Tamil Nadu HSE Second Year Basic Electrical Engineering common mistakes
Tamil Nadu HSE Second Year Basic Electrical Engineering · Understand the traps and how to avoid them.
Adding resistances in parallel by simple summation (R = R1 + R2) instead of using the reciprocal formula, and treating a series-parallel network as if every resistor were in series.
The series rule is learned first and is reinforced by habit; candidates memorize 'add them up' without attaching it to the physical idea that parallel branches share voltage and offer extra paths, so total resistance must fall below the smallest branch.
What to do instead
Before substituting numbers, redraw the network and label each junction, then mark series pairs (same current) and parallel pairs (same voltage) explicitly. Use 1/RT = 1/R1 + 1/R2 for two branches, or the product-over-sum shortcut R1R2/(R1+R2), and sanity-check that the answer is smaller than the smallest branch resistor. Practise three-resistor mixed networks until the reduction order becomes automatic.
Applying Ohm's law to a whole circuit while ignoring the source's internal resistance, or misapplying Kirchhoff's laws by assigning the wrong sign to an emf or a voltage drop around a loop.
Textbook problems often quote terminal voltage, and candidates treat it as identical to emf; with Kirchhoff's voltage law they follow current direction inconsistently, so terms that should be subtracted get added. Sign discipline is a procedural skill that is rarely drilled separately.
What to do instead
Write terminal voltage as V = E - Ir every time a cell or battery drives current, and state I = E/(R + r) before solving. For loop equations, fix a traversal direction on the diagram, mark each emf with an arrow from negative to positive terminal, and adopt one written convention (rise = positive, drop = negative) that you apply mechanically. Check the result by verifying that power delivered by sources equals I squared R losses.
Computing equivalent capacitance with the resistor rules, or forgetting that inserting a dielectric multiplies capacitance by the relative permittivity while the stored charge or voltage behaviour depends on whether the capacitor is isolated or still connected to the supply.
Capacitors look like resistors on a diagram, and the reciprocal relationship (series capacitors add like parallel resistors) is counter-intuitive. The dielectric case has two distinct physical situations that are usually summarized in one line, so candidates apply the wrong one.
What to do instead
Memorize the inversion explicitly: capacitors in parallel add (C = C1 + C2), capacitors in series follow 1/C = 1/C1 + 1/C2. For dielectrics, first ask whether the battery is still connected (V fixed, Q rises, C becomes kC) or disconnected (Q fixed, V falls to V/k, C becomes kC). Track energy as half C V squared and confirm it changes consistently with the situation you assumed.
Using the wrong hand rule or the wrong angle when finding the force on a current-carrying conductor, the direction of induced emf, or the flux linkage of a coil, and dropping the number of turns N from the emf formula.
Fleming's left-hand and right-hand rules are visually similar and are often memorized as pictures without the mapping to motor and generator action. The sine of the angle between conductor and field is frequently replaced by the angle given in the question without checking what it measures.
What to do instead
Anchor each rule to its application: left hand for motor effect (force), right hand for generator effect (induced emf), and state the mapping aloud (thumb = motion or force, first finger = field, second finger = current). Always write e = -N dPhi/dt and F = BIl sin(theta), identifying theta as the angle between the conductor and the field, so a conductor parallel to the field gives zero force. Verify direction with Lenz's law as a cross-check.
Treating impedance as a plain arithmetic sum (Z = R + XL + XC) rather than a phasor combination, and reporting the power factor as cos of the wrong angle or as the ratio of reactance to impedance.
Impedance is introduced right after DC resistance, and the phasor diagram is often drawn once and not practised. Candidates also confuse the phase angle of the impedance with the phase difference between supply voltage and current when both R and C or L are present.
What to do instead
Draw the impedance triangle for every AC problem: R on the horizontal, the net reactance (XL - XC) on the vertical, and Z as the hypotenuse, so Z = sqrt(R squared + (XL - XC) squared) and tan(phi) = (XL - XC)/R. Compute XL = 2 pi f L and XC = 1/(2 pi f C) first, note which dominates to decide whether the circuit is inductive or capacitive, then state power factor = R/Z = cos(phi) with the lead/lag description.
Mixing line and phase quantities in three-phase problems, for example applying VL = sqrt(3) Vph to a delta-connected load, or computing total three-phase power as Vph times Iph without the sqrt(3) factor for a star load.
Star and delta relationships are mirror images, and the two sets of formulas are memorized side by side without a diagram. Candidates also forget that the given supply voltage is almost always the line voltage.
What to do instead
Sketch the connection before calculating. For star: Iline = Iphase and Vline = sqrt(3) Vphase. For delta: Vline = Vphase and Iline = sqrt(3) Iphase. Use total power = sqrt(3) Vline Iline cos(phi) for a balanced load regardless of connection, and confirm by computing per-phase power and multiplying by three. State which quantity the question gives before substituting.
Assuming a transformer's secondary voltage follows the turns ratio exactly under load, or confusing the current ratio with the voltage ratio, and treating copper loss and iron loss as interchangeable when finding efficiency.
The ideal transformer equation is learned first and then applied to real machines; candidates forget that leakage reactance and winding resistance cause regulation, and they invert the current ratio because 'step-up' suggests everything increases.
What to do instead
Write both ratios together: V2/V1 = N2/N1 = I1/I2, noting that voltage and current move in opposite directions. For efficiency use output/(output + copper loss + iron loss), remembering iron loss is approximately constant at fixed voltage and frequency while copper loss varies as the square of load current. For regulation, compare no-load and full-load secondary terminal voltages and express the difference as a percentage of the no-load value.
Confusing the functions of earth wire, neutral and live, and selecting fuse or earthing conductor sizes by guesswork rather than by the current rating of the circuit; also describing earthing as a return path for normal operating current.
In domestic wiring the neutral is bonded to earth at the supply, so the two appear connected on a diagram; candidates then conclude that earth carries current in normal operation and underrate its importance as a safety conductor.
What to do instead
State the role of each conductor explicitly: live brings current at high potential, neutral completes the circuit and is near earth potential, earth carries current only under fault conditions and must have low resistance to operate the protective device quickly. Size fuses and MCBs above the normal load current but below the cable's current-carrying capacity, and describe the earth conductor as providing a low-resistance fault path that causes the fuse to blow.
Reading an analog meter without accounting for the range multiplier, parallax error, or the difference between deflection-type and null-type instruments, and quoting ammeter or voltmeter readings as if the meter were ideal.
Scale reading is treated as a trivial step, so the multiplying factor marked on the range switch is ignored, and the effect of meter resistance on the circuit is never considered because ideal meters are assumed in earlier problems.
What to do instead
For every reading, record three things: the scale value at the pointer, the range multiplier, and the least count. Position the eye directly above the pointer to avoid parallax. Add the meter's own resistance to the circuit when accuracy matters: an ammeter's low resistance is in series and a voltmeter's high resistance is in parallel, so state whether the reading is slightly low and why. For bridge and potentiometer questions, identify them as null methods where no current is drawn at balance.
Describing battery capacity and state of charge without reference to discharge rate, and confusing the action of a rectifier diode with that of a zener diode or a transistor in a regulated power supply.
Capacity in ampere-hours is quoted as a single number, so candidates treat it as fixed regardless of current drawn; similarly, all diodes are treated as identical one-way valves, ignoring reverse breakdown as the intended operating region of a zener.
What to do instead
State capacity as ampere-hours at a specified discharge rate, and use the relationship capacity equals current multiplied by time to find backup duration, noting that higher currents reduce usable capacity. For supplies, describe the rectifier diode as conducting in forward bias and blocking in reverse, the zener as operating in controlled reverse breakdown to hold a steady voltage, and the filter capacitor as smoothing the ripple. Trace the signal path from transformer to rectifier to filter to regulator when explaining a circuit.