Transformers MCQs 2026

The EMF induced in a transformer primary winding is given by the equation

1. A E = 2π f N Φm ✓
2. B E = 4.44 f N Φm ✓
3. C E = 4.44 f N I ✓
4. D E = f N Φm ✓

Maximum flux Φm in a transformer core is directly proportional to

1. A current and turns ✓
2. B frequency squared ✓
3. C applied voltage and inversely proportional to frequency ✓
4. D core area only ✓

The turns ratio of an ideal transformer is defined as

1. A Ns/Np ✓
2. B Vp/Is ✓
3. C Ep × Is ✓
4. D Np/Ns ✓

For an ideal transformer, if primary has 1000 turns and secondary has 100 turns, the turns ratio is

1. A 0.1 ✓
2. B 10 ✓
3. C 100 ✓
4. D 100000 ✓

In an ideal step-down transformer, the secondary current compared to primary current is

1. A higher ✓
2. B lower ✓
3. C equal ✓
4. D zero ✓

The voltage ratio of an ideal transformer equals

1. A inverse of turns ratio squared ✓
2. B turns ratio ✓
3. C square of turns ratio ✓
4. D independent of turns ✓

Open-circuit (OC) test on a transformer is performed on the

1. A low-voltage winding ✓
2. B high-voltage winding ✓
3. C both windings simultaneously ✓
4. D tertiary winding only ✓

The open-circuit test of a transformer primarily determines

1. A copper losses ✓
2. B full-load efficiency only ✓
3. C short-circuit impedance ✓
4. D core (iron) losses and no-load current ✓

Short-circuit (SC) test on a transformer is normally conducted on the

1. A low-voltage winding ✓
2. B neutral winding ✓
3. C high-voltage winding ✓
4. D open secondary ✓

The short-circuit test of a transformer is used to determine

1. A iron losses ✓
2. B copper losses and equivalent impedance ✓
3. C turns ratio ✓
4. D no-load power factor ✓

During OC test, wattmeter reading represents mainly

1. A I²R copper loss at full load ✓
2. B dielectric loss only ✓
3. C stray load loss ✓
4. D hysteresis and eddy current losses ✓

During SC test, the applied voltage is approximately

1. A 5 to 10 percent of rated voltage ✓
2. B 50 percent of rated voltage ✓
3. C 100 percent of rated voltage ✓
4. D 0.1 percent of rated voltage ✓

All-day efficiency of a distribution transformer is also called

1. A commercial efficiency ✓
2. B maximum efficiency ✓
3. C energy efficiency ✓
4. D regulation efficiency ✓

Commercial efficiency of a transformer is defined at

1. A no load ✓
2. B full load ✓
3. C half load only ✓
4. D quarter load only ✓

Maximum efficiency of a transformer occurs when

1. A copper loss is zero ✓
2. B iron loss is twice copper loss ✓
3. C load is zero ✓
4. D copper loss equals iron loss ✓

If iron loss is 2 kW and full-load copper loss is 8 kW, load fraction for maximum efficiency is

1. A 0.25 ✓
2. B 1.0 ✓
3. C 0.5 ✓
4. D 0.707 ✓

At maximum efficiency, the variable copper loss equals

1. A constant iron loss ✓
2. B half the iron loss ✓
3. C twice the iron loss ✓
4. D zero ✓

A transformer has Pi = 1 kW and Pc = 4 kW. Maximum efficiency occurs at

1. A 50% load ✓
2. B 25% load ✓
3. C 100% load ✓
4. D 75% load ✓

Star-star (Y-Y) connection of three-phase transformers gives

1. A 30° phase shift ✓
2. B 180° phase shift ✓
3. C 60° phase shift ✓
4. D zero phase shift between primary and secondary line voltages ✓

Delta-delta (Δ-Δ) connection is preferred when

1. A minimum copper is needed ✓
2. B phase shift of 30° is required ✓
3. C one transformer can be removed for maintenance (open delta) ✓
4. D neutral is mandatory ✓

Star-delta (Y-Δ) connection introduces a phase shift of

1. A 0° ✓
2. B 30° lagging of secondary ✓
3. C 60° leading ✓
4. D 180° ✓

Delta-star (Δ-Y) connection produces a phase shift of

1. A 30° lagging ✓
2. B 0° ✓
3. C 30° leading on secondary ✓
4. D 45° ✓

Scott connection uses two transformers to convert

1. A three-phase to two-phase ✓
2. B single-phase to three-phase ✓
3. C DC to AC ✓
4. D two-phase to single-phase only ✓

In Scott connection, one transformer is called teaser and has

1. A 100% same rating ✓
2. B 50% rating ✓
3. C 173% rating ✓
4. D 86.6% of main transformer voltage rating ✓

ONAN cooling classification means

1. A Oil Forced Air Forced ✓
2. B Oil Natural Air Natural ✓
3. C Oil Natural Air Forced ✓
4. D Oil Forced Air Natural ✓

OFAF cooling in transformers stands for

1. A Oil Natural Air Forced ✓
2. B Oil Forced Air Natural ✓
3. C Oil Forced Air Forced ✓
4. D Oil Free Air Forced ✓

As transformer load increases, voltage regulation generally

1. A decreases ✓
2. B remains constant ✓
3. C becomes negative always ✓
4. D increases ✓

Voltage regulation of a transformer is defined as

1. A (V2 no-load − V2 full-load) / V2 rated × 100% ✓
2. B (V2 full-load − V2 no-load) / V2 rated ✓
3. C V2 full-load / V1 ✓
4. D I2 / I1 ✓

Negative voltage regulation in a transformer can occur when

1. A load is purely resistive ✓
2. B load power factor is leading ✓
3. C load power factor is lagging unity ✓
4. D transformer is open-circuited ✓

The approximate equivalent circuit of a transformer is referred to

1. A both sides simultaneously without referral ✓
2. B the core only ✓
3. C the tank ✓
4. D one side (primary or secondary) ✓

In the exact equivalent circuit, R0 and X0 represent

1. A series leakage impedance ✓
2. B load resistance ✓
3. C no-load (core loss and magnetizing) branch ✓
4. D tap changer resistance ✓

Series components R1 and X1 in transformer equivalent circuit represent

1. A core loss ✓
2. B winding resistance and leakage reactance ✓
3. C magnetizing current ✓
4. D cooling system ✓

Buchholz relay is installed in

1. A transformer conservator pipe / main tank pipe ✓
2. B circuit breaker panel ✓
3. C secondary bus bar ✓
4. D lightning arrester ✓

Buchholz relay operates on the principle of

1. A gas accumulation and oil flow displacement ✓
2. B overcurrent in secondary ✓
3. C overvoltage on primary ✓
4. D external temperature only ✓

A Buchholz relay provides protection against

1. A external line faults only ✓
2. B lightning on transmission towers ✓
3. C incipient internal faults ✓
4. D metering errors ✓

On-load tap changer (OLTC) allows

1. A only off-load tapping ✓
2. B frequency change ✓
3. C phase reversal ✓
4. D voltage adjustment without interrupting load ✓

Off-load tap changer must be operated when

1. A at full rated load ✓
2. B transformer is de-energized or unloaded ✓
3. C during short circuit ✓
4. D only at maximum tap ✓

An auto-transformer differs from a two-winding transformer because

1. A it has no core ✓
2. B it cannot step up voltage ✓
3. C it has no losses ✓
4. D primary and secondary share a common winding ✓

Savings in copper of an auto-transformer compared to two-winding type is approximately

1. A (1 − Vlow/Vhigh) × 100% ✓
2. B always 50% ✓
3. C zero always ✓
4. D twice the rating ✓

Auto-transformers are NOT used when

1. A economy is important ✓
2. B small voltage change is needed ✓
3. C galvanic isolation is required ✓
4. D starting large motors ✓

An isolation transformer has turns ratio of

1. A 10:1 ✓
2. B 1:1 ✓
3. C 1:10 ✓
4. D variable only ✓

Primary purpose of an isolation transformer is to

1. A maximize efficiency at 1:10 ratio ✓
2. B provide galvanic isolation and safety ✓
3. C eliminate all losses ✓
4. D increase frequency ✓

A current transformer (CT) is connected in

1. A parallel with the line ✓
2. B across the bus bar in shunt ✓
3. C series with the line ✓
4. D only in neutral ground ✓

Standard secondary current of most instrument CTs is

1. A 5 A or 1 A ✓
2. B 110 V ✓
3. C 415 V ✓
4. D 0.1 A only ✓

A potential transformer (PT) is connected in

1. A series with load current ✓
2. B inside transformer tank ✓
3. C only on secondary of CT ✓
4. D parallel (shunt) across the line ✓

The secondary of a CT must never be

1. A open-circuited on load ✓
2. B shorted ✓
3. C grounded ✓
4. D connected to relay ✓

CT ratio 200/5 means secondary current of 5 A when primary carries

1. A 5 A ✓
2. B 2000 A ✓
3. C 200 A ✓
4. D 20 A ✓

Per-unit system for transformers uses base quantities such that

1. A all values are in amperes only ✓
2. B per-unit values are dimensionless ratios ✓
3. C per-unit has physical units ✓
4. D only voltage has per-unit ✓

Base impedance in per-unit system is given by

1. A Sbase / Vbase ✓
2. B Vbase × Ibase² ✓
3. C Ibase / Vbase ✓
4. D Vbase² / Sbase ✓

Transformer per-unit impedance is independent of

1. A the base MVA ✓
2. B the turns ratio ✓
3. C which side it is referred to ✓
4. D the frequency ✓

Hysteresis loss in transformer core depends on

1. A load current only ✓
2. B maximum flux density and frequency ✓
3. C secondary voltage squared only ✓
4. D tap position only ✓

Eddy current loss in transformer laminations varies approximately with

1. A flux density only ✓
2. B frequency only ✓
3. C load power factor ✓
4. D flux density squared and frequency squared ✓

Stacking laminations reduces eddy current loss because

1. A they increase flux density ✓
2. B they eliminate hysteresis ✓
3. C thin sheets increase resistance to eddy paths ✓
4. D they increase core volume ✓

The no-load current of a power transformer is typically

1. A 50 to 60 percent ✓
2. B 2 to 5 percent of rated current ✓
3. C equal to full-load current ✓
4. D zero always ✓

Silica gel breather in a transformer prevents

1. A moisture entering transformer oil ✓
2. B oil overheating ✓
3. C short circuit ✓
4. D phase unbalance ✓

Conservator tank on a transformer maintains

1. A secondary voltage constant ✓
2. B three-phase balance ✓
3. C zero iron loss ✓
4. D positive oil pressure and accommodates expansion ✓

Transformers are rated in

1. A kW only ✓
2. B kVA or MVA ✓
3. C kVAR only ✓
4. D hp only ✓

Iron losses in a transformer are

1. A constant from no load to full load ✓
2. B proportional to square of load ✓
3. C zero at no load ✓
4. D only at short circuit ✓

Copper losses in a transformer vary as

1. A load current linearly ✓
2. B inversely with load ✓
3. C independent of load ✓
4. D square of load current ✓

Equivalent resistance referred to secondary equals

1. A R1 only ✓
2. B R2 only ✓
3. C R2 + R1 × (Ns/Np)² ✓
4. D R1 × Np/Ns ✓

Leakage flux in a transformer links

1. A both windings equally ✓
2. B neither winding ✓
3. C only one winding ✓
4. D only the core ✓

Percentage impedance of a transformer indicates

1. A iron loss in watts ✓
2. B no-load current ✓
3. C turns ratio ✓
4. D voltage drop at rated current ✓

Two transformers can operate in parallel if they have

1. A same voltage ratio, phase shift, percent impedance, and polarity ✓
2. B different voltage ratios ✓
3. C opposite polarity only ✓
4. D any impedance ✓

Polarity test of a transformer determines

1. A core material type ✓
2. B relative direction of induced EMF in windings ✓
3. C oil dielectric strength ✓
4. D cooling class ✓

Subtractive polarity means

1. A opposite side terminals same polarity ✓
2. B no polarity exists ✓
3. C HV and LV terminals on same side have same instantaneous polarity ✓
4. D only delta connection possible ✓

Three single-phase transformers in bank can replace

1. A only auto-transformer ✓
2. B one three-phase transformer ✓
3. C only Scott connection ✓
4. D no other configuration ✓

Open-delta connection delivers

1. A 57.7% of full delta-delta capacity ✓
2. B 100% capacity ✓
3. C 33% capacity ✓
4. D 86.6% capacity ✓

Tertiary winding in Y-Y-Δ transformer bank is often

1. A open circuited always ✓
2. B used only for metering ✓
3. C connected to primary only ✓
4. D delta connected unbalanced load carrier ✓

Zero-sequence impedance of Y-Δ transformer is

1. A zero on all sides ✓
2. B equal to positive sequence ✓
3. C very high on Y side ✓
4. D negative value ✓

Instrument PT secondary is normally grounded for

1. A increasing voltage ✓
2. B reducing iron loss ✓
3. C changing turns ratio ✓
4. D safety and relay reference ✓

CT accuracy class 5P20 means

1. A 20% error at 5 A ✓
2. B 5% error up to 20 times rated current ✓
3. C 5 VA burden only ✓
4. D 20 kV insulation ✓

Magnetizing inrush current occurs when transformer is

1. A switched on at voltage zero crossing unfavorably ✓
2. B at full load steady state ✓
3. C during tap change off load ✓
4. D with secondary shorted ✓

To reduce switching inrush, transformers may use

1. A open secondary permanently ✓
2. B remove core laminations ✓
3. C increase frequency only ✓
4. D controlled switching or series resistance ✓

Core saturation during inrush causes

1. A large magnetizing current with harmonics ✓
2. B zero current ✓
3. C only fundamental at rated value ✓
4. D decreased iron loss ✓

Distribution transformers use

1. A only step-up for generation ✓
2. B only isolation 1:1 ✓
3. C step-down voltage for consumers ✓
4. D DC conversion ✓

Power transformer impedance is designed mainly for

1. A maximizing inrush ✓
2. B fault current limitation and system stability ✓
3. C eliminating regulation ✓
4. D zero copper loss ✓

Transformer oil serves as

1. A only structural support ✓
2. B only flux path ✓
3. C insulation and coolant ✓
4. D only current carrier ✓

Dissolved gas analysis (DGA) in transformers detects

1. A external lightning only ✓
2. B incipient insulation breakdown ✓
3. C meter calibration ✓
4. D tap position ✓

Winding hot-spot temperature limit for oil transformers is typically guided by

1. A insulation aging (IEEE/IEC standards) ✓
2. B only ambient air ✓
3. C secondary current alone ✓
4. D tap number ✓

FR3 or ester oil in transformers offers

1. A lower insulation than mineral oil ✓
2. B no cooling function ✓
3. C increased flammability ✓
4. D higher fire point and biodegradability ✓

Transformer tank is earthed to

1. A provide safety and fault current path ✓
2. B increase secondary voltage ✓
3. C block fault detection ✓
4. D eliminate Buchholz ✓

Lightning arrester on transformer protects against

1. A steady state overload ✓
2. B low power factor ✓
3. C negative regulation ✓
4. D surge overvoltages ✓

Surge impedance loading (SIL) relates to

1. A transformer iron loss ✓
2. B CT ratio ✓
3. C transmission line reactive power balance ✓
4. D tap changer steps ✓

A welding transformer is designed for

1. A constant voltage like PT ✓
2. B high drooping volt-ampere characteristic ✓
3. C 1:1 isolation only ✓
4. D zero leakage reactance ✓

Pulse transformer design emphasizes

1. A high iron loss ✓
2. B fast rise time and low leakage inductance ✓
3. C large magnetizing current ✓
4. D ONAN cooling only ✓

Three-phase transformer vector group Dy11 means

1. A star primary delta secondary 0° ✓
2. B delta-delta 30° lag ✓
3. C star-star 11° ✓
4. D delta primary, star secondary, 11 o'clock shift (−30°) ✓

Vector group Yd1 indicates phase displacement of

1. A −30° ✓
2. B 0° ✓
3. C +30° (1 o'clock) ✓
4. D 180° ✓

Parallel operation of Yy0 and Dd0 transformers is

1. A not possible due to phase shift mismatch ✓
2. B always ideal ✓
3. C required for Scott ✓
4. D same as Yd11 parallel ✓