171. When st ar t i ng a di ffer ent i al l y connect ed compound motor, it is desirable to short-circuit the series field winding to

1. 1. avoid very high starting time

   
   
   

   2. avoid excessive starting speed

   
   
   

   3. prevent the motor from starting in the reverse direction

   
   
   

   4. avoid heavy in-rush of current

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: B

   NA

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172. A dc shunt gener at or when driven without connecting field winding shows as open circuit terminal voltage of 12 V. When field winding is connected and excited the terminal voltage drops to zero because

1. 1. field resistance is higher than critical resistance

   
   
   

   2. there is no residual magnetism in the field circuit

   
   
   

   3. field winding has got wrongly connected

   
   
   

   4. there is a fault in armature circuit

   
   
   

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1. View Hint View Answer Discuss in Forum

   The wrongly connected field circuit can destroy the residual magnetism.

   Correct Option: C

   The wrongly connected field circuit can destroy the residual magnetism.

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173. An electric motor with “constant output power” will have a torque-speed characteristic in the form of a

1. 1. straight line through the origin

   
   
   

   2. straight line parallel to the speed axis

   
   
   

   3. circle about the origin

   
   
   

   4. rectangular hyperbola

   
   
   

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1. View Hint View Answer Discuss in Forum

   
   E_αI_α = Tω
   ⇒ Tω = constant
   ⇒ Tω = C

   Correct Option: B

   
   E_αI_α = Tω
   ⇒ Tω = constant
   ⇒ Tω = C

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174. A 200 V, 2000 rpm, 10A, separately excited dc motor has an armature resistance of 20Ω. Rated dc voltage is applied to both the armature and field winding of the motor. If the armature draws 5 A from the source, the torque developed by the motor is

1. 1. 4.30 Nm

   
   
   

   2. 4.77 Nm

   
   
   

   3. 0.45 Nm

   
   
   

   4. 0.50 Nm

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: B

   NA

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175. Neglecting all losses, the developed torque (T) of a d.c. separately excited motor, operating under constant terminal voltage, is related to its output power (P) as

1. 1. T ∝ √P

   
   
   

   2. T ∝ P

   
   
   

   3. T² ∝ P³

   
   
   

   4. T is independent of P

   
   
   

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1. View Hint View Answer Discuss in Forum

   From the relation,
   2πN × T= E_b × I_α = power developed in armature
   where, N = speed of motor
   E_b = induced back emf
   I_α = armature current
   ∴  T ∝ power developed in armature.
   Neglecting all losses, power output P will be equal to power developed in armature.
   ∴  T ∝ P

   Correct Option: C

   From the relation,
   2πN × T= E_b × I_α = power developed in armature
   where, N = speed of motor
   E_b = induced back emf
   I_α = armature current
   ∴  T ∝ power developed in armature.
   Neglecting all losses, power output P will be equal to power developed in armature.
   ∴  T ∝ P

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