Generator and Alternator Principles Flashcards

What is the primary function of slip rings in an AC alternator?

Answer: Transfer current to the rotating field winding

Slip rings provide a continuous electrical connection to the rotating field winding, supplying DC excitation current without interruption.

In a synchronous generator, what happens to the output frequency if the rotor speed increases by 10%?

Answer: Frequency increases by 10%

Output frequency is directly proportional to rotor speed (f = P×N/120), so a 10% speed increase produces a 10% frequency increase.

Which type of rotor is most commonly used in high-speed steam turbine-driven generators?

Answer: Cylindrical (round) rotor

Cylindrical rotors are used in high-speed (3600 RPM) turbo-generators because their smooth surface minimizes windage losses and mechanical stress.

What is meant by 'armature reaction' in a synchronous generator?

Answer: Effect of armature current magnetomotive force on the main field flux

Armature reaction is the distortion and weakening of the main magnetic field caused by the magnetomotive force produced by load current in the armature windings.

A 4-pole alternator must rotate at what speed (RPM) to produce 60 Hz output?

Answer: 1800 RPM

Using f = P×N/120: N = 120×60/4 = 1800 RPM for a 4-pole machine at 60 Hz.

What is the purpose of damper windings (amortisseur windings) in a synchronous generator?

Answer: Dampen rotor oscillations and suppress hunting

Damper windings are short-circuited bars embedded in rotor pole faces that produce braking torque to suppress rotor hunting and oscillations.

How does increasing field excitation beyond the unity power factor point affect a synchronous generator connected to an infinite bus?

Answer: It causes the generator to supply reactive power (over-excitation)

Over-excitation in a synchronous generator causes it to supply reactive power (act as a capacitor) to the bus, boosting system voltage.