basic electrical engineering code: 1fy1-08
TRANSCRIPT
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.