Q. 1
Electrical Engineering
Difficulty: Easy
(1 Mark)
Permanent Magnet Moving Coil (PMMC) instruments can be used to measure:
💡
Step-by-Step Explanation & Concept Rationale
PMMC instruments operate on Lorentz force (Td = B*I*N*A). With AC current, the deflecting torque reverses direction every half-cycle, resulting in zero average torque (pointer merely vibrates around zero). Therefore, PMMC inherently measures only DC (average value).
Q. 2
Electrical Engineering
Difficulty: Easy
(1 Mark)
Why is the scale of a PMMC measuring instrument perfectly linear and uniform?
💡
Step-by-Step Explanation & Concept Rationale
In a PMMC instrument with cylindrical soft-iron core and curved pole shoes, magnetic flux density B is uniform and radial. Deflecting torque Td = B*N*A * I. Since spring control torque Tc = C * theta, at balance Td = Tc -> theta ∝ I, resulting in a strictly linear, evenly-spaced scale.
Q. 3
Electrical Engineering
Difficulty: Easy
(1 Mark)
Moving Iron (MI) instruments (attraction or repulsion type) can measure:
💡
Step-by-Step Explanation & Concept Rationale
In moving iron instruments, deflecting torque is proportional to the square of current (Td ∝ I^2 * dL/dtheta). Because I^2 is always positive regardless of current direction, deflecting torque is unidirectional for both AC and DC, indicating true RMS value.
Q. 4
Electrical Engineering
Difficulty: Easy
(1 Mark)
Why is the scale of a Moving Iron instrument non-linear (cramped at the lower end and spread out at the upper end)?
💡
Step-by-Step Explanation & Concept Rationale
Because deflecting torque varies with current squared (Td ∝ I^2), deflection theta ∝ I^2. Thus, the scale follows a square-law distribution, cramped closely at low current values and open at higher current values.
Q. 5
Electrical Engineering
Difficulty: Easy
(1 Mark)
To convert a basic PMMC galvanometer with internal resistance Rm into an ammeter capable of measuring higher currents, one must connect:
💡
Step-by-Step Explanation & Concept Rationale
To extend current range, a low-resistance shunt Rsh is connected in parallel with the meter: Rsh = Rm / (m - 1), where multiplying factor m = I / Im. The majority of load current bypasses the delicate meter movement through the low-resistance shunt.
Q. 6
Electrical Engineering
Difficulty: Easy
(1 Mark)
To extend the voltage measurement range of a basic PMMC meter movement to form a voltmeter, one must connect:
💡
Step-by-Step Explanation & Concept Rationale
To extend voltage range, a high resistance multiplier Rs is connected in series: Rs = Rm * (m - 1), where m = V / Vm. The series resistor drops the bulk of the applied voltage, limiting current through the coil to its safe full-scale deflection value Im.
Q. 7
Electrical Engineering
Difficulty: Medium
(1 Mark)
What material is standard for manufacturing ammeter shunts and voltmeter multiplier resistors due to its near-zero temperature coefficient of resistance?
💡
Step-by-Step Explanation & Concept Rationale
Manganin has a negligible temperature coefficient of resistance (approx. 0.00001 / deg C) and low thermal EMF against copper. This ensures that instrument calibration does not drift with ambient or I^2*R self-heating temperature changes.
Q. 8
Electrical Engineering
Difficulty: Medium
(1 Mark)
Which AC bridge is specifically used for the precision measurement of an unknown capacitance and its dielectric dissipation factor (tan delta)?
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Step-by-Step Explanation & Concept Rationale
The Schering Bridge is the industry-standard bridge for measuring capacitance and dielectric loss angle (dissipation factor tan delta = omega * C1 * r1) of high-voltage cables, transformer bushings, and insulating materials.
Q. 9
Electrical Engineering
Difficulty: Medium
(1 Mark)
Maxwell's Inductance-Capacitance Bridge is most suitable for measuring the inductance of coils having:
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Step-by-Step Explanation & Concept Rationale
Maxwell's L-C bridge balances an unknown inductance against a standard variable capacitor. It is ideal for medium-Q coils (1 to 10). For high-Q coils (Q > 10), Hay's Bridge is used; for low-Q coils (Q < 1), Anderson's Bridge is preferred.
Q. 10
Electrical Engineering
Difficulty: Medium
(1 Mark)
Hay's Bridge is preferred over Maxwell's Bridge for measuring inductances with:
💡
Step-by-Step Explanation & Concept Rationale
In Hay's bridge, the resistance and capacitance in the standard arm are connected in series rather than parallel. The balance equation contains term (1 / [1 + (1/Q)^2]), which simplifies cleanly for high-Q coils (Q > 10) where (1/Q)^2 is negligible.
Q. 11
Electrical Engineering
Difficulty: Hard
(1 Mark)
Anderson's Bridge is an advanced modification of Maxwell's Bridge designed to measure:
💡
Step-by-Step Explanation & Concept Rationale
Anderson's bridge introduces an additional resistance in the capacitor branch, allowing smooth independent balance adjustments for low-Q coils while utilizing a fixed standard capacitor instead of an expensive calibrated variable capacitor.
Q. 12
Electrical Engineering
Difficulty: Medium
(1 Mark)
Wien's Bridge is widely utilized in audio instrumentation and measurement for:
💡
Step-by-Step Explanation & Concept Rationale
Wien's bridge balance condition is f = 1 / (2 * pi * sqrt(R1 * R2 * C1 * C2)). When R1 = R2 = R and C1 = C2 = C, f = 1 / (2 * pi * R * C). It is widely used as a precision frequency meter and distortion analyzer.
Q. 13
Electrical Engineering
Difficulty: Medium
(1 Mark)
Kelvin's Double Bridge is the standard laboratory instrument for precision measurement of:
💡
Step-by-Step Explanation & Concept Rationale
Kelvin's Double Bridge incorporates a second set of ratio arms. When the two sets of ratio arms are matched (P/Q = p/q), the effect of connecting lead resistance (r) between standard and unknown low resistors is mathematically eliminated, measuring resistances down to 0.00001 ohm.
Q. 14
Electrical Engineering
Difficulty: Medium
(1 Mark)
A Megger (insulation resistance tester) incorporates a high-voltage hand-driven generator or DC-DC converter and an ohmmeter movement based on a:
💡
Step-by-Step Explanation & Concept Rationale
A Megger utilizes a ratio meter movement with two coils (current coil and pressure coil) wound on a common axle in a non-uniform magnetic field. Deflection depends strictly on the ratio of V to I (R = V/I), making the reading independent of hand-cranking generator voltage fluctuations.
Q. 15
Electrical Engineering
Difficulty: Medium
(1 Mark)
The Guard Terminal (G) on a 3-terminal Megger is provided to:
💡
Step-by-Step Explanation & Concept Rationale
Surface moisture and dirt create surface leakage currents that bypass bulk insulation. Wrapping a bare wire around the cable sheath and connecting it to the Guard terminal intercepts surface leakage and diverts it around the current-measuring coil.
Q. 16
Electrical Engineering
Difficulty: Easy
(1 Mark)
Creeping in an induction-type energy meter refers to:
💡
Step-by-Step Explanation & Concept Rationale
To overcome mechanical pivot friction, shading loops on the shunt magnet provide a small forward driving torque. If over-compensated or if supply voltage rises, this friction-compensating torque causes the disc to rotate continuously (creep) on no-load.
Q. 17
Electrical Engineering
Difficulty: Easy
(1 Mark)
How is creeping prevented in single-phase induction energy meters?
💡
Step-by-Step Explanation & Concept Rationale
Two diametrically opposite holes drilled in the disc cause reluctance variation. When a hole arrives beneath the shunt magnet pole, the disc experiences a reluctance counter-torque that arrests no-load rotation.
Q. 18
Electrical Engineering
Difficulty: Medium
(1 Mark)
Braking torque in an induction-type energy meter is provided by a permanent magnet and is directly proportional to:
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Step-by-Step Explanation & Concept Rationale
As the aluminum disc rotates across the field of the permanent brake magnet, eddy currents are induced in the disc proportional to speed N. These eddy currents interact with the permanent field to generate a braking torque Tb = K * B^2 * N * d. At steady-state, Td = Tb, ensuring disc speed is strictly proportional to power.
Q. 19
Electrical Engineering
Difficulty: Easy
(1 Mark)
An electrodynamometer wattmeter can measure:
💡
Step-by-Step Explanation & Concept Rationale
The electrodynamometer wattmeter has fixed coils carrying load current (I) and a moving pressure coil carrying current proportional to voltage (V). Deflecting torque Td = (1/R) * V * I * (dM/dtheta) * cos(phi), accurately indicating true active average power in Watts for both AC and DC.
Q. 20
Electrical Engineering
Difficulty: Easy
(1 Mark)
Why is power factor improvement economically essential for industrial consumers?
💡
Step-by-Step Explanation & Concept Rationale
For constant real power P = V*I*cos(phi), line current is I = P / (V * cos(phi)). A low power factor (e.g., 0.6 lagging) forces the system to supply 67% more current than at unity PF, overloading cables and transformers, increasing I^2*R losses, causing excessive voltage drop, and incurring heavy utility billing penalties.
Q. 21
Electrical Engineering
Difficulty: Easy
(1 Mark)
Static shunt capacitors improve the power factor of inductive loads by:
💡
Step-by-Step Explanation & Concept Rationale
Inductive loads (induction motors, transformers) require lagging reactive power Q_L to maintain magnetic fields. Shunt capacitors draw leading reactive current (Q_C = V^2 * omega * C), supplying these magnetizing VARs locally so they do not have to be transmitted from the distant generating station.
Q. 22
Electrical Engineering
Difficulty: Medium
(1 Mark)
The rating of a shunt capacitor bank required to improve load power factor from cos(phi1) to cos(phi2) for an active load P (in kW) is given by:
💡
Step-by-Step Explanation & Concept Rationale
Initial reactive power is Q1 = P * tan(phi1). Desired final reactive power is Q2 = P * tan(phi2). The leading reactive kVAR required from the shunt capacitor bank is Q_cap = Q1 - Q2 = P * [tan(phi1) - tan(phi2)] kVAR.
Q. 23
Electrical Engineering
Difficulty: Hard
(1 Mark)
Phase advancers are installed on large slip-ring induction motors to improve their power factor by:
💡
Step-by-Step Explanation & Concept Rationale
A phase advancer is an AC exciter connected to the rotor circuit of a slip-ring motor. Injecting leading MMF at rotor slip frequency provides the motor's magnetizing ampere-turns from the rotor rather than the stator, raising stator power factor to near unity or even leading.
Q. 24
Electrical Engineering
Difficulty: Medium
(1 Mark)
A Cathode Ray Oscilloscope (CRO) uses an electrostatic deflection system where deflection sensitivity is defined as:
💡
Step-by-Step Explanation & Concept Rationale
Deflection sensitivity S = D / Vd = (L * l) / (2 * d * Va), where D is trace deflection, Vd is deflecting voltage, L is screen distance, l is plate length, d is plate separation, and Va is accelerating anode potential. Its inverse is deflection factor G = 1 / S (V/mm).
Q. 25
Electrical Engineering
Difficulty: Easy
(1 Mark)
Lissajous figures displayed on an oscilloscope screen when two sinusoidal signals of identical frequency are applied to the horizontal and vertical plates form a circle when:
💡
Step-by-Step Explanation & Concept Rationale
Applying x = A * sin(omega*t) and y = B * sin(omega*t + phi) with A = B yields x^2 + y^2 = A^2 when phi = 90 or 270 degrees, tracing a perfect circle. A phase difference of 0 or 180 degrees traces a straight line.
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