Operating Principles Of DC Generators Flashcards
What are generators?
Machines that provide electrical energy
What principle do generators work under?
Faradays Law of electric magnetic induction
What does “field excitation” refer to?
Various methods of establishing and controlling the magnetic field so that induction occurs
What do small generators provide better than permanent magnets?
A much higher and more controllable flux density
How can the current required to excite the field windings or coils be obtained?
- From a DC external source (separately excited)
- From the armature of the generator itself (self excited)
How can field coils be connected to the armature of a generator?
- Series
- Parallel (shunt)
How are series field windings connected to the armature?
In series
Series windings must be of sufficient gauge to carry….
Full armature current
Series windings are made of….
Few turns of large gauge wire
True or False. Shunt field windings do not need to carry high current.
True
Shunt field windings are made of…
Large number of turns of small gauge wire
What causes the low current in the shunt field?
High coil resistance
The low current in the shunt field is sufficient to provide…
A magneto-motive force (ampere turns) capable of establishing the necessary magnetic field
How can generator voltage be controlled?
A rheostat in the shunt field circuit
What external sources for field current in separately excited generators derive from?
Battery, rectifier circuit, separate DC generator
DC generators that derive the field current from the armature of the machine itself are called ____________.
Self excited generators
In self excited generators, at the moment the generator starts to rotate, ______________ is available for the field current
No voltage
The buildup of voltage in a self excited generator requires…..
Residual magnetism of the field core
What is Faradays law of electromagnetic induction?
e=Blv
What does the voltage induced in a conductor depend on?
Rate at which it is cutting flux
The emf generated in the voltage is a function of….
- number of poles
- flux per pole
- rotational speed of armature
- number of paths
- total number of active conductors on the armature
For any given machine, what values are fixed?
Number of poles, number of paths, active conductors
Generated voltage is directly proportional to the product of _________ and _________
Flux, speed
At the instant of startup, the speed of the generator is…..
Zero
The absence of residual magnetism can be rectified by _________.
flashing the field
Before flashing the field, disconnect ___________ from ________
shunt field, low resistance armature
Voltage buildup requires that the ________ due to residual magnetism be sufficient to _________ the strength of the magnetic field poles.
generated voltage, increase
The field current is dependent on ____________ and __________
voltage across field coils, resistance of the field circuit
A critical value of field resistance exists that will __________ and prevent ________
limit field current, voltage buildup
For build up to occur, the ___________ curve must lie below the ______________ curve.
field resistance curve, saturation curve
It is common to control the generated voltage with a _______ in the field circuit.
rheostat
As resistance in the field circuit is increased, the slope or steepness of the field resistance curve ________ and may move to the ______ of the saturation curve.
increases, left
To ensure voltage buildup during the start up process, set the field rheostat to their _______________
minimum value of resistance
Generated voltage is proportional to the product of ______
flux speed
Too ______ speed may also result in failure of voltage buildup
low
What is a significant factor that influences the terminal voltage of a DC generator?
armature reaction
What can be impaired by armature reaction?
commutation, by severely affecting the physical conditions of the brushes and commutator
What happens if the magnetic field surrounding the armature conductor is incorrect?
distorts main field established by field poles
