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  1. #1
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    Default Speed Control of a Three Phase Motor

    Been trying to understand how to speed control a 3-phase motor for a belt grinder. I think I understand all the theory and everything points towards the old VFD. I originally thought VFD's were only used to convert 1-phase to 3-phase but perhaps I have that wrong?

    I recently acquired a high quality (I doubt...) Conon Motor 3HP 3-phase 2 pole motor (https://cononmotor.com.au/product/2-...ase-gl-415v-2/). This is rated at 2800rpm (being a 2 pole).

    My original intention was to run this off my rotary phase converter with some mechanism to vary the speed. I therefore looked at one of the usual eBay cheapie VFD's being this.

    My questions;
    • seeing as a VFD can be used to convert single to three phase would it be better to run this motor through this VFD off single phase (10amp, 15amp or 20amp circuit required?)
    • can this VFD be used with an input of 3-phase from my rotary converter and still vary the motor speed? I think it only accepts single phase input?



    I find it very easy to get confused when it comes to electrics and I certainly don't want to introduce any fire hazards into my workshop!!

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    Default

    Given its only a 3HP motor I wouldn't bother using a 3P to 3P VFD and just use the SP to 3P VFD because you can then plug it into any 10 or 15A GPO.

  3. #3
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    Quote Originally Posted by BobL View Post
    Given its only a 3HP motor I wouldn't bother using a 3P to 3P VFD and just use the SP to 3P VFD because you can then plug it into any 10 or 15A GPO.
    The motor has the same power/torque using 1 phase through VFD?

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Mk1_Oz View Post
    The motor has the same power/torque using 1 phase through VFD?
    Yep.

  5. #5
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    Quote Originally Posted by Mk1_Oz View Post
    The motor has the same power/torque using 1 phase through VFD?
    Depends.
    Because you will be using what was a 3 phase 415v motor in star connection and rewiring/ reconnecting it to delta 3 phase to use it on a single phase to 3 phase VSD, the motor will be running on 240V 3 phase.
    This means the motor will not have the same torque as if it was running directly of mains 3 phase.
    The torque will also depend on the speed that the motor is running at.

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Sparksy View Post
    Depends.
    Because you will be using what was a 3 phase 415v motor in star connection and rewiring/ reconnecting it to delta 3 phase to use it on a single phase to 3 phase VSD, the motor will be running on 240V 3 phase.
    This means the motor will not have the same torque as if it was running directly of mains 3 phase.
    That's not what motor name plates say.
    If torque T is in ftlbs
    HP = T x rpm / 5252

    Provided the "Y to ∆" conversions have been done right the motor will produce the same HP at 240V as at 415V, and it won't matter if the initial supply is 240V SP.
    At the same frequency the motor should also produce the same RPM so therefore it will produce the same torque.

    What happens is Power = Current x Voltage, so rewiring from 415V ∆ to 240V Y results in the motor drawing more current to make up for the reduced V.

    I have tested a bunch of motor in different connections and found this to be the case.

  7. #7
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    Quote Originally Posted by Sparksy View Post
    Depends.
    Because you will be using what was a 3 phase 415v motor in star connection and rewiring/ reconnecting it to delta 3 phase to use it on a single phase to 3 phase VSD, the motor will be running on 240V 3 phase.
    This means the motor will not have the same torque as if it was running directly of mains 3 phase.
    The torque will also depend on the speed that the motor is running at.
    I hope your forum nickname doesn't have anything to do with being an electrician.... because this is VERY wrong information!

    PS: I put this blunt response here not to embarrass anyone, but to warn off future search engine finders of that advice. No offense intended!
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

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