Line M-3 Flashcards
What is the main difference in construction of a DC generator and a DC motor?
The type of enclosure
The rotating part of a DC generator is called ________
The rotor/armature
What are the three basic types of armature windings?
Frog leg windings
Lap windings
Wave windings
What is the electrical function of the commutator?
To maintain the current in one direction to the external circuit
What common materials are used in making brushes?
Carbon
Electro-graphite
Graphite
Copper Graphite
Suitable brush pressure is usually between what psi?
1 and 2 psi
What is the function of field poles?
To provide the necessary magnetic field
What two kinds of bearings do DC machines use?
Sleeve or Ball Bearings
Grease nipples in the end of shields indicate that the machine uses what kind of bearings?
Open bearings
What is a dynamo?
A rotating machine that converts mechanical energy into electrical energy
What applications are frog leg windings used in?
Moderate current and moderate voltage applications
What applications are lap windings used in?
High current and low voltage applications
What applications are wave windings used in?
Low current and high voltage applications
What function do brushes provide?
They conduct current to or from the armature coils
What is the purpose of frame and end shields
To provide mechanical support for the machine
Which way does the neutral plain shift in a generator?
With the generator
Which way does the neutral plain shift in a motor
Against the motor
Define Generator
A machine that converts mechanical energy into electrical energy
What are magnetos?
Small generators that use permanent magnets for excitation
Which two ways are field coils connected in?
Series, parallel (shunt)
Which field windings are made of a low number of turns of large gauge wire?
Series windings
What device in the shunt field circuit is used to control generator voltage?
A rheostat
The buildup of voltage in a self-excited generator requires ______________
Residual magnetism
What five factors influence generated voltage?
Number of poles
Flux per poles
Rotational speed of the armature
Number of paths
Total number of active conductors in the armature
Which factors can be varied in an operating generator?
Flux per pole and rotational speed of the armature
What is the effect of generated voltage if the speed of the prime mover is decreased by one-third?
Generated voltage is reduced by one-third
Voltage buildup occurs until the generator reaches it’s ___________
No-load terminal voltage
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
Define armature reaction
Distortion of the main field flux caused by current flowing from the armature to the conductors
Interpole winds consist of _______________
Small number of turns of large-gauge wire
Interpole coils are connected in ____________
Series with the armature
The effects of armature reaction can be minimized by placing windings called __________________ in the main pole faces
Compensating windings
List the three types of power loss in a generator
Mechanical
Electrical
Magnetic
Rotational losses include ____________ and ____________
Bearing friction, windage
How is the effect of eddy currents minimized in the construction of DC machines?
By laminating the core materials
Heat produced by current flowing through the armature and field coils is called ________________
Copper losses
Define efficiency
The ratio of power out to power in
When referring to Generators, what do A, F and S stand for?
A=Armature
F=Shunt field coils
S=Series field coils
What is flat compound?
Where the terminal voltage is flat or constant with respect to the load
What is over compound?
The rise in voltage at the terminals of the generator compensates for the line drop at the load
What application is differential compound most commonly used in?
DC arc welding
What determines the amount of current that flows through an armature
The impedance of the connected load
List the 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 current
Is the series generator a practical machine?
No
Why would it not be practical to control the output of a series generator with a field rheostat?
The rheostat would have to carry 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, no current flows through the series coils
In a series generator, why does the terminal voltage rise with increasing load?
As armature current increases, the strength of the series field increases
Describe how the voltage characteristic of series field coils could be useful
The rising characteristics is used to offset the decrease in terminal voltage associated with shunt field excitation in a compound generator
A compound generator contains both _________ and ________ fields
Series, shunt
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
In long shunt connections, the shunt field is connected across the line. In short shunt connections, the shunt field is connected across the armature
Line drop can be compensated for by using a generator that is ____________
Over compound
A compound generator that has a relatively constant terminal voltage characteristic is called _____________
Flat compound
What generator has the most severe drop in terminal voltage with load?
Differential compound
With respect to long versus short shunt connections, what is a general rule?
Connect the shunt field across the source of emf
