EE MCQ

SSC JE Electrical 2019 with solution SET-2
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Lower efficiency, lower power factor, are the features of which of the following motors?

The efficiency and power factor of single-phase motors are usually low. Thus at full-load a 186 W motor (1/4 hp) has efficiency and poor factor of about 60 percent. For the same size, the single-phase induction motors develop about 50% of the output as that of three-phase induction motors. The starting torque is also low for asynchronous motors / single phase induction motor.

The low power factor is mainly due to the large magnetizing current, which ranges, between 70 percent and 90 percent of full-load current. Consequently, even at no-load these motors have a substantial temperature rise. The power factor of 3-phase motors is of the order of 0.85 lagging, whereas it is 0.55 to 0.6 for single-phase motors.

The relatively low efficiency and power factor of single-phase induction motors is a consequence of their fractional horsepower ratings. The efficiency of 3-phase motors is quite high of the order of 80 to 90 %, while single-phase motors hardly work with an efficiency of 55 to 60 %.

Related Question

SSC JE Electrical 2019 with solution SET-2
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Form Factor × Peak Factor =?

Form factor: The ratio of r.m.s (or effective) value to average value is the form factor (Kf) of the Waveform. It has used in voltage generation and instrument correction factors.

Peak factor: The ratio of maximum value to the r.m.s value is the peak factor (Kp) of the waveform.

Form Factor × Peak Factor = (RMS Value / Average Value) * (Maximum Value / RMS Value) = Maximum Value / Average Value

SSC JE Electrical 2019 with solution SET-2
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The scheduled speed of a train is______

**Average speed:-** The distance covered between two stops divided by the actual time of the run is known as average speed.

Average speed = (Distance between stops)/(Actual time of run)**Schedule speed:-** It is defined as the ratio of distance covered between two stops and the total time of run including the time of the stop.

Schedule speed = (Distance between stops) /(Actual time of run + Stop time)

This shows that schedule speed is always smaller than the average speed. The difference is large in the case of urban and suburban services and is negligibly small in the case of main-line service.

SSC JE Electrical 2019 with solution SET-2
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While estimating the overhead lines, if the number of poles required is 14, then what is the estimated number of earth sets required?

While estimating the overhead lines, the first and last pole is always earth connected and after every 3rd electrical pole, the fourth pole is earthed. Therefore, the approximate number of required ground set = 4