MPPSC SES Electrical common mistakes
MPPSC SES Electrical · Understand the traps and how to avoid them.
Deriving transformer EMF, regulation and efficiency from first principles each time instead of applying the standard relations, then losing 3–4 minutes per numerical and making sign or ratio errors under time pressure.
Candidates memorise only the open-circuit and short-circuit test procedures but not the compact result forms (E1/E2 = N1/N2, regulation ≈ (I·R cosφ ± I·X sinφ)/V, η = output/(output + Pcu + Pfe) with Pcu scaling as the square of load fraction). Without the packaged forms they rebuild every step and drift on lagging versus leading sign conventions.
What to do instead
Build a one-page formula card per machine: EMF equation, voltage regulation with + for lagging and − for leading, condition for maximum efficiency (Pcu = Pfe), all-day efficiency, and SCA/OCA test calculations. Practise 15 numericals per formula until each takes under 90 seconds, and always state the load power factor sign explicitly before substituting.
Applying the wrong per-unit base or forgetting to change base when referring quantities across a transformer in a power system single-line diagram, producing protection settings and fault currents off by the turns ratio squared.
Per-unit analysis is taught in the machines course and in the power systems course with different base conventions; candidates carry impedance values from one base (kV, MVA) to another without rescaling, and mix ohmic and per-unit values in the same network reduction.
What to do instead
Adopt one rule: Z_pu,new = Z_pu,old × (S_new/S_old) × (V_old/V_new)². Before any fault study, tabulate every component with its base MVA and base kV, convert all to a single common base, and only then reduce the network. Do 10 fault problems (symmetrical and LG/LL/LLG) starting from a raw single-line diagram and check that the total per-unit impedance lies in a plausible range.
Confusing the role of the field winding and armature in DC machines during motoring versus generating, so back-EMF direction, torque direction and the effect of field weakening are answered with reversed signs.
The same machine equations (E = kφω, T = kφIa) are used for both modes, and candidates memorise 'opposes supply' without tracking which quantity is the cause. Speed-control questions on field weakening then get answered as 'speed falls' rather than 'speed rises'.
What to do instead
Anchor on one sentence: in a motor the back-EMF opposes the applied voltage and is the regulating variable; in a generator the induced EMF drives the current. Write the armature circuit equation V = E + IaRa for the motor and E = V + IaRa for the generator side by side, then work through field weakening (φ↓ so ω↑ for fixed V) and armature control (V↓ so ω↓) as two distinct experiments.
Treating the synchronous machine V-curves, power-angle relation and excitation effects as isolated facts, so questions on over-excited/under-excited operation, synchronising power and hunting are answered by guesswork.
Candidates learn the OCC and SCC tests but never connect excitation to reactive power flow. They cannot state whether an over-excited alternator on infinite bus delivers lagging or leading VARs, and they confuse the power-angle curve with the load characteristic.
What to do instead
Draw the phasor diagram for one machine at unity, lagging and leading power factor with the infinite bus fixed, and derive P = (EV/X) sinδ yourself once. Then map excitation to VARs: over-excited = supplies lagging VARs (acts like a capacitor to the system). Rehearse the V-curve shape (armature current versus field current) and the meaning of the stability limit at δ = 90°.
Solving control-system problems by substituting numbers into the Routh array or root-locus rules without first checking the standard form (monic characteristic polynomial, positive leading coefficient), so the number of right-half-plane poles is miscounted.
Routh–Hurwitz requires the polynomial in descending powers with a positive leading coefficient; candidates feed in a negative or non-monic polynomial and read the sign-change count from the wrong column. Special cases (zero first-column element, row of zeros) are skipped entirely.
What to do instead
Always normalise the characteristic polynomial to monic form before building the array, and handle the two special cases explicitly: replace a zero first element with ε and take the limit, and for a row of zeros use the auxiliary polynomial from the previous row and differentiate it. Verify every Routh answer by a quick root-locus sketch or by checking the closed-loop poles numerically.
Designing or analysing a converter with the wrong conduction mode assumption, applying continuous-conduction transfer ratios (buck: D, boost: 1/(1−D), buck-boost: D/(1−D)) to a discontinuous-conduction circuit and ignoring the diode drop and inductor ripple.
Textbook derivations assume ideal switches and continuous conduction; candidates memorise the three ratios as a table and never verify the boundary condition or the ripple equation. They also forget that the average inductor voltage over a cycle must be zero, which is the real derivation tool.
What to do instead
Derive each ratio once using volt-second balance on the inductor and charge balance on the capacitor, and note the validity condition for continuous conduction. For a given L, f, and load, compute the critical inductance or critical load and check the mode before choosing a ratio. Include switch and diode drops as a fixed voltage offset in the balance equation.
Using the wrong instrument for the measurement: connecting an ammeter in parallel or a voltmeter in series, using a wattmeter with the current coil on the wrong side, or reading an energy meter constant without converting revolutions to kWh correctly.
Candidates learn instrument types by name but not their connection topology or their loading effect. The wattmeter pressure-coil/current-coil polarity and the two-wattmeter method sign rule for power factor below 0.5 are frequent blind spots, as is the CT/PT ratio in high-voltage measurement.
What to do instead
For every instrument write its ideal impedance (ammeter ≈ 0, voltmeter ≈ ∞), its connection, and its loading error. Drill the two-wattmeter method: total power = W1 + W2, power factor = cos(tan⁻¹(√3(W1 − W2)/(W1 + W2))), and a negative reading means pf < 0.5. For energy meters, verify the meter constant K = revolutions per kWh and convert with kWh = revolutions/K.
Mixing up microprocessor and microcontroller architecture facts and instruction-level behaviour: addressing modes, stack operation, interrupt vector handling, and the difference between memory-mapped and I/O-mapped peripherals.
The syllabus spans 8085/8086 and 8051, and candidates learn pin diagrams without the execution model. They cannot trace a PUSH/POP sequence through the stack pointer, confuse the CALL/RET pair, and cannot say how an interrupt saves the program counter.
What to do instead
For each processor, write the register set, the stack growth direction, the interrupt sequence (save PC, jump to vector), and the timing of one instruction cycle. Trace small hand-written programs: a subroutine call with nested PUSH/POP, a delay loop with cycle counting, and one interrupt service routine. Tabulate 8085 versus 8051 differences (multiplexed bus, on-chip RAM, ports, timers) in a single comparison table.
Skipping the sign and boundary conventions in electromagnetic field problems (Gauss's law surface orientation, Ampere's law direction, boundary conditions at a dielectric interface, and Poynting vector direction), leading to answers with the right magnitude but the wrong sign or the wrong normal component.
Field problems are geometry-heavy and candidates jump to the formula without drawing the surface or the closed loop. They also forget that the tangential E and normal B are continuous across a boundary while normal D and tangential H jump by surface charge and surface current.
What to do instead
Before substituting, sketch the Gaussian surface or Amperian loop and mark the outward normal and the integration direction. Memorise the four boundary conditions as a two-by-two table (E_t continuous, D_n jumps by σs, H_t jumps by K, B_n continuous) and test each answer against them. For Poynting vector problems, confirm E × H points in the direction of power flow.
Treating engineering aptitude, general studies and ethics as a last-week reading task, so answers on work culture, professional responsibility, environmental policy and Madhya Pradesh-specific schemes are given from general impression rather than from a defensible principle.
These sections are perceived as non-technical and low-yield, so candidates defer them; they then face situational-judgement items on conflict of interest, public safety versus cost, and ethical use of authority without any prepared framework, and they cannot recall state-specific facts that carry easy marks.
What to do instead
Prepare a short ethics framework: identify the stakeholders, the duty owed, the applicable rule or law, the least-harm option, and the transparency test, and apply it to at least 30 case vignettes. Maintain a dated one-page fact sheet on Madhya Pradesh institutions, schemes and current appointments, revised monthly, and do 20 aptitude problems (ratio, time-work, data interpretation) daily under timed conditions rather than in bulk at the end.