Discussion in 'General Electronics Chat' started by Rookie2002, Apr 30, 2009.

  1. Rookie2002

    Thread Starter New Member

    Apr 30, 2009
    I'm wondering if this is true. A co-worker ( Heating and Ac trade) told me the other day that a 240 volts motor has more poles than a 120 volts one. So, it would rotate slower than the 120 volts. one. I know that speed is a inversely proportional to the number of poles in the stator.More poles ,slower thew RPM. But, Is that true? a 240 volts motor really has more ( He said " twice as much ') poles than the 120 volts one.? Where in the name tag of a motor can I read the number of poles a motor has?
    We were troubleshooting a blower in a furnace which was not moving enough air. Come on! I also know that we always play by the 10% rule. You know + or - 10%. He said that an enexperinece tech. installed a 240 volts blower motor instead of a 120 volts one. By the way, this motor was installed in 2006 , how come it lasted so long?
    I'm very new in this field. He's all the years of expeirence in this. Was him pulling my leag?
    Please could you educate me on this subject or point me to a web site that I can read about it.
    Thanks Rookie
  2. mik3

    Senior Member

    Feb 4, 2008
    The number of the poles is determined by the desired output speed of the motor. Usually a 240V system works with 50Hz and a 120V system with 60Hz. This may be a reason to change the number of poles in the motors but I don't think 50Hz motors have twice the number of poles of 60Hz motors.
  3. SgtWookie


    Jul 17, 2007
    In North America, almost all AC power is 60Hz except for aircraft, which is 400Hz.

    Frequently, AC motors designed for this market have jumpers so that they can be operated from either 120v or 240v, as long as the jumpers are set properly.
  4. greendean


    Dec 20, 2008

    more poles, less rpm.

    Synchronous Speed = (120 x frequency) / # of poles.

    All power in the US is 60hz. Whether it be 120, 208, 277, 480, 347, or 600 volts.

    So a 208 volt motor with 2 poles will spin twice as fast as a 208 volt motor with 4 poles.

    So he is right that more poles less speed.
    But he is wrong saying voltage has anything to do with it.

    There is so mouch to know about motors and the control of them that this post could go on for hours.
    But I kept it to a bare minimum.
    Very Bare!

    Pick up a book on motors. It is quite interesting, and it will help you in your trade.

    Pick up a book called "Electricity 1-7".
    It is out of publication now so you'll probably have to find it on ebay.
    If you read it cover to cover you will know 20 times more than most 40 year experienced electricians know.
  5. sarwiz

    New Member

    Mar 9, 2009
    What you have is a motor wired for 240 running on 120. You don't necessarily need a new motor, but just rewire the one that is in there to run properly on 120. There should be a tag on the motor for this purpose. And yes, the before mentioned book is a good one, and can still be found. As long as it is an inductive single phase ac motor, it should run fine on 50 or 60 Hz. The number of poles is in the design of the motor, and yes more poles = lesser rpm.