Ques.71. The number of brushes required for a 4 pole, lap wound DC machine is
8
1
4
2
Answer.3. 4
Explanation:-
Lap Winding:- In a lap winding the number of parallel paths (a) is always equal to the number of poles (p) and also to the number of brushes. In the given question number of poles is 4 hence the number of brushes will be 4.
Wave Winding:- In wave windings, the number of parallel paths (a) is always two and there may be two or more brush positions.
Ques.72. Which of the following motors can be connected across a single-phase AC supply?
DC shunt motor
Differential compound motor
Cumulative compound motor
DC Series Motor
Answer.4. DC Series Motor
Explanation:-
A DC motor can be operated from a single-phase AC supply.
The direction of the torque produced by a DC machine is determined by the direction of current flow in the armature conductors and by the polarity of the field.
Torque is developed in a DC machine because the commutator arrangement permits the field and armature currents to remain in phase, thus producing torque in a constant direction.
A similar result can be obtained by using an AC supply, and by connecting the armature and field windings in series.
A series DC motor connected in this configuration can therefore operate on a single-phase AC supply and is referred to as a universal motor. An additional consideration is that, because of the AC excitation, it is necessary to reduce AC core losses by laminating the stator; thus, the universal motor differs from the series DC motor in its construction features.
Since the load current is sinusoidal and therefore reverses direction each half-cycle; however, the torque generated by the motor is always in the same direction, resulting in a pulsating torque, with nonzero averaging value.
In a dc series motor when the direction of current changes, the direction of the field flux also changes. Thus by Fleming’s left-hand rule, we can see that the force is always unidirectional thus generating unidirectional torque.
A choke coil is simply a coil having a large inductance, but a small resistance. A device that can regulate the current in an A.C. circuit without much loss of energy is called as ‘choke coil’.
If the choke coil were not used the voltage across the resistor would be the same as the applied voltage. Thus by using the choke coil the voltage across the resistor is reduced by a factor $\frac{R}{{\sqrt {{R^2} + {\omega ^2}{L^2}} }}$ which is the power factor (cos φ) for practical inductance coil.
The ohmic resistance of a practical choke coil is not exactly zero. This is the reason, that some electrical energy is always lost in the form of heat energy.
The choke coil can be used only in A.C. circuits but not in D.C. circuits because for direct current (ω = 0) the inductive part of reactance (= ωL) of the coil is zero and only ohmic resistance is left which is already very small.
Ques.74. Which of the following is NOT an expression to find self-inductance?
Let L be the length of a long solenoid and the total number of turns are N. The number of tuns per unit length is N/I. When a current I flows through it, the magnetic field inside the solenoid,
$B = {\mu _o}{\mu _R}\dfrac{{NI}}{l}$
Where
μo = permeability of free space
μr = Relative permeability
If A is the area of each turn (cross-section area), then the magnetic flux through each turn,
${\Phi _B} = BA = {\mu _o}{\mu _R}A\dfrac{{NI}}{l}$
∴ total magnetic flux linked through the solenoid is
$N{\Phi _B} = BA = {\mu _o}{\mu _R}A\dfrac{{{N^2}I}}{l}$
Ques.76. If ‘V’ is the voltage phasor and ‘I’ is the current phasor, then VI represents
Active power
Total power
Reactive power
Apparent power
Answer.4. Apparent power
Explanation:-
Apparent Power (S): It is defined as the product of r.m.s value of voltage (V) and current (1). It is denoted by S.
S = V/I Volt Ampere
Note:- Active power (VI cosφ) is actually used up in the circuit to do a useful work e.g. producing torque in motors and supplying heat, light etc. On the other hand, reactive power (= VI sinφ) is neither consumed nor it does any useful work in the circuit.
Active power is unidirectional i.e. it flows from source to load only. However, reactive power flows back and forth between the source and reactance (L or C).
Active power. It is the power that is actually consumed or utilized in the circuit. It is also defined as the product of apparent power and power factor. It is denoted by P and measured in unit watt or kilowatt (kW) P= VI cos (φ).
Ques.77. If a circular conductor carries a current of ‘I’ ampere having radius ‘r’ meter, then the magnetizing force at the center of the coil is given by
1.) $\dfrac{1}{r}At/m$
2.) $\dfrac{1}{2r}At/m$
3.) $\dfrac{1}{2r}At/Wb$
4.) $\dfrac{1}{4r}At/m$
Answer.2.$\dfrac{1}{2r}At/m$
Explanation:-
Magnetic Field Strength (H) gives the quantitative measure of strongness or weakness of the magnetic field.
H = B/μo
Where
B = Magnetic Flux Density
μo = Vacuum Permeability
The magnetic Field strength at the center of circular loop carrying current I is given by
B = μoI/2r
B/μo = I/2r
H = I/2r At/m
Where r = Radius
Ques.78. While estimating domestic wiring cost, if the length of the PVC conduit required is 62 meters, then what is the length of the cable required?
124 meters
62 meters
186 meters
100 meters
Answer.3. 186 meters
Explanation:-
“Conduit” means rigid or flexible metallic tubing or mechanically strong and fire-resisting non-metallic tubing into which a cable or cables may be drawn for the purpose of affording it or them mechanical protection.
Since the size of the conduit is 62 meters. In conduit, there is a 2-phase wire and 1 neutral wire. Therefore the total number of conducting wire is 3 in a conduit
Total length of cable = 62 × 3 = 186 meters.
Ques.79. In an alternator, if α = 60°, then find the pitch factor?
√3/2
1/2
1
0
Answer.1. √3/2
Explanation:-
The factor by which, the induced E.M.F gets reduced due to short pitching is called pitch factor or coil span factor denoted by Kc. It is given as
Kc = cosα/2
given
α = 60°
Kc = cos60/2
Kc = cos30°
Kc = √3/2
Ques.80. Which of the following decides the direction of rotation of a shaded pole motor?
Frequency of supply
Stator poles
Supply voltage
Shaded and unshaded portion of the pole
Answer.4. Shaded and unshaded portion of the pole
Explanation:-
SHADED-POLE MOTORS
The shaded-pole motor has the lowest starting torque of all the single-phase motor. It is relatively inexpensive and is used to turn very small fan blades connected directly to the shaft of the motor. The basic construction of this motor is very simple since no start winding is present. The imbalance in the magnetic field needed to produce rotation is obtained by shading a portion of the run winding with heavy copper wire or band.
When the motor is energized, the strength of the magnetic field is different in the area of the main poles than in the area of the shaded poles, allowing the rotor to turn.
The direction of rotation of a shaded-pole motor is determined by the orientation of the main poles and the shaded poles. The rotor will turn in the direction indicated by the arrows in Figure. Shaded-pole motors are used for very small applications and are rated in watts instead of horse-power. They normally range in size from 6 watts to roughly 35 watts. This is the equivalent of 1/125 horse-power to 1/20 horsepower. The direction of rotation is from the unshaded portion of the pole to the shaded portion of the pole.