GLOBAL INSTITUTE OF TECHNOLOGY, JAIPUR

RTU Paper Solution Dec 2019

Basic Electrical Engineering

Code: 1FY1-08

GLOBAL INSTITUTE OF TECHNOLOGY, JAIPUR

RTU Paper Solution Dec 2019

Basic Electrical Engineering Code: 1FY1-08

Solutions BEE RTU PAPER DEC2019

PART A

Q1. Explain the concept of voltage and current source transformation with example.

A1. The voltage and current source are mutually transferable or in other words the

source transformation i.e. voltage to current source and current to voltage source can

be done. Let us understand this by considering a circuit given below

Q2. What is meant by power factor of the AC circuit? What is its minimum and

maximum value?

A.2. In AC circuits, the power factor is the ratio of the real power that is used to do

work and the apparent power that is supplied to the circuit.

The minimum value of power factor is zero. It occurs in a purely inductive circuit.

The maximum value of power factor is one. It occurs in a pure resistor circuit.

Q3. What is eddy current loss and how can this loss be reduced?

A.3 When an alternating magnetic field is applied to a magnetic material an emf is

induced in the material itself according to Faraday’s Law of Electromagnetic

induction. Since the magnetic material is a conducting material, these EMFs

circulates currents within the body of the material. These circulating currents are

called Eddy Currents. They will occur when the conductor experiences a changing

magnetic field. This loss can be reduced by reducing the eddy currents. It is done by

usind laminated stamping instead of solid core.

Q4. What is meant by slip of an induction motor?

A4. The slip in an induction motor is the difference between the main flux speed and

their rotor speed. The symbol S represents the slip. It is expressed by the percentage

of synchronous speed.

Q5. Distinguish between a rectifier and an inverter.

A5. An inverter and a rectifier perform opposite functions in electronic circuits.

Both acts as electric power converters; a rectifier changes current from alternating

current (AC) to direct current (DC), while an inverter converts DC to AC.

PART -B

Q4. Drive EMF equation of single-phase transformer. Explain also transformer is

known as constant flux device.

EMF Equation of The Transformer

Let, N1 = Number of turns in primary winding

N2 = Number of turns in secondary winding

Φm = Maximum flux in the core (in Wb) = (Bm x A)

f = frequency of the AC supply (in Hz)

As, shown in the fig., the flux rises sinusoidally to its maximum value Φm from 0.

It reaches to the maximum value in one quarter of the cycle i.e in T/4 sec (where, T

is time period of the sin wave of the supply = 1/f).

Therefore,

average rate of change of flux = Φ

m /(T/4) = Φ

m /(1/4f)

Therefore,

average rate of change of flux = 4f Φm .............. (Wb/s).

Now,

Induced emf per turn = rate of change of flux per turn

Therefore, average emf per turn = 4f Φm ............ (Volts).

Now, we know, Form factor = RMS value / average value

Therefore, RMS value of emf per turn = Form factor X average emf per turn.

As, the flux Φ varies sinusoidally, form factor of a sine wave is 1.11

Therefore, RMS value of emf per turn = 1.11 x 4f Φm = 4.44f Φm.

RMS value of induced emf in whole primary winding (E1) = RMS value of emf per

turn X Number of turns in primary winding E1 = 4.44f N1 Φm

As the net flux is always at a constant level in the transformer. The core loss is

constant for all loads.

Therefore, a transformer is a constant flux device.

Q.5 Explain in detail the construction and working principle of three phase

induction motor.

Construction of A Three-phase Induction Motor

An induction motor essentially consists of two main

parts Stator and Rotor. Stator:

The stator of an induction motor is in principle, the same as that of a

synchronous motor (or) generator.

It is made up of a number of stampings, which are slotted to receive

the windings.

The stator carries a 3-phase winding and is fed from a 3-phase

supply.

It is wound for a definite number of poles, the exact number of poles

being determined by the requirements of speed.

The number of poles are higher, lesser the speed and vice-versa. The stator winding, when supplied with a 3-phase currents, produce

a magnetic flux, which is of constant magnitude but which revolves

at synchronous speed (Ns = 120 x f / p).

This revolving magnetic flux induces emf in rotor by mutual induction

Rotor

Squirrel cage Rotor: Motors employing this type of rotor are known as squirrel

cage induction motor.

(ii). Phase wound (or) slip-ring Rotor: Motors employing this type of rotor

are widely known as “phase-wound” motors or wound motor or “slip-ring”

motors.

SQUIRREL CAGE ROTOR:

The Rotor consists of cylindrical laminated core with parallel slots for carrying the

rotor conductors which, it should be noted clearly, are not wires but consists of heavy

bars of copper, aluminum or alloys.

One bar is placed in each slot; rather the bars are inserted from the end when semi-

enclosed slots are used. The rotor bars are brazed or electrically welded or bolted to

two heavy and stout short- circuiting end-rings, thus giving us, what is called a

squirrel cage construction

PHASE-WOUND ROTOR

The Rotor is wound for as many poles as the number of stator poles and

is always wound 3-phase even when the stator is wound for two-phase.

The three phases are shorted internally.

The other three winding terminals are slip-rings mounted on the shaft

with brushes resting on them.

These three brushes are further externally connected to a 3-phase star

connected Rheostat.

This makes possible the introduction of additional resistance in the rotor

circuit during the starting period for increasing the starting torque of the

motor.

When running under normal conditions, slip-rings are automatically

short circuited by means of a metal collar, which is pushed along the shaft

and connects all the rings together.

Frame:

Made of close-grained alloy cast iron.

Stator and Rotor core:

Built from high quality low loss silicon steel laminations and flash enameled on

both sides.

Stator and Rotor windings:

Have moisture proof tropical insulation and embodying mica and high quality

varnishes.

These are rigidly braced to withstand centrifugal forces and any short circuit

stresses.

Air gap:

The stator rabbets and bore are machined carefully to ensure uniformity of air gap.

Shaft and Bearings:

Ball and roller bearings are used to suit heavy duty, trouble free running and for

enhanced service life.

Fans:

Light aluminum fans are used for adequate circulation of cooling air and are

securely keyed onto the Rotor shaft.

Slip-Rings and Slip-Ring Enclosures:

Slip rings are made of high-quality phosphor bronze and are of molded construction

Working principle ■ According to Faraday’s law an emf induced in any circuit is due to the rate of

change of magnetic flux linkage through the circuit. ■ As the rotor winding and cut the stator rotating magnetic field, an emf is

induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

■ Relative speed between the rotating flux and static rotor conductor is the cause of current generation; hence as per Lenz’s law, the rotor will rotate in the same direction.

■ Thus from the working principle of three phase induction motor, it may be observed that the rotor speed should not reach the synchronous speed produced by the stator.

Q 6. Explain the working principle of single-phase full bridge inverter with the help of circuit diagram and output voltage waveform.

A6. Single phase Full bridge inverter

Circuit Description:-

Four thyristor are used in full bridge inverter. Thyristor S1 and S2 are

used along with two feedback diode D1 and D2 and thyristor S3 and S4

are used along with another two feedback diode D3 and D4 respectively.

Fesistive load is connect between point A and B,as shown in fig:-

DC voltage source is applied to circuit.

Fig of the single phase full bridge inverter is given below:

Fig: single phase full bridge inverter

Mode 1 (0 to T/2):-

During this mode switch S1 and switch S2 are ON and switch S3 and

switch S4 are OFF From period 0 to T/2.

Current flowing path during this mode is Vdc – S1- P -R(load reistor) – Q

– S2 – Vdc.

Voltage across the load resistor is positive Vdc

Mode 2 (T/2 to T):-

During this mode switch S3 and switch S4 are ON and switch S1 and

switch S2 are OFF From period T/2 to T.

Current flowing path during this mode is Vdc – S3 – Q – R(load reistor) –

P – S4 – Vdc. Voltage across the load resistor is negative Vdc.

fig : conducting mode 2

1. Load is resistive hence it does not store any charge. therefore, feedback

diode D1, D2, D3 and D4 are not effective here.

Waveform of Output Voltage Thyristor Current With Resistive Load Are Shown In Fig:

PART C

Q1.bDistinguish between active power, reactive power and apperent power with thehelp of powerbtraingle.

A1b The active power is the real power consumes by the load. Whereas, the reactive power is the useless power. The active power is the product of the voltage, current and the cosine of the angle between them. Whereas, the reactive power is the product of voltage and current and the sine of the angle between them.

Q2a What is SCR ? Sketch static I-V characteristic of a thyrister voltage and opertaing mode.

A silicon-controlled rectifier or semiconductor-controlled rectifier is a four-layer

solidstate current-controlling device. The name "silicon-controlled rectifier" is

General Electric's trade name for a type of thyristor.

SCRs are mainly used in electronic devices that require control of high voltage and

power. This makes them applicable in medium and high AC power operations such

as motor control function.

An SCR conducts when a gate pulse is applied to it, just like a diode. It has four

layers of semiconductors that form two structures namely; NPNP or PNPN. In

addition, it has three junctions labeled as J1, J2 and J3 and three terminals anode,

cathode and agateanode, cathode and agate. An SCR is diagrammatically

represented as shown below.

The anode connects to the P-type, cathode to the N-type and the gate to the P-type

as shown below.

Q.2 b Explain the torque- speed scharacteristics and speed control of seperately excited DCmotor.

A2.b

When T = 0 the corresponding speed N =V /(K Φ) is the no-load speed. The motor d 0 a e

speed decreases as the torque developed increases, resulting in a drooping

characteristic

( )

Speed N= K

From above equation it can be seen that speed control of separately excited DC motor can be achieve by changing any one of the parameter such as V/R/ .Following methods are generally used for speed control of separately excited DC motor.

1. Field Control Method

2. Armature Resistance Control Method

3 . Armature Voltage Control Method

4. Ward Leonard Method of Speed Control.

Q3a Why protective devices are used for overload and short circuit protections?Why do we use an ELCB in an electrical installations?

We use the protective device for overload and short ckt protection to avoid the damage of the equipments which are design for normal parameteric conditions.

The ELCB is usec in electrical installation for followinfg purposes.

The ELCB is used to protect the circuit from the electrical leakage.

When someone gets an electric shock, then this circuit breaker cuts off the

power at the time of 0.1 secs for protecting the personal safety

ELCB is a security device used in electrical system with high Earth impedance

to avoid shock.

The main principle of Earth leakage protectors is to stop injury to humans and

nature due to electric shock.

This circuit breaker is a specialized kind of latching relay that has structures

incoming mains power connected through its switching contacts so that this

circuit breaker disconnects the power supply in an unsafe condition.

The ELCB notices fault currents from live to the ground wire inside the

installation it guards.

If enough voltage emerges across the sense coil in the circuit breaker, it will

turn off the supply, and stay off until reset by hand.