AC/DC Chapter 31
What are the three factors that determine the output voltage of an alternator? 23. 24. 25.
23. the length of the armature or stator conductors 24. the strength of the magnetic field of the rotor 25. the speed of the rotation of the rotor
What two factors determine the output frequency of an alternator? 26. 27.
26. The number of stator poles 27. the speed of rotation of the rotor
In Fig. 1 identify the components of this three-phase armature. 29. 30. 31.
29. Shaft 31. Windings 30. Sliprings
In Fig 4 identify the sections of this brushless exciter circuit. 32. 33. 34.
32. Field 33. Rectifier 34. Armature
______________ operate on the same principle of electromagnetic induction as DC generators, but they have no commutator. A. Alternators B. Generators C. Oscillators D. Convertors
A. Alternators (p. 766)
The direct current that is an input to the alternator's rotor is the _____________ current. A. excitation B. starting C. synchronizing D. initiating
A. excitation (p. 769)
The output frequency of an alternator is controlled by the ____________. A. number of stator poles and rotor rotation speed B. input voltage C. number of stator poles and number of rotor bars D. input voltage and torque
A. number of stator poles and rotor rotation speed (p. 773)
The direction of magnetic field rotation is known as ____________ rotation. A. phase B. loop
A. phase (p. 775)
The non-rotating part of an alternator is called the ____________. A. stator B. exciter C. rotor D. synchronizer
A. stator (p. 766)
A(n) ____________ is used to measure the difference in voltage and frequency between two alternators. A. synchronoscope B. Ohmmeter C. Volt-meter D. Diode
A. synchronoscope (p 776)
An alternator containing six poles per phase is turning at 800 RPM. What is the output frequency? A. 30 Hz B. 40 Hz C. 50 Hz D. 60 Hz
B. 40 Hz
Because some of the input energy to the alternator end up bing converted into heat, often cooling is required. The main types of cooling media are ____________. A. gas and liquid B. air and hydrogen C. air and water D. hydrogen and oxygen
B. Air and hydrogen (p. 772)
The higher power output alternators use a revolving _____________. A. armature B. field C. stator D. exciter
B. field (p 766)
The revolving armature type of alternator is capable of ____________ power output and is, therefore, ____________ often used than the revolving field type. A. less, more B. less, less C. more, less D. more, more
B. less, less (p. 766)
If one alternator does not produce sufficient power to supply a load, another alternator can be connected into the circuit. The second alternator is connected in ____________. A. series B. parallel
B. parallel (p. 775)
Using a(n) ____________ is one method of preventing induced voltage from becoming excessive. A. paralleling switch B. inductive bypass C. field discharge resistor D. capacitive resistor.
C. field discharge resistor (p 777)
In order to increase the efficiency of the cooling, some alternators operate in an atmosphere of ____________. A. helium B. oxygen C. hydrogen D. nitrogen
C. hydrogen (p. 772)
The rotating part of an alternator is called the ____________. A. stator B. exciter C. rotor D. synchronizer
C. rotor (p. 769)
Connection to the armature of an alternator is normally done by using ____________. A. commutators B. diodes C. sliprings D. wires
C. sliprings (p766)
A three-phase alternator containing six poles is turning at 1200 RPM. What is the output frequency? A. 30 Hz B. 40 Hz C. 50 Hz D. 60 Hz
D. 60 Hz
An alternator containing eight poles per phase is turning at 900 RPM. What is the output frequency? A. 30 Hz B. 40 Hz C. 50 Hz D. 60 Hz
D. 60 Hz
An alternator that has a separate small alternator mounted on the shaft of its rotor is equipped with a(n) _____. A. electric starter B. primary exciter C. brushless energizer D. brushless exciter
D. brushless exciter (p. 770)
T or F: All alternators are three-phase.
False (p 766)
T or F: Alternators operate on the same principle of electromagnetic induction as DC generators, and they do not have a commutator.
True (p 766)
T or F: The DC current supplied to the field of an alternator is called excitation current.
True (p 769)
Using a(n) _____ is one method of preventing induced voltage from becoming excessive.
field-discharge resistor
A diode, when used in field discharge protection, is ____________ biased when DC excitation current is removed in order to protect the circuit.
forward (p 778)