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Thread: varible speed drives

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    varible speed drives

    is it more cost effective to use 230 volt single phase to 230 3 phase drives...or 400 to 400 v drives.

    please explain your answer.

    my thoughts are if you use a 230 - 230 drive you will save on panel size and wiring...not having to install 3 phase circuit breakers and 3 phase supply wiring...but maybe i am missing something...like the cost of the drives or capabilities of the 230 v drives.

    another way of looking at it...the single phase side would require bigger cables.

    anyone ever done a costing?

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    Diamond Member Justloadit's Avatar
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    The problem with 230V drives is that the 3 phase motor must be a 230V phase to phase motor, which is not a popular voltage in RSA. Also the single phase supply must supply all three phases of the motor, limiting the maximum motor size you can connect on that specific line.

    A 400V drive is far superior, as you can use a standard motor on the drive. You do have the situation that you have to supply it with 3 phase.

    Only you can make the decision - depends on the site circumstances.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    I've never come across a scenario where it's been an option. Usually the motor will only run satisfactorily on either 230v or 380v 3-phase, not on both as the rotation speed will usually vary.

    That aside I would always specify a three phase system in the interests of a balanced load, also it gives an option to run the system without the drive should the need ever arise.
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    the single - 3 phase drive is a bit cheaper.i think they manufacture to about 2,2kw.
    not sure if i am stating the obvious ,but if you take any 3 phase motor where you have access to all 6 connections ( not internally star connected) then you change the connection from star to delta and you still get 230 volt across each coil.not sure how efficient it is ,but they do have there uses. you can plug it into a normal 16 amp socket outlet.

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    my thoughts...

    if i were to install lets say 6 up to 2.2 kw 3 phase motors.

    by using a single phase drive...i save on circuit breakers R70 as appososed to R350
    The wiring is only single phase...so i save on wire...i need to use slightly bigger but still save on quantity.
    the size of the control panel can be smaller because now instead of making space for the 3 phase breakers i only need single phase and balance 2 motors per phase...balancing it.
    most 3 phase motors have a 230/400 facility by just changing the motor from star - delta or delta - star
    if you are using single motors...like in a printing company...and only require small motors to run...but need to control the speed...running single phase socket outlets is far cheaper than installing 3 phase circuit breakers...cabling and isolators.
    you can also change make the motor work for you by changing the parameters of the drive...for more torque...braking etc for safety...and and what they are designed for speed control with a small pot mounted at any location for ease of operation...or you can control it from your pc.
    for bigger motors...you dont have the choice...3 phase is the only option.

    another advantage is if you need a 3 phase motor at home and only have single phase...you just mount a drive next to the motor and plug it into a single phase socket outlet...this worked for a rural project i was involved in for mixing paint...we mounted the drive in a box...the paint was non flamable...so you take it home plug it into a single phase socket and start your own paint bussiness...mixing from a powder.


    am i missing something out...or am i on the right track.

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    Diamond Member Justloadit's Avatar
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    Hi Murdock,

    At the end of the day, the application will dictate what is the best drive.
    Yes the single phase drive will be a far cheaper solution if the application allows it.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Diamond Member AndyD's Avatar
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    You're talking about wiring a motor in star 220v 3-phase and trying to achieve the same mechanical output characteristics as if it was configured delta & 380v 3-phase by compensating with the VFD settings.

    My initial thoughts are that this wouldn't end well. I'm not even sure it's possible, if the drive (and motor) isn't specifically designed to do this you're going to run into flux over-saturation, copper losses will go through the roof and efficiency will plummet. The end result will be an extremely hot rotor and probable catastrophic winding failure.

    You're kind of in the twilight zone with this sort of set up. I'm not saying it's not possible but the whole ensemble would need to be designed to run like this from the ground up. Another problem is that even if you can achieve a balance where everything works for a given load, small changes in the mechanical shaft load could have very large effects on the electrical fundamentals of the motor and drive. These effects wouldn't be predictable, it would be trial and error and again possibly a costly exercise.

    Quote Originally Posted by murdock View Post
    ...running single phase socket outlets is far cheaper than installing 3 phase circuit breakers...cabling and isolators.
    I don't think so if you take in account the RCD protection requirements of plugs and sockets, the larger cable sizes involved and the motor would still require an isolator.
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    dedicated sockets for electronics...and stationary machine...

    the load of a 230 v vsd is double the load of a 3 phase vsd...

    thanks for the input...i had to think a little the other day while doing a DC to AC drive and motor conversion...havent had to think for a while...ended up with a pile of wire on the floor after modifying the power and controls...when the customer walked past he must have been wondering if his machine would ever be the same again...

    modern technology has come a long way...size and application.

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