166. ‘Crawling’ in an induction motor is due to

1. 1. time harmonics in supply

   
   
   

   2. slip ring rotor

   
   
   

   3. space harmonics produce by winding currents

   
   
   

   4. insufficient starting torque

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: C

   NA

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167. If load current and flux of d.c motor are held constant and voltage applied across it’s armature is increase by 5%, then speed of motor will

1. 1. increase by about 5%

   
   
   

   2. reduce by about 5%

   
   
   

   3. remain unaltered

   
   
   

   4. depends on other factors

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: A

   NA

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168. A 230-V d.c. series motor has an armature circuit resistance of 0.2Ω and field resistance of 0.1Ω. At rated votage, the motor draws a line current of 40 amps and runs at speed of 1000 r.p.m. What is the speed of the motor for a line current of 20 A at 230 V? Assume that the flux at 20 A line current is 60% of the flux at 40 A line current.

1. 1. 1213 rpm

   
   
   

   2. 1513 rpm

   
   
   

   3. 1713 rpm

   
   
   

   4. 1913 rpm

   
   
   

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1. View Hint View Answer Discuss in Forum

   For 40 A line current,
   E_α1 = V_t − I_α (r_α + r_s)
   = 230 – 40 (0.2 + 0.1) = 218 V
   For a line current of 20 A,
   E_α2 = 230 – 20 (0.3) = 224 volts
   Flux at 20 A,
   φ₂ = 0.6 times the flux at 40 A,
   i.e. φ₂ = 0.6 φ₁
   

   ∴	Eα1	=	n1φ1
   	Eα1		n2φ2

   
   

   ⇒	218	=	(1000)(φ1)
   	224		n2(0.6φ1)

   
   

   ⇒   n2 =	1000 × 224
   	218 × 0.6

   
   = 1713 r.p.m.

   Correct Option: C

   For 40 A line current,
   E_α1 = V_t − I_α (r_α + r_s)
   = 230 – 40 (0.2 + 0.1) = 218 V
   For a line current of 20 A,
   E_α2 = 230 – 20 (0.3) = 224 volts
   Flux at 20 A,
   φ₂ = 0.6 times the flux at 40 A,
   i.e. φ₂ = 0.6 φ₁
   

   ∴	Eα1	=	n1φ1
   	Eα1		n2φ2

   
   

   ⇒	218	=	(1000)(φ1)
   	224		n2(0.6φ1)

   
   

   ⇒   n2 =	1000 × 224
   	218 × 0.6

   
   = 1713 r.p.m.

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169. For a level compounded dc generator to run at constant speed, the series field mmf must effectively compensate

1. 1. armature reaction mmf

   
   
   

   2. armature resistance voltage drop

   
   
   

   3. brush contact voltage drop

   
   
   

   4. none of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: A

   NA

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170. Armature mmf of a dc machine has

1. 1. triangular space distribution and rotates at the speed of armature

   
   
   

   2. trapezoidal space distribution and is stationary in space

   
   
   

   3. stepped distribution and rotates at the speed of the armature

   
   
   

   4. triangular space distribution and is stationary in space

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: D

   NA

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