Chapter 8: AC Theory and Machines Flashcards
Current transformers are:
Select one:
a.
Power step-up transformers
b.
Current step-up transformers
c.
Rectifying transformers
d.
High power factor transformers
e.
Voltage step-up transformers
e.
Voltage step-up transformers
Modern alternators consist of a rotor on which:
Select one:
a.
The AC voltage producing windings are placed
b.
A DC excitation winding is placed
c.
The DC voltage producing windings are placed
d.
An AC excitation winding is placed
e.
An AC excitation voltage is produced
b.
A DC excitation winding is placed
Modern transformers are very efficient devices with large industrial transformers, often better than _____ efficient.
Select one:
a.
95%
b.
85%
c.
75%
d.
65%
e.
55%
a.
95%
Line impedance starting:
Select one:
a.
Gives higher starting torque
b.
Gives a lower starting torque
c.
Is a more costly method than others
d.
Is used only on rotating switch starters
e.
Is a relatively expensive method
b.
Gives a lower starting torque
Three-phase systems often use banks of _____ transformers to replace three-phase transformers.
Select one:
a.
Double-phase
b.
Single-phase
c.
Step-down
d.
Step-up
e.
Auto-
b.
Single-phase
The most common type of rotor used in induction motors is the _____ rotor.
Select one:
a.
Squirrel cage
b.
Wound
c.
Slip ring
d.
Shaded pole
e.
Compound wound
a.
Squirrel cage
Star-delta starting:
Select one:
a.
Is used for any type of starting.
b.
Is used on motors designed to run as delta connected machines.
c.
Is used only for synchronous motors.
d.
Can be used for only DC machines.
e.
Can be used for AC or DC generators.
b.
Is used on motors designed to run as delta connected machines.
Synchronous motors are started by:
Select one:
a.
The amortisseur winding
b.
A shunt winding
c.
A capacitor
d.
A delta winding
e.
A wye winding
a.
The amortisseur winding
Auto-transformer starting is:
Select one:
a.
A type of starting used only on automobiles
b.
A method that connects a motor to a higher voltage tap
c.
Not used much because of poor starting torque
d.
Expensive method but capable of giving better starting torque
e.
Are used for starting AC generators
d.
Expensive method but capable of giving better starting torque
Salient pole rotors:
Select one:
a.
Are used on high-speed alternators
b.
Are used on alternators not exceeding 1800 rpm
c.
Require no maintenance
d.
Are used on only single-phase generators
e.
Are used on only three-phase generators
b.
Are used on alternators not exceeding 1800 rpm
When taking an alternator off the line in a system that involves two or more alternators:
Select one:
a.
Reduce the driving torque until the alternator is supplying no current
b.
Lower the boiler pressure to minimum
c.
Slow the unit down to the same speed as the other alternators
d.
Increase the excitation current
e.
Reduce the frequency to zero
a.
Reduce the driving torque until the alternator is supplying no current
One of the main advantages of the synchronous motor is that it can be run:
Select one:
a.
At lagging power factor
b.
At a leading power factor
c.
As a DC generator
d.
As an AC generator
e.
As a DC motor
b.
At a leading power factor
Wound rotor induction motors:
Select one:
a.
Require less maintenance than squirrel cage motors
b.
Are less complicated in construction than other motors
c.
Are more reliable
d.
Are not very reliable
e.
Are used to control speed
e.
Are used to control speed
The synchronous motor is:
Select one:
a.
Self-starting
b.
Not self-starting
c.
Used to create a lagging power factor
d.
A split phase type motor
e.
Used to decrease the power factor
b.
Not self-starting
Single-phase transformers:
Select one:
a.
Use two common forms of construction known as the core type and the shell type.
b.
High-voltage coil is nearest the iron core.
c.
Are used in all power supplies.
d.
Use two common forms of construction known as the internal type and the external type.
e.
Are very low in efficiency.
a.
Use two common forms of construction known as the core type and the shell type.
