RVUNL Electrical Machines Revision Quiz

Consider the following statements regarding DC series motors:

1) Efficiency of dc series motor cannot be determined by swinburne’s test as series motor runs at dangerously high speed at no-load.

2) During rheostatic braking of dc series motor, motor operates as a generator.

Consider the following statements regarding commutation in DC machines:

1) In a dc machine the ripples in emf generated are reduced by employing commutator with large number of segments.

2) In dc machine the number of commutator segments is equal to armature turns.

3) The purpose of commutator in dc generator is to convert dc into ac.

Which of the above statements is/are NOT correct?

Core of the transformer should have______ permeability and ______ reluctance.

A 3-ϕ 50 Hz, 8 pole, star connected alternator has 120 stator slots and each slot has 10 conductors. If the coil span is 12 slots, then the value of pitch factor will be-

A 3-ϕ, 14.5 kW, 400V, four pole, 60 Hz, IM delivers full output power to a load connected to its shaft and running at speed of 1728 rpm. The windage and friction loss of the motor is 500 W. The value of rotor copper loss of motor will be-

The slip at which maximum Power developed for 3-phase IM, If 120V, 60 Hz, 6-Pole, (Δ) 3 phase IM has stator impedance = (0.1 + j0.15) Ω/phase and rotor impedance = (0.1 + j0.25) Ω/phase (at standstill).

A 10 kW, 200 V DC shunt motor has armature resistance of 2 Ω and shunt field resistance of 50 Ω. At no-load condition, the motor draws 10 A from 200V supply and runs at 1000 rpm. Thus, value of total copper loss at no-load of the machine will be-

A DC Generator whose external characteristic is given by equation, V_t = 20 – 0.5I_L, where I_L represents the load current. If the connected load takes 200 W, then the resistance of the load will be-

When two generators are operating together, an increase in governor set points on one of them:

1) Decrease the supply frequency

2) Increase in Power supply by that generator, while reducing the power supplied by the other one.

Which of the above statements is/are correct?

Which of the following DC generator is not suitable for parallel operation.

What is the increase in the torque expressed as percentage of initial torque. If the current drawn by a dc series motor is increased from 9A to 11A (Neglect saturation)?