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Thread: Any Electrical Engineers out there who can explain something?

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    Quote Originally Posted by onotoman View Post
    Great, so I'll just go for a 240V Single Phase Input to 240V 3 Phase Output and wire as before.

    Good thing I didn't go for a 380Volt output of the motor would have took off like a jet..

    Many thanks guys.
    That is correct. The notations can be a bit ambiguous from time to time.
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    Quote Originally Posted by onotoman View Post
    If my motor is 380 Volt, how did this work - or is the 3 Phase the key - the VSD was working fine until the dust issue.

    Do I not need a 240 Volt Single Phase input/380 Volt 3 Phase Output VSD?

    Some suppliers say 240 Volt 3 phase output is fine, while others say 380 Volt 3 Phase Output.

    Any advice appreciated.
    I'm not an electrical engineer, but I can explain the math/physics.

    First, electrical voltage is measured on 'interval scale' i.e. it is always relative voltage or difference between two points.

    Three-phase electrical power/voltage comes with three active leads, which means that the voltage difference between each of them and the 'ground' is the same, in your case 220V. But that 220V is the peak or the amplitude, the actual voltage oscillates back and forth with a 50Hz or 60Hz frequency (i.e. switches 50 or 60 times per second).
    Now, these three separate leads are also shifted in phase, so that they get to the peak voltage not at the same time but a third of the oscillation cycle from each other.

    This means that even though each goes from -220V to +220V relative to the same 'ground' point, the difference between any two of them oscillates not with amplitude 220V but with (after simple trigonometry) 2*sin(60deg)*220V=380V (still at the same frequency though).

    So, in short, the voltage between the ground and any of the 'live' wires goes back and fort from -220V to +220V 50 times a second, but between one 'live' wire and another live wire the voltage oscillates between -380V and +380V 50 times a second.

    Most likely this is what the 380V notation on your motor means, but it's always best to consult with a professional who would know for sure.

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    Actually AC voltages are measured RMS [root mean square] so 220 volts AC will dissipate the same energy as 220 DC through the same resistance. The peak voltage of a 220 AC supply will be 1.41 times the RMS voltage or 310 volts. None of this has anything to do with the OPs question.

    Induction motors are designed to operate at a given voltage at a specified frequency. The labeling on the motor does not make any sense to me as the speed is listed at 2800 and the freq. specified as 60 hz.. A 2 pole induction motor has a synchronous speed of 3600 at 60hz and with the slip at full load it is usually around 3450 or so. The same motor will run around 2800 at 50hz. The VFD label doesn't really make sense either as the voltage should vary proportionately with the frequency. Running an induction motor at a voltage lower than specified for a given freq. will increase the slip at load and can over heat the motor by drawing too much amperage. It's possible that excess current was the cause of the failure of the VFD.

    Were you loading the grinder heavily [sensing slow down] around the time of the failure?

    In my mind the VFD should have a spec. in volts/hz that matches the volts/hz of the motor.
    Last edited by bluesman7; 11-12-2017 at 01:39 AM.
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    Quote Originally Posted by bluesman7 View Post
    Actually AC voltages are measured RMS [root mean square] so 220 volts AC will dissipate the same energy as 220 DC through the same resistance. The peak voltage of a 220 AC supply will be 1.41 times the RMS voltage or 310 volts. None of this has anything to do with the OPs question.
    That's very true, I forgot that it's rescaled in order to dumb down basic integrals to simple multiplication

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    Last edited by gugi; 11-12-2017 at 01:54 AM.
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    Hi Bluesman7,

    Thanks for the information. I'd been using the grinder for around 60 minutes, hollow grinding, when the VFD went Pop.

    Quote Originally Posted by bluesman7 View Post
    Actually AC voltages are measured RMS [root mean square] so 220 volts AC will dissipate the same energy as 220 DC through the same resistance. The peak voltage of a 220 AC supply will be 1.41 times the RMS voltage or 310 volts. None of this has anything to do with the OPs question.

    Induction motors are designed to operate at a given voltage at a specified frequency. The labeling on the motor does not make any sense to me as the speed is listed at 2800 and the freq. specified as 60 hz.. A 2 pole induction motor has a synchronous speed of 3600 at 60hz and with the slip at full load it is usually around 3450 or so. The same motor will run around 2800 at 50hz. The VFD label doesn't really make sense either as the voltage should vary proportionately with the frequency. Running an induction motor at a voltage lower than specified for a given freq. will increase the slip at load and can over heat the motor by drawing too much amperage. It's possible that excess current was the cause of the failure of the VFD.

    Were you loading the grinder heavily [sensing slow down] around the time of the failure?

    In my mind the VFD should have a spec. in volts/hz that matches the volts/hz of the motor.

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    Quote Originally Posted by onotoman View Post
    Hi Bluesman7,

    Thanks for the information. I'd been using the grinder for around 60 minutes, hollow grinding, when the VFD went Pop.
    Were you hollow grinding with high load or low? Could you sense that you were slowing the motor? I'm suspecting a poor match between the motor and the VFD.
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