21. Given the maximum efficiency of a transformer of a 500 kVA, 3300/500V, 50 Hz single-phase transformer is 97% and occurs at 3/4 of full-load, unity power factor. If the impedance is 10%, the regulation at full-load, power factor 0.8 lagging will be _____ % .
    

1. 1. 8.25%

   
   
   

   2. 17.52%

   
   
   

   3. 7.52%

   
   
   

   4. 10.52%

   
   
   

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   η = 0.97 =		500 ×	3	× 1
   			4
   	500 ×	3	+ Pcore + Pcu ×	9
   		4		16

   
   

   ⇒ Pcore + Pcu ×	9	= 11.60
   	16

   
   ⇒ P_cu = 10.31 kW (at full-load)
   

   ∴ re (p.u.) =	Pcu	=	10.31	= 0.02062
   	kVArated		500

   
   x_e (p.u.) = √Z_e² - r_e²
   = √0.1² - (0.02062)² = 0.0978
   ∴ Regulation = r_e cos θ + x_e sin θ
   = 0.02062 × 0.8 + 0.0978 × 0.6 = 7.52%
   

   Correct Option: C

   η = 0.97 =		500 ×	3	× 1
   			4
   	500 ×	3	+ Pcore + Pcu ×	9
   		4		16

   
   

   ⇒ Pcore + Pcu ×	9	= 11.60
   	16

   
   ⇒ P_cu = 10.31 kW (at full-load)
   

   ∴ re (p.u.) =	Pcu	=	10.31	= 0.02062
   	kVArated		500

   
   x_e (p.u.) = √Z_e² - r_e²
   = √0.1² - (0.02062)² = 0.0978
   ∴ Regulation = r_e cos θ + x_e sin θ
   = 0.02062 × 0.8 + 0.0978 × 0.6 = 7.52%
   

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22. A 3– phase delta/star transformer is supplied at 6000 V on the delta-connected side. The terminal voltage on the secondary side when supplying full load at 0.8 lagging power-factor is 415 V. The equivalent resistance and reactance drops for the transformer are 1% and 5% respectively. The turns ratio of the transformer is _______ .
    

1. 1. 4

   
   
   

   2. 24

   
   
   

   3. 12

   
   
   

   4. All of the above

   
   
   

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   Turn ratio =	Phase voltage on primary side
   	Phase voltage on secondary side

   
   

   =	6000 × √3	= 24
   	415

   
   

   Correct Option: B

   Turn ratio =	Phase voltage on primary side
   	Phase voltage on secondary side

   
   

   =	6000 × √3	= 24
   	415

   
   

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23. A 400 V/ 200 V/ 200 V, 50 Hz three winding transfor mer is connected as shown in the given figure. The reading of the voltmeter,V will be _______ V.
    

1. 1. 0 V

   
   
   

   2. 1 V

   
   
   

   3. 2 V

   
   
   

   4. 3 V

   
   
   

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   NA

   Correct Option: A

   NA

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24. The low voltage winding of a 400 / 230V, 1– phase, 50 Hz transformer is to be connected to a 25 Hz, the supply voltage should be _______ V .

1. 1. 75 V

   
   
   

   2. 315 V

   
   
   

   3. 115 V

   
   
   

   4. 65 V

   
   
   

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   NA

   Correct Option: C

   NA

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25. For a p-pole machine, the relation between electrical (θ_e) and mechanical (θ_m) degree is given by
    

1. 1. θe =	2
      	p.θm

   
   
   

   2. θ_e = p.θ_m

   
   
   

   3. θ_e = θ_m

   
   
   

   4. θe =	p
      	2θm

      
      

   
   
   

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   NA

   Correct Option: D

   NA

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