3 phase motor speed control

Discussion in 'The Projects Forum' started by john mackay, Oct 26, 2011.

  1. john mackay

    john mackay Thread Starter New Member

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    I came across a 3 phase fan today at work, the starter is marked slow and fast but only contains three interconnected contactors with no other internal or external controls. There are six wires out to the motor ie three windings. I thought this is a mistake which should read for / rev so I asked someone to watch the fan asI tried it both ways and it is indeed slow / fast. I`m baffled, how is the speed changed with this contactor arrangement?.
  2. GetDeviceInfo

    GetDeviceInfo Senior Member

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    have no idea what your contactor arrangement is, but you have a couple of options. Your motor may be wired with seperate windings to give you two independant speeds, or the single speed windings may be sectionally reversed giving you a 'consequent pole' configuration. Google is your friend.
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  3. gremlin

    gremlin New Member

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    it will i would imagen
    switch the 6 wires from your 3 phase motor into
    either star or delta configuratuion which will give you two speeds
    this is quite comon on mixer agitators for the food industry

    http://www.google.co.uk/url?sa=t&rct=j&q=star%20delta%202%20speed%20motor&source=web&cd=1&ved=0CEMQFjAA&url=http%3A%2F%2Fwww.mandc.co.za%2Fpdfs%2FStar_Delta_Starting_and_Dual_Voltage_Motors_Explained.pdf&ei=i4mpTu7pC8_d8QOpk_X5Cw&usg=AFQjCNEOHRbe7dfr8gLj31yPccLtYiWH5g

    hope this helps

    google star delta motor configuration
    big three phase motors have to use bot star and delta one to start and one to run switched over after a few seconds of running
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  4. GetDeviceInfo

    GetDeviceInfo Senior Member

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    Let's flush out this misconception;

    the only difference between star (wye) and delta, is the voltage across the windings. There is no change in speed. The poles are fixed and you use the same poles regardless of delta or wye. There is typically a loss in speed regulation at load with a wye connection compared to delta, however a low or no load, wye gives improved efficiencies.

    The wye connection gives you 1/3 power output compared to it's delta configuration. This is what the 'soft start' concept is based upon.

    Double check your linked doc, specifically the RPM ratings at the different connections.

    Spending the last two days rebuilding moisture damaged contactors for a 37kw wye/delta starter.
    Last edited: Oct 27, 2011
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  5. john mackay

    john mackay Thread Starter New Member

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    Thanks for the replies, GetDeviceinfo has the correct answer. On further investigation I found that the motor does indeed contain two separate windings, one of which has such a low resistance it does not register on my multi-meter.
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