Electrical Machines - Online Test
Q1. For a single phase, two winding transformer, the supply frequency and voltage are both increased by 10%. The percentage changes in the hysteresis loss and eddy current loss, respectively, are
Answer : Option A
Explaination / Solution:
Q2. Find the transformer ratios a and b that the impedance (Zin) is resistive and equal
2.5Ω when the network is excited with a sine wave voltage of angular frequency of 5000 rad/s.
Answer : Option B
Explaination / Solution:
Q3. A 4-pole, separately excited, wave wound DC machine with negligible armature resistance is rated for 230 V and 5 kW at a speed of 1200 rpm. If the same armature coils are reconnected to forms a lap winding, what is the rated voltage (in volts) and power (in kW) respectively at 1200 rpm of the reconnected machine if the field circuit is left unchanged?
Answer : Option B
Explaination / Solution:
In wave wound, no. of parallel paths is 2.
In lap wound, no of parallel paths is A = p = 4.
Rated voltage = (230 × 2)/4 = 115 V
As no. of parallel paths is divided into four from two, the voltage reduces to half but power remains the same.
Q4. In a constant V/f induction motor drive, the slip at the maximum torque
Answer : Option C
Explaination / Solution:
Q5. Consider a system consisting of a synchronous generator working at a lagging power factor, a synchronous motor working at an overexcited condition and a directly grid-connected induction generator. Consider capacitive VAr to be a source and inductive VAr to be a sink of reactive power. Which one of the following statements is TRUE?
Answer : Option A
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Q6. A 4-pole, lap-connected, separately excited dc motor is drawing a steady current of 40 A while running at 600 rpm. A good approximation for the waveshape of the current in an armature conductor of the motor is given by
Answer : Option C
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Q7. If an ideal transformer has an inductive load element at port 2 as shown in the figure below, the equivalent inductance at port 1 is
Answer : Option B
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Q8. The direction of rotation of a single-phase capacitor run induction motor is reversed by
Answer : Option C
Explaination / Solution:
No Explaination.
Q9. Three single-phase transformers are connected to form a delta-star three-phase transformer of 110 kV/ 11 kV. The transformer supplies at 11 kV a load of 8 MW at 0.8 p.f. lagging to a nearby plant. Neglect the transformer losses. The ratio of phase currents in delta side to star side is
Answer : Option A
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Q10. A 4 pole induction machine is working as an induction generator. The generator supply frequency is 60 Hz. The rotor current frequency is 5 Hz. The mechanical speed of the rotor in RPM is
Answer : Option C
Explaination / Solution:
