**Ques.21.** The short-circuit test in the transformer is used to determine:-

- The iron loss at any load
- The copper loss at any load or at full load
- The hysteresis loss
- The eddy current loss

**Answer.2. ****The copper loss at any load or at full load**

**Explanation:-**

**Short-Circuit Test**

In case of short-circuit test, all the test instruments are connected on the high-voltage side and the low-voltage side is short-circuited. Now, the voltage across the HV side is gradually increased from a very low value. Since the transformer is short-circuited, the rated full-load current will appear at a much lower voltage. Low voltage (5 to 10%) is applied to the primary and slowly increase till full load current are flowing both in primary and secondary. Over and above, the rated full-load current on the high-voltage side is lower than that on the low-voltage side. So, the lower-rated instruments on the primary side can serve the purpose of testing.

Now at a low voltage, the exciting current will be much lower. So the effect of iron loss in this test can be neglected. Hence the wattmeter reads the copper loss in the transformer.

**Ques.22.** Power diode is

- A three terminal semiconductor devices
- A two-terminal semiconductor device
- A four terminal semiconductor device
- A three terminal analog device

**Answer.2. A two-terminal semiconductor device**

**Explanation:-**

Power diode is a two-terminal semiconductor device with a relatively large single P-N junction. It consists of a two-layer silicon wafer attached to a substantial copper base. The base acts as a heat-sink, support for the enclosure and also one of the electrical terminals of the diode. The other surface of the wafer is connected to the other electrical terminal. The enclosure seals the silicon wafer from the atmosphere and provides adequate insulation between the two terminals of the diode. The two terminals of a diode are called the anode (A) and the cathode (K).

A power diode is more complicated in structure and operating characteristics than low power diode Usually, power diodes have high-forward-current carrying capability and high reverse breakdown voltage. The area of the PN junction in a power diode is very high compared to low power signal diodes. Therefore, the structural modification of a low-power signs diode is required to make them suitable for high power applications.

**Ques.23.** In industries which electrical braking is preferred?

- Regenerative braking
- Plugging
- Dynamic Braking
- All of the above

**Answer.4. All of the above**

**Explanation:-**

Electrical braking is the most important requirement of quick-speed control of a running motor in modern electric drives. The quickness and accuracy of the operations in controlling the speed determine the quality and productivity of the product. Electrical braking is very smooth compared to mechanical braking. The direction of rotation and direction of the developed torque are exactly reverse to each other in electrical braking. The speed-torque characteristics are totally dependent on the method of braking. The three methods of braking are as follows:

- Regenerative braking,
- Plugging or countercurrent braking and
- Dynamic braking

**Regenerative braking:-** In regenerative braking, the motor, instead of being disconnected from the supply, remains connected to the supply and returns the braking energy to the supply line. So, the wastage of kinetic energy in the plugging and rheostatic braking methods is prevented in the regenerative braking. However, by this method, the drives cannot be brought to a standstill. It reduces the speed to a minimum permissible value.

**Dynamic braking:**-The supply to the field winding is maintained, but the armature is disconnected from the supply voltage and reconnected to an external resistor. The machine now acts as a generator, converting kinetic energy stored in its moving parts to electrical energy, which is dissipated as heat in the resistor. This method of braking is called the dynamic rheostatic braking.

**Plugging:- **Plugging or counter-current braking is a more commonly used method. In this method, the direction of rotation of the machine can be reversed only after reducing the speed to zero. Interchange of any two supply leads to the staler results in plugging. This reverses the direction of the rotation of the stator mmf, with rotor still running in the initial condition. The developed electromagnetic torque works in a direction opposite to the direction of rotation of the rotor. This helps to bring the motor at rest quickly. If the supply is not removed at zero speed, the motor runs in the opposite direction.

These three types of braking techniques preferred to stop a DC motor and used widely in industrial applications.

**Ques.24.** The steady-state stability of the power system can be increased by

- Using machines of high impedance
- Connecting lines in series
- Connecting lines in parallel
- Reducing the excitation of the machine

**Answer.3. Connecting lines in parallel**

**Explanation:-**

**Stability Limit**

According to the definition, power system stability is the ability of the power system to remain in a balanced condition during normal operation of the system and to bring back balanced conditions within the minimum possible time after the occurrence of disturbance.

**Steady-State Stability**

Steady state stability refers to system power stability in response to small disturbances and continuous changes in the load. Steady state stability can be improved by

Reduction of transfer reactance A power system which has a lower value of transfer reactance can have better steady-state stability limit. This can be achieved by:

- Use of parallel lines
- Use of series capacitors

If the power has to be transferred through long-distance transmission lines, the use of parallel lines reduce transfer reactance as well as improve voltage regulations. Similarly, series capacitors are some times employed in lines to get the same features.

**Ques.25.** An AC servomotor operated on the principle of operation of

- AC generator
- Synchronous motor
- Single phase induction motor
- Two-phase induction motor

**Answer.4. Two-phase induction motor**

**Explanation:-**

The servomotor is also called a control motor. These motors are used in the feedback control system as output actuators unlike large industrial motors, they are not used for continuous energy conversion. The basic principle of operation of the motor is the same as that of other electromagnetic motors. The rating of the motor varies from a fraction of watt to a few hundred watts.

Servomotors are of two types

- A.C servomotor
- D.C servomotor

**A.C servomotor**

Most of the servomotors used in the low power servomechanism are A.C. servomotors. The a.c servomotor is two-phase two phase induction motor. The output power of a.c. servomotor varies from fraction of watts to few hundred of watts. The operating frequency is 50 Hz to 400 Hz.

**Construction:** The two phase a.c. the servomotor is basically consist of two parts:

1. Stator

2. Rotor.

**Stator:** The stator is the stationary part of the servomotor. The stator carries two windings, uniformly distributed and displaced by 90° in space, from each other. One winding is called as main winding or fixed winding or reference winding. The reference winding is excited by a constant voltage a.c. supply.

The other winding is called as control winding. It is excited by the variable control voltage, which is obtained from a servo amplifier. The windings are 90° away from each other and the control voltage is 90° out of phase with respect to the voltage applied to the reference winding. This is necessary to obtain a rotating magnetic field.

To reduce the loading on the amplifier, the input impedance i.e., the impedance of the control winding is increased by using a tuning capacitor in parallel with the control winding.

**Rotor:** The rotor is generally of two types. The two types of rotors are:

(a) Squirrel cage rotor

(b) Drag cup type rotor

**(a) Squirrel Cage Rotor:** The usual squirrel cage rotor has aluminum bars which are shorted at the ends with the help of the end rings. The overall construction looks like a cage. The construction is similar to the squirrel cage rotor used for the three-phase induction motors.

This has a small diameter and large length This is because to reduce inertia. Aluminum conductors are used to keep weight small Its resistance is high to keep torque-speed characteristics as linear as possible. Ai gap is kept very small which reduces the magnetizing current.

**(b) Drag Cup Type Rotor:** To reduce the inertia further, drag cup type of rotor construction is used. There are tY air gaps in this construction. The drag cap is made up of non-magnetic material like copper, aluminium or alloy. The slotted rotor laminations are replaced by a s of stationary ring shaped laminations in this construction.

These are wound for as many numbers of poles as possible so that the operating speed of the motor is very low. Such a construction is used in very low power applications.

**Ques.26.** The number of unpaired electron in the chromium atom is

- 7
- 5
- 6
- 4

**Answer.3. 6**

**Explanation:-**

The atomic number of chromium is 24 and the number of unpaired electron in the chromium atom is six.

1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^{1}

**Ques.27.** A transformer has 1000 primary turns. It is connected to 250 V A.C supply. Find the mumber of secondary turns to get secondary voltage of 400 volts.

- 625
- 1600
- 400
- 1250

**Answer.2. 1600**

**Explanation:-**

Given

Primary turns N_{1} = 1000

Primary voltage V_{1} = 250 V

Secondary Turn N_{2} = ?

Secondary voltage V_{2} = 400 V

According to the transformer Ratio

N_{1}/N2 = V_{1}/V_{2}

1000/N_{2} = 250/400

**N _{2} = (1000 × 400)/250 = 1600**

**Ques.28.** Calculate the total resistance between point A and B

- 4.05Ω
- 8.67Ω
- 0.26Ω
- 0Ω

**Answer.1. 4.05Ω**

**Explanation:-**

The resistance R_{1} and R_{2} are in parallel therefore

R = R_{1} || R_{2}

R = (1 × 1)/(1 + 1)

R = 0.5Ω

Now resistance R and R_{3} are in series hence

R_{eqv} = 4 + 0.5 = 4.05Ω

**R _{eqv} = 4.05Ω**

**Ques.29.** If 4V is supplied to a 5F capacitor. Find the charge stored in the capacitor.

- 10 C
- 20 C
- 30 C
- 40 C

**Answer.2. 20 C**

**Explanation:-**

Given

V = 4 V

Capcitance = 5F

Charge stored in the capacitor

**Q = CV = 5 × 4 = 20 V**

**Ques.30.** What happens to the effective inductance when a ferromagnetic core is introduced?

- Increases
- Decreases
- Remain the same
- Becomes Zero

**Answer.1. Increases**

**Explanation:-**

An inductor is an electrical component formed by a coil of wire. Inductors store energy in a magnetic field that is set up when current flows through the coil of wire. The magnetic field returns to zero when current ceases to flow through the inductor. An inductor’s ability to set up a magnetic field is called inductance and is measured in Henrys (H). Inductance is represented by the letter L.

**Construction**

An inductor is usually made from a coil of conductive wire wrapped around a core material. The core performs two functions:

(1) it stabilizes the wire and provides support around which to wind the wire

(2) it increases the inductance, depending on the type of core material.

One type of core is an air core which consists of a cylinder of cardboard, wood, or ceramic that may be hollow. Basically, it is a form for winding the wire and does not affect the value of inductance. Another type of core is made of a ferromagnetic material such as iron, nickel, or iron ferrite. This type of core material concentrates the magnetic field and increases the inductance of the inductor.

**Factors Affecting Inductance**

Inductance is a function of the number of wire turns, length of the coil, the cross-sectional area of the coil, and permeability of the core material. The inductance of a cylindrical inductor can be calculated by the proportion

**L = µ.N ^{2}.A/L**

where

L is inductance (Henrys)

µ is core permeability (Henrys per meter)

A is a cross-sectional area (square meters)

L is the length of the core (meters)

N is the number of turns of wire.

Permeability is a measure of how easily a magnetic field can be generated in a material. The higher a material’s permeability, the easier it is to establish a field. Note that inductance increases by the number of turns of wire squared, so the number of turns has a significant effect on the inductance of the inductor.

Ferromagnetic materials have a considerable effect on magnetic fields. Magnetic fields tend to concentrate in high-permeability materials. For example, in the ferromagnetic-core inductor, we indicated that the magnetic flux was confined to the ferromagnetic core.