DMRC JE Electrical 10th- April- 2018 Paper With Solution and Explanation

Shaded-pole motor has a salient pole staler similar to the stator of a dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in fig.

A heavy, short-circuited copper ring, called the shading coil, placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-thirds section of the pole is referred to as the unshaded portion. The main single-phase winding is wound on the entire pole section. The rotor used is similar to the rotor of any other single-phase induction motor.

When a single-phase supply is fed to the main winding, an alternating flux is produced in the pole. A portion of this flux links with the shading coil and induces a voltage in it. As a shading coil is a short-circuited coil, a large current flows in it.

The current in the shading band causes the flux in the shaded portion of the pole to lag the flux in the unshaded portion of the pole. Thus the flux in the shaded portion reaches its maximum value after the flux in the unshaded portion reaches its maximum. This is equivalent to a progressive shift of the flux from the unshaded to the shaded portion of the pole, that is it is similar to a rotating field moving from the unshaded to the shaded portion of the Pole. Hence. the motor reproduces a starting torque.

Because of the small phase of displacement of the currents in the mail and the auxiliary winding and because of the winding misalignment lower than π/2 the start torque is very low.

Therefore shaded-pole motors are especially suited for small fans and pumps. Other applications are juice presses, clothe dryers, grills, simple butterfly control waves, massage apparatus, hot-air stoves, and cabinet fans. Drives for reversing duties can be built with two motors assembled homologously. Shaded-pole motors are low-cost motors. Because of their low efficiency, they mostly need intensive cooling.

Unbalanced loading is not a direct fault, does not apply any harm but the cause of unbalanced is harmful like the grounding of one phase, or short circuit of phase to phase.

Unbalanced loading means the circulation of the different phase current ratios in three phases of an alternator. In normal conditions, the difference in phase current varies +/- 5 %, but when this difference exceeds then it becomes an unbalanced condition.

The reasons for the unbalanced load conditions are,

1. The occurrence of unsymmetrical faults near the generating station.
2. The failure of circuit breaker near Me generating station in clearing all the three phases.

To sense and protect the unbalanced loading a simple conception is used to compare the value of two currents i.e Incoming current and the Outgoing current of the stator coil. In normal conditions, the value of two currents will be equal, during fault conditions there will be some difference, if the value is not in the tolerable range then this circuit can send a trip signal.

As shown in the above circuit the secondaries of three CTs are shorted, so the sum of normal phase currents is zero, with no current in the trip coil. In some instances if there is some unbalanced, then there will be some current in the secondary, which will eventually send a trip signal.

The unbalanced loading of the generator results in the circulation of negative sequence currents. These currents produce a rotating magnetic field. This rotating magnetic field rotates at a synchronous speed with respect to the rotor. The direction of rotation of this magnetic field is opposite to that of the rotor. Hence effectively the relative speed between the two is double the synchronous speed.

Thus the e.m.f. gets induced, having double the normal frequency in the rotor winding. The circulating currents due to the induced e.m.f. are responsible to overheat the rotor winding as well as rotor stampings. A continuous unbalanced load of more than 10% of the rated load causes tremendous heating which is dominant in the case of the cylindrical rotor of turbo-alternators.

• Varistors are voltage-dependent resistors whose resistance decreases sharply with an increase in voltage.
• Metal oxide varistors are ceramic semiconductive devices having highly nonlinear current-voltage characteristics.
• Solid-state power supplies are protected from high surge voltage due to lightning using a silicon carbide resistor.
• They are solid-state devices that significantly decrease their internal resistance once a specific voltage threshold has been reached across the connecting terminals.
• Once activated, varistors do not short instead, they clamp voltages to some maximum level while allowing surge currents to pass through the device.
• The most common type of varistor used in surge protection is the metal oxide varistor; in addition, there are other types of varistor also available that can provide high current protection in the range of 70,000 A for power applications. Varistors can be made to provide surge protection at all levels.

• The zinc oxide-based varistors are primarily used for protecting solid-state power supplies with low and medium size voltage in the supply line.
• Silicon carbide varistor provides protection against high-voltage surges caused by lightning and by the discharge of electromagnetic energy stored in the magnetic fields of a large coil.
• The practical voltage range of zinc oxide varistor is as wide as six to several thousand volts.

Silicon Carbide Varistor or Carborundum crystal Varistor:- These varistors are made from silicon carbide mixed with a suitable ceramic binder material. The mixture la prefixed to the desired shape and then wintered (compressed under heat) under controlled conditions to produce the hard ceramic body.

Silicon-carbide varistors have a high power-handling capability and are used in high-voltage surge (lightning) arresters. The devices tend to draw considerable current in the normal state, and so they are commonly used in series with a gap that provides an open circuit until a surge occurs. This property makes silicon-carbide varistors unsatisfactory for low-voltage clamping operation. Newer zinc-oxide lightning arresters have better nonlinear characteristics and can be used without a gap. They are essentially crowbar devices but perform almost like clamping devices.

Cerment Resistor

A cermet is a composite material composed of ceramic and metal materials.

A cermet is ideally designed to have the optimal properties of both a ceramic, such as high-temperature resistance and hardness, and those of a metal, such as the ability to undergo plastic deformation.

The cermet resistor can be obtained by depositing highly stable metallic solutions (like silver/gold with palladium or chromium with silicon monoxide or oxides of rhodium/zirconium) in an organic solvent on the ceramic substrate. This is done by micro screen printing, roller printing, or dipped proceed. When the substrate la heated (1000°C), the organic particles evaporate and the metal fuses into the ceramic substrate.

Advantaged Cermet resistors are offering the following advantages.

1. These resistors offer higher stability.
2. It produces low Noise

Cermet or Metal Glaze Resistors

The metal glaze resistor is also a fixed resistor, similar to the metal film resistor. This resistor is made by combining metal with glass. The compound is then applied to a ceramic base as a thick film. The resistance is determined by the amount of metal used in the compound. Tolerance ratings of 2% and 1% are common.

For most materials, a very high sheet resistance can only be obtained by using an extremely thin film. By using a composite material such as a metal glaze, a thick and therefore more stable high ohmic film can be made. Such a film is produced by dipping the ceramic rod in a paste and drying and firing the layer.

As cylindrical resistors, cermet or metal glaze resistors are mostly used for high-resistance values and high voltages. The available resistance range is 100 kΩ to 100 MΩ. The stability is good and the current noise is acceptable for such high values.

The resistance of the metal glaze resistor depends on the amount of metal and glass added
Metal glaze film with a higher amount of metal particles and a lower amount of glass provides low resistance to the electric current. Hence, a large amount of electric current flows through the metal glaze resistor.

Coefficient of Magnetic Coupling

During Mutual Inductance between two coils, we assume that the flux produced by one coil is entirely linked with the other coil. But practically it is not true, the certain portion of the flux of one coil does not link with the other coil.

The ratio of the portion of the flux of one coil linked with the other coil to the entire flux produced by the coil is referred to as the coefficient of magnetic coupling.

Consider two coils having self-inductance L₁ and L₂ placed very close to each other. Let the number of turns of the two coils be N₁ and N₂ respectively. Let coil-1 carry current I₁ and coil- 2 carry current i₂.

Due to current i₁, the flux produced is Φ₁ which links with both the coils. Then the mutual inductance between two coils can be written as

$M = \dfrac{{{N_1}{\Phi _{21}}}}{{{I_1}}}$

Where Φ₂₁ is the part of the flux Φ₁ linking with the coil 2.

Now as per given question

Number of turns N₁ = 500

Current I = 5A

Flux Produced in X Φ₁ = 10 mWb = 1 × 10^-2 Wb

Fluxed Linked with Y Φ₂₁ = 1 × 10^-2 × 0.8 = 0.8 × 10^-2 Wb

$\begin{array}{l}M = \dfrac{{500 \times 0.8 \times {{10}^{^{ – 2}}}}}{5}\\\\M = 0.8H\end{array}$

Voltage Transformation Ratio (K)

The constant K is called as Voltage transformation ratio and it is given as:

For an ideal transformer, there is no voltage drop in the windings, therefore, E₁ = E₂and V₁ = V₂.

If N₂ > N₁ and K >1 then the transformer is called a step-up transformer.
If N₁ > N₂ and K< 2 then the transformer is called a step-down transformer.

There are no losses in an ideal transformer therefore volt-ampere input of the primary will be always equal to the volt-ampere of the secondary.

$\begin{array}{l}\dfrac{{{I_2}}}{{{I_1}}} = \dfrac{{{N_1}}}{{{N_2}}} = \dfrac{{{V_1}}}{{{V_2}}}\\\\Hence\\\\{I_1} = \left( {\dfrac{{{N_2}}}{{{N_1}}}} \right) \times {I_2}\end{array}$

From the above equation, it is clear that current is inversely proportional to the voltage i.e if we will increase the value of voltage then the value of current will decrease.

At the normal operating conditions of the DC series motor. The field winding is connected in series with the armature winding. This means that power will be applied to one end of the series field winding and to one end of the armature winding (connected at the brush). When the voltage is applied, the current begins to flow from negative power supply terminals through the series winding and armature winding.

Now if the field winding of the DC series motor gets open then we have cut the supply off the motor as the field is in series with the armature. Hence the current will stop flowing in the entire circuit and the motor will not start at all.

Any polyphase load in which the impedances in one or more phases differ from the impedance of other phases is said to be an unbalanced load.

The order in which the individual phase voltages attain their respective maximum values in a three-phase system is called phase sequence. So whether it is a balanced system or an unbalanced system the phase voltage depends on the phase sequence.

Conventionally the three phases are designated as red-R, yellow-Y, and blue-B phases.
The phase sequence is said to be RYB if R attains its peak or maximum value first with respect to the reference as shown in the counterclockwise direction followed by Y phase 120° later and B phase 240° later than the R phase.

The phase sequence is said to be RBY if R is followed by B phase120° later and Y phase 240° later than the R phase. By convention, RYB is considered positive while the sequence RBY is negative.

Then Red phase  V_RN = Vm sinθ
Yellow Phase V_YN = Vm sin(θ – 120°)
Blue Phase V_BN = Vm sin(θ – 240°)
or V_BN = Vm sin (θ + 240°)

The effects of phase sequence of the source voltages are not of considerable importance for applications like lighting, heating, etc. but in the case of a three-phase induction motor, reversal of sequence results in the reversal of its direction.

In the case of an unbalanced polyphase system, a reversal of the voltage phase sequence will, in general, cause certain branch currents to change in magnitude as well as in phase position. Even though the system is balanced, the readings of the two wattmeters in the two wattmeter method of measuring power interchange when subjected to a reversal of phase sequence when two or more three-phase generators are running parallel to supply a common load, reversing the phase sequence of any one machine causes severe damage to the entire system.

A Bipolar Junction Transistor (BJT) is a three-layer, two junction semiconductor device consisting of either two N-type and one P-type layer of material (NPN transistor) or two P-type and one N-type layer of material (PNP transistor). If a thin layer of N-type Silicon is sandwiched between two layers of P-type silicon. This transistor is referred to as PNP. Alternatively, in an NPN transistor, a layer of P-type material is sandwiched between two layers of N-type material. In the common collector configuration of bipolar junction transistor (BJT), the output voltage is in phase with the input voltage.

The term bipolar is to justify the fact that holes and electrons participate in the injection process into the oppositely polarised material. If only one carrier is employed (electron or hole). it is considered a unipolar device.

CIRCUIT CONFIGURATION OF A BIPOLAR JUNCTION TRANSISTOR

A transistor has three terminals: an emitter (E), base (B), and collector (C) and two junctions as emitter-base junction and collector-base junction. Each of these junctions requires two terminals for a biasing arrangement. When a transistor is connected to a circuit, it requires four terminals, i.e. two input terminals and two output terminals. Therefore, one of the three transistor terminals (emitter, base, and collector) is used as a common terminal between input and output. Depending upon the common terminal, the transistor may be connected to the three different configurations such as common emitter (CE), common base (CB), and common collector (CC) configurations.

Common-Collector Configuration

In a common-collector configuration, the collector terminal of a transistor is connected as a common terminal between input and output as shown in fig. The base and collector terminals are used to apply the input signal whereas the output signal is obtained from the emitter and collector terminal. The CC configuration is also commonly known as the emitter follower or the voltage follower. This is because the output signal across the emitter is almost the replica of the input signal with little loss.

Since the emitter is the output terminal, it can be noted that the output voltage from a common collector circuit is the same as its input voltage. Hence the phase shift between the input voltage and the output voltage is 0°.

In other words, the output voltage follows the input voltage and hence the voltage gain will be a maximum of one. The common-collector configuration is used primarily for impedance-matching purposes since it has a high input impedance and low output impedance, opposite to that of the common-base and common-emitter configuration.

Note:- 

• In the COMMON-BASE configuration, there is no PHASE SHIFT between input and output. It is not necessary for the feedback network to provide a phase shift. Voltage and power gain are high enough to give satisfactory operation in an oscillator circuit.
• In the COMMON-EMITTER configuration, there is a 180-degree PHASE SHIFT between input and output. The feedback network must provide another phase shift of 180 degrees. It has a high power gain.

One-wattmeter Method

In this method, only one single-phase wattmeter can be used to measure the total three-phase power. this method, the current coil (CC) of the wattmeter is connected in series with any phase and the pressure coil (PC) is connected between that phase and the neutral as shown in Fig.

The one-wattmeter method has a demerit that even a slight degree of unbalance in the load produces a large error in the measurement. ln this method one wattmeter will measure only the power of one phase. Hence, the total power is taken as three times the wattmeter reading.

Total Power =  3 × V_ph. I_ph. cosφ

Note:-  The two and three-wattmeter method can be used for balanced as well as unbalanced load.

In star connected system the Line voltage is root three times the phase voltage and line current remain same as the phase current

V_L = √3 V_ph

V_L = √3 × 150

I_L = I_ph

I_L = 30√3

According to two wattmeter method

⇒ Reading of wattmeter 1
W₁ = V_L I_L cos (30 − φ)

⇒ Reading of wattmeter 2
W₂ = V_L I_L cos (30 + φ)

⇒ Sum of the two wattmeter

W₁ + W₂ =  √3V_L I_L cosφ

⇒ Difference of the two wattmeter

W₁ − W₂ =  V_L I_L sinφ

Hence

√3 × 150 × 30 × √3 × 0.707

W₁ − W₂ = 9544.5 W

W₁ − W₂ = 9.54 kW