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Maharashtra HSC Electrical Technology (Vocational) study materials
Maharashtra HSC Electrical Technology (Vocational) study resources

Maharashtra HSC Electrical Technology (Vocational) practice questions

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Question 1Basic Electrical Concepts and Fundamentals

In a DC circuit, the unit of electrical resistance is the ohm. Which symbol is used to represent the ohm in circuit diagrams and formulae?

  1. μ
  2. λ
  3. Φ
  4. Ω Correct answer.

Explanation

The ohm is the SI derived unit of electrical resistance and is written with the Greek capital letter omega, Ω. It is defined as the resistance that allows one ampere of current when one volt is applied across it. The other symbols belong to unrelated quantities such as magnetic flux and wavelength.

Question 2Basic Electrical Concepts and Fundamentals

A lamp rated 60 W is connected across a 230 V supply and operates normally. What current does it draw from the supply?

  1. 26 A
  2. 13.8 A
  3. 0.26 A Correct answer.
  4. 2.6 A

Explanation

For a resistive load the power relation is P = V × I, so I = P / V. Substituting gives I = 60 / 230 ≈ 0.26 A. This small current is typical of a domestic filament lamp on a 230 V supply.

Question 3Basic Electrical Concepts and Fundamentals

Which statement correctly describes the direction of conventional current in a metallic conductor?

  1. It flows from the negative terminal to the positive terminal through the external circuit.
  2. It flows only inside the battery from positive to negative.
  3. It flows from the positive terminal to the negative terminal through the external circuit. Correct answer.
  4. It alternates direction every half cycle in a DC circuit.

Explanation

Conventional current is defined as the direction in which positive charge would move, so it leaves the positive terminal and returns to the negative terminal through the external circuit. Actual electron drift is opposite to this convention. In a steady DC circuit the direction does not alternate.

Question 4Basic Electrical Concepts and Fundamentals

Two resistors of 6 Ω and 3 Ω are joined in parallel. What is the equivalent resistance of the combination?

  1. 2 Ω Correct answer.
  2. 0.5 Ω
  3. 4.5 Ω
  4. 9 Ω

Explanation

For two resistors in parallel, the equivalent resistance is the product divided by the sum: (6 × 3) / (6 + 3) = 18 / 9 = 2 Ω. The reciprocal formula 1/R = 1/6 + 1/3 also gives 1/R = 0.5, so R = 2 Ω. The result is always smaller than the smallest branch resistance.

Question 5Basic Electrical Concepts and Fundamentals

A copper conductor of uniform cross-section is stretched so that its length doubles while its volume stays constant. How does its resistance change?

  1. It becomes half the original value.
  2. It becomes twice the original value.
  3. It becomes four times the original value. Correct answer.
  4. It remains unchanged.

Explanation

Resistance is R = ρL/A. If the volume stays constant while the length doubles, the cross-sectional area must halve. The length factor doubles the resistance and the halved area doubles it again, giving a fourfold increase.

Question 6Basic Electrical Concepts and Fundamentals

Which instrument is connected in series with a circuit element to measure the current through it?

  1. Wattmeter
  2. Voltmeter
  3. Ammeter Correct answer.
  4. Megger

Explanation

An ammeter must carry the circuit current, so it is connected in series and is designed to have very low resistance to avoid disturbing the circuit. A voltmeter is connected in parallel across the element. A wattmeter has both a current coil and a voltage coil.

Question 7Basic Electrical Concepts and Fundamentals

In a series circuit containing a 12 V battery and three equal resistors, what is the voltage across each resistor?

  1. 6 V
  2. 12 V
  3. 4 V Correct answer.
  4. 36 V

Explanation

In a series circuit the same current flows through every element, so equal resistances develop equal voltage drops. The supply voltage divides equally among the three resistors: 12 / 3 = 4 V. The sum of the drops equals the supply voltage, as Kirchhoff's voltage law requires.

Question 8Basic Electrical Concepts and Fundamentals

Which material is classified as a semiconductor at room temperature?

  1. Germanium Correct answer.
  2. Mica
  3. Copper
  4. Nichrome

Explanation

Germanium is a Group IV element whose resistivity lies between that of conductors and insulators, making it a semiconductor used in diodes and transistors. Copper is a good conductor, while mica is an insulator and nichrome is a resistance alloy.

Question 9Basic Electrical Concepts and Fundamentals

A 100 W, 250 V lamp is operated on a 200 V supply. Assuming its resistance stays constant, what power does it actually consume?

  1. 64 W Correct answer.
  2. 125 W
  3. 100 W
  4. 80 W

Explanation

First find the resistance from rated values: R = V²/P = 250²/100 = 625 Ω. On 200 V the power is P = V²/R = 200²/625 = 64 W. Power varies with the square of the applied voltage for a fixed resistance.

Question 10Basic Electrical Concepts and Fundamentals

Which law states that the current through a conductor is directly proportional to the potential difference across it, provided temperature stays constant?

  1. Faraday's law
  2. Lenz's law
  3. Coulomb's law
  4. Ohm's law Correct answer.

Explanation

Ohm's law expresses the linear relationship V = IR for a conductor at constant temperature. Faraday's and Lenz's laws concern electromagnetic induction, and Coulomb's law concerns electrostatic force between charges.

Question 11Basic Electrical Concepts and Fundamentals

A charge of 60 coulombs flows past a point in a conductor in 4 minutes. What is the average current?

  1. 0.25 A Correct answer.
  2. 240 A
  3. 15 A
  4. 4 A

Explanation

Current is the rate of flow of charge, I = Q/t, and time must be in seconds: 4 minutes = 240 s. Then I = 60/240 = 0.25 A. Forgetting to convert minutes to seconds is the common error.

Question 12Basic Electrical Concepts and Fundamentals

Three cells, each of EMF 1.5 V and internal resistance 0.5 Ω, are connected in series to a 4.5 Ω load. What current flows?

  1. 0.9 A
  2. 3.0 A
  3. 1.0 A
  4. 0.75 A Correct answer.

Explanation

In series the EMFs add to 4.5 V and the internal resistances add to 1.5 Ω, giving a total circuit resistance of 1.5 + 4.5 = 6 Ω. The current is I = EMF/R = 4.5/6 = 0.75 A. Internal resistance must always be included in the total.

Question 13Basic Electrical Concepts and Fundamentals

What is the SI unit of electric charge?

  1. Joule
  2. Ampere
  3. Coulomb Correct answer.
  4. Volt

Explanation

The coulomb is the SI unit of electric charge and equals the charge transported by one ampere in one second. The ampere is the unit of current, the volt of potential difference, and the joule of energy.

Question 14Basic Electrical Concepts and Fundamentals

A resistor of 20 Ω carries a current of 0.5 A. How much heat energy does it dissipate in 10 seconds?

  1. 200 J
  2. 10 J
  3. 50 J Correct answer.
  4. 100 J

Explanation

The power dissipated is P = I²R = 0.5² × 20 = 0.25 × 20 = 5 W. Energy is power multiplied by time: 5 × 10 = 50 J. Using P = I²R avoids the intermediate step of finding voltage.

Question 15Basic Electrical Concepts and Fundamentals

Which effect of electric current is used in the working of an electric bell?

  1. Chemical effect
  2. Heating effect
  3. Photoelectric effect
  4. Magnetic effect Correct answer.

Explanation

An electric bell uses an electromagnet that attracts an armature; the movement interrupts the circuit, the magnet releases, and the cycle repeats to strike the gong. This depends on the magnetic effect of current, not on heating or chemical action.

Question 16Basic Electrical Concepts and Fundamentals

In a household wiring installation, what is the main purpose of the earth wire?

  1. To improve the power factor of the installation
  2. To provide a low-resistance path that operates the protective device if a live part touches the metal body Correct answer.
  3. To carry the normal load current back to the supply
  4. To reduce the supply voltage at the appliance

Explanation

The earth wire connects exposed metalwork to the general mass of earth, so a fault from live to the body produces a large fault current that quickly trips the fuse or MCB. It carries current only under fault conditions. It does not carry normal load current or alter voltage.

Question 17Basic Electrical Concepts and Fundamentals

Two capacitors of 4 μF and 12 μF are connected in parallel. What is the total capacitance?

  1. 3 μF
  2. 16 μF Correct answer.
  3. 48 μF
  4. 8 μF

Explanation

Capacitances in parallel add directly, just as resistances in series do: 4 + 12 = 16 μF. The voltage across each capacitor is the same and the total charge is the sum of the individual charges.

Question 18Basic Electrical Concepts and Fundamentals

A conductor of length 0.5 m carries a current of 4 A at right angles to a uniform magnetic field of flux density 0.2 T. What force acts on the conductor?

  1. 0.8 N
  2. 0.04 N
  3. 4.0 N
  4. 0.4 N Correct answer.

Explanation

The force on a current-carrying conductor is F = B I L sin θ. With θ = 90°, sin θ = 1, so F = 0.2 × 4 × 0.5 = 0.4 N. The force is greatest when the conductor is perpendicular to the field.

Question 19Basic Electrical Concepts and Fundamentals

What happens to the resistance of a pure metallic conductor such as copper when its temperature rises?

  1. It decreases steadily.
  2. It first falls to zero and then rises.
  3. It increases. Correct answer.
  4. It remains practically constant.

Explanation

In metals the number of free electrons is essentially fixed, but rising temperature increases lattice vibrations, which scatter the electrons more strongly. This raises resistivity and therefore resistance, giving a positive temperature coefficient.

Question 20Basic Electrical Concepts and Fundamentals

A 12 V battery delivers 2 A to an external load for 5 hours. What energy is supplied to the load?

  1. 120 Wh Correct answer.
  2. 24 Wh
  3. 0.12 kWh
  4. 240 Wh

Explanation

Power delivered is P = V × I = 12 × 2 = 24 W. Energy is power multiplied by time: 24 W × 5 h = 120 Wh, which equals 0.12 kWh. Keeping time in hours gives the answer directly in watt-hours.

Question 21Electrical Materials and Wiring

In a typical Indian domestic wiring installation, which conductor colour is specified for the earth (protective) conductor in a flexible cord?

  1. Solid black
  2. Green with yellow stripe Correct answer.
  3. Solid blue
  4. Solid red

Explanation

Indian wiring convention reserves green with a yellow stripe for the protective earth conductor so that it is instantly identifiable and never confused with live or neutral. Red, yellow and blue are used for phase conductors and black for neutral in older colour codes. The earth conductor must never be used to carry load current.

Question 22Electrical Materials and Wiring

A fuse wire in a domestic distribution board is rated 10 A. Which property of the fuse element is chiefly responsible for its current-limiting action?

  1. Its low melting point combined with its small cross-sectional area Correct answer.
  2. Its very high resistivity combined with a large cross-sectional area
  3. Its high melting point combined with a long physical length
  4. Its magnetic permeability combined with its insulation resistance

Explanation

A fuse works on the heating effect of current, so the element must melt at a modest temperature and have a small cross-section that offers enough resistance to heat quickly. Low melting point plus small area gives fast, reliable operation at the rated current. High melting point or large cross-section would delay melting and fail to protect the circuit.

Question 23Electrical Materials and Wiring

Which material is most commonly used as the conductor in Indian house wiring cables because of its good conductivity and lower cost?

  1. Electrolytic-grade aluminium
  2. Annealed copper Correct answer.
  3. Nichrome
  4. Soft iron

Explanation

Annealed copper combines high conductivity with good ductility, so it bends easily in conduit and gives low voltage drop at moderate cost. Aluminium is cheaper per kilogram but needs a larger cross-section for the same current and is harder to terminate. Iron and nichrome have far too high a resistivity for wiring conductors.

Question 24Electrical Materials and Wiring

A single-phase circuit carries 13 A through a 2.5 mm² copper conductor of length 20 m. Taking copper resistivity as 0.0175 Ω·mm²/m, the voltage drop in the conductor is nearest to

  1. 0.9 V
  2. 7.2 V
  3. 3.6 V
  4. 1.8 V Correct answer.

Explanation

Resistance equals resistivity times length divided by area, giving 0.0175 × 20 ÷ 2.5 = 0.14 Ω. Multiplying by 13 A gives about 1.82 V. Remember that a single-phase circuit has both a phase and a neutral conductor, so the total loop drop would be twice this value.

Question 25Electrical Materials and Wiring

A 240 V, 2 kW geyser is wired with a circuit that must be protected. Ignoring diversity and power factor, the minimum fuse rating from the standard series 6, 10, 16, 20, 25, 32 A is

  1. 20 A
  2. 6 A
  3. 16 A
  4. 10 A Correct answer.

Explanation

The load current is 2000 ÷ 240 ≈ 8.33 A. The protective device must carry this continuously, so the next standard rating above 8.33 A is 10 A. Choosing 6 A would cause nuisance blowing, while larger ratings would leave the cable under-protected.

Question 26Electrical Materials and Wiring

In a conduit wiring system, why are loop-in (joint-box-free) connections preferred over joints made inside conduit?

  1. They eliminate the need for an earth conductor in the circuit
  2. They reduce the number of junction boxes and keep joints accessible for inspection Correct answer.
  3. They permit the use of thinner conductors for the same load
  4. They allow the conduit to be used as the neutral return path

Explanation

Loop-in wiring carries the circuit through the accessories so that no joint is buried inside conduit, and any connection remains accessible at a box or ceiling rose. Burying joints in conduit makes inspection and fault-finding impossible and is prohibited. Accessibility, not conduit earthing or conductor sizing, is the governing reason.

Question 27Electrical Materials and Wiring

Which wiring system is most suitable for a workshop where machines are frequently relocated and the wiring must be quickly altered?

  1. Casing-and-capping wiring
  2. Bus-bar trunking with tap-off boxes Correct answer.
  3. Conduit wiring buried in the wall
  4. Cleat wiring

Explanation

Bus-bar trunking allows tap-off boxes to be added or shifted along the run, which suits layouts that change frequently. Cleat and casing-and-capping systems are surface methods that are awkward to modify and offer limited protection. Concealed conduit is fixed within the structure and cannot be altered without breaking the wall.

Question 28Electrical Materials and Wiring

A 1.5 mm² copper conductor of resistivity 0.0175 Ω·mm²/m has a resistance of 0.35 Ω. Its length is nearest to

  1. 30 m Correct answer.
  2. 60 m
  3. 10 m
  4. 120 m

Explanation

From R = ρL/A, the length is L = RA/ρ = 0.35 × 1.5 ÷ 0.0175 = 30 m. Rearranging the resistance formula is a standard skill for checking cable runs against voltage-drop limits. Larger lengths would give proportionally higher resistance.

Question 29Electrical Materials and Wiring

Which property of PVC makes it the preferred insulation for general-purpose domestic wiring cables?

  1. Very high continuous temperature rating above 150 °C
  2. Excellent resistance to sustained high-temperature soldering
  3. High dielectric strength with good flexibility and flame retardance Correct answer.
  4. High thermal conductivity for heat dissipation

Explanation

PVC compounds used for cable insulation combine adequate dielectric strength with flexibility and self-extinguishing behaviour, which suits domestic wiring. Their continuous temperature limit is modest, typically around 70 °C for general-purpose grades. High-temperature resistance is a property of materials such as silicone rubber or PTFE, not ordinary PVC.

Question 30Electrical Materials and Wiring

In a three-pin domestic socket outlet, the earth pin is made longer and thicker than the live and neutral pins. The main purpose of this design is to

  1. allow the plug to be inserted in either orientation
  2. increase the current-carrying capacity of the neutral pin
  3. reduce the contact resistance of the live pin
  4. ensure the earth connection is made first and broken last Correct answer.

Explanation

The longer earth pin enters the socket first on insertion and leaves last on withdrawal, so the appliance is earthed before it can be energised. The thicker pin also prevents it from being bent easily and makes wrong insertion difficult. The live and neutral pins carry load current, so earth-pin dimensions do not affect their rating.

Question 31Electrical Materials and Wiring

A circuit is protected by a 16 A MCB of type C. Compared with a type B device of the same rating, the type C device will

  1. give no thermal overload protection at all
  2. provide a lower breaking capacity
  3. tolerate a higher short-time inrush current before tripping magnetically Correct answer.
  4. trip magnetically at a lower multiple of its rated current

Explanation

The letter on an MCB indicates the magnetic trip range: type B trips magnetically at roughly 3–5 times rated current, while type C trips at roughly 5–10 times. Type C is therefore chosen for loads with moderate inrush, such as small motors. Both types retain thermal overload protection and similar breaking capacities.

Question 32Electrical Materials and Wiring

The main reason for using a miniature circuit breaker instead of a rewirable fuse in a modern domestic distribution board is that the MCB

  1. has a lower purchase cost per way
  2. provides a higher earth-loop impedance
  3. can be reset and gives more repeatable tripping characteristics Correct answer.
  4. requires no earthing of the enclosure

Explanation

An MCB trips on a calibrated thermal-magnetic mechanism, so its characteristic stays consistent, and it can be reset after clearing a fault. A rewirable fuse depends on the skill of the person replacing the wire and needs a spare element. MCBs are generally costlier than fuse units and do not remove the need for earthing.

Question 33Electrical Materials and Wiring

Which of the following is the correct function of the earth continuity conductor in a metal conduit installation?

  1. To reduce the voltage drop along the phase conductor
  2. To improve the power factor of the installation
  3. To carry the normal return current of the circuit
  4. To provide a low-impedance path so that fault current operates the protective device quickly Correct answer.

Explanation

The earth continuity path must have low impedance so that a fault from live to metalwork produces enough current to trip the protective device promptly. It never carries normal load current. Voltage drop and power factor are determined by the phase and neutral conductors and the load, not by the earthing system.

Question 34Electrical Materials and Wiring

A metal water pipe is used as an earth electrode. Which practice is required to make this acceptable?

  1. Use the pipe only as a neutral return to save a conductor
  2. Connect the earth lead to any convenient indoor tap
  3. Bond the pipe near the point of entry and confirm continuity with the supply earth Correct answer.
  4. Rely on the pipe alone without any supplementary electrode

Explanation

Where a metallic water service pipe is used for earthing, it must be bonded close to its entry point and its electrical continuity verified, because joints and plastic inserts can break the path. Connections must be made to a clean, accessible part of the pipe, not to a tap. A water pipe must never serve as a neutral return conductor.

Question 35Electrical Materials and Wiring

A 2.5 mm² PVC-insulated copper cable is run through thermal insulation in a roof space. The effect on its current-carrying capacity compared with free-air clipping is that the capacity

  1. remains unchanged because the conductor size is fixed
  2. increases because the ambient temperature falls
  3. decreases because heat cannot escape from the conductor Correct answer.
  4. increases because the insulation protects the cable

Explanation

Current rating depends on how quickly heat generated in the conductor can escape. Surrounding the cable with thermal insulation traps heat, so the permissible current must be derated. The conductor cross-section is unchanged, but its usable capacity is reduced by the installation condition.

Question 36Electrical Materials and Wiring

Which test is performed on a completed installation to verify that the resistance of the protective conductor path is low enough for automatic disconnection?

  1. Polarity test
  2. Earth continuity test Correct answer.
  3. Insulation resistance test
  4. Earth electrode resistance test

Explanation

The earth continuity test measures the resistance of the protective conductor from the main earthing terminal to exposed metalwork, confirming a low-impedance path for fault current. Insulation resistance checks isolation between live parts and earth, and polarity confirms correct connection of switches and sockets. Earth electrode resistance applies to the electrode itself, not to circuit protective conductors.

Question 37Electrical Materials and Wiring

A 3-core cable feeds a single-phase appliance with a metal case. The green-yellow core becomes disconnected at the plug. The most serious consequence is that

  1. the supply frequency will change
  2. the neutral conductor will overheat
  3. the appliance will draw more current than its rating
  4. the metal case may become live if a fault develops inside the appliance Correct answer.

Explanation

Without a continuous protective earth, a fault from live to the metal case leaves the case at line potential with no low-impedance path to operate the protective device. Anyone touching the case and earth would then receive a severe shock. Current, neutral heating and frequency are not affected by loss of the earth connection.

Question 38Electrical Materials and Wiring

Which fire-safety property of a cable insulation material is assessed by its oxygen index?

  1. The rate at which the material absorbs moisture
  2. The voltage at which the insulation breaks down
  3. The temperature at which the insulation softens
  4. The minimum oxygen concentration needed to sustain burning of the material Correct answer.

Explanation

The limiting oxygen index expresses the lowest oxygen percentage in the surrounding atmosphere that will support combustion of the material. A higher index indicates a material that is harder to burn in normal air. Softening temperature, breakdown voltage and moisture absorption are measured by different tests.

Question 39Electrical Materials and Wiring

A socket outlet circuit is protected by a 32 A ring final circuit with 2.5 mm² conductors. A spur is taken from a socket to supply a single 13 A outlet. The protective device for the spur is

  1. no device, since the spur is short
  2. a 32 A MCB at the distribution board only
  3. a 6 A fuse in the distribution board
  4. a 13 A fuse in the plug of the appliance or a fused connection unit Correct answer.

Explanation

The 2.5 mm² spur conductor is protected against overload by the 13 A fuse in the plug or by a fused connection unit, because the 32 A ring device alone would not protect it. The ring itself is protected at 32 A, but the spur needs local protection. A short spur length does not remove the requirement.

Question 40Electrical Materials and Wiring

A 240 V single-phase load of 12 A is supplied by a cable whose total loop resistance (phase plus neutral) is 0.8 Ω. The percentage voltage drop at the load is nearest to

  1. 0.4 %
  2. 2.0 %
  3. 4.0 % Correct answer.
  4. 8.0 %

Explanation

The loop voltage drop is current times loop resistance, 12 × 0.8 = 9.6 V. Expressed as a percentage of 240 V, this is 9.6 ÷ 240 × 100 = 4.0 %. Indian practice generally limits such drops to about 3–5 % for a final circuit, so this value is close to the limit.

Question 41Electrical Materials and Wiring

Two 100 W, 240 V lamps are connected in series across a 240 V supply. Compared with one lamp alone, the total power consumed is

  1. 200 W
  2. 100 W
  3. 50 W Correct answer.
  4. 25 W

Explanation

Each lamp has a resistance of V²/P = 240² ÷ 100 = 576 Ω, so two in series give 1152 Ω. The total power is then 240² ÷ 1152 = 50 W. Series operation halves the voltage across each lamp and reduces the total power to half that of a single lamp.

Question 42Electrical Materials and Wiring

A distribution board uses HRC fuses instead of rewirable fuses for the main incomer. The chief advantage claimed for the HRC fuse is its

  1. lower cost of the fuse element
  2. ability to be reset after a fault without replacement
  3. ability to limit normal load current
  4. high rupturing capacity that clears heavy fault currents without case damage Correct answer.

Explanation

A high rupturing capacity fuse is filled with quartz and has a constricted element, so it can interrupt very large prospective fault currents safely without the body bursting. It is not resettable and must be replaced after operation. It does not limit normal load current, only fault current.

Question 43Electrical Materials and Wiring

Which of the following best explains why aluminium conductors require special bimetallic connectors when joined to copper terminals?

  1. Aluminium is magnetic and interferes with the terminal
  2. Direct contact causes galvanic corrosion and a high-resistance joint that can overheat Correct answer.
  3. Copper and aluminium have identical thermal expansion, so no clamping is possible
  4. Aluminium has a lower melting point than copper, so it melts during soldering

Explanation

Copper and aluminium form a galvanic couple in the presence of moisture, and the aluminium side corrodes, raising joint resistance and risking overheating. Aluminium also creeps under clamping pressure, which loosens joints over time. Bimetallic connectors separate the metals and maintain a stable, low-resistance joint.

Question 44Electrical Materials and Wiring

A switch is required to control a lamp from two positions at the ends of a corridor. Which arrangement is correct?

  1. Two two-way switches connected by two strapper wires Correct answer.
  2. Two one-way switches connected in series
  3. Two one-way switches connected in parallel
  4. One two-way switch and one intermediate switch

Explanation

Two-way switching uses two two-way switches linked by a pair of strapper (traveller) wires so that either switch can make or break the circuit. One-way switches in parallel or series cannot give independent control from both ends. An intermediate switch is added only when a third or further control point is needed.

Question 45Electrical Machines and Transformers

In a single-phase transformer, the winding connected to the supply is called the primary winding. Which winding delivers power to the load?

  1. Field winding
  2. Compensating winding
  3. Tertiary winding
  4. Secondary winding Correct answer.

Explanation

The primary winding absorbs power from the AC supply and the secondary winding delivers power to the load. Tertiary windings exist only in some three-phase units for harmonic suppression, and field windings belong to rotating machines.

Question 46Electrical Machines and Transformers

The core of a practical power transformer is laminated rather than solid. What is the main purpose of this lamination?

  1. To increase the mechanical strength of the core
  2. To lower the copper loss of the windings
  3. To reduce eddy-current loss in the core Correct answer.
  4. To raise the saturation flux density of the core

Explanation

Eddy currents circulate in the core and their loss grows with the square of lamination thickness. Thin, insulated laminations break up these paths and reduce eddy-current loss, while copper loss depends on winding resistance.

Question 47Electrical Machines and Transformers

A transformer has 500 primary turns and 100 secondary turns. If the primary is connected to a 230 V supply, what is the approximate secondary voltage on no load?

  1. 46 V Correct answer.
  2. 23 V
  3. 230 V
  4. 1150 V

Explanation

The turns ratio gives secondary voltage as primary voltage times the ratio of secondary to primary turns, that is 230 times 100 divided by 500. This equals 46 V, which is a step-down result consistent with fewer secondary turns.

Question 48Electrical Machines and Transformers

Which statement correctly describes the frequency of the induced emf in a transformer secondary compared with the supply frequency?

  1. It is half the supply frequency
  2. It is twice the supply frequency
  3. It equals the supply frequency Correct answer.
  4. It depends on the load power factor

Explanation

Transformer action is purely electromagnetic induction, so the secondary emf alternates at exactly the same frequency as the primary supply. Load power factor changes the phase angle between voltage and current but never the frequency.

Question 49Electrical Machines and Transformers

A transformer is rated 10 kVA at 2000 V on the primary. What is the approximate full-load primary current?

  1. 0.2 A
  2. 20 A
  3. 5 A Correct answer.
  4. 50 A

Explanation

For a single-phase transformer the full-load current is rating in volt-amperes divided by voltage, giving 10000 divided by 2000. This equals 5 A, which is the current the primary winding carries at rated load.

Question 50Electrical Machines and Transformers

In a transformer, the power factor on the primary side at no load is very low. What is the main reason?

  1. The primary resistance is very high
  2. The secondary is short-circuited
  3. The no-load current is largely magnetising current Correct answer.
  4. The core reluctance is zero

Explanation

At no load the primary draws a small current that is mostly reactive magnetising component needed to establish flux, with only a small in-phase loss component. The resulting power factor is therefore low, typically below 0.3.