The synchronizing power between two alternators depends on

 

91. The open-circuit test of an alternator is conducted to determine

A) Leakage reactance
B) Armature resistance
C) Relationship between field current and generated EMF
D) Core losses only
Answer: C
Explanation:
In the open-circuit test, the alternator runs at rated speed with no load. The field current and generated voltage are recorded to obtain the O.C.C. (Open Circuit Characteristic).

---

92. The short-circuit test of an alternator is conducted to determine

A) Efficiency
B) Synchronous impedance
C) Voltage regulation only
D) Load characteristics
Answer: B
Explanation:
In the short-circuit test, the terminals are shorted, and the field current is adjusted to get rated current. Using this and open-circuit data, synchronous impedance (Zs) is determined.

---

93. The two main losses occurring in an alternator are

A) Copper and mechanical losses
B) Copper and iron losses
C) Magnetic and dielectric losses
D) Eddy current and hysteresis losses
Answer: B
Explanation:
Alternator losses include copper losses (I²R) in the armature and iron losses (hysteresis + eddy current) in the magnetic core.

---

94. The efficiency of an alternator depends mainly on

A) Load power factor
B) Voltage regulation
C) Frequency
D) Shaft speed
Answer: A
Explanation:
Alternator efficiency = Output power / Input power. Since output depends on power factor, efficiency is highest at unity PF.

---

95. The phasor representing internal generated voltage (E) in an alternator always

A) Leads terminal voltage at lagging PF
B) Lags terminal voltage at leading PF
C) Is equal to terminal voltage at unity PF
D) Both A and B
Answer: D
Explanation:

• For lagging PF → E leads V

• For leading PF → E lags V

• For unity PF → E ≈ V (same phase)

---

96. If the excitation of a synchronous generator fails suddenly, it will

A) Continue to generate voltage
B) Act as an induction generator
C) Stop generating voltage immediately
D) Burn out
Answer: B
Explanation:
When excitation fails, the rotor loses DC magnetism. The machine continues to run as an induction generator using residual magnetism and grid excitation.

---

97. The alternator output voltage waveform mainly depends on

A) Rotor shape
B) Flux distribution in the air gap
C) Speed of the prime mover
D) Number of poles only
Answer: B
Explanation:
Sinusoidal air-gap flux distribution leads to a sinusoidal EMF waveform.

---

98. Voltage regulation is defined as

A) Change in voltage from no-load to full-load as a % of full-load voltage
B) Change in current from no-load to full-load
C) Power factor variation
D) Efficiency variation
Answer: A
Explanation:

$$\text{Voltage Regulation} = \frac{E_{no-load} - V_{full-load}}{V_{full-load}} \times 100\%$$

---

99. If the load on a synchronous generator is removed suddenly, the terminal voltage

A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: A
Explanation:
On removing load, armature current → 0 → voltage drop in reactance disappears → terminal voltage rises (especially in over-excited condition).

---

100. The synchronizing power between two alternators depends on

A) Load angle (δ)
B) Excitation
C) Synchronous reactance
D) All of these
Answer: D
Explanation:
Synchronizing power $P_s = \frac{EV}{X_s} \cos δ$. It depends on excitation (E), reactance (Xs), and load angle (δ).