UPPCL JE Electrical question paper with Explanation 2018

Fleming’s left-hand rule.

 

• When a current carrying conductor such as a wire attached to a circuit moves placed in a magnetic field, an electric current is induced in the wire due to Faraday’s law of induction.
• The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other than
• The thumb is pointed in the direction of the motion of the conductor relative to the magnetic field i.e direction of the force.
• The forefinger is pointed in the direction of the magnetic field.
• The middle finger represents the direction of the induced or generated current within the conductor.

Thyristor protection or SCR Protection 

The transient overvoltages can switch on the thyristor. In some cases, the thyristor can be damaged due to these transient voltages. These transient voltages are very common when the converter is having inductive loads.

Snubber circuit: The thyristors can be protected against transient voltages by an RC network This RC network is connected in parallel across the thyristor. It is called snubber circuit.

The Fig. shows snubber(RC) circuit. The resistance has the value of few hundred ohms. Whenever there is a large spike or voltage transient across the thyristor, it is absorbed by the RC circuit. The RC circuit (snubber) acts as a lowpass filter for this voltage transient.A low-pass filter (LPF) is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency.

The resistance has normally low value so that the transient is absorbed by the capacitor quickly. Thus the thyristor is protected against voltage transients. The RC snubber circuit very commonly used for protection of thyristors against transient voltage High-frequency voltage spikes).

SCR is a unidirectional device and like a diode, it allows current to flow in one direction only. It can be used as a switch. The switching of SCR is controlled by the additional input called gate and biasing conditions. It can be used as a rectifier to convert AC signals to DC signals.

Construction

The SCR is a four-layer p-n-p-n device, where p and n layers are alternately arranged. The outer layers are heavily doped. There are three p-n junctions called J₁, J₂, and J₃. The outer P-layer is called anode and outer n-layer is called cathode, middle P-layer is called the gate. The terminals are taken out from their layers. Anode must be positive with respect to the cathode so that SCR will be forward biased. To turn on an SCR, a current is to be passed through the gate terminal. Hence it is a current operated device.

Two Transistor Model of SCR

The operation of an SCR can also be explained in a very simple way by considering it in terms of two transistors. This is known as the two transistor analogy of the SCR. The SCR can be considered as an NPN and a PNP transistor, where the collector of one transistor is attached to the base of the other and vice versa. This model is obtained by splitting the two middle layers of the SCR into two separate parts.

In the above figure, two middle layers of the SCR is split into two separate parts i.e PNP transistor T₁ and NPN transistor T₂. The collector terminal of NPN transistor T₂ drives the base of the upper transistor PNP T₁, while the collector terminal of PNP transistor T₁ drives the base of NPN transistor T₂.

The relation between the anode current and the current gain of transistor t₁ and t₂ is given as

I_a = (α₂I_g) ⁄ [1 − (α₁ + α₂)

Where

I_a = Anode current

I_g = Gate current

α₁ = Current gain of transistor t₁

α₂ = Current gain of transistor t₂

From the above, equation if, (α₁ + α₂) = 1, then anode current becomes infinite, i.e. it suddenly attains very high value approaching infinity. The device latches into conduction (ON) state from nonconducting (OFF) state. This action of SCR is called the regenerative action. As the gate current I_G is of such a value that (α₁ + α₂) approaches unity, the SCR gets turned on.

Carborundum is also known as silicon carbide and it is used to make varistor.

The speed of the synchronous motor is given as

Ns = 120f/P

So the speed of the synchronous motor can be changed by changing either the number of poles or the frequency.

Constantan: Constantan is a copper alloy containing 40 percent nickel, 1.5 percent manganese, and the rest copper. The main feature of Constantan: is its high electrical resistivity which remains almost constant upon variations in temperature. Constantan is used for rheostats, therms couples and heating devices operating at moderately high temperatures.

Note:- 

Brass is a metallic alloy that is made of copper and zinc.

Tin-silver-copper (SnAgCu, also known as SAC), is a lead-free (Pb-free) alloy commonly used for electronic solder

Peltier Effect

In 1834, Jean Charles Athanase Peltier, a French watchmaker turned scientist discovered the thermoelectric effect which is the converse of the Seebeck effect. He discovered that when current is passed through the junction between two conductors, heat is either evolved or absorbed. When the direction of the current is the heating effect is also reversed.

The absorption or evolution of heat at a junction of two dissimilar metals when current is passed is known as Peltier effect.

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of solid materials. This effect relates to the energy transfer at the junction of two dissimilar materials when the electric current is passing through it. When the junction is maintained at a given temperature an equivalent heat addition or heat removal is present, so-called the Peltier heat, which is reversible and proportional to the electric current. Semiconductor materials are most often used as thermoelectric elements. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments. The thermoelectric device is characteristic for the lack of moving parts and is easily controlled and reversible.

Span is defined as the algebraic difference between the upper and lower range values.

Upper Limit = 200 bar

Lower Limit = 20 bar

Span = 200 − 20 = 180 bar

When 1 Coulomb charge flows through a wire in 1 second then the current through the wire is 1 AMPERE.

I = Q/T

Where I is current in A

Q is the charge and t is the time.

1 electron has a charge of 1.6 × 10⁻¹⁹ Coulombs of charge.

so 5 × 10¹⁹ electrons has a charge of 5 × 10¹⁹ × 1.6 × 10⁻¹⁹  = 8 coulomb

Current I = Q/T = 8/1 = 8 Ampere

Gauge factor (GF) or strain factor of a strain gauge is the ratio of relative change in electrical resistance R, to the mechanical strain ε.

GF=change in resistance/(resistance × strain)

Change in resistance = 5 ohm

Resistance = 152 Ohms

Strain = 5000 microstrain = 5 × 10⁻³

Gauge factor = 5/(152 × 5 × 10⁻³)

Gauge factor = 6.58