81. The number of brushes required for a 4 pole, lap wound DC machine is ?
If the machine is “wave wound” there will be two brushes. All of the rotor windings are in series to provide a high voltage at a low current.
If the machine is “lap wound” the number of brushes will be equal to the number of poles.
There will be many low voltage high current paths through the rotor. Consider that each “pole” represents a physical magnet in the stator.
A pair of brushes is required for each pair of stator magnets.
82. Which of the following motors can be connected across single phase AC supply ?
DC Series Motors Operated on AC Power The field winding of a dc series motor has fewer turns and larger wire than the field winding of the dc shunt motor.
Therefore, its inductive reactance is less.
The strength of the magnetic field is about the same as when the motor runs on dc power.
The field winding and the armature winding are connected series.
Therefore, the current lags behind the potential difference by the same amount in both windings.
As a result, the current in the two windings is in phase.
The current in the armature winding is in phase with the magnetic field produced by the field winding.
However, a dc series motor does not operate well on ac power.
The motor has a low-efficiency power factor and runs with considerable sparking a low the brushes. The low efficiency is caused by high hysteresis loss and high eddy current loss.
The power factor is caused by the inductive reactance of the field and armature windings.
83. A choke can be represented by a ?
All electrical coils, relays, solenoids and transformers will have a certain amount of resistance no matter how small associated with the coil turns of wire being used. This is because copper wire has resistivity. Then we can consider our inductive coil as being one that has a resistance, R in series with an inductance, L producing what can be loosely called an “impure inductance”.
84. Which of the following is NOT an expression to find self-inductance ?
Self inductance of a coil
Self-inductance or simply inductance is the process by which an emf is induced inside a coil of wire as a result of an alternating current traveling through that coil.
This induced emf (as per Lenz's Law) will create an induced current that will tend of oppose the change in magnetic flux.
L = - Nφ/I
Where:
L is in Henries
N is the Number of Turns
Φ is the Magnetic Flux
Ι is in Amperes
self-inductance for an air cored coil (solenoid)
L = μo $N^2$ A / ℓ
Where:
L is in Henries
μο is the Permeability of Free Space (4.π.$10^-7$)
N is the Number of turns
A is the Inner Core Area (π$r^2$) in $m^2$
ℓ is the length of the Coil in metres
Magnetic flux (φ) = MMF/ Reluctance = NI/S
and L = Nφ/I
L = (N*NI/S)I = $N^2$/S
85. A coil of 360 turns is linked by a flux of 200 µWb. If the flux is reversed in 0.01 second, then find the EMF induced in the coil ?
Given
N = 360 turns
φ = 200 μWb
Induced EMF
e=N dΦ/dt
dφ = φ2 − φ1
dφ = 200 − (−200)……(since the flux is reversing)
dφ = 400 × $10^−6$ Wb
L = 360
time (t) = 0.01 sec = $10^−2$ sec
e=( 360×400×$10^−6$ )/$10^−2$
e = 36 × 0.4 = 14.4 V
86. If `V' is voltage phasor and `I' is current phasor, then VI represents ?
87. If a circular conductor carries a current of `I' ampere having radius `r' metre, then the magnetizing force at he centre of the coil is given by ?
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
88. While estimating domestic wiring cost, if the length of the PVC conduit required is 62 metres, then what is the length of the cable required ?
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.
89. In an alternator, if α · 600, then find the pitch factor ?
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
90. Which of the following decides the direction of rotation of a shaded pole motor ?
Shaded Pole Induction Motor
The shaded pole induction motor is simply a self-starting single-phase induction motor whose one of the pole is shaded by the copper ring.
The copper ring is also called the shaded ring.
This copper ring act as a secondary winding for the motor.
The shaded pole motor rotates only in one particular direction, and the reverse movement of the motor is not possible.
The direction of rotation is from the unshaded side to the shaded (ring) side of the pole.
Since the phase angle between the shaded and unshaded sections is small, shaded-pole motors produce only a small starting torque relative to torque at full speed.
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