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SSC JE Electrical 2019 with solution SET-1
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What would the total loss of the 2kVA transformer corresponding to maximum efficiency be, provided the transformer has the iron of 150 W and full-load copper loss of 250W?

When variable loss becomes equal to the constant loss, efficiency is maximum.

Losses = Pi + Pc

Since copper loss is a variable loss therefore

Losses = Pi + Pi = 2pi

Thus at a maximum efficiency of this transformer total loss

= 150 x 2 = 300 W

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.

SSC JE Electrical 2019 with solution SET-1
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In an AC network, the load connected is (10 + j10). The phase relation between the voltage applied and the current through the load is:

L0ad impedance Z = R + jX

z = 10 + 10j

Phase angle θ= tan−1(IL/IR)

θ= tan−1(10/10)

tanθ = 1

tanθ =45°

SSC JE Electrical 2019 with solution SET-1
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Which of the following lamps has the shortest/less life span in working hours?

The lifespan of a bulb is referred to as “Average Rated Lifetime Hours” (ARL). Incandescent bulbs generally have the shortest lifespans. The average incandescent bulb light span is approximately 750-2,000 hours. Fluorescent lights generally have a life expectancy of 10,000 to 20,000 hours and other high-intensity lamps, such as high-pressure sodium, have greater than 20,000 hours of life expectancy.