EEE-360 Energy Systems and Power Electronics Midterm
Exam 2|Questions and Answers Latest Updated
2026/2027 (Graded A+)- ASU
1. For a given synchronous machine, corresponding to a stator current value of 𝐼𝑠𝑡𝑎, the
value of the armature reaction is 𝑋𝑎𝑟𝑚. For everything else remaining the same, if the
stator current now becomes 2 × 𝐼𝑠𝑡𝑎, what will be the new value of armature reaction?
a. 𝑋𝑎𝑟𝑚
b. ½ × 𝑋𝑎𝑟𝑚
c. 2 × 𝑋𝑎𝑟𝑚
d. Cannot be determined
Correct Answer: c. 2 × 𝑋𝑎𝑟𝑚
Rationale: Armature reaction is directly proportional to the stator (armature) current under linear
magnetic conditions. Doubling the stator current doubles the magnetomotive force (MMF)
produced by the armature, hence doubling the armature reaction effect. Saturation effects are
neglected as per “everything else remaining the same.”
2. As the speed of an alternator increases:
a. the frequency increases
b. the frequency decreases
c. the frequency remains constant, but power factor decreases
d. none of the above
Correct Answer: a. the frequency increases
Rationale: The frequency of an alternator is given by ( f = \frac{PN}{120} ), where ( P ) is the
number of poles and ( N ) is speed in rpm. Since poles are fixed, increasing speed directly
increases frequency proportionally.
3. The driving power from the prime mover driving the alternator is lost but the alternator
remains connected to the supply network and the field supply also remains on. The
alternator will
a. get burnt
b. behave as an induction motor but will rotate in the opposite direction
c. behave as a synchronous motor and will rotate in the same direction
d. behave as a synchronous motor but will rotate in a reverse direction to that
corresponding to generator action
, Correct Answer: c. behave as a synchronous motor and will rotate in the same direction
Rationale: When mechanical input is removed but the machine remains connected to an
energized grid with excitation intact, it draws power from the grid and operates as a synchronous
motor. The rotational direction remains unchanged due to synchronism with the grid frequency.
4. Maximum electric power output of a synchronous generator is:
a. ( \frac{E_{gen} V_t}{X_{syn}} )
b. ( \frac{V_t^2}{X_{syn}} )
c. ( \frac{E_{gen}^2}{X_{syn}} )
d. ( \frac{E_{gen} V_t}{2X_{syn}} )
Correct Answer: a. ( \frac{E_{gen} V_t}{X_{syn}} )
Rationale: The power output of a synchronous generator is ( P = \frac{E V}{X_s} \sin\delta ).
Maximum power occurs when ( \delta = 90^\circ ), hence ( \sin\delta = 1 ). Therefore, maximum
power is ( \frac{EV}{X_s} ).
5. Consider the following statements about a three-phase synchronous generator:
6. Typically, the power angle of the synchronous generator is about 30 degrees
7. In order to synchronize and interconnect to the network, the synchronous generator has
just to be started by a mechanical device and connected to the network
8. Reactive power is regulated by the regulation of the induced voltage by the rotor DC
excitation current
9. The brushless excitation systems use an AC alternator and rectifiers mounted on the
generator shaft to produce DC needed for the field windings on the rotor.
Regarding the above statements:
a. 1 alone is correct
b. 2 alone is incorrect
c. 2 and 4 are incorrect
d. All four are correct
Correct Answer: b. 2 alone is incorrect
Rationale:
• Statement 1 is correct: practical operation keeps power angle around 20–30° for stability.
• Statement 2 is incorrect: synchronization requires matching voltage, frequency, phase
sequence, and phase angle—not just starting and connecting.
• Statement 3 is correct: excitation controls internal EMF and hence reactive power.
• Statement 4 is correct: brushless excitation uses shaft-mounted alternator and rectifier
assembly.
Exam 2|Questions and Answers Latest Updated
2026/2027 (Graded A+)- ASU
1. For a given synchronous machine, corresponding to a stator current value of 𝐼𝑠𝑡𝑎, the
value of the armature reaction is 𝑋𝑎𝑟𝑚. For everything else remaining the same, if the
stator current now becomes 2 × 𝐼𝑠𝑡𝑎, what will be the new value of armature reaction?
a. 𝑋𝑎𝑟𝑚
b. ½ × 𝑋𝑎𝑟𝑚
c. 2 × 𝑋𝑎𝑟𝑚
d. Cannot be determined
Correct Answer: c. 2 × 𝑋𝑎𝑟𝑚
Rationale: Armature reaction is directly proportional to the stator (armature) current under linear
magnetic conditions. Doubling the stator current doubles the magnetomotive force (MMF)
produced by the armature, hence doubling the armature reaction effect. Saturation effects are
neglected as per “everything else remaining the same.”
2. As the speed of an alternator increases:
a. the frequency increases
b. the frequency decreases
c. the frequency remains constant, but power factor decreases
d. none of the above
Correct Answer: a. the frequency increases
Rationale: The frequency of an alternator is given by ( f = \frac{PN}{120} ), where ( P ) is the
number of poles and ( N ) is speed in rpm. Since poles are fixed, increasing speed directly
increases frequency proportionally.
3. The driving power from the prime mover driving the alternator is lost but the alternator
remains connected to the supply network and the field supply also remains on. The
alternator will
a. get burnt
b. behave as an induction motor but will rotate in the opposite direction
c. behave as a synchronous motor and will rotate in the same direction
d. behave as a synchronous motor but will rotate in a reverse direction to that
corresponding to generator action
, Correct Answer: c. behave as a synchronous motor and will rotate in the same direction
Rationale: When mechanical input is removed but the machine remains connected to an
energized grid with excitation intact, it draws power from the grid and operates as a synchronous
motor. The rotational direction remains unchanged due to synchronism with the grid frequency.
4. Maximum electric power output of a synchronous generator is:
a. ( \frac{E_{gen} V_t}{X_{syn}} )
b. ( \frac{V_t^2}{X_{syn}} )
c. ( \frac{E_{gen}^2}{X_{syn}} )
d. ( \frac{E_{gen} V_t}{2X_{syn}} )
Correct Answer: a. ( \frac{E_{gen} V_t}{X_{syn}} )
Rationale: The power output of a synchronous generator is ( P = \frac{E V}{X_s} \sin\delta ).
Maximum power occurs when ( \delta = 90^\circ ), hence ( \sin\delta = 1 ). Therefore, maximum
power is ( \frac{EV}{X_s} ).
5. Consider the following statements about a three-phase synchronous generator:
6. Typically, the power angle of the synchronous generator is about 30 degrees
7. In order to synchronize and interconnect to the network, the synchronous generator has
just to be started by a mechanical device and connected to the network
8. Reactive power is regulated by the regulation of the induced voltage by the rotor DC
excitation current
9. The brushless excitation systems use an AC alternator and rectifiers mounted on the
generator shaft to produce DC needed for the field windings on the rotor.
Regarding the above statements:
a. 1 alone is correct
b. 2 alone is incorrect
c. 2 and 4 are incorrect
d. All four are correct
Correct Answer: b. 2 alone is incorrect
Rationale:
• Statement 1 is correct: practical operation keeps power angle around 20–30° for stability.
• Statement 2 is incorrect: synchronization requires matching voltage, frequency, phase
sequence, and phase angle—not just starting and connecting.
• Statement 3 is correct: excitation controls internal EMF and hence reactive power.
• Statement 4 is correct: brushless excitation uses shaft-mounted alternator and rectifier
assembly.
