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Electrical Machines-I: Transformers, DC Motors, Induction & Synchronous Machines: 20 MCQs with answers

All 20 multiple-choice questions from this kit, free and with no sign-up.

Full kit · everything Flashcards · 30 Quick Revision · 25

Warm-up · 8 questions

Q1. The Open Circuit test on a single-phase transformer is performed to determine which quantity?

Beginner
A Leakage reactance B Core (iron) loss C Full-load copper loss D Winding resistance
Show answer & explanation

Core (iron) loss

The OC test energises one winding at rated voltage with the other open, so only core loss is measured.

Q2. In the Short Circuit test, the wattmeter reading primarily represents which loss?

Beginner
A Windage loss B Full-load copper loss C Core loss D Friction loss
Show answer & explanation

Full-load copper loss

At reduced voltage the flux is tiny, so core loss is negligible and the wattmeter reads copper loss.

Q3. During a polarity test, the voltmeter reads $V = V_1 - V_2$. What is the polarity?

Beginner
A Subtractive polarity B Reversed polarity C Zero polarity D Additive polarity
Show answer & explanation

Subtractive polarity

When the voltmeter reads the difference of the two voltages, the polarity is subtractive.

Q4. The torque equation of a DC motor is $T = \frac{P \phi Z I_a}{2\pi A}$. What does $A$ represent?

Beginner
A Number of poles B Number of parallel paths C Armature current D Number of conductors
Show answer & explanation

Number of parallel paths

In the torque equation, $A$ is the number of parallel paths in the armature winding.

Q5. For a DC series motor, torque is proportional to which quantity?

Beginner
A $I_a^2$ B $1/I_a$ C $\sqrt{I_a}$ D $I_a$
Show answer & explanation

$I_a^2$

In a series motor, flux $\phi \propto I_a$, so torque $T \propto \phi I_a \propto I_a^2$.

Q6. At standstill, what is the frequency of rotor currents in a 50 Hz induction motor?

Beginner
A 25 Hz B 50 Hz C 0 Hz D 100 Hz
Show answer & explanation

50 Hz

At standstill $s = 1$, so $f_r = sf = 50$ Hz.

Q7. Star-delta starting reduces the starting current to what fraction of the direct-delta value?

Beginner
A One-half B One-sixth C One-fourth D One-third
Show answer & explanation

One-third

In star, phase voltage is $1/\sqrt{3}$ of delta, so line current is reduced to one-third.

Q8. The OC test is performed at rated voltage on the LV side because core loss depends on flux, which depends on applied voltage; the SC test is performed at reduced voltage (5–10% of rated) to circulate full-load current without core loss.

Beginner
A True B False
Show answer & explanation

True

The OC test is performed at rated voltage on the LV side because core loss depends on flux, which depends on applied voltage; the SC test is performed at reduced voltage (5–10% of rated) to circulate full-load current without core loss.

Building up · 7 questions

Q9. A 10 kVA transformer has its maximum efficiency at 90% of rated output. At what load does maximum efficiency occur?

Intermediate
A At 50% of rated output B At no load C At full load D At 90% of rated output
Show answer & explanation

At 90% of rated output

Maximum efficiency occurs when copper loss equals core loss, which happens at the stated 90% load.

Q10. Which condition is NOT required for parallel operation of three-phase transformers?

Intermediate
A Same polarity B Same kVA rating C Same percentage impedance D Same voltage ratio
Show answer & explanation

Same kVA rating

Transformers of different kVA ratings can be paralleled; the other conditions must match.

Q11. A DC shunt motor's speed is to be increased above its rated value. Which method is used?

Intermediate
A Reduce field flux using a field rheostat B Insert resistance in armature circuit C Increase armature voltage D Increase load on the motor
Show answer & explanation

Reduce field flux using a field rheostat

Since $N \propto (V - I_a R_a)/\phi$, reducing flux $\phi$ increases speed.

Q12. Armature reaction in a DC machine causes which of the following?

Intermediate
A Reduction in armature current B Elimination of sparking C Increase in field flux D Shifting of the magnetic neutral axis
Show answer & explanation

Shifting of the magnetic neutral axis

Armature reaction distorts the main field and shifts the MNA, causing sparking at brushes.

Q13. A 4-pole, 50 Hz induction motor runs at a slip of 0.04. What is the rotor speed?

Intermediate
A 1500 rpm B 1440 rpm C 1350 rpm D 1400 rpm
Show answer & explanation

1440 rpm

$N_s = 120f/P = 1500$ rpm; $N = (1-s)N_s = 0.96 \times 1500 = 1440$ rpm.

Q14. In a 3-phase induction motor, maximum torque occurs at which slip?

Intermediate
A $s = X_2/R_2$ B $s = R_2/X_2$ C $s = 0$ D $s = 1$
Show answer & explanation

$s = R_2/X_2$

The torque-slip equation peaks when $s = R_2/X_2$.

Q15. The no-load test on a 3-phase induction motor gives which losses?

Intermediate
A Core loss plus friction and windage loss B Stray load loss only C Rotor copper loss only D Copper loss only
Show answer & explanation

Core loss plus friction and windage loss

At no load, the wattmeter reads core loss plus friction and windage loss.

Topper level · 5 questions

Q16. The synchronous impedance method of finding voltage regulation is called pessimistic because it:

Advanced
A Overestimates copper loss B Assumes zero armature resistance C Ignores saturation effects D Uses DC excitation
Show answer & explanation

Ignores saturation effects

The method ignores saturation, so it gives a higher (pessimistic) regulation value.

Q17. In the context of a single-phase transformer, if the Open Circuit (OC) test is performed, which of the following parameters is NOT directly derived from this test?

Advanced
A No-load current $I_0$ B Shunt branch parameters $R_0$ and $X_0$ C Full-load copper loss $P_{cu}$ D Core loss $P_i$
Show answer & explanation

Full-load copper loss $P_{cu}$

The OC test measures core loss and provides shunt branch parameters, but it does not measure full-load copper loss, which is determined during the Short Circuit (SC) test.

Q18. Given the torque equation for a DC motor, $T = rac{P \phi Z I_a}{2\pi A}$, if the armature current $I_a$ is doubled while keeping all other parameters constant, what will be the effect on the torque $T$?

Advanced
A Torque will remain the same B Torque will increase by a factor of four C Torque will decrease D Torque will double
Show answer & explanation

Torque will double

Since torque $T$ is directly proportional to the armature current $I_a$, doubling $I_a$ will result in doubling the torque, hence $T$ increases by a factor of two.

Q19. In a three-phase transformer, if the percentage impedance of two transformers is different, what is the most likely consequence when they are operated in parallel?

Advanced
A Circulating currents B Equal load sharing C Increased efficiency D No effect on operation
Show answer & explanation

Circulating currents

Different percentage impedances will lead to unequal load sharing, resulting in large circulating currents even at no load, which can cause overheating and inefficiency.

Q20. If a synchronous motor operates at a leading power factor, which of the following statements about the armature current $I_a$ and field current $I_f$ is true?

Advanced
A Armature current leads field current B Armature current lags field current C Both currents are in phase D Both currents are equal
Show answer & explanation

Armature current leads field current

In a synchronous motor operating at a leading power factor, the armature current $I_a$ leads the field current $I_f$, which is characteristic of over-excited operation.

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