No account needed. This session stays in this browser tab; sign in for timed tests, bookmarks and saved performance.
Question 1 · Electrical Engineering (SSC JE) · Electrical Machines A 4-pole DC generator has a wave winding with 500 conductors and a flux of 0.02 Wb per pole. At 1200 rpm, the generated emf is
Choose your answer for question 1 A. 200 V B. 800 V C. 400 V D. 100 VAnswer and explanation Correct answer: C
E = Φ Z N P / (60 A). For wave winding A = 2: E = 0.02 × 500 × 1200 × 4 / (60 × 2) = 400 V.
Question 2 · Electrical Engineering (SSC JE) · Electrical Machines A DC motor is supplied at 220 V and takes an armature current of 20 A. The armature resistance is 0.5 Ω. The back emf is
Choose your answer for question 2 A. 230 V B. 219 V C. 210 V D. 200 VAnswer and explanation Correct answer: C
Eb = V − Ia Ra = 220 − 20 × 0.5 = 210 V.
Question 3 · Electrical Engineering (SSC JE) · Electrical Machines The speed of a DC shunt motor can be increased above its normal speed by
Choose your answer for question 3 A. increasing the armature resistance B. reducing the armature voltage C. increasing the field flux D. reducing the field fluxAnswer and explanation Correct answer: D
N ∝ Eb/Φ, so weakening the field raises the speed (field control).
Question 4 · Electrical Engineering (SSC JE) · Electrical Machines A DC series motor should never be started without a load because
Choose your answer for question 4 A. its torque would be too low B. its speed would become dangerously high C. it would draw no current D. its field would burn outAnswer and explanation Correct answer: B
With no load, the armature current and the flux are both small, and the speed rises to a very high value.
Question 5 · Electrical Engineering (SSC JE) · Electrical Machines The emf equation of a transformer is
Choose your answer for question 5 A. E = 2.22 f N Φm B. E = 4.44 f N Φm C. E = 1.11 f N Φm D. E = 4.44 f N² ΦmAnswer and explanation Correct answer: B
E_rms = 4.44 f N Φm, where Φm is the maximum core flux.
Question 6 · Electrical Engineering (SSC JE) · Electrical Machines A transformer has a primary voltage of 11 kV and a secondary voltage of 400 V. If the primary has 1100 turns, the secondary has
Choose your answer for question 6 A. 400 turns B. 30 turns C. 40 turns D. 4 turnsAnswer and explanation Correct answer: C
N2 = N1 × V2/V1 = 1100 × 400/11 000 = 40 turns.
Question 7 · Electrical Engineering (SSC JE) · Electrical Machines The open circuit test on a transformer gives
Choose your answer for question 7 A. the regulation at unity power factor B. the copper loss C. the iron (core) loss D. the efficiency at full loadAnswer and explanation Correct answer: C
At no load, the current is small, so copper loss is negligible and the input power is the core loss.
Question 8 · Electrical Engineering (SSC JE) · Electrical Machines The efficiency of a transformer is maximum when
Choose your answer for question 8 A. copper loss is zero B. iron loss is zero C. copper loss equals iron loss D. the load is zeroAnswer and explanation Correct answer: C
Iron loss is constant and copper loss varies with the square of the load. The efficiency peaks when the two are equal.
Question 9 · Electrical Engineering (SSC JE) · Electrical Machines An autotransformer is most economical compared to a two-winding transformer when the transformation ratio is
Choose your answer for question 9 A. close to 1 B. zero C. exactly 10 D. much greater than 10Answer and explanation Correct answer: A
With K = V1/V2 (greater than 1), the copper saved is a fraction 1/K of that in the two-winding transformer. The saving is therefore greatest when K is close to 1.
Question 10 · Electrical Engineering (SSC JE) · Electrical Machines An induction motor runs at 1440 rpm on a 50 Hz supply with 4 poles. The slip is
Choose your answer for question 10 A. 2.5 percent B. 6 percent C. 4 percent D. 40 percentAnswer and explanation Correct answer: C
Ns = 120 × 50/4 = 1500 rpm. s = (1500 − 1440)/1500 = 0.04.
Question 11 · Electrical Engineering (SSC JE) · Electrical Machines The synchronous speed of a 6-pole machine on a 50 Hz supply is
Choose your answer for question 11 A. 1000 rpm B. 3000 rpm C. 1500 rpm D. 500 rpmAnswer and explanation Correct answer: A
Ns = 120 f / P = 120 × 50 / 6 = 1000 rpm.
Question 12 · Electrical Engineering (SSC JE) · Electrical Machines The frequency of the rotor current of an induction motor at a slip of 0.04 on a 50 Hz supply is
Choose your answer for question 12 A. 1 Hz B. 50 Hz C. 2 Hz D. 4 HzAnswer and explanation Correct answer: C
fr = s f = 0.04 × 50 = 2 Hz.
Question 13 · Electrical Engineering (SSC JE) · Electrical Machines Star-delta starting of a three-phase induction motor reduces the starting line current to
Choose your answer for question 13 A. one-half of the direct on-line value B. equal to the direct on-line value C. one-ninth of the direct on-line value D. one-third of the direct on-line valueAnswer and explanation Correct answer: D
In star, the phase voltage is 1/√3 of the line voltage, so the line current falls to 1/3.
Question 14 · Electrical Engineering (SSC JE) · Electrical Machines An over-excited synchronous motor operates at
Choose your answer for question 14 A. zero power factor lagging only B. a lagging power factor C. a leading power factor D. a power factor of zeroAnswer and explanation Correct answer: C
An over-excited motor supplies reactive power to the system, so it acts like a capacitor (synchronous condenser).
Question 15 · Electrical Engineering (SSC JE) · Electrical Machines The voltage regulation of an alternator is highest for a load of
Choose your answer for question 15 A. lagging power factor B. zero power C. leading power factor D. unity power factorAnswer and explanation Correct answer: A
For a lagging load, armature reaction is demagnetising, and the terminal voltage falls the most.
Question 16 · Electrical Engineering (SSC JE) · Electrical Machines A single-phase induction motor is not self-starting because the stator field is
Choose your answer for question 16 A. zero at all times B. pulsating and not rotating C. direct current D. rotating at twice the speedAnswer and explanation Correct answer: B
A pulsating field can be resolved into two equal and opposite rotating fields that give no net torque at standstill.
Question 17 · Electrical Engineering (SSC JE) · Electrical Machines A universal motor can operate on
Choose your answer for question 17 A. ac supply only at a fixed frequency B. three-phase supply only C. both ac and dc supplies D. dc supply onlyAnswer and explanation Correct answer: C
It is a series-wound motor, whose torque direction does not change when the supply polarity reverses.
Question 18 · Electrical Engineering (SSC JE) · Electrical Machines Interpoles in a DC machine are provided to
Choose your answer for question 18 A. increase the main flux B. reduce the iron loss C. increase the number of poles D. improve commutation and reduce sparkingAnswer and explanation Correct answer: D
They produce a flux that induces an emf in the coil under commutation which opposes the reactance voltage.
Question 19 · Electrical Engineering (SSC JE) · Electrical Machines A DC shunt generator delivers 40 A at 220 V. The field current is 2 A and the armature resistance is 0.1 Ω. The generated emf is
Choose your answer for question 19 A. 216 V B. 224.2 V C. 228 V D. 220 VAnswer and explanation Correct answer: B
Ia = 40 + 2 = 42 A. E = V + Ia Ra = 220 + 42 × 0.1 = 224.2 V.
Question 20 · Electrical Engineering (SSC JE) · Electrical Machines A DC motor delivers 10 kW at 1000 rpm. The shaft torque is about
Choose your answer for question 20 A. 10 N·m B. 9.55 N·m C. 95.5 N·m D. 955 N·mAnswer and explanation Correct answer: C
T = P / ω = 10 000 × 60 / (2π × 1000) = 95.5 N·m.