What should the voltage be across phases on a 3-phase supply (VFD)?

  1. skotl

    skotl Forum Supporter

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    My VFD and motor appear to be working fine, but I'm confused about the voltage across any 2 of the three phases.

    My mutimeter tells me it's around 70V, however I expected it to be at least 230V :dontknow:

    Of course, it could be the multimeter not reading correctly, given it's seeing PWM rather than a sine wave...


    ----

    In case anyone is interested in the why... I bought the too-cheap version of the VFD so there's no output pins on it, meaning I can't (for example) power an external fan on the motor while the motor is running. So I figured I'd put a relay across two of the phases and use that to trigger the fan.
     
  2. the snooper

    the snooper getting older by the day

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    Usually 400 between 2 phases and 230 between any phase and neutral
     
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  3. Brad93

    Brad93 M J B Engineering

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    Phase to Neutral would be 230V.
    Phase to Phase would be 400v or there abouts
     
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  4. the snooper

    the snooper getting older by the day

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    Didn't have the meter on DC or something by mistake did you ?
     
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  5. skotl

    skotl Forum Supporter

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    It's auto-ranging and auto DC/AC.

    What you and @Brad93 said are exactly what I expected so, given the motor itself is working, I reckon it's down to the meter getting confused.

    I'll stick the oscilliscope on it tomorrow (with less wine!) - that should confirm it, I'd think.

    Back to my "why are you doing this?", do you think it's reasonable to put a 400VAC relay across two phases to trigger a fan while the motor is running?

    Edit: Would have been cheaper and easier to but the more advanced VFD...
     
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  6. daleyd

    daleyd Member

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    Is this on a single phase input vfd(ie not a step up one that gives 400v out for 230 v in?)

    If so then the max you will get out phase to phase will be 230/240 volts - BUT only at 50 hz.....

    The vfd gives out a set voltage per hz so for example at 25 hz you would expect to see around 115 v ish (on more advanced vfd it can be different, you can alter this relationship to give different characteristics).

    What might complicate it is that it’s not a “neat” waveform so for an accurate reading you will probably need a meter that will give “true rms “ readings.

    It wouldn’t be recommended to put a relay across an output due to the varying voltage ( as the volts drop it would start to pull more current, and below a certain voltage just stop working) - and a 400v one wouldn’t work anyway, if it’s a 230v vfd.
     
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  7. daedalusminos Member

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    As previously mentioned, a DMM will struggle to measure the PWM constructed output of the VFD accurately - you might have more luck with an analogue meter.

    Nevertheless, that won't solve your problem.... To there must be some outputs on the VFD or, what input is being used to command motor drive?
     
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  8. awemawson Forum Supporter

    Another case where a genuine Avo will give you more meaningful readings than a more ‘modern’ digital meter.
     
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  9. daleyd

    daleyd Member

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    An analogue meter in itself won’t help in accurately measuring a non sinusoidal wave - look for a digital meter that says “true rms” if you need an accurate reading of this type.
     
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  10. minimutly Member

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    Most vfds will have a running feedback as well as fault - surely yours does?
     
  11. Agroshield Member

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    Like he says, the varying voltage would be a challenge to a relay directly-connected. All you really need to do is see if there is a phase to phase voltage and use that as a trigger. Your volt meter (extremely high impedence) is such a device, so something working on a similar principle could do - very high resistance, minimal current draw. Perhaps an LED and resistor across the phases and a photocell at the other side could do. As a way of doing it without even touching the power electronics, magnet(s) on the shaft and a coil of wire surrounding it/them could generate enough to switch a transistor (Google 'shaft rotation sensor').
     
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  12. daleyd

    daleyd Member

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    I think he bought the bargain basement version!:laughing:

    If it really hasn’t got any outputs and you wanted to drive something on when running I would double up the contacts on your switch or button that you use to select run on the drive and do something that way (relay, arduino or something?). Trying to cobble something together using the output side of the vfd, while it might be do-able would be tricky due to the varying nature of the output (in terms of both volts and frequency, as well as not being a very nice wave)
     
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  13. minimutly Member

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    Hmm, no I think doable, maybe just use a diode and capacitor, with a Schmitt trigger swich, with suitable resistors to limit current it should work - someone mentioned an opto isolator - @Agroshield - probably a good idea, you would then use a dc supply and relay to provide a pair of contacts. Simple electronic circuit design - takes me back a bit....
     
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  14. daleyd

    daleyd Member

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    Oh yeah I didn’t say it wasn’t doable, just to me it seems there’s an easier way to achieve the same result ...
     
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  15. daedalusminos Member

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    The problem is the voltage waveform is likely to be a chopped DC waveform, the current waveform in the motor would more closely approximate a sine wave - an analogue meter would have a natural filtering.
     
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  16. daleyd

    daleyd Member

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    I wouldn’t expect to see much difference between an analogue and a standard DMM on the output side to be honest, given the vfd will be putting out 4 kHz (or more) carrier signal - on AC measurement a DMM would not see that sort of frequency so you would just get a basic peak to peak measurement, which is perfectly ok if you just want to do some comparisons or basic fault finding. What neither will do is give you a “true” voltage if the waveform is not a perfect sine wave - for the vast majority of the time this won’t matter of course as you just want to make comparisons or to know whether you have “some” voltage there.
     
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  17. the snooper

    the snooper getting older by the day

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    What you need is one of those power sensing things that was around a few years ago, name escapes me, plug the vfd into its main socket then your fan into the other, when it senses the current draw it powers up the other sockets
     
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  18. m_c Member

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    Most multimeters won't give an accurate reading from a VFD output. I've tried various, and reading vary from totally inaccurate, to a scrambled mess.
    I'm guessing it's because the switching frequency screws with the multimeter circuitry, so it just can't get a good reading. If you really want to measure the output, I've found an oscilloscope is the only accurate method.

    To answer @skotl 's problem, the output voltage varies.
    It will only be at 230V (assuming you've got a 230V in/ 230V out VFD), when the motor/VFD is running at the motor's rated speed, as output voltage is proportional to speed.

    Easy option would be to buy a better VFD, as to to do what you want will likely involve a bit bodging.
    Failing that, some kind of voltage detector that triggers at low voltage but can handle high voltage (transistor, resistor, and zener diode would do the job) that then feeds into a time delay that triggers for at least a second or two whenever the transistor triggers, which then controls the cooling relay.
     
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  19. skotl

    skotl Forum Supporter

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    Thanks all - understood on measuring and using the output voltage.

    If I end up doing something on this then I'll use the input switch, as Dale suggests.

    And yeah - next time, don't buy the cheapest VFD on Amazon :) (albeit the one with the highest reviews)
     
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  20. mtt.tr

    mtt.tr Member

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    Likely a miss leading reading as they are fast switching
     
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