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.
The most common frequency of AC generators in North America is:
Select one:
a.
40 hertz
b.
50 hertz
c.
60 hertz
d.
110 hertz
e.
120 hertz
c.
60 hertz
Paralleling alternators require the following conditions:
1. It must be in phase with the system.
2. The incoming alternator must be the same voltage as the system.
3. Its phase sequence must be the same as the system.
4. Alternator frequency and system frequency must be the same.
Select one:
a.
1, 3, 4
b.
1, 2, 3
c.
2, 3, 4
d.
1, 2, 4
e.
1, 2, 3, 4
e.
1, 2, 3, 4
If the number of secondary turns is such that the secondary voltage is smaller than the primary voltage, then the transformer is a _____ transformer.
Select one:
a.
Small scale
b.
Ramp up
c.
Quick step
d.
Step-up
e.
Step down
e.
Step down
The best method of synchronizing an alternator to a system is:
Select one:
a.
The synchroscope
b.
The dark lamp
c.
One dark and two bright rotating brushes
d.
The phase indicator
e.
All dark rotating brushes
a.
The synchroscope
An induction motor starting when directly connected to the source:
Select one:
a.
Requires very little current for start-up
b.
Needs slightly more current for start-up
c.
Needs only normal current and voltage for start-up
d.
Requires up to six times its full-load current for start-up
e.
Requires a starting motor
d.
Requires up to six times its full-load current for start-up
When paralleling alternators, the incoming alternator must be at the same _____ as the system voltage.
Select one:
a.
Impedence
b.
Conductance
c.
Voltage
d.
Amperage
e.
Current
c.
Voltage
Transformers are used for:
1. Stepping up AC voltage
2. Stepping down AC voltage
3. Transforming AC voltage to DC voltage
4. Rectifying DC voltage to AC voltage
Select one:
a.
1, 2
b.
1, 2, 3
c.
1, 2, 4
d.
1, 3, 4
e.
1, 2, 3, 4
a.
1, 2
Transformers function on the principle of:
Select one:
a.
Conductance
b.
High resistance to DC current
c.
Interaction of residual magnetism
d.
Mutual inductance
e.
Mutual conductance
d.
Mutual inductance
The stator of the induction motor is identical to that of the:
Select one:
a.
Three-phase alternator
b.
DC alternator
c.
Single-phase alternator
d.
DC generator
e.
Squirrel cage motor
a.
Three-phase alternator
Transformers are cooled by:
1. Spraying water over the coils
- Placing the core in an oil-filled tank
- Forced cooling, using fans
- Using pyrene as a coolant
Select one:
A.
1, 2, 3
B.
1, 3, 4
C.
2, 3
D.
2, 3, 4
E.
1, 2, 3, 4
C.
2, 3
Wound-rotor motor starters:
Select one:
a.
Allow for high resistance in the stator circuit for starting
b.
Reduce the resistance for starting
c.
Use split-phase for starting
d.
Insert external resistances into the rotor circuit during starting
e.
Increase the starting current
d.
Insert external resistances into the rotor circuit during starting
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
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
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
If the number of secondary turns is such that the secondary voltage is smaller than the primary voltage, then the transformer is a _____ transformer.
Select one:
a.
Small scale
b.
Ramp up
c.
Quick step
d.
Step-up
e.
Step down
e.
Step down
