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Electrical Machines

Electrical Machines
In DC machine torque depends on which of the following?

 flux (φ)
 armature current (I)
 both A and B
 speed

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Electrical Machines
A 240 V dc series motor takes 40 A when giving its rated output at 1500 rpm. Its resistance is 0.3 Ω. The value of resistance which must be added to obtain rated torque at 1000 rpm is

 6 Ω
 1.9 Ω
 5.7 Ω
 2.2 Ω

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Electrical Machines
A 200 V DC machine supplies 20 A at 200 V as a generator. The armature resistance is 0.2 Ω. If the machine is now operated as a motor at same terminal voltage and current but with the flux increased by 10%, the ratio of motor speed to generator speed is

 0.95
 0.96
 1.06
 0.87

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Electrical Machines
Induction motors normally use cast aluminium rotor because aluminium is

  lighter in weight.
  cheaper in cost.
  of low resistivity.
  easy to cast owing to its low melting point and is easily available.

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Electrical Machines
On switching a DC motor to DC supply mains, it is found to rotate in the direction opposite to that for which it is designed. The DC motor is

 Series shunt.
 Cummulative compound.
 have an equal number of electrons and protons.
  Differentical compound.

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Electrical Machines
The low voltage winding of a 400/230 V single phase 50 Hz transformer is to be connected to keep the magnetization current at the same level in both the cases the voltage at 25 Hz should be

 65 V
 460 V
 115 V
 230 V

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