Deck 16: Alternators

Question 1

Alternators operate on the same principle of electromagnetic induction as direct current generators, but they have no commutator to change the alternating current produced in the armature into direct current.

Answer 1

True

Question 2

The magnets of a rotor are electromagnets and require some source of external direct current to excite the alternator. This direct current is known as rotation current.

Answer 2

False

Question 3

Several conditions must be met before parallel alternators can be used, including the following: ____.

A) The output voltage of the two alternators should be the same.
B) Phase A of one machine must be in sequence with phase A of the other machine, but phases B and C in each machine may be different.
C) The phases must be connected in such a manner that the phase rotation of each machine is different.
D) Phases A and B of one machine must be in sequence with phases A and B of the other machine, but phase C in each machine must be different.

Answer 3

A

Question 4

The following formula can be used to determine the output frequency of an alternator when the poles and RPM (revolutions per minute) are known: f = PS / 120.

Question 5

Alternators of small kVA rating are generally air-cooled.

Question 6

When a DC excitation current is disconnected, the collapsing magnetic field can induce a high voltage in the rotor winding.

Question 7

Most large alternators use an exciter that contains no ____.

A) brushes
B) rotor
C) electromagnet
D) DC supply

Question 8

Large-capacity alternators are often enclosed and operate in a(n) ____ atmosphere.

A) helium
B) oxygen
C) hydrogen
D) nitrogen

Question 9

The most common method of detecting when the phase rotation of one alternator is matched to the phase rotation of the other is with the use of two lights.

Question 10

Since the number of stator poles is constant for a particular machine, the ____ frequency is controlled by adjusting the speed of the rotor.

A) input
B) output
C) operational
D) index

Question 11

An instrument often used for paralleling two alternators is the paralleloscope.

Question 12

What is the output frequency of an alternator that contains six poles per phase and is turning at a speed of 1000 RPM?

A) 0.72 Hz
B) 7.2 Hz
C) 50 Hz
D) 20,000 Hz

Question 13

The following formula can be used to compute the amount of voltage induced in the stator winding: E = ____/108.

A) BLv
B) BL
C) Lv
D) B + Lv

Question 14

A rotor provides the ____ needed to induce voltage into the stator windings.

A) spin
B) magnetism
C) speed
D) momentum

Question 15

The revolving armature type alternator is the more used of the two basic types.

Question 16

A synchroscope measures the difference in voltage and frequency of two alternators. The pointer of the synchroscope is free to rotate in a ____-degree arc.

A) 90
B) 120
C) 180
D) 360

Question 17

One of the factors that determines the amount of induced voltage is the length of the conductor. This factor is often stated as number of turns of wire in the stator.

Question 18

The revolving field type alternator uses a ____ armature called the stator and a rotating magnetic field.

A) circulating
B) rotating
C) revolving
D) stationary

Question 19

Three factors determine the amount of output voltage of an alternator: length of the armature or stator conductors (number of turns), ____ of the magnetic field of the rotor, and speed of rotation of the rotor.

A) physical size
B) depth
C) direction
D) strength

Question 20

The output frequency of an alternator is determined by two factors, the size of stator poles and the speed of rotation of the rotor.