241. The efficiency of a 100 kVA transform is 0.98 at full as well as at half load. For this transformer at full load the copper loss is

1. 1. less than core loss

   
   
   

   2. equal to core loss

   
   
   

   3. more than core loss

   
   
   

   4. none of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   For 100 kV transformer, we have
   At full load (η = 0.98),
   

   0.98 =	100
   	100 + Wcu + Wco

   
   

   ⇒ Wcu + Wco + 100 =	100
   	0.98

   
   ⇒ W_cu + W_co = 2.04 ...(i)
   At half load,
   

   0.98 =	50
   	50 + Wcu/4 + Wco

   
   

   ⇒	Wcu	+ Wco =	50	− 50
   	4		0.98

   
   

   ⇒	Wcu	+ Wco = 1.02 ...(ii)
   	4

   
   From equations (i) and (ii), we get
   

   Wcu	= 1.02
   4

   
   ⇒ W_cu = 1.36 kW
   W_core = 0.68 kW
   Hence at full load copper loss is more than core loss.

   Correct Option: C

   For 100 kV transformer, we have
   At full load (η = 0.98),
   

   0.98 =	100
   	100 + Wcu + Wco

   
   

   ⇒ Wcu + Wco + 100 =	100
   	0.98

   
   ⇒ W_cu + W_co = 2.04 ...(i)
   At half load,
   

   0.98 =	50
   	50 + Wcu/4 + Wco

   
   

   ⇒	Wcu	+ Wco =	50	− 50
   	4		0.98

   
   

   ⇒	Wcu	+ Wco = 1.02 ...(ii)
   	4

   
   From equations (i) and (ii), we get
   

   Wcu	= 1.02
   4

   
   ⇒ W_cu = 1.36 kW
   W_core = 0.68 kW
   Hence at full load copper loss is more than core loss.

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242. The windings of a Q kVA, V₁ / V₂ volt, three-phase, Delta connected, core type transformer are reconnected to work as a single phase transformer. The maximum voltage and the power ratings of the new configuration are

1. 1. 3V₁ / 3V₂, 3Q

   
   
   

   2. V₁ / V₂, 3Q

   
   
   

   3. √3V₁ / √3V₂, 2Q
      

   
   
   

   4. 2V₁ / 2V₂, 2Q/3

   
   
   

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1. View Hint View Answer Discuss in Forum

   Current will be same but voltage become 2V (V/2,V/2,V) due t o maximum flux density constraint.

   Correct Option: D

   Current will be same but voltage become 2V (V/2,V/2,V) due t o maximum flux density constraint.

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243. The fundament al and t he second-har moni c component of the field-flux waves (having their respective positive-maximum at the same spatial position) induce the maximum emf of E₁ and E₂ in each side of a full-pitch coil respectively when the coil is rotated at certain speed. At the instant when the sides of the coil are cutting the maximum flux, the result voltage across the coil terminals will be

1. 1. E₁ + E₂

   
   
   

   2. 2(E₁ + E₂)

   
   
   

   3. 2√E₁² + E₂²

   
   
   

   4. 2E₁

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: C

   NA

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244. A 1 k VA, 230 V/ 100V, single phase, 50 H z transformer having negligible winding resistance and leak age induct ance i s oper at ing under saturation, while 250V, 50 Hz sinusoidal supply is connected to the high voltage winding. A resistive load is connected to the low voltage winding which draws rated current. Which one of the following quantities will not be sinusoidal?

1. 1. Voltage induced across the low voltage winding

   
   
   

   2. Core flux

   
   
   

   3. Load current

   
   
   

   4. Current drawn from the source

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: D

   NA

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245. A single-phase transformer is to be switched to the supply to have minimum inrush current. The switch should be closed at

1. 1. maximum supply voltage

   
   
   

   2. zero supply voltage

   
   
   

   3. 1	maximum supply voltage
      √2

   
   
   

   4. 1	maximum supply voltage
      2

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: A

   NA

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