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Applied Electricity Paper 2 , May/June 2009  
Questions:   1 2 3 4 5 6   Main
General Comments
Weakness/Remedies
Strength








Question 6

(a)List two applications of a three-phase a.c.motor.    

(b)In fig. 8, calculate the:
(i) turn ratio;
(ii)number of turns on the primary if the secondary has 100 turns;

iii)current in the primary if the current in the lamp is 2A.

_____________________________________________________________________________________________________
Observation

The expected answers were:

(a)Lifts; Conveyors;
Industrial air conditioners;
Industrial drives.

(b)(i)  Turns ratio  = Ns / Np = Vs/ Vp
                           = 12 / 240  =   1 //20
             Np  :  Ns  =  20:1
  (ii)    Np  =  (Ns * Vp) / Vs
                = (100 * 240) / 12    = 2000 turns
 (iii)    Is / Ip = Vs / Vp
          Ip  = (12 * 12) / 240 = 0.1A
This question was on electrical machines (A.C.).  The question was popular.  Many candidates got turns ratio, primary turns and current correctly but a few did not which may be due to inadequate preparation and insufficient practice.

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