EE MCQ

SSC JE Electrical 2019 with solution SET-1
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A three-phase induction machine, when it is operating like a motor, the range of slip is given by

Torque speed characteristic of induction motor

Motoring mode: 0 ≤ s ≤ 1

For this range of slip, the load resistance in the circuit is positive, i.e. torque developed is in the direction in which the rotor rotates.

In this region the value of slip lies between 0 to 11 i.e., slip is positive.

The motor rotates in the same direction as that of a rotating magnetic field.

At s = 0 (synchronous speed), the torque produced by the motor is zero because the induced voltage in the rotor is zero when N= Ns.

The torque increases as the slip increases while the air gap flux remains constant.

The torque-slip characteristic from no-load to somewhat beyond full-load is almost linear.

Generating mode: s <0

In this operating mode, the slip s is negative i.e., s < 0. The slip will be negative if and only if the rotor speed N is greater than the synchronous speed Ns (N> Ns). However, the rotor and R.M.F both rotate in the same direction.

In this region, the motor acts as a generator and return the power back to a.c. source.

Braking or plugging Mode: s > 1

In this region, the value of slip is greater than 1 and the rotor rotates in the opposite direction of the rotating magnetic field.

This is achieved by interchanging any two phases of the stator supply.

Related Question

SSC JE Electrical 2019 with solution SET-1
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The meter constant of an energy meter will be given in:

Meter constant = No. of revolution/kWh

Hence the number of revolutions made by the disc for 1 kWh of energy consumption is called meter constant.

The meter constant is always written on the name plates of the energy meters installed m homes, commercial and industrial establishments. If the meter constant of an energy meter is 1500 rev/kWh, it means that for the consumption of 1 kWh, the disc will make 1500 revolutions.

SSC JE Electrical 2019 with solution SET-1
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Which motor is referred to as a universal motor?

Universal motors may be powered by either AC or DC power sources.

The universal motor is constructed in the same way as a series-wound DC motor. However, it is designed to operate with either AC or DC applied.

The series-wound motor is the only type of DC motor that will operate with AC applied.

The windings of shunt-wound motors have inductance values that are too high to allow the motor to function with AC applied.

The series-wound motors have windings that have low inductances (few turns of large diameter wire), and they, therefore, offer a low impedance to the flow of AC.

The universal motor is one type of AC motor that has concentrated or salient field windings. These field windings are similar to those of all DC motors.

SSC JE Electrical 2019 with solution SET-1
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In the circuit shown, if R = 0, then the phase angle between v(t) and i(t) is

In the given circuit if R = 0 then the circuit becomes purely inductive. So the phase angle between v(t) and i(t) is 90°.

SSC JE Electrical 2019 with solution SET-1
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Reflected Light/Incident Light =

The ratio of luminous flux leaving the surface (Reflected Light) to the luminous flux incident on it (Incident Light) is known as the reflection factor.

Reflection Factor = Reflected light/Incident light

The value of the Reflection factor is always less than 1.