Synchronous Machines MCQs 2026

Hunting can be caused by sudden change in mechanical load or fault conditions on the power system

1. A false ✓
2. B true ✓
3. C only by incorrect pole number ✓
4. D only during motor starting never on generators ✓

Increasing damper winding resistance always improves hunting damping in all designs

1. A true ✓
2. B damper resistance has no effect ✓
3. C false ✓
4. D damper windings increase hunting ✓

V-curves of a synchronous motor plot armature current Ia versus field current If at constant input power

1. A true ✓
2. B false ✓
3. C Ia versus load torque at variable If ✓
4. D If versus speed at constant Ia ✓

The minimum point on the V-curve of a synchronous motor corresponds approximately to unity power factor

1. A false ✓
2. B zero power factor lagging ✓
3. C maximum armature current always ✓
4. D true ✓

Over-excitation of a synchronous motor causes it to operate at leading power factor

1. A false ✓
2. B lagging power factor ✓
3. C true ✓
4. D unity power factor only ✓

Under-excited synchronous motors operate at lagging power factor and draw reactive power from the bus

1. A true ✓
2. B false ✓
3. C leading power factor ✓
4. D zero reactive power always ✓

Inverted V-curves plot power factor versus field current If for a synchronous motor at constant load

1. A false ✓
2. B plot speed versus If ✓
3. C plot torque versus Ia only ✓
4. D true ✓

The inverted V-curve of a synchronous motor has unity power factor at the crest for a given load

1. A false ✓
2. B true ✓
3. C zero pf at the crest ✓
4. D leading pf at minimum If always ✓

A synchronous motor runs at constant speed determined by supply frequency and number of poles

1. A false ✓
2. B speed varies directly with load like an induction motor ✓
3. C speed is controlled only by field current ✓
4. D true ✓

Synchronous motors are not self-starting with dc field excitation alone on three-phase supply

1. A true ✓
2. B false ✓
3. C self-start like induction motors always ✓
4. D start without any auxiliary means ✓

Pony motor starting brings the synchronous motor rotor near synchronous speed before energizing the main motor field

1. A false ✓
2. B starts synchronous motor from rest with full field ✓
3. C true ✓
4. D replaces the need for three-phase supply ✓

Using amortisseur windings helps a synchronous motor develop starting torque as an induction motor

1. A false ✓
2. B true ✓
3. C prevents any starting torque ✓
4. D is used only on alternators never motors ✓

Reduced-voltage starting of synchronous motors limits inrush current during the induction starting phase

1. A false ✓
2. B increases inrush current ✓
3. C is not applicable to synchronous machines ✓
4. D true ✓

Dark lamp synchronizing method requires lamps dark when voltage, frequency, and phase sequence match

1. A false ✓
2. B true ✓
3. C lamps brightest at synchronism ✓
4. D lamps flicker at twice line frequency when synchronized ✓

In the bright lamp method of synchronizing, lamps are brightest at the instant of correct synchronism

1. A false ✓
2. B darkest at synchronism ✓
3. C true ✓
4. D never illuminate during procedure ✓

A synchroscope indicates whether the incoming alternator is running fast or slow relative to the bus

1. A true ✓
2. B false ✓
3. C measures only field current ✓
4. D replaces the need to match voltage magnitude ✓

Before paralleling an alternator, its voltage magnitude should be adjusted to match the bus voltage

1. A false ✓
2. B true ✓
3. C voltage should differ by 20% ✓
4. D only frequency must match not voltage ✓

When paralleling alternators, the phase sequence of the incoming machine must match the bus

1. A false ✓
2. B phase sequence is irrelevant ✓
3. C true ✓
4. D only one phase need match ✓

Two identical synchronous generators in parallel share load according to their speed-load characteristics and governor settings

1. A false ✓
2. B always equally regardless of settings ✓
3. C by armature resistance only ✓
4. D true ✓

For parallel operation of alternators on an infinite bus, frequency is fixed by the bus

1. A true ✓
2. B false ✓
3. C each machine sets its own frequency ✓
4. D frequency equals sum of individual speeds ✓

Reactive load sharing between parallel alternators is controlled mainly by adjustment of field excitation

1. A false ✓
2. B true ✓
3. C by prime mover throttle only ✓
4. D by armature resistance mismatch only ✓

Potier triangle method is used to determine voltage regulation and armature leakage reactance of alternators

1. A false ✓
2. B measures rotor inertia only ✓
3. C determines number of poles ✓
4. D true ✓

In a synchronous motor, increasing mechanical load at constant excitation increases armature current and torque angle

1. A false ✓
2. B true ✓
3. C decreases torque angle ✓
4. D changes speed proportionally to load ✓

If a synchronous generator loses excitation while on load, it may continue to deliver reduced power as an induction generator via damper action

1. A false ✓
2. B immediately stops delivering any power ✓
3. C speed becomes zero instantly ✓
4. D true ✓

The frequency of generated EMF in an alternator depends on rotor speed and number of pole pairs

1. A false ✓
2. B depends only on field current ✓
3. C true ✓
4. D depends only on load power factor ✓

Slip rings are required on the rotor of a conventional brushed synchronous machine to feed dc field current

1. A false ✓
2. B true ✓
3. C ac field is fed through the stator only ✓
4. D slip rings carry three-phase armature current to rotor ✓

A four-pole alternator running at 1500 rpm generates EMF at 50 Hz

1. A true ✓
2. B false ✓
3. C 25 Hz ✓
4. D 100 Hz ✓

In a salient-pole alternator the air gap is minimum along the direct axis

1. A false ✓
2. B true ✓
3. C uniform around the periphery ✓
4. D maximum along the direct axis ✓

Round-rotor alternators are typically used in turbo generators because they withstand high mechanical stress at high speed

1. A true ✓
2. B false ✓
3. C because they have salient poles for low-speed hydro plants ✓
4. D because round rotors have larger air gaps for hydro head ✓

The number of poles in an alternator is related to speed and frequency by P = 120f/N

1. A false ✓
2. B P = 60f/N ✓
3. C P = 120N/f ✓
4. D true ✓

In a hydro alternator the rotor is usually salient-pole type running at lower speed

1. A false ✓
2. B cylindrical rotor at 3000 rpm ✓
3. C true ✓
4. D squirrel-cage rotor ✓

The generated EMF per phase of an alternator is proportional to the flux per pole

1. A false ✓
2. B true ✓
3. C inversely proportional to flux per pole ✓
4. D independent of flux per pole ✓

The standard EMF equation of an alternator per phase is E = 4.44 f φ T_kw

1. A true ✓
2. B false ✓
3. C E = 2.22 f φ T_kw ✓
4. D E = 4.44 f φ / T_kw ✓

Increasing the speed of an alternator while flux remains constant increases the generated EMF

1. A true ✓
2. B false ✓
3. C decreases EMF ✓
4. D has no effect on EMF ✓

The winding factor of an alternator accounts for distributed winding and short-pitched coils

1. A false ✓
2. B only rotor skew ✓
3. C true ✓
4. D only stator slot leakage ✓

Synchronous reactance Xs of an alternator is the sum of armature reactance and armature leakage reactance

1. A false ✓
2. B only field winding reactance ✓
3. C rotor resistance only ✓
4. D true ✓

Synchronous reactance is measured by the open-circuit and short-circuit test on an alternator

1. A false ✓
2. B true ✓
3. C only by insulation resistance test ✓
4. D only by no-load saturation curve ✓

The synchronous impedance of an alternator is represented as Zs = Rs + jXs in the per-phase equivalent circuit

1. A false ✓
2. B true ✓
3. C Zs = jXs only always ✓
4. D Zs = Rs only ✓

Power angle δ in a synchronous machine is the angle between excitation EMF E and terminal voltage V

1. A true ✓
2. B false ✓
3. C angle between Ia and V only ✓
4. D angle between field flux and armature current ✓

For a cylindrical-rotor alternator supplying active power to an infinite bus, power is approximately P = (EV/Xs) sin δ

1. A false ✓
2. B P = (EV/Xs) cos δ ✓
3. C true ✓
4. D P = EVXs sin δ ✓

Maximum power transfer from a synchronous generator to an infinite bus occurs at δ = 90° in the simplified model

1. A true ✓
2. B false ✓
3. C δ = 0° ✓
4. D δ = 45° only for all machines ✓

If load on a synchronous generator increases, the rotor angle δ increases to deliver more power

1. A false ✓
2. B δ decreases ✓
3. C true ✓
4. D δ remains fixed regardless of load ✓

Pull-out torque or power of a synchronous motor corresponds to operation near maximum power angle

1. A false ✓
2. B occurs at δ = 0 ✓
3. C occurs when excitation is zero only ✓
4. D true ✓

In the power-angle equation, increasing excitation EMF E for fixed δ increases active power output

1. A false ✓
2. B true ✓
3. C decreases active power ✓
4. D affects only reactive power never active ✓

Damper or amortisseur windings are copper bars embedded in the pole faces of salient-pole alternators

1. A false ✓
2. B placed only on the stator teeth ✓
3. C true ✓
4. D made of permanent magnet material ✓

The primary purpose of damper windings in a synchronous alternator is to damp rotor oscillations during transients

1. A true ✓
2. B false ✓
3. C to increase synchronous speed ✓
4. D to replace the main field winding ✓

Damper windings help suppress hunting when load on an alternator changes suddenly

1. A false ✓
2. B cause hunting ✓
3. C eliminate the need for governor action entirely ✓
4. D true ✓

Hunting in synchronous machines refers to periodic oscillations of rotor angle or speed about the synchronous value

1. A false ✓
2. B true ✓
3. C continuous acceleration above sync speed ✓
4. D steady-state load sharing only ✓

Active load sharing between parallel alternators is controlled mainly by the prime mover governor setting

1. A false ✓
2. B by field excitation only ✓
3. C true ✓
4. D by synchronous reactance alone ✓

The short-circuit ratio of an alternator is the ratio of field current for rated open-circuit voltage to field current for rated short-circuit current

1. A false ✓
2. B ratio of short-circuit power to rated power ✓
3. C ratio of Xs to armature resistance ✓
4. D true ✓

A higher short-circuit ratio in an alternator generally means better steady-state voltage regulation

1. A true ✓
2. B false ✓
3. C poorer voltage regulation ✓
4. D no relation to voltage regulation ✓

Short-circuit ratio is inversely related to per-unit synchronous reactance in approximate design relations

1. A false ✓
2. B directly proportional to Xs always ✓
3. C unrelated to machine reactance ✓
4. D true ✓

Brushless excitation in alternators uses an ac exciter and rotating rectifiers on the main rotor shaft

1. A false ✓
2. B true ✓
3. C uses slip rings and carbon brushes on the main field ✓
4. D eliminates the need for any excitation ✓

A disadvantage eliminated by brushless excitation is brush maintenance and contamination at the main field

1. A false ✓
2. B need for automatic voltage regulator ✓
3. C true ✓
4. D all excitation losses ✓

Turbo alternators have smaller diameter and longer axial length compared to hydro alternators of similar rating

1. A true ✓
2. B false ✓
3. C larger diameter and shorter length always ✓
4. D identical proportions for same kVA ✓

Hydro alternators typically have more poles than turbo alternators of the same frequency

1. A false ✓
2. B true ✓
3. C fewer poles because speed is higher ✓
4. D same pole count for same frequency ✓

Turbo alternators are usually hydrogen-cooled in large ratings to reduce windage loss and improve cooling

1. A false ✓
2. B always air-cooled only ✓
3. C true ✓
4. D oil-filled stator only ✓

In a salient-pole machine, reluctance torque contributes to total torque in addition to excitation torque

1. A false ✓
2. B only in induction motors ✓
3. C absent in all synchronous machines ✓
4. D true ✓

Two-reaction theory is used to analyze salient-pole synchronous machines with separate direct and quadrature reactances

1. A true ✓
2. B false ✓
3. C applies only to dc machines ✓
4. D replaces the need for any phasor diagram ✓

Cylindrical-rotor alternators have nearly equal direct-axis and quadrature-axis synchronous reactances

1. A false ✓
2. B Xd much greater than Xq always ✓
3. C zero quadrature reactance ✓
4. D true ✓

Armature reaction in an alternator at unity power factor is cross-magnetizing in effect

1. A false ✓
2. B purely demagnetizing always ✓
3. C true ✓
4. D purely magnetizing always ✓

At lagging power factor load, armature reaction in an alternator is partially demagnetizing

1. A true ✓
2. B false ✓
3. C magnetizing ✓
4. D has no effect on air-gap flux ✓

At leading power factor load, armature reaction in an alternator is partially magnetizing

1. A false ✓
2. B true ✓
3. C demagnetizing ✓
4. D cross-magnetizing only for all pf ✓

Synchronous motors are used in industry for power factor correction when over-excited

1. A false ✓
2. B only for variable speed drives ✓
3. C only at lagging pf when under-excited ✓
4. D true ✓

A synchronous condenser is an over-excited synchronous motor running without mechanical load to supply reactive power

1. A false ✓
2. B an under-excited generator only ✓
3. C true ✓
4. D a type of capacitor bank with rotor windings ✓

The capability curve of a synchronous machine limits simultaneous real and reactive power by heating and stability

1. A true ✓
2. B false ✓
3. C shows only iron loss ✓
4. D applies to induction machines only ✓

When an alternator is synchronized onto a bus, the closing switch should be made near zero phase difference

1. A false ✓
2. B true ✓
3. C at maximum phase difference ✓
4. D phase difference is unimportant if voltages equal ✓

Circulating current between paralleled alternators can flow if their terminal voltages are not equal

1. A true ✓
2. B false ✓
3. C only if frequencies differ never for voltage mismatch ✓
4. D prevented entirely by synchronous reactance ✓

An alternator operating on an infinite bus cannot change system frequency by itself

1. A false ✓
2. B raises frequency by increasing excitation ✓
3. C true ✓
4. D sets bus frequency to its no-load speed ✓