EGSA Generator and Alternator Principles Questions and Answers — Questions and Answers
Question 1: A technician is troubleshooting a self-excited generator that fails to build up any voltage when started. The prime mover is confirmed to be operating at the correct speed. Which of the following is the most likely initial cause for this failure?
- The automatic voltage regulator (AVR) is faulty.
- The generator's field windings are open.
- Loss of residual magnetism in the field poles. (Correct answer)
- The exciter circuit breaker has tripped.
Correct answer: Loss of residual magnetism in the field poles.
For a self-excited generator to begin producing voltage, there must be a small amount of magnetism remaining in the iron cores of the field poles. This 'residual magnetism' creates a weak initial magnetic field that induces a small voltage in the armature windings as the generator spins. This small voltage then supplies current to the field windings, which strengthens the magnetic field, inducing more voltage, and so on, until the rated output voltage is achieved. Without residual magnetism, this process cannot start.
Question 2: What is the primary function of an Automatic Voltage Regulator (AVR) in an alternator?
- To adjust the prime mover's speed to control frequency.
- To maintain a constant output voltage under varying loads. (Correct answer)
- To protect the alternator from overcurrent conditions.
- To ensure the phase sequence of the output is correct.
Correct answer: To maintain a constant output voltage under varying loads.
The Automatic Voltage Regulator (AVR) continuously monitors the alternator's output voltage. It maintains a constant voltage level by adjusting the DC current supplied to the exciter field, which in turn controls the strength of the main rotor's magnetic field. As the electrical load on the generator changes, the AVR compensates to keep the output voltage stable.
Question 3: A 4-pole alternator is operating at a speed of 1800 RPM. What is the frequency of the generated AC voltage?
- 50 Hz
- 120 Hz
- 45 Hz
- 60 Hz (Correct answer)
Correct answer: 60 Hz
The relationship between frequency (f), the number of poles (P), and the synchronous speed (N) in RPM is given by the formula: f = (N * P) / 120. In this case, N = 1800 RPM and P = 4 poles. So, f = (1800 * 4) / 120 = 7200 / 120 = 60 Hz.
Question 4: Which of the following principles is used to determine the direction of induced current in the conductors of a generator's armature?
- Fleming's Left-Hand Rule
- Lenz's Law
- Fleming's Right-Hand Rule (Correct answer)
- Ohm's Law
Correct answer: Fleming's Right-Hand Rule
Fleming's Right-Hand Rule is specifically for generators. It relates the direction of motion of a conductor, the direction of the magnetic field, and the direction of the induced current. The thumb represents the direction of motion, the forefinger represents the magnetic field (North to South), and the middle finger indicates the direction of the induced current.
Question 5: In a synchronous generator, what is the purpose of the damper windings (or amortisseur windings) located in the rotor pole faces?
- To increase the main field strength for higher voltage output.
- To provide the main excitation for the rotor.
- To cool the rotor during high-load operation.
- To dampen rotor oscillations and assist in starting. (Correct answer)
Correct answer: To dampen rotor oscillations and assist in starting.
Damper windings are squirrel-cage-like windings that oppose changes in rotor speed relative to the synchronous speed. Their primary function is to dampen oscillations (a phenomenon called 'hunting') caused by sudden load changes, thereby improving system stability. They also provide the starting torque for synchronous motors, allowing them to start as induction motors.
Question 6: A technician observes that the terminal voltage of an alternator drops significantly as the load increases, even though the AVR appears to be functioning. Which of the following is a primary internal factor causing this voltage drop?
- A decrease in prime mover speed.
- The alternator's internal synchronous reactance. (Correct answer)
- An increase in the excitation current.
- A change in the ambient temperature.
Correct answer: The alternator's internal synchronous reactance.
The main internal factors causing voltage drop in an alternator under load are the armature resistance, leakage reactance, and armature reaction. These are collectively represented as synchronous reactance. As load current increases, the voltage drop across this internal impedance (Synchronous Reactance) increases (V_drop = I * Z), leading to a lower terminal voltage. The AVR works to counteract this effect, but the synchronous reactance is the underlying cause of the drop.
EGSA Generator and Alternator Principles Questions and Answers
A technician is troubleshooting a self-excited generator that fails to build up any voltage when started.
The prime mover is confirmed to be operating at the correct speed.
Which of the following is the most likely initial cause for this failure?