EE Terms 2

When a thyristor is negatively biased,

Outer junctions are negatively biased and the inner junction is positively biased.

The junction near the anode is negatively biased and the one near the cathode is positively biased.

Outer junctions are positively biased and the inner junction is negatively biased.

All the three junctions are negatively biased.

At one instant an electron is moving in the positive x direction along the x axis in a region where there is a uniform magnetic field in the positive z direction. When viewed from a point on the positive z axis, it subsequent motion is:

In the positive z direction

Counterclockwise around a circle in the xy plane

Straight ahead

Clockwise around a circle in the xy plane

According to Gauss’ law for magnetism, magnetic field lines:

Form closed loops

Start at north poles and end at south poles

Start at both north and south poles and end at infinity

Start at south poles and end at north poles

In a DC machine, if the field current I_f increase to 2I_f, what will the new torque be as a function of the initial torque, Tm? Neglect saturation.

Tm²

2Tm

4Tm

A transformer has maximum efficiency at 3/4 load. The ratio of iron loos and full load copper loss is _____.

9/16

3/4

16/9

4/3

A three-phase alternator has three armature coils, each rated at 1200 V and 120 A. What is most nearly the kVA rating is equal to the power output.

520 kVA

440 kVA

540 kVA

430 kVA

1 kVA, 230 V, 50 Hz, single phase transformer has an eddy current loss of 30 watts. The eddy current loss when the transformer is excited by a dc source of same voltage will be _______.

More than 30 W

Less than 30 W

30 W

Zero watt

A single-phase induction motor is not self-starting. Instead, auxiliary methods must be used, such as varying inductance, resistance and capacitance. Which of the following is FALSE regarding this situation?

A capacitor motor uses capacitance to split the phase, resulting in two phases almost 90° apart.

If the capacitor remains in the circuit, the power factor will have a value close to unity.

Capacitor motors have lower starting torque than comparably sized single-phase induction motor.

To obtain a higher reactance, a capacitor can be used when starting and then be switched out of the circuit by mechanical means.

A string insulator has 4 units. The voltage across the bottom most unit is 33.33% of total voltage. Its string efficiency is

66.66%

75%

33.33%

25%

Three phase transformers which cannot be connected in parallel are

Y – Δ with Y - Δ

Y – Δ with Δ – Y

Y – Y with Δ – Δ

Y – Y with Y – Δ

It is better to send 10,000 kW of electric power long distances at 10,000 V rather than at 220 V because:

The insulation is more effective at high voltages

More current is transmitted at high voltages

The resistance of the wires is less at high voltages

There is less heating in the transmission wires

The Buchholz relay is used to protect the:

Synchronous motors against all types of faults

Alternators transformers against all internal faults

Transmission line against all types of faults

Oil immersed transformers against all internal faults

Which statement is TRUE?

Magnetic flux lines do not have sources or sinks.

Magnetic flux lines have both sources and sinks.

Magnetic flux lines have sources only.

Magnetic flux lines have sinks only.

The power transmission capacity of the transmission line is:

Directly proportional to the voltage

Inversely proportional to the voltage

Proportional to the square of the voltage

Proportional to the cube of the voltage

A hydrogen atom that has lost its electron is moving east in a region where the magnetic field is directed from south to north. It will be deflected:

North

South

Down

In the positive z direction

Counterclockwise around a circle in the xy plane

Straight ahead

Clockwise around a circle in the xy plane

The magnetic torque exerted on a flat current-carrying loop of wire by a uniform magnetic field B is:

Dependent on the shape of the loop for a fixed loop area

Maximum when the plane of the loop is parallel to B

Maximum when the plane of the loop is perpendicular to B

Independent of the orientation of the loop

If the transmission voltage increased, the percentage resistance drop will:

Remain unchanged

None of these

Increase

Decrease

A cylindrical region contains a uniform electric field that is along the cylinder axis and is changing with time. If r the distance from the cylinder axis the magnitude of the magnetic field within the region is proportional to _____.

1/r²

1/r

R²

Before illuminating a P-N junction photodiode, it has to be ________.

Switched OFF

Reverse-biased

Forward-biased

Switched ON

Iron, rather than copper, is used in the core of transformers because iron:

Can withstand a higher temperature.

Makes a good permanent magnet.

Has a very high permeability.

Has a greater resistivity.

In which of the following voltage regulators the effect of dead zero is found?

All of the choices

Electronic type using integrated circuits

Magnetic amplifier

Electromagnetic type

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