking out loud!

    Mount the VFD at the motor, and cutting it into the high leads.

    I've never used a vfd in an "always run" mode. I have serviced a few systems, where user routinely opened line circuit to stop motor. I warned against it, but it never caused a problem.

    Is that a legit setting?

    IOW set it to ramp up after boot.

    These run 24-7. How damaging will power loss be?

    If bad, I'll have to reconfigure control circuit, and pull more wires.

    It would sure be a clean modification if I could just add it to the high speed side (at motor location) and use low speed as vfd bypass.

    Most of the other pumps have vfd bypass built in.
     
  15. MaxHeadRoom

    Expert

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    You can open the input side, I would not suggest the motor side for sure.
    Also on power up for the VFD to go into run, you would need to use the FWD or RUN input permanently made, and also fit an external pot if the operators want to control the speed.
    The alternative is to use the preset speeds feature, most have up to 6 or 8 presets that can be set by a selection by switch on the inputs etc.
    You could use the present contactor on the input side for E-stop purposes.
    What switches them at the moment when the bacterial count changes?
    Manual or automatic?
    Max.
     
  16. MaxHeadRoom

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    Yes, Inductor, reactor, 3ph choke all the same in this context.
    Max.
     
  17. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    It's manual control.
    Operator just sets to high or low.
    All they really want is an available slower speed.
    I first suggested a gear ratio change, but was told full speed is needed. And lower loading will only make pf problems worse.

    Operator said it would be good to mount vfd by motor. Preferring a pot control. I'm sure key entry is fine.

    Initially I down played the idea, being outdoors, and control wiring issues.

    If there are not strong objections to breaking line under load, it's starting to look like a good idea. There is little inertial loading during stop.
    I'll add a minute delay-make to the contactor controlling the vfd. And another delay start if it can be programmed into vfd. To prevent any rapid cycling.

    What makes this attractive, is not having to redraw plant control diagrams, nor make any real changes to operation sequences.

    Other vfds are in outdoor enclosures. I can copy their design for heating. (if any)

    Now that I think of it. Running 24-7, condensation shouldn't be an issue.

    I'll have to verify, but all 4 motors are running any time I've been to the plant.
     
    Last edited: Apr 26, 2014
  18. MaxHeadRoom

    Expert

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    If just two speeds, why is the VFD needed? Were these running in this application before?
    Max.
     
  19. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    An interesting side note to this.

    A year or so ago I was called to the plant in middle of the night.

    The two main pumps were down. Operators had cycled the pumps on high temp. limit for a couple hours before calling.

    Exactly the same problem today, except they called right away.

    It's not easy to shut down the pumping system with all the redundancy and vfd bypass.

    What we found was the exact same foreign body jamming both 7.5hp pumps.

    Same items as last time and nothing normally found in waste water.:eek:
    So effective the pump body has to be disassembled to clear.

    Operators heard of a similar problem in a nearby city.
    It could be intentional vandalism. Or incredibly stupid act million to one.
    But who would know what item to use? It's the only thing these pumps can't handle.

    I won't post it. Might give ideas.
     
  20. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    They need speeds lower than the low speed winding delivers. Manually selected.
    Any pf improvement will be a bonus.
     
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