DC Generators Flashcards
A _____ is a machine that converts mechanical energy into electrical energy by using the principle of Faraday’s law of electromagnetic induction and Lenz’s law.
generator
T or F. When a conductor is moved within a magnetic field or vice versa, it cuts across magnetic lines of flux and thus, current is generated in the conductor.
FALSE.
VOLTAGE
The amount of voltage generated (Eg) depends on:
- magnetic flux density (B)
- angle at which the conductor cuts the magnetic field (θ)
- length of the conductor within the magnetic field (L)
- speed at which the conductor is moved (v)
e = BLvsinθ
Related to the effective area cutting the flux
angle, length
Related to the amount of flux, Φ
flux density, angle, length
T or F. The stronger the field or the more flux lines are cut for a given period of time, the larger the induced EMF.
TRUE
The polarity of the generated voltage depends on the direction of the magnetic flux and the direction of movement of the conductor (Lenz’s law). To determine the direction of the current in a given situation, _______ for generators is used.
Fleming’s Right Hand Rule
Fleming’s Right Hand Rule
thumb - ______
forefinger - _______
middle finger - ______
motion
field
EMF & current
An ______ consists of a loop of wire placed so that it can be rotated in a stationary magnetic field. This will produce an induced emf in the loop.
elementary generator
provide the magnetic field
pole pieces
usually made of carbon, with wires attached to them, ride against the rings
brushes
loop of wire that rotates thru the field
armature
the conductor ends of the armature loop are connected to rings called ____.
(AC Generator)
slip rings
T or F. Number of poles of any generator is not always an even number.
False
always
S or R
pole pieces
stator
S or R
armature
rotor
S or R
brushes
stator
S or R
slip rings
rotor
S or R
pole pieces (High AC generators)
rotor
S or R
armature (High AC generators)
stator
(AC Generator) At 0° position, armature loop is ______ to the magnetic field
perpendicular
When motion is parallel to the field
- no flux lines cut
- no emf produced (0V)
another term for Neutral Plane
Magnetic Neutral Axis (MNA)
(AC Generator) At 90° position, ______ has been cut
maximum flux
(AC Generator) At 90° position, induced emf at gold conductor is in _____ w/ the induced emf with the black conductor
series
T or F. (AC Generator) At 180° position, coil is moving parallel to the direction of the flux, thus, no flux cutting.
True
(AC Generator) From 0° to 180° the conductors of the armature loop have been moving in the same direction thru the magnetic field. Therefore, the polarity of the induced emf is _____.
the same (+)
(AC Generator) At 180° to 360° (back to initial point) position, the terminal voltage will be the same as it was from 0° to 180° but the polarity is _____.
reversed (-)
(DC generator) Each terminal is connected to a segment of a two-segment metal ring called ________.
commutator or split rings
The ______ 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.
commutator
T or F. In commutation, commutator changes the generated DC voltage to an AC voltage.
False. AC to DC
The resulting induced voltage at the armature windings is always ______ regardless if the generator is AC or DC.
AC
(DC Generator) At 90° position, voltage reaches its ______ value.
maximum
(DC Generator) At 180° position, each brush makes contact with both segments of the commutator. Thus, the generated voltage and the current _____.
fall to zero
(DC Generator) As the armature loop rotates to 270° position, a voltage is again induced in the loop but of ______.
opposite polarity
the voltage across the brushes
pulsating DC or unidirectional
The pulsating DC voltage varies twice during each revolution between zero and maximum. This variation is called ______.
Ripple
Ways to lower the ripple voltage of DC generators
- add more armature coils
- add more magnetic poles
Nearly all practical generators use ________ instead of the permanent magnets used in the elementary generator.
electromagnetic poles
Advantages of using electromagnets as field poles
- increases field strength
- means of controlling the field strengths
T or F. In general, electromagnets are cheaper than permanent magnets.
False. Permanent magnets are cheaper
The brief period during which the coil remains short-circuited
commutation period
current reversal (+I -> 0 -> -1) is completed by the end of commutation period
ideal commutation
when current reversal is not completed by the end of commutation period, _____ in the brushes occurs (non-ideal commutation)
sparking
other term for non-ideal commutation
improper commutation
Possible causes of improper commutation
- Defective brush holder
- Worn out carbon brushes
- Defective armature winding
- Uncompensated armature reaction
For proper commutation, the coil short-circuited by the brushes must be in the ________.
neutral plane
the shift in the neutral plane caused by the interaction between the magnetic field produced by the current in the armature and the main field.
armature reaction
T or F. To prevent arcing, the brushes must be shifted to a new neutral plane.
True
used to address the issue of armature reaction
Compensating Winding
or
Interpoles
consists of a series of coils embedded in slots in the pole faces and these coils are connected in series with the armature.
Compensating Windings
The compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to ______ the effects of armature reaction.
cancel
small auxiliary poles between the main field poles. They have a few turns of large wire and are connected in series with the armature.
interpoles
other term for interpoles
commutating poles
T or F. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole behind of it, in the direction of rotation.
False.
ahead
T or F. The amount of shift caused by the interpoles will equal the shift caused by armature reaction.
True
A current-carrying conductor experiences a force when placed in a magnetic field.
Lorentz Law
formula to compute the magnitude of the Lorentz force
F = BILsinθ
When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature.
Motor Reaction
The system/device that provides the turning force applied to the armature.
Prime mover
T or F. The prime mover must be able to overcome the armature reaction force in the generator.
False.
motor reaction force
Examples of Prime mover
- Electric motor
- Gasoline engine
- Steam turbine
Losses in DC Generator
- Copper Losses
- Eddy-Current Losses
- Hysteresis Losses
- Mechanical Losses
- Stray Power Losses (SPL)
The mechanical input of generator is the same as the
power output of prime mover
power lost as heat in the windings
Copper Losses
T or F. Copper loss is minimized in armature windings by using small diameter wire.
False
large
The currents that are induced in the generator armature core when it is rotated in a magnetic field.
Eddy Currents
Eddy current losses are kept low when the core material is made up of many ______.
thin sheets of metal
Laminations in a small generator armature may be as thin as _______.
1/64 inch (equiv. to 4 thin sheets of paper)
To compensate for hysteresis losses, heat-treated ______ laminations are used in most dc generator armatures
Silicon steel
Rotational or mechanical losses are caused by:
- Friction between bearings and armature shaft
- Friction between brushes and commutator segments
- Windage/air friction
These are additional losses that cannot be easily accounted for
Stray Power Losses (SPL)
SPL typically occur when the machine is _____
loaded
On small machines, SPL are usually ____
ignored
On large machines, SPL are computed as ______
1% Pout
T or F. Gramme-ring Armature DC Generator differs in construction with elementary DC generator
True
The windings of a Gramme-Ring Armature DC Generator are placed on an _____
Iron ring
T or F. Gramme-ring is more efficient than drum-type
False
drum type > gramme-ring
T or F. Drum-type armature dc generator has a more complex construction than Gramme-Ring
True
used in dc generators designed for high current applications
LAP winding
LAP windings are connected to provide several _____ paths for current in the armature
parallel
used in dc generators designed for high voltage applications
WAVE Winding
T or F. WAVE winding only requires one pair of brushes.
True
When a dc voltage is applied to the field windings of a dc generator, current flows thru the windings and sets up a steady magnetic field.
Field Excitation
Field winding is energized from a source of DC external to the machine called exciter
Separately-excited DC Generator
T or F. Exciter may be a battery or another dc generator of bigger capacity
False
smaller
Field winding is energized by its own armature
Self-excited DC generator
T or F. Self-excited DC generator is possible only if the the field pole pieces have retained a slight amount of permanent magnetism called remanent/residual magnetism
True
Armature of dc generator is laminated to reduce _______ losses
Eddy-current
Types of Self-Excited DC Generator
- Series-wound DC Generator
- Shunt-wound DC Generator
- Compound-wound DC Generator (Long-shunt & Short-shunt)
uses very low resistance field coils which consists of a few turns of large diameter wire
Series-wound DC generator
T or F. In a series generator, changes in load current do not affect the generator output voltage.
False
drastically affect
A series generator has a ____ voltage regulation, thus, it is not used for _______ loads
poor, fluctuating
T or F. Series-wound generator has the poorest voltage regulation among DC generators.
True
used for constant voltage application like in electric welding
Shunt-wound DC Generator
The field coils used in shunt generator have _____ resistance, consisting of many turns and ____ wire
high, small
T or F. Current in the field windings of a shunt-wound generator is dependent of the load current
False
independent
In actual use, the output voltage in a DC shunt-wound generator varies ______ as load current varies.
inversely
Series-wound and shunt-wound generators have a disadvantage in that changes in load current cause changes in output voltage. One means to supply a stable output voltage is by using a ______ generator.
Compound
Compound-wound DC Generators have both the series and shunt field windings connected in either _______ or ________ configurations.
long-shunt, short-shunt
T or F. The two windings of the compound generator are made such that their magnetic fields will either aid or oppose one another.
True
The series field flux opposes the shunt field flux
Differentially Compounded Compound-wound DC Generator
The series field flux aids the shunt field flux
Cumulatively Compounded Compound-wound DC Generator
Types of Cumulatively Compounded Compound-wound DC Generator
- Under-compounded
- Flat-compounded
- Over-compounded
Full-load VT < No-load VT
Under-compounded
is used when the load is located near the Cumulatively Compounded Compound-wound DC Generator
Under-compounded
Full-load VT = No-load VT
Flat-compounded
is used when the load is at a medium distance from the Cumulatively Compounded Compound-wound DC Generator
Flat-compounded
Full-load VT > No-load VT
Over-compounded
is used when the load is far from the Cumulatively Compounded Compound-wound DC Generator
Over-compounded
refers to the voltage change that takes place when the load changes
Voltage Regulation
Voltage Regulation is usually expressed as the change in voltage from a ______ to a ______, and is expressed as a percentage of _______.
no-load condition -> full-load condition, full-load
T or F. The higher the percent of voltage regulation, the better the generator.
False
lower
T or F. It is better when the no-load VT is almost the same (or fairly constant) as the full-load VT.
True
The 3 factors that may be able to vary generated voltage (Eg)
- Number of conducting loops in series (in the armature)
- Speed of the rotor/armature
- Magnetic field strength
_______ is the internal action that is occurring within the generator whenever the load changes
Voltage Regulation
______ is an imposed action, usually thru an external adjustment for the purpose of increasing or decreasing the voltage
Voltage Control
Voltage control can be provided ______ and ______.
manually, automatically
A type of voltage control where hand-operated field rheostat is connected in series with the field.
Manual Voltage Control
Most field rheostats for generators use resistors of _____ wire.
alloy
(copper, nickel, manganese, chromium)
T or F. Some very large generators use cast-iron grids (in place of rheostats), and motor-operated mechanisms to provide voltage control.
True
It senses changes in output voltage and causes a change in field resistance to keep output voltage constant.
Automatic voltage control device
The shunt field winding current is ______ when the resistance of rheostat is low
high
(I=V/R)
When two or more generators are supplying a common load, they are said to be operating in ______.
parallel
Parallel operation of generators is done on ________
Compound wound & Shunt Generators
Purposes of Parallel operations of generators
- Greater supply capacity
- Redundancy
A disabled or faulty generator may be taken off-line and replaced without interrupting normal operations
Redundancy
Conditions for parallel operation of DC Generators
- The terminal voltage is the same
- The polarities of the generator must be identical
- The prime movers driving the armature of the generators must be similar and stable rotational characteristics (same speed regulations)
- The change of voltage with the change of load must have the same character (same drooping voltage characteristics, same percent voltage regulation)
If generators are used in the purpose of redundancy, maximum load power must be equal to the power rating of the _______ generator.
smallest
