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The AMOUNT of voltage generated depends on what 4 factors?
- 1) The strength of the magnetic field
- 2) The angle at which the conductor cuts the magnetic field
- 3) The speed at which the conductor is moved
- 4) The length of the conductor within the magnetic field
The POLARITY of the voltage depends on what 2 factors?
- 1) The direction of the magnetic lines of flux
- 2) The direction of movement of the conductor
What is the simplest elementary generator that can be built?
An AC generator
An elementary generator consists of a wire loop placed so that it can be rotated in a ____________
- Stationary magnetic field.
- Note: This will produce an induced emf in the loop. Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf.
In the elementary AC generator, what is the armature?
The loop of wire that rotates through the field
To what are the ends of the armature loop connected?
In the elementary AC generator, what do the slip rings and brushes do?
The slip rings rotate with the armature. The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these brushes.
In the elementary AC generator, what is the position of the armature loops at 0 degrees?
And what is this position called?
- Perpendicular to the magnetic field
- This position is called the neutral plane
The elementary DC generator
What replaces the slip rings of the AC generator?
- A commutator
- Note: This is the main difference in construction of the DC generator vs. the AC generator
What is the process of commutation?
The commutator mechanically reverses the armature loop connections to the external circuit. This occurs at the same instant that the polarity of the voltage in the armature loop reverses. Through this process, the commutator changes the generated AC voltage to a pulsating DC voltage.
How many commutator segments are required in a 2-coil generator?
Instead of the permanent magnets used in the elementary generator, what do nearly all practical generators use?
Of what do electromagnetic field poles consist?
Coils of insulated copper wire wound on soft iron cores
What are the 2 main advantages of using electromagnetic poles?
- 1) Increased field strength
- 2) A means of controlling the strength of the fields
- Note: By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled.
Sparking between the brushes and the commutator is an indication of what?
What is the main cause of improper commutation?
Improper brush placement
The magnetic field produced by current in the armature of a DC generator affects the flux pattern and distorts the main field.
This distortion causes a shift in the neutral plance, which affects commutation.
What is this change in the neutral plane and the reaction of the magnetic field?
In small generators, how are the effects of armature reaction reduced?
By actually mechanically shifting the position of the brushes
In larger generators, what 2 means are used to eliminate armature reaction?
- 1) Compensating windings
- 2) Interpoles
Of what do compensating windings consist?
A series of coils embedded in slots in the pole faces. These coils are connected in series with the armature.
What is motor reaction?
When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature.
What is the device that provides the turning force applied to the armature?
- The prime mover
- Note: The prime mover may be an electric motor, a gasoline engine, a steam turbine, or any other mechanical device that provides turning force.
In DC generators, certain forces act to decrease the efficiency.
These forces, as they affect the armature, are considered as losses and may be defined as follows:
- 1) I2R, or copper loss in the winding
- 2) Eddy current loss in the core
- 3) Hysteresis loss (a sort of magnetic friction)
What causes copper losses?
The power lost in the form of heat in the armature winding of a generator is known as copper loss. Heat is generated any time current flows in a conductor. Copper loss is an I2R loss, which increases as current increases.
How do we minimize copper loss in armature windings?
By using large diameter wire