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AC & DC Electrical Motors

  1. Jun 23, 2004 #1
    May Ask Help ...

    How I can know if the electrical motor is AC or DC ... In the case if i didn't find any ( Name Plate ) on it ... ?

    Also Asking If I Can Know ...

    For Wich Cases We Should Use ( AC ) OR ( DC ) ... Electrical Motors ???
    I Mean Wich Kind Of Machines ...

    Because I'm Mechanical Engineer ... Working In The Field Of Machine Desiging (( Industrial Machinery )) ...

    If There Is Some Websites Or Tables That Helping Me To Get More Informations About The Practical Use Of Those Kind Of E. M. ... Inform Me About It ... Please

    Wish The Best Always ...


    Eng. Luqman H. H.
  2. jcsd
  3. Jun 24, 2004 #2


    User Avatar
    Science Advisor

    Well the design specifications would be the key and drive the motor choice. The AC induction motor is simple and works well within a narrow operating range making it a good choice for a tabletop fan blowing air. A permanent magent brushed DC motor is also fairly simple design that works well over a wide RPM range with good torque, coupled with an optical encoder and now it can function as a servo for rotary or linear motion. A universal motor can run on either DC or AC.

    Then there should be a series of questions on power output requirements, control, operating environment, et cetera. A lot of engineering criteria that would be hard to source online, how could a website offer enough insight to provide a suitable match?

    To test your motor, first off describe how many leads. Does it have brushes? Does it have a big capacitor stuck on the case or nearby? Then use a DMM and measure the resistance of the leads. Rotate the shaft slowly one full turn and note if the resistance changes at any point. Then a good guess can be made. One set of leads that doesn't change, AC induction. One set of leads that changes twice and brushes, universal or DC. Two sets of leads where neither changes, AC induction with starter..... with the exception to exceptions.

  4. Jun 26, 2004 #3
    If you are able to open or dismental the motor u can differentiate by construction easly or else by the terminals. If its 3 phase AC motor u'll find 6 terminals in the terminal bord for star-delta connection. And if its single phase AC motor u'll find only two terminals. And if its DC motor you may find 4 terminals in the terminal board
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