Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
46 Cards in this Set
- Front
- Back
|
What is the main difference in the construction of a DC Generator and DC Motor? |
The type of surrounding enclosure |
|
|
The rotating part of a DC Generator is called the.... |
Armature |
|
|
3 basic types of armature windings? |
Frog Leg Lap Wave |
|
|
What is the electrical function of the commutator? |
To maintain the current in 1 direction to the external circuit |
|
|
Common materials used in making brushes? |
Carbon, Electro-Graphite, Graphite, Copper Graphite |
|
|
Suitable brush pressure is usually between _____ and _____ psi |
1 and 2 |
|
|
Describe the function of field poles |
They provide the magnetic field in which the armature rotates |
|
|
Grease nipples in the end shields indicate that the machine contains _______ bearings |
Open Bearings |
|
|
Small generators that use permanent magnets for field excitation are called... |
Magnetos |
|
|
The difference between separately excited and self excited fields are... |
Separately excited gets field current from an external source ie battery whereas Self Excited gets FC from the armature itself |
|
|
2 ways in which field coils are connected in self-excited generators? |
Series and Parallel(Shunt) |
|
|
Which field windings are made of a low number of turns of large gauge wire? |
Series field windings |
|
|
What device in the shunt field circuit is used to control generator voltage? |
Rheostat |
|
|
The buildup of voltage in a self excited generator requires.... |
Residual Magnetism |
|
|
Voltage buildup occurs until the generator reaches its.... |
No load Voltage Eg |
|
|
Appropriate residual magnetism can be established on the field poles by a process commonly called.... |
Flashing the Field |
|
|
The value of field resistance that will limit field current to a value insufficient for voltage buildup is called.... |
Critical field resistance |
|
|
A generator characteristic curve shows the effect of ______ on ________ |
Connected Load, Terminal Voltage |
|
|
Describe the conditions that must be satisfied to obtain the desired performance when connecting a generator |
Field coils must be properly connected with respect to the armature and each other |
|
|
List the letters that are used in the NEMA system to identify the armature, shunt field and series field |
Armature = A Series = S Shunt = F |
|
|
The terminal voltage of a shunt generator is controlled by.... |
A rheostat in the field circuit |
|
|
What determines the amount of current that flows through an armature? |
The impedance of the connected load |
|
|
Factors that cause a reduction in terminal voltage with increasing load? |
IR drop of the armature, Armature reaction Loss of field current due to a reduction in generated voltage |
|
|
Is the series generator a practical machine? |
No (main use is an arc welder) |
|
|
Why would it not be practical to control the output of a series generator with a field rheostat? |
Because the rheostat would have to carry the high armature current, resulting in high wattage losses |
|
|
Why is the no-load voltage of a series generator extremely low? |
In the absence of a connected load (open circuit) no current flows through series field coils |
|
|
In a series generator, why does the terminal voltage rise with increasing load? |
As armature current increases, strength of the series field increases |
|
|
A compound generator contains both _____ and ______ fields |
Series, Shunt |
|
|
The condition under which the series field adds to the shunt field is called |
Cumulative Compound |
|
|
The condition under which the magnetic flux of the series and shunt fields oppose each other is called.... |
Differential Compound |
|
|
Describe the difference between long shunt and short shunt connections |
Long shunt: shunt field is connected across the line Short shunt: shunt field is connected across the armature |
|
|
A compound generator that has a relatively constant terminal voltage characteristic is called.... |
Flat Compound |
|
|
Line drop can be compensated for by using a generator that is.... |
Over-Compound |
|
|
With respect to long versus short shunt connections, what is the general rule? |
Connect the shunt field across the source of emf |
|
|
What is the effect on generated voltage if the speed of the prime mover is decreased by one third? |
Decreases by one third |
|
|
Define Armature Reaction |
Armature reaction is a distortion of the main field flux caused by current flowing in armature conductors |
|
|
List 2 effects of armature reaction on the operation of a generator |
-A reduction in terminal voltage with increasing load -Impaired Commutation |
|
|
Interpole coils consist of a.... |
Small number of turns of Large Gauge wire |
|
|
Interpole coils are connected... |
In series with the Armature |
|
|
The effects of armature reaction can also be minimized by placing windings called ______ in the main pole faces |
Compensating windings |
|
|
List 3 types of power loss in a generator |
Mechanical loss Electrical Loss Core Loss |
|
|
Rotational losses include... |
Bearing friction, movement of air |
|
|
How is the effect of eddy currents minimized in the construction of DC machines? |
Laminating core materials |
|
|
Heat produced by current flowing through the armature and field coils is called ____ |
Copper Losses |
|
|
Define efficiency |
Ratio of Power out to power in |
|
|
DC machines use ____ or ____ bearings |
Sleeve or Ball Bearings |
