Three-Phase Induction Motors MCQs 2026

The slip of a three-phase induction motor is defined as

1. A (Nr - Ns) / Nr ✓
2. B Nr / Ns ✓
3. C Ns / Nr ✓
4. D (Ns - Nr) / Ns ✓

Synchronous speed of a 50 Hz, 4-pole induction motor is

1. A 3000 rpm ✓
2. B 750 rpm ✓
3. C 1500 rpm ✓
4. D 1000 rpm ✓

At standstill, slip of an induction motor is

1. A 0 ✓
2. B 0.5 ✓
3. C infinity ✓
4. D 1 ✓

At synchronous speed, slip of an induction motor is

1. A 1 ✓
2. B 0.5 ✓
3. C 0 ✓
4. D negative ✓

Rotor frequency in a running induction motor equals

1. A f / s ✓
2. B f + s ✓
3. C 2 × f ✓
4. D s × f ✓

For a 50 Hz motor at 4% slip, rotor frequency is

1. A 50 Hz ✓
2. B 4 Hz ✓
3. C 0.5 Hz ✓
4. D 2 Hz ✓

The torque-slip curve of an induction motor is approximately linear in the region

1. A low slip (stable operating region) ✓
2. B high slip near breakdown ✓
3. C only at synchronous speed ✓
4. D only at standstill ✓

Breakdown torque of an induction motor occurs at

1. A zero slip ✓
2. B synchronous speed ✓
3. C a specific slip beyond which torque falls sharply ✓
4. D infinite slip only ✓

Starting torque of a squirrel-cage motor is typically

1. A 1.5 to 2.5 times rated torque ✓
2. B equal to breakdown torque always ✓
3. C less than 0.5 times rated torque ✓
4. D zero until rated speed ✓

Direct-on-line (DOL) starting applies

1. A reduced voltage through rotor resistance ✓
2. B only half the poles initially ✓
3. C DC excitation to rotor ✓
4. D full line voltage to stator terminals at start ✓

Main disadvantage of DOL starting for large motors is

1. A no starting torque ✓
2. B permanent speed reduction ✓
3. C need for slip rings ✓
4. D very high starting current (5-7 times FLC) ✓

Star-delta starting reduces starting current to approximately

1. A one-third of DOL value ✓
2. B one-half ✓
3. C one-tenth ✓
4. D same as DOL ✓

Star-delta starter is applicable only when

1. A motor is designed for delta running with six leads accessible ✓
2. B motor has wound rotor ✓
3. C motor always runs in star ✓
4. D supply is single phase ✓

Autotransformer starting reduces voltage to motor by

1. A inserting resistance in rotor circuit ✓
2. B changing number of poles ✓
3. C reversing two phases ✓
4. D tapping a fraction of line voltage ✓

Rotor resistance starting is used with

1. A wound-rotor (slip-ring) induction motors ✓
2. B squirrel-cage motors only ✓
3. C synchronous motors only ✓
4. D single-phase shaded-pole motors ✓

Adding external resistance in wound-rotor circuit at start

1. A increases starting torque and reduces starting current ✓
2. B reduces both torque and current ✓
3. C increases speed above synchronous ✓
4. D eliminates slip ✓

Squirrel-cage rotor construction uses

1. A slip rings and brush gear ✓
2. B copper or aluminum bars short-circuited by end rings ✓
3. C separate DC winding ✓
4. D permanent magnets on rotor ✓

Wound-rotor motors differ from cage motors mainly by

1. A higher synchronous speed ✓
2. B absence of stator winding ✓
3. C accessible rotor winding via slip rings for external resistance ✓
4. D operation only at unity slip ✓

Crawling in induction motors is caused by

1. A harmonics (especially 7th) producing forward torque at sub-synchronous speeds ✓
2. B excessive load only ✓
3. C unity power factor ✓
4. D open circuit in one phase ✓

Cogging in induction motors occurs when

1. A rotor runs above synchronous speed ✓
2. B rotor and stator slot numbers create locking tendency at low voltage ✓
3. C frequency is doubled ✓
4. D motor is overloaded ✓

Ratio of starting torque to full-load torque is called

1. A starting torque ratio or Tst/TL ✓
2. B slip ratio ✓
3. C breakdown factor only ✓
4. D service factor ✓

High-resistance rotor bars increase

1. A synchronous speed ✓
2. B starting torque but may reduce running efficiency ✓
3. C breakdown torque always to infinity ✓
4. D power factor at full load only ✓

DOL starting torque is proportional to

1. A square of slip only ✓
2. B inverse of frequency cubed ✓
3. C square of applied voltage ✓
4. D rotor external resistance only ✓

Star connection at start compared to delta run gives starting torque

1. A three times delta torque ✓
2. B equal torque ✓
3. C nine times delta torque ✓
4. D one-third of delta-start torque ✓

Autotransformer starter set at 70% tap gives approximately

1. A 70% torque ✓
2. B 49% of full-voltage starting torque ✓
3. C 30% torque ✓
4. D 100% torque ✓

For maximum starting torque with rotor resistance, total rotor circuit resistance should equal

1. A stator resistance R1 only ✓
2. B zero ohms ✓
3. C infinite ohms ✓
4. D rotor standstill reactance X2 ✓

Slip rings on wound-rotor motors are used to

1. A feed DC to stator ✓
2. B connect external resistance or recovery equipment to rotor winding ✓
3. C measure air-gap flux only ✓
4. D cool the bearings ✓

Maintenance of squirrel-cage motors is generally lower because

1. A rotor spins without any magnetic field ✓
2. B stator has no winding ✓
3. C rotor has no brushes or external connections ✓
4. D they never heat up ✓

To reduce cogging, designers often choose

1. A equal stator and rotor slot counts ✓
2. B rotor slots skewed and slot combinations avoiding equal numbers ✓
3. C zero air gap ✓
4. D single-phase supply ✓

Skewing rotor bars helps reduce

1. A synchronous speed ✓
2. B rated voltage ✓
3. C number of poles ✓
4. D cogging and noise ✓

Outer cage in double-cage rotor has

1. A no function at start ✓
2. B lower resistance than inner cage always ✓
3. C direct DC connection ✓
4. D higher resistance and lower leakage inductance for starting ✓

Inner cage of double-cage rotor primarily carries

1. A only harmonics at crawl speed ✓
2. B running current at low rotor frequency ✓
3. C stator current directly ✓
4. D excitation for synchronous speed ✓

Below base speed in V/f drive, increasing frequency without raising voltage causes

1. A flux increase ✓
2. B speed above synchronous mechanically ✓
3. C zero slip operation ✓
4. D flux weakening and reduced torque capability ✓

Above base speed, V/f drives often operate in

1. A constant torque with rising voltage unlimited ✓
2. B constant power region with weakened flux ✓
3. C blocked rotor mode ✓
4. D pure DOL connection ✓

Pole-changing from 4 poles to 8 poles at same 50 Hz halves

1. A supply frequency ✓
2. B rotor resistance ✓
3. C synchronous speed ✓
4. D stator slots ✓

Two separate windings for two speeds is

1. A impossible on induction machines ✓
2. B same as rotor resistance start ✓
3. C only for DC motors ✓
4. D less economical than Dahlander but flexible in ratio ✓

Static Kramer drive inserts

1. A resistance only permanently ✓
2. B controlled converter in rotor circuit to recover slip energy ✓
3. C DOL contactor in stator ✓
4. D permanent magnet excitation ✓

Air-gap power Pg in an induction motor is approximately

1. A only stator copper loss ✓
2. B only windage loss ✓
3. C power transferred from stator to rotor across air gap ✓
4. D output shaft power directly ✓

Efficiency of induction motor is highest near

1. A no-load always ✓
2. B rated load where copper and iron losses are balanced ✓
3. C locked rotor always ✓
4. D slip equal to 1 ✓

Stator copper loss in equivalent circuit is represented by

1. A I1²R1 ✓
2. B I2²R2/s only ✓
3. C mechanical output ✓
4. D s×Pg term alone ✓

Referred rotor leakage reactance X2 in equivalent circuit is

1. A stationary-rotor reactance scaled to stator side ✓
2. B variable with mechanical output only ✓
3. C equal to synchronous reactance of supply ✓
4. D zero at all slips ✓

No-load current in circle diagram is mainly

1. A magnetizing component with small real loss part ✓
2. B purely resistive short-circuit current ✓
3. C rotor blocked-rotor current ✓
4. D DC bias current ✓

Blocked-rotor point on circle diagram corresponds to

1. A s = 0 operation ✓
2. B maximum efficiency point ✓
3. C field weakening region ✓
4. D s = 1 and starting impedance condition ✓

NEMA design A motors have

1. A low breakdown torque ✓
2. B external rotor resistance required ✓
3. C no cage rotor ✓
4. D high breakdown torque and normal starting current ✓

IP23 enclosure typically indicates

1. A dust-tight and jet-proof ✓
2. B no protection at all ✓
3. C oil-immersed construction ✓
4. D protected against fingers and dripping water up to 15° from vertical ✓

Double-cage rotor design improves

1. A synchronous speed ✓
2. B power factor at no load only ✓
3. C need for slip rings ✓
4. D starting performance while maintaining good running efficiency ✓

Deep-bar or double-cage rotors increase starting torque because

1. A effective rotor resistance is higher at low rotor frequency ✓
2. B stator poles are doubled ✓
3. C supply frequency is increased at start ✓
4. D slip is forced to zero ✓

V/f control of induction motors maintains

1. A approximately constant flux below base speed ✓
2. B constant rotor resistance ✓
3. C synchronous speed fixed ✓
4. D unity slip always ✓

Pole-changing speed control alters

1. A rotor resistance only ✓
2. B line frequency only ✓
3. C synchronous speed by changing effective stator poles ✓
4. D air-gap length ✓

Dahlander winding arrangement allows

1. A only wound-rotor starting ✓
2. B synchronous speed above 3000 rpm always ✓
3. C two-speed operation with single winding ✓
4. D DC injection braking only ✓

Slip energy recovery (Scherbius/Kramer system) returns

1. A slip power from wound-rotor circuit to supply or load ✓
2. B stator copper loss to rotor ✓
3. C mechanical power to slip rings ✓
4. D all losses as heat only ✓

Rotor copper loss in an induction motor equals

1. A s × Pg ✓
2. B Pg / s ✓
3. C (1-s) × Pg ✓
4. D s × Pout ✓

If air-gap power is 10 kW and slip is 0.04, rotor copper loss is

1. A 4 kW ✓
2. B 40 W ✓
3. C 400 W ✓
4. D 9.6 kW ✓

Mechanical power developed by rotor equals

1. A s × Pg ✓
2. B (1 - s) × Pg ✓
3. C Pg / (1-s) ✓
4. D s × Pout ✓

The per-phase approximate equivalent circuit of an induction motor includes

1. A only stator resistance ✓
2. B stator resistance, leakage reactance, and referred rotor branch R2/s + jX2 ✓
3. C DC field winding ✓
4. D commutator segments ✓

In the equivalent circuit, R2/s represents

1. A rotor resistance and electromechanical power conversion ✓
2. B fixed stator leakage only ✓
3. C friction loss alone ✓
4. D supply frequency ✓

Circle diagram of an induction motor graphically shows

1. A only hysteresis loop of core ✓
2. B commutation sparking ✓
3. C open-circuit stator curve only ✓
4. D current locus and torque/power relations under varying slip ✓

From circle diagram, maximum torque corresponds to

1. A zero stator current ✓
2. B infinite slip only visually ✓
3. C a specific current and power factor point on the locus ✓
4. D synchronous speed point ✓

NEMA design B squirrel-cage motors typically have

1. A very high starting torque, high current ✓
2. B normal starting torque and normal starting current ✓
3. C low starting torque, low current ✓
4. D no starting torque ✓

NEMA design D motors are characterized by

1. A low breakdown torque ✓
2. B high starting torque with high slip at rated load ✓
3. C synchronous operation ✓
4. D wound-rotor construction ✓

IP55 motor enclosure rating means

1. A dust-protected and water jets from any direction will not harm motor ✓
2. B totally submersible without limit ✓
3. C open drip-proof only ✓
4. D explosion-proof for mines ✓

Induction motors are preferred for constant-speed industrial drives because

1. A they always run above synchronous speed ✓
2. B they require commutators ✓
3. C they are rugged, low cost, and need little maintenance ✓
4. D rotor must be fed with DC ✓

A 6-pole, 50 Hz motor has synchronous speed of

1. A 1000 rpm ✓
2. B 1500 rpm ✓
3. C 750 rpm ✓
4. D 3000 rpm ✓

If a 4-pole, 50 Hz motor runs at 1455 rpm, slip is

1. A 0.05 ✓
2. B 0.97 ✓
3. C 0.03 ✓
4. D 0.15 ✓

Frequency of rotor currents at standstill equals

1. A zero always ✓
2. B twice supply frequency ✓
3. C mechanical speed in Hz ✓
4. D supply stator frequency ✓

As motor accelerates, rotor frequency

1. A decreases proportionally with slip ✓
2. B increases linearly with speed ✓
3. C remains equal to supply frequency ✓
4. D becomes negative ✓

Operating point of motor on torque-slip curve at rated load is

1. A beyond breakdown torque ✓
2. B at zero torque always ✓
3. C on the stable side left of breakdown torque peak ✓
4. D only at slip equal to 1 ✓

If load torque exceeds breakdown torque, the motor

1. A speeds up above Ns ✓
2. B stalls or slows drastically with excessive slip ✓
3. C maintains rated speed automatically ✓
4. D becomes a generator without change ✓

NEMA design C motors provide

1. A low starting torque ✓
2. B operation only as generator ✓
3. C wound rotor only ✓
4. D high starting torque with moderate starting current ✓

Service factor of 1.15 on a motor nameplate means

1. A efficiency is 15% ✓
2. B slip is fixed at 0.15 ✓
3. C motor can operate at 15% overload for short periods under specified conditions ✓
4. D speed is 15% above Ns ✓

IP44 enclosure offers protection against

1. A complete submersion ✓
2. B no dust entry ever without test ✓
3. C solid objects over 1 mm and splashing water from any direction ✓
4. D internal explosions ✓

Totally enclosed fan-cooled (TEFC) motors dissipate heat by

1. A open ventilation through windings ✓
2. B external shaft-mounted fan blowing over ribbed frame ✓
3. C oil circulation only ✓
4. D rotor slip rings cooling water ✓

Induction motors driving centrifugal pumps are often selected with

1. A constant heavy overload at start only ✓
2. B cubic torque-speed load characteristic consideration ✓
3. C synchronous speed operation required ✓
4. D commutator maintenance schedule ✓

Conveyor belt applications may require motors with

1. A zero breakdown torque ✓
2. B high starting torque to overcome static friction ✓
3. C synchronous speed above 6000 rpm ✓
4. D single-phase capacitor start only ✓

Crane hoist duty often uses wound-rotor motors because

1. A they cannot reverse ✓
2. B they eliminate slip ✓
3. C cage rotors are banned ✓
4. D rotor resistance control gives high starting torque and speed control ✓

Large compressor motors may use soft starters to

1. A increase synchronous speed ✓
2. B limit inrush current while providing adequate starting torque ✓
3. C remove need for bearings ✓
4. D operate as synchronous machines ✓

A 2-pole, 60 Hz motor synchronous speed is

1. A 3600 rpm ✓
2. B 1800 rpm ✓
3. C 1200 rpm ✓
4. D 7200 rpm ✓

At 3% slip, rotor speed of 4-pole 50 Hz motor is approximately

1. A 1500 rpm ✓
2. B 1410 rpm ✓
3. C 1455 rpm ✓
4. D 1470 rpm ✓

Negative slip in induction motor operation indicates

1. A motor is off ✓
2. B regenerative braking with rotor faster than synchronous speed ✓
3. C normal motoring at full load ✓
4. D locked rotor test ✓

In motoring mode, rotor always turns

1. A faster than synchronous speed ✓
2. B at exactly Ns under load ✓
3. C slower than synchronous speed (positive slip) ✓
4. D backward relative to field always ✓

Torque is maximum on stable operating region when

1. A load demand matches motor torque at intersection before breakdown ✓
2. B slip is unity at rated speed ✓
3. C voltage is zero ✓
4. D rotor is open circuited ✓

Pull-out torque is also known as

1. A starting torque ✓
2. B full-load torque only ✓
3. C breakdown or maximum torque ✓
4. D cogging torque ✓

Increasing supply voltage to a running induction motor generally

1. A increases torque and may increase speed slightly (lower slip) ✓
2. B decreases flux always ✓
3. C forces synchronous speed change ✓
4. D opens rotor circuit ✓

Inserting too much rotor resistance at full speed causes

1. A speed above Ns in motoring ✓
2. B excessive slip, poor efficiency, and reduced speed ✓
3. C unity power factor automatically ✓
4. D elimination of heating ✓

Thermal limit during repeated DOL starts is critical because

1. A rotor and stator heat accumulates from high I² during low-speed start ✓
2. B starting uses no current ✓
3. C slip is zero during start ✓
4. D no losses occur in rotor ✓

Phase sequence reversal of three-phase supply to motor

1. A changes number of poles ✓
2. B doubles synchronous speed ✓
3. C reverses direction of rotating field and rotor rotation ✓
4. D converts motor to generator only ✓

Single phasing during operation can

1. A improve efficiency ✓
2. B damage motor due to unequal currents and loss of torque ✓
3. C increase breakdown torque safely ✓
4. D raise power factor to unity ✓

Core loss in induction motor depends mainly on

1. A rotor slip only ✓
2. B shaft diameter only ✓
3. C supply voltage and frequency (flux density) ✓
4. D external rotor resistance ✓

Friction and windage loss component increases approximately with

1. A slip only at start ✓
2. B square of voltage alone ✓
3. C rotor resistance setting ✓
4. D speed (higher at higher rotor speed) ✓

Power factor of induction motor at light load is usually

1. A unity leading ✓
2. B low lagging due to dominant magnetizing current ✓
3. C unity lagging always ✓
4. D zero ✓