RRB JE Electrical (CBT II) Mock Test- 5 - Electrical Engineering (EE) MCQ

The total reluctance is equal to the ratio of the MMF in a passive magnetic circuit and the magnetic flux in this circuit. In an AC field, the reluctance is the ratio of the amplitude values for a sinusoidal MMF and magnetic flux.
R = F/Φ

On December 30, 1906, Muslim league was formed under the leadership of Aga Khan, the Nawab of Dhaka.

The torque equation of a three-phase induction motor is given by,

Torque is maximum when slip, s = R2/X2
At this condition, maximum torque is given by

The maximum torque is independent of slip.

Given that, line current (I_L) = 51 A
Shunt field resistance (R_sh) = 120 Ω
Voltage (V) = 120 V
Shunt current (I_sh) = 120/120 = 1 A
Armature current (l_a) = I_L - I_sh = 51 - 1 = 50 A

When the voltage of a circuit is measured by a voltmeter whose input impedance is low as compared to the output impedance of the circuit, the instrument gives less accurate results. This is known as the error due to the loading effect.

Belted Cables: The conductors (usually three) are bunched together and then bounded with an insulating paper ‘belt’. In such cables, each conductor is insulated using paper impregnated with a suitable dielectric. The gaps between the conductors and the insulating paper belt are filled with a fibrous dielectric material such as Jute or Hessian.

Maximum power transfers to R_L­ when R­_L = R_th

To find Thevenin's equivalent resistance, we need to short circuit the voltage source and find the equivalent resistance across load resistance.

R_L = R_th = 2 + 3 = 5 Ω

In DC machines, maximum efficiency occurs when variable losses are equal to constant losses.

Variable losses = Constant losses

Constant losses include windage losses, brush losses, hysteresis losses and eddy current losses.

A single phase 3 wire AC system is made up of 2 conductor wire and 1 neutral wire

A single phase 2 wire AC system is made up of 1 conductor wire and 1 neutral wire

Nonbiodegradable chemicals are most harmful environmental pollutants as these cannot be broken down into simpler, harmless substances in nature. Whereas, biodegradable pollutants can be rapidly decomposed into simpler and harmless substances by natural processes.

• At any speed other than synchronous speed, its rotor would experience an oscillating torque of zero average value as the rotating magnetic field repeatedly passes the slower moving rotor
• Normally, a short-circuited winding similar to that of an induction machine is added to the rotor to provide starting torque

We know that for 3ϕ system
 Total power = √3 V_L I_L cos Ø
∴ power factor = cos Ø = P/√3 V_L I_L = 20 x 10³/√3 x 400 x 100/√3 = 0.5 
PF = cos Ø = 0.5 leading 

In the armature-voltage control method of speed control of a DC shunt motor, as the speed is increased the torque remains constant and the power increases which is shown in above figure.

Given J is the current density at a given location in a resistive material, E is the electric field at that location, and σ is a material-dependent parameter called the conductivity, Ohm’s law can be expressed as

J = σE

Where,

j → is the current density

E → is the electric field inside the conductor

And σ is the electrical a conductivity of the material

• The process of reducing the magnetic flux density to zero is called demagnetization of the core
• As the current becomes negative, H becomes negative, but B does not become negative instantaneously
• The value of magnetic field strength required to wipe out the residual flux density is called as the coercive force H_C

An armature control method is used in the DC shunt motor.
In a DC motor, the back emf is directly proportional to speed and flux.
E_b ∝ Nϕ
Where N is speed and ϕ is flux.|
In a DC shunt motor, flux is constant.
So, the back emf is directly proportional to speed.
Important Point:
Armature resistance control:
We know that,
N ∝ E_b/ø
N ∝ V- I _a(R_a + R)/ø
We can control the speed by increasing armature resistance. This method gives only below base speeds. This method is a constant torque and variable power drive.
Field control method:
We know that
N ∝ E_b/ø
N ∝ V- I _a(R_a + R)/ø
By varying flux, we can increase the speed more than its base speed. This method is constant power and variable torque drive.
The characteristics are shown below

Voltage regulation is the change in secondary terminal voltage from no load to full load at a specific power factor of load and the change is expressed in percentage.
E2 = no-load secondary voltage
V2 = full load secondary voltage
Voltage regulation for the transformer is given by the ratio of change in secondary terminal voltage from no load to full load to no load secondary voltage.
Voltage regulation E₂- V 2/E2
It can also be expressed as,

Regulation

+ sign is used for lagging loads and
- ve sign is used for leading loads

From the above graph, the poorest voltage regulation of a transformer at full load is at 0.8 lagging power factor.

The SI unit for the magnetic moment Newton-meter/Tesla

Tesla is a unit of flux

Newton-meter is unit of torque

Important:

The magnetic moment of a magnet is a quantity that determines the torque it will experience in an external magnetic field.

It is considered to be a vector having a magnitude and direction. The direction of the magnetic moment points from the South Pole to the North Pole of the magnet.

The magnetic field produced by the magnet is proportional to its magnetic moment.

(3 + j4) and (3 – j4) are connected in parallel and this combination is in series with 17/6 Ω resistance.
Total impedance (Z) = 17/6 + [(3 + j4) ||(3 – j4)]

Concept:

Core loss (P_C) = VI­_C

I_C = I cos ϕ

V is induced emf

I is current

ϕ is phase difference between induced emf and current

Calculation:

Given that, induced emf (V) = 230 V

Current (I) = 2 A

Phase difference (ϕ) = 60°

I­_C = I cos ϕ = 2 cos 60° = 1 A

• A capacitor start capacitor run motor is used in a ceiling fan
• It essentially consists of a running winding and a starting winding
• The capacitor is connected in series with the starting winding to make it start
• Secondary winding surrounds the primary winding and at starting these both are connected in parallel

Dummy coils:

• The wave winding is possible only with number of conductor or coils.
• But sometimes the armature slots available in the armature winding do not meet the requirement of the winding. (It means that the available number of slots is more than the required number of conductors).
• Some slots are kept without armature winding in that case and dummy coil or coils are employed in that slots.
• The dummy coils are similar to other coils except their ends are cut, short and taped. They do not connect with the commutator bars.
• The dummy coils are simply to provide mechanical balance for the armature.
• As they do not connect with commutator bars, they do not affect the electrical characteristics of the winding.

The advantages of the parallel operation of DC generators are -

Continuity of service: If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will be shut down. If power is supplied from a number of small units operating in parallel, then in case of failure of one unit, the continuity of supply can be maintained by other healthy units.

Efficiency: Generators run most efficiently when loaded to their rated capacity. Electric power costs less per kWh when the generator is efficiently loaded. Therefore, when load demand on power plant decreases, one or more generators can be shut down and the remaining units can be efficiently loaded.

Initially the sodium is in the form of a sloid, deposited on the walls of inner tube. When sufficient voltage is impressed across the electrodes, the discharge starts in the inert gas (neon). It operates as a low-pressure neon lamp with pink colour.