UPPCL JE Electrical question paper with Explanation 2018

Electromagnets use electricity to create their power of magnetism. Electricity moving along the straight wire creates a magnetic field. This magnetic field gets weaker as it moves away from the wire. But if the wire is coiled, the magnetic field is forced back toward the wire and it becomes stronger. Electromagnets are made of coils of wire called as solenoids.

A current through the wire creates a magnetic field which is concentrated in the hole in the center of the coil. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful magnet.

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be quickly changed by controlling the amount of electric current in the winding. However, unlike a permanent magnet that needs no power, an electromagnet requires a continuous supply of current to maintain the magnetic field.

Electromagnets are widely used as components of other electrical devices, such as motors, generators, electromechanical solenoids, relays, loudspeakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment. Electromagnets are also employed in the industry for picking up and moving heavy iron objects such as scrap iron and steel.

Direct-axis and Quadrature-axis

The total mmf in a synchronous machine may be split up into two components—one along the pole axis or direct axis and the other at right angles to this or the quadrature axis. In the case of cylindrical rotor machines, the air gap is uniform and the reluctance of the magnetic circuit along both the axis is the same. The effect of armature reaction fluxes and the voltages induced constant for all the positions of field poles with respect to the armature.

In the case of salient pole construction, however, the reluctances of the magnetic circuits on which the mmfs act are different along the direct axis and the quadrature axis. The reluctance of the direct axis magnetic circuit is due to the yoke and teeth of the stator, airgap, pole, and core of the rotor. In the quadrature axis, the reluctance is mainly due to the large airgap in the interpolar space.

The armature currents produce the same fundamental mmf wave regardless of the angular, position of the rotor, but the fundamental flux varies with the rotor position. If the rotor is rotated that the pole axis or the direct axis stays in line with the crest of the rotating mmf, wave lo reluctance or high permeance is offered and the fundamental flux wave has its greatest magnitude for a given armature current. Air has more reluctance than any laminations of cast iron/silicon steel. It is clear that in d-axis, the air gap is less. If the air gap is less, the reluctance is low. Similarly, the air gap in the q-axis is more hence the reluctance is high.

As the air gap is nonuniform, the reactance offered also varies and hence current drawn the armature also varies cyclically at twice the slip frequency.

The r.m.s. current is minimum when machine reactance is Xd and it is maximum when machine reactance is Xq. As the reactance offered varies due to the non-uniform air gap, the voltage drops also varies cyclically. Hence the impedance of the alternator also varies cyclically. The terminal voltage also varies cyclically. The voltage at terminals is maximum when current and various drops are minimum while the voltage at terminals is minimum when current and various drops are maximum. The waveforms of the voltage induced in the rotor, terminal voltage and current drawn by armature are shown in the Figure below.

The direct axis and the quadrature axis synchronous reactances can be defined as follows:

The direct axis synchronous reactance is the ratio of the fundamental component of the reactive armature voltage due to the fundamental direct axis component of armature current to this component of current under steady-state conditions and at the rated frequency. Direct axis air gap between rotor and stator is less than that of the quadrature axis air gap, so the current required for magnetization is less in case of the direct axis.

X_d=V_max/I_min

The quadrature axis synchronous reactance is the ratio of the fundamental component of reactive armature voltage due to the fundamental direct axis component of armature current to this component of current under steady-state conditions and at the rated frequency. When the rotor and stator rotating magnetic field aligns along with the quadrature axis, air gap reluctance is maximum. Therefore more magnetizing current is drawn by the stator to maintain the flux and the voltage is minimum.
∴ Quadrature axis synchronous reactance Xq = Vmin / Imax.

Expression of current in RL circuit as shown in the figure is

$\begin{array}{l}i(t) = \dfrac{V}{R}\left( {1 – {e^{ – t\frac{R}{L}}}} \right)\\\\100 \times {10^{ – 3}} = \dfrac{{200}}{{20}}\left( {1 – {e^{ – t\frac{{20}}{{0.5}}}}} \right)\end{array}$

Solving the value of t

t = 100.5 μs

The synchronous motor is a truly constant speed motor. This is the specialty of this motor. Yet, it has very limited applications. To develop a steady torque, its rotor must be rotating at synchronous speed, Ns. This is the major defect of synchronous motors. Either it runs at synchronous speed, or it does not run at all.

The stator field rotates at synchronous speed due to the three-phase currents supplied to its windings.  In order to develop a continuous unidirectional torque, it is necessary that the stator and rotor poles do not move with respect to each other. This is possible only if the rotor also rotates at the synchronous speed. Therefore Magnetic Locking between the poles is necessary to do so.

The concept of Magnetic Locking

• Synchronous motor work on the principle of magnetic locking.
• When two unlike strong unlike magnets poles are brought together, there exists a tremendous force of extraction between those two poles. In such condition, the two magnets are said to be magnetically locked.

• If now one of the two magnets is rotated, the other magnets also rotate in the same direction with the same speed due to the strong force of attraction.
• This phenomenon is called as magnetic locking 

For magnetic locking condition, there must be two unlike poles and magnetic axes of this two poles must be brought very nearer to each other.

• Consider a synchronous motor whose stator is wound for 2 poles.
• The stator winding is excited with 3 phase A.C supply and rotor winding with D.C supply respectively. Thus two magnetic fields are produced in the synchronous motor.
• When the 3 phase winding is supplied by 3 phase A.C supply than the rotating magnetic field or flux is produced.
• This magnetic field or flux rotates in a space at a speed called synchronous speed.
• When the rotor speed is about synchronous, stator magnetic field pulls the rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked. Then the rotor continues to rotate with a speed equal to synchronous speed.
• The rotating magnetic field or rotating flux has fixed relationship between, the number of poles, the frequency of a.c supply and the speed of rotation.
• The rotating magnetic field creates an effect which is similar to the physical rotation of magnets in space with a synchronous speed.
• So for rotating magnetic field
  
  Where f = supply frequency
  P = Number of poles

• Suppose the stator poles are N₁ and S₁ which are rotating at a speed of N_s  and the direction of rotation be clockwise.
• When the field winding on a rotor is excited by the D.C source, it produces the two stationary poles i.e N₂ and S_2.
• To establish the magnetic locking between the stator and rotor poles the, unlike poles N1 and S2 or N2 and S1 should be brought near to each other.
• As stator poles are rotating and due to magnetic locking the rotor poles will rotate in the same direction of rotating magnetic field as that of stator poles with the same speed N_s.
• Hence synchronous motor rotates at only one speed that is synchronous speed.
• The synchronous speed depends on the frequency therefore for constant supply frequency synchronous motor speed will be constant irrespective of the load changed.

At zero speed or at any other speed lower than synchronous speed, the rotor poles rotate slower than the stator field. Therefore, in one cycle of rotation of the stator field, the N-pole of the rotor is for some time nearer to N pole of the stator and for some other time nearer to the S-pole of the stator. As a result, the torque developed is for some time clockwise and for some other time anticlockwise. Consequently, the average torque developed remains zero. Hence the Synchronous Motor Run either on Synchronous Speed Or Not at all.

Eddy current loss: In transformer we provide alternating current in primary which produces alternating flux in core ,this flux links with secondary of transformer and induces emf in secondary, it may be possible that flux also links with some other conducting parts of transformer such as  ferromagnetic core or iron body and induces local emf in these parts of transformer which will cause a circulating current to flow in these parts causing heat loss. these currents are called eddy current and this loss is called eddy current loss.

Eddy current losses in the transformer are given by

Eddy current losses = K_e × B_m² × f² × V ×t²

Where 
K_e = Eddy current constant 
t = thickness of the core
V = Volume of material 
B_m = Maximum flux density
f = frequency 

Synchronous condenser or capacitor

A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behave as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

Now let V is the voltage applied and I_L is the current lagging V by angle φ_L. This power factor φ_L, is very low, lagging.

The synchronous motor acting as a synchronous condenser is now connected across the same supply. This draws a leading current of I_m.

The total current drawn from the supply is now phasor sum of I_L and I_m. This total current I now lags V by smaller angle φ due to which effective power factor gets improved. This is how the synchronous motor as a synchronous condenser is used to improve the power factor of the combined load. 

Advantages

1. A synchronous condenser has an inherently sinusoidal waveform and the voltage does not exist.
2. It can supply as well as absorb kVAr.
3. The PF can be varied in smoothly.
4. It allows the overloading for short periods.
5. The high inertia of the synchronous condenser reduces the effect of sudden changes in the system load and improves the stability of the system.
6. It reduces the switching surges due to sudden connection or disconnection of lines in the system.
7. The motor windings have the high thermal stability to short-circuit currents.
8. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless control of power factor.
9. The faults can be removed easily.

Ferromagnetic materials the most useful magnetic materials. These derive their name from iron (Ferrum) as the most common of the ferromagnetic materials. The relative permeability of ferromagnetic materials is much larger than 1 and can be in the thousands or higher. Some typical ferromagnetic materials are iron, cobalt, and nickel.

Ferromagnetic materials tend to magnetize in the direction of the magnetic field and some of them retain this magnetize lion after the external magnetic field has been removed. When they do so, and the magnetization is permanently retained, the material becomes a permanent magnet. An additional important property of ferromagnetic materials is the dependence o magnetization on the level of the external field. Thus, magnetization in ferromagnetic materials is a nonlinear process.

When a ferromagnetic substance is kept in the magnetic field, the permanent alignment of the domain due to a strong interaction (force) takes place between adjacent atoms in regions called magnetic domains. In these domains, large numbers of atoms are aligned parallel to each other so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnetized state, the domains are nearly randomly organized and the net magnetic field is zero.

 ANTI-FERROMAGNETIC MATERIALS

If the alignment of the dipoles is compensatory so as to give zero net moments, the material is termed anti-ferromagnetic. Such a phenomenon is termed anti-ferromagnetism ). For example, MnO is anti-ferromagnetic.

FERRIMAGNETIC MATERIALS

When the magnetic dipoles are oriented in the parallel and antiparallel direction in unequal numbers, so that there is a net magnetic moment, ferrimagnetism is observed. 

All ferromagnetic, anti-ferromagnetic and ferrimagnetic substances transform to the paramagnetic material at certain characteristic temperatures. This happens due to the randomization of the spins at higher temperatures.

The power triangle or Impedance triangle of the AC circuit is shown Below

Cosφ = Base ⁄ Hypotenuse = Active Power  ⁄ Apparent Power

Active Power = Apparent power × Cosφ

Apparent Power = Active Power/Cosφ

Active Power = 120 W

Power factor = 0.68

Apparent Power = 120/0.68 = 176.47 VA

Luminous flux of the lamp F = 4πL

Where L = Luminous Intensity in candela

F = 4 × π × L ( taking π = 3.14)

F = (4 × 45 × 3.14) = 565.2 Lumen

GB stands for Gigabyte and 1 Gigabyte is equal to 1024 MB