Free Practice Questions for Bac F3 Niger

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Key Facts: Bac F3 Niger Exam

OBEECS

The national examination board in Niamey that oversees the Baccalauréat F3 in Niger

Office du Baccalauréat du Niger (obeecsniger.com)

8 + 6 papers

Technical, general, and language papers listed across the official first and second groups

OBEECS official paper bank

3–4 hours

Duration range of the official F3 technical papers reviewed

OBEECS official past papers

Free independent practice for Niger's Baccalauréat Série F3 (Électrotechnique): English-language MCQs covering single-phase and three-phase transformers, induction motors, synchronous alternators, DC machines, power converters (rectifiers, choppers, inverters), earthing systems (TT, TN, IT), protection discrimination, and PLC automation.

Sample Bac F3 Niger Practice Questions

Try these sample questions to review concepts for the Bac F3 Niger exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1What is the transformation ratio m of an ideal single-phase transformer with N1 = 1,000 primary turns and N2 = 100 secondary turns?

A.m = 10 (Step-up transformer)

B.m = 0.1 (Step-down transformer)

C.m = 1.0 (Isolation transformer)

D.m = 0.01

Explanation: The transformation ratio m is defined as the ratio of secondary turns to primary turns: m = N2 / N1 = U20 / U1. With N2 = 100 and N1 = 1,000, m = 100 / 1,000 = 0.1. Since m < 1, the transformer steps down voltage from primary to secondary.

2In a standard transformer open-circuit test (essai à vide) conducted at rated primary voltage U1n, what does the active power P10 measured by the primary wattmeter represent?

A.The reactive magnetizing power required by the air gap

B.The full-load copper losses in both the primary and secondary windings

C.The mechanical friction and windage losses of the stationary core

D.The iron (core) losses: hysteresis plus eddy-current losses

Explanation: During an open-circuit test, the secondary winding is open (I2 = 0), and the primary draws only a very small no-load current I10 (typically 1–5% of rated current). The Joule losses in the primary winding are negligible (proportional to I10²), meaning the wattmeter reading P10 almost exclusively measures core losses (pertes magnétiques fer = hysteresis + eddy currents) at rated flux.

3What is the synchronous speed Ns of a 4-pole three-phase induction motor supplied by a 50 Hz industrial power grid?

A.1,500 rpm

B.1,000 rpm

C.750 rpm

D.3,000 rpm

Explanation: The synchronous speed of the rotating magnetic field is given by Ns = (60 * f) / p, where f is supply frequency and p is the number of pole pairs. For a 4-pole motor, p = 4 / 2 = 2 pole pairs. Therefore, Ns = (60 * 50) / 2 = 3,000 / 2 = 1,500 rpm.

4If a three-phase induction motor has a synchronous speed Ns = 1,500 rpm and rotates under full load at an actual rotor speed N = 1,440 rpm, what is its fractional slip s?

A.0.06 (6%)

B.0.40 (40%)

C.0.04 (4%)

D.0.96 (96%)

Explanation: Rotor slip s is defined as the relative difference between synchronous speed and actual rotor speed: s = (Ns - N) / Ns. Here, s = (1,500 - 1,440) / 1,500 = 60 / 1,500 = 0.04 = 4%.

5What is the frequency fr of the induced rotor currents in an induction motor with slip s = 0.05 supplied at a stator grid frequency f = 50 Hz?

A.50 Hz

B.47.5 Hz

C.10 Hz

D.2.5 Hz

Explanation: The frequency of the alternating EMF and currents induced in the rotor windings is proportional to the slip: fr = s * f. With s = 0.05 and f = 50 Hz: fr = 0.05 * 50 = 2.5 Hz.

6In a three-phase transformer with a 'Dyn11' vector group designation, what do the individual characters signify?

A.Delta HV winding, star LV winding with neutral out, LV lagging HV by 330° (index 11)

B.Double HV winding with natural oil cooling and an 11 kV insulation class

C.Star HV winding, delta LV winding without neutral, LV lagging HV by 110°

D.Delta HV winding, star LV winding without neutral, and a displacement of 11°

Explanation: In standard IEC vector group coding: uppercase 'D' denotes a Delta (triangle) high-voltage primary winding; lowercase 'y' denotes a Star (étoile) low-voltage secondary; lowercase 'n' indicates the neutral terminal is accessible; and the clock hour index '11' indicates a phase angle displacement of 11 * 30° = 330° (or +30° lead).

7When starting a three-phase squirrel-cage induction motor using star-delta starting (démarrage étoile-triangle), how are the line starting current and starting torque affected compared to direct-on-line (DOL) starting?

A.Starting current is divided by sqrt(3), while starting torque is divided by 3

B.Starting current is doubled while starting torque is halved

C.Starting current and starting torque are both divided by 3

D.Starting current is divided by 3, but starting torque remains unchanged at 100%

Explanation: In star connection, the voltage across each phase winding is reduced by 1/sqrt(3) compared to delta line voltage (V = U / sqrt(3)). The phase current is thus reduced by 1/sqrt(3), and the line current is reduced by (1/sqrt(3)) * (1/sqrt(3)) = 1/3 of the DOL delta line current. Since induction motor electromagnetic torque is proportional to the square of the applied phase voltage (C ~ V²), torque is reduced by (1/sqrt(3))² = 1/3.

8What is the electromagnetic power Pem transmitted across the air gap of an induction motor running at slip s with rotor Joule losses Pjr?

A.Pem = Pjr * s

B.Pem = Pjr / s

C.Pem = Pjr / (1 - s)

D.Pem = Pjr * (1 - s)

Explanation: Rotor Joule losses are directly proportional to slip and transmitted air-gap power: Pjr = s * Pem. Rearranging gives the fundamental relationship Pem = Pjr / s. The remaining electromechanical power converted into mechanical rotation is P_meca = (1 - s) * Pem.

9What is the back-electromotive force (back-EMF) E induced in the armature of a direct current (DC) motor having armature constant k, magnetic flux per pole Phi, and rotational speed Omega (in rad/s)?

A.E = k * Phi / Omega

B.E = k * Phi * Omega

C.E = k * Omega / Phi

D.E = (1/2) * k * Phi² * Omega

Explanation: According to Faraday's law applied to rotating armatures, the induced counter-electromotive force is directly proportional to the total field flux per pole and the angular speed: E = k * Phi * Omega.

10Why is a DC series motor (moteur à courant continu série) never operated without a mechanical load (à vide)?

A.At no load the current, and therefore the field flux, collapses, so the speed runs away

B.The commutator brushes immediately arc and weld permanently to the copper segments

C.The motor cannot start because series field windings develop zero starting torque

D.The counter-EMF reverses polarity and charges the supply line with hazardous overvoltages

Explanation: In a DC series motor, the field winding is connected in series with the armature, so field flux is proportional to load current (Phi ~ I). At zero mechanical load, the motor draws minimal current, causing the flux Phi to approach zero. Since speed Omega = (U - R*I) / (k * Phi), dividing by near-zero flux causes the motor speed to reach catastrophic runaway levels (emballement), destroying the rotor by centrifugal force.

About the Bac F3 Niger Exam

Série F3 is Niger's technical secondary baccalauréat specializing in electrotechnology and electrical systems engineering, organized by OBEECS under the Ministry of Higher Education. It provides the pathway to higher technical studies (BTS Électrotechnique, DUT, university engineering faculties) and technical industry careers. This independent bank provides conceptual English-language MCQ practice covering electrical machines, power electronics, distribution networks, protective systems, and automation.

Exam sponsor: Office du Baccalauréat, des Équivalences et des Examens et Concours du Supérieur (OBEECS). The requirements and fees below concern the certification or admission exam, separate from our free practice resources.

Assessment

OBEECS's official paper bank lists first-group papers in Électrotechnique, Étude d'équipement, Schéma de commande, Dessin, Mathématiques, Français, Anglais, and Arabe. The second group lists Électrotechnique, Machines—mesures et essais, Technologie d'électricité, Construction, Sciences physiques, and Mathématiques. The technical papers reviewed are in French.

Time Limit

Official papers reviewed: Électrotechnique 3 hours; Étude d'équipement 4 hours

Passing Score

Not published in the current OBEECS public materials reviewed; the papers carry subject coefficients.

Exam / Certification Fees

Latest official schedule located (4 April 2024): 2,500 FCFA public-school official; 10,000 FCFA private-school official or independent; 25,000 FCFA foreign candidate. No separate 2026 fee notice was found.

Exam sponsor website

Fees, eligibility, and exam policies can change. Confirm them with the exam sponsor before applying or paying.

Official sources

Our practice resources: topics covered

We aim to reflect publicly available exam outlines and topic information in our study resources. Coverage, format, and difficulty may differ from the actual exam, and we cannot guarantee that every detail is accurate or current. Confirm exam requirements, fees, and policies with the official exam sponsor.

25% of this local bank (25 questions)

Electrical Machines and Transformers

Transformers (losses, efficiency, vector groups), three-phase induction motors (slip, torque-speed curve, starting methods), synchronous alternators, and DC motors

20% of this local bank (20 questions)

Power Electronics and Energy Conversion

Single-phase and three-phase diode/thyristor rectifiers, step-down and step-up choppers, autonomous inverters, and power semiconductor characteristics

20% of this local bank (20 questions)

Electrical Distribution, Earthing, and Protection

Régimes de neutre (TT, TN-C, TN-S, IT), fault current loops, circuit breaker tripping curves, residual current devices, cable sizing, and power factor correction

18% of this local bank (18 questions)

Industrial Control and Automation

Motor starter schematic diagrams (direct-on-line, star-delta, reversing), contactors, thermal relays, PLCs, ladder logic, GRAFCET charts, and sensors

17% of this local bank (17 questions)

Circuit Analysis and Applied Physics

Three-phase power calculations (P, Q, S), power factor improvement, Boucherot theorem, complex impedance, and magnetic circuit laws (Hopkinson)

Preparing for the Bac F3 Niger Exam

What You Need to Know

Passing score: Not published in the current OBEECS public materials reviewed; the papers carry subject coefficients.
Assessment: OBEECS's official paper bank lists first-group papers in Électrotechnique, Étude d'équipement, Schéma de commande, Dessin, Mathématiques, Français, Anglais, and Arabe. The second group lists Électrotechnique, Machines—mesures et essais, Technologie d'électricité, Construction, Sciences physiques, and Mathématiques. The technical papers reviewed are in French.
Time limit: Official papers reviewed: Électrotechnique 3 hours; Étude d'équipement 4 hours
Exam / certification fees: Latest official schedule located (4 April 2024): 2,500 FCFA public-school official; 10,000 FCFA private-school official or independent; 25,000 FCFA foreign candidate. No separate 2026 fee notice was found. Official sources

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Bac F3 Niger: Suggested Study Strategy

1Practise three-phase power calculations: P = sqrt(3) * U * I * cos(phi) and Q = sqrt(3) * U * I * sin(phi), and compute the required capacitor rating Qc = P * (tan(phi) - tan(phi')).

2Know the slip formula s = (Ns - N) / Ns and rotor frequency fr = s * f for induction motors.

3Understand the operation and fault isolation principle for TT, TN, and IT earthing schemes.

4Review rectifier average voltage calculations for half-wave, full-wave diode bridge, and controlled thyristor converters.

5Practise translating GRAFCET specifications into PLC ladder diagrams and relay control schematics.

Frequently Asked Questions

Is the Niger Bac F3 exam multiple-choice?

No. The official Série F3 materials reviewed use written technical problems, equipment studies, calculations, and schematic work in French. This bank is an independent English-language MCQ study adaptation for revision; it is not an official translation or format simulation.

What are the main earthing systems (régimes de neutre) examined in Série F3?

Candidates must master the three international IEC/NFC earthing systems: TT (neutral grounded, exposed metalwork grounded), TN (neutral grounded, exposed metalwork connected to neutral via TN-C or TN-S), and IT (neutral isolated or impedance-earthed, exposed metalwork grounded), including fault current and touch voltage calculations for each.

What electrical machines are covered in the Série F3 syllabus?

The syllabus covers single-phase and three-phase static transformers, three-phase squirrel-cage and wound-rotor induction motors, three-phase synchronous generators (alternators), and direct current machines (shunt, series, and separate excitation).

Free Practice Questions for Bac F3 Niger

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