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Why efficiency of induction motor is low for low HP?

  1. Mar 5, 2016 #1
    why there is a decrease in efficiency of induction motor when power rating is low???? example 1 HP motor efficiency is say 85% then 1/4HP motor efficiency is less than 85%??? please clear me this doubt?
     
  2. jcsd
  3. Mar 10, 2016 #2
    This might help :smile:
     
  4. Mar 11, 2016 #3
    Indeed, according to most of catalogues the efficiency and power factor decreases if the rated power decreases [and also if the rated velocity decrease].The slip increases. The weight per power unit increases.
    In my opinion this could be the explanation:
    The motor dimensions-the stator inside diameter at the air gap- is stated according to motor
    rated torque.
    D=K*T^(1/3)
    For the same rated rpm the torque increases proportionally with the rated power.
    T=K*P/rpm then:
    Dint=K*P^(1/3)
    Actually Weight1/Weight2~=(P1/P2)^(a/3) a=1 to 2.5
    That means the weight/hp is less for high hp and then mechanical losses[friction, ventilation] and
    magnetic losses are less for more power rated. Also the slip decreases if power increases, so the electrical losses Plosses=Pm*(1-s) will decrease.
     
  5. Mar 17, 2016 #4
    Correction:
    electrical losses=~k*Pmec*s.Then electrical losses/HP=~s.Less slip less electrical losses per HP.
     
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