Machines(2); Alternators And Generators Flashcards
What does a dynamo do?
Mechanical energy —> electrical energy (dc generator)
OR
Electrical energy —> mechanical energy (dc motor)
What IS a dynamo?
Rotating electrical machine
What kind of electrical machine converts mechanical energy to electrical energy?
Generator
How are generators rated?
Kilo watts (or mega watts) of power they deliver at rated voltage and speed without overheating.
What is another name for an alternator?
Synchronous generator
What kind of current is created by an alternator?
Ac current
How are alternators rated?
VA, KVA, or MVA they deliver at a specific speed without overheating.
How would you describe the speed of an alternator?
Synchronous
What is a prime mover?
Mechanical machine that supplies rotational mechanical force to turn the rotor of an alternator or generator
What are common prime movers?
Turbines (water, steam,wind)
Diesel engines
Natural gas engines
What is a motor?
Rotating electrical machine that converts electrical energy into mechanical energy
How are motors rated?
The power in KW or horsepower that they deliver at rated speed and voltage without overheating
What is the stationary part of an electrical machine?
Stator
What is the rotating part of an electrical machine?
Rotor
How do ac and dc machines differ in terms of field winding location?
DC - on stator
AC - on rotor
What is the role of field windings?
Create magnetic flux
What is the function of armature windings?
DC generator = Develop voltage
DC motor = develop torque
Where are armature windings located?
DC - rotor
AC - stator
How are coil terminals connected to the field windings in an ac alternator or synchronous motor?
Slip rings
Where are slip rings mounted?
On the shaft of the ac machine, rotating with coil and shaft
What is the difference in constriction between a slip ring and a split ring?
Slip = continuous
Split = segmented
Another name for split ring?
Commutator
What is the purpose of a commutator?
1) Convert applied ac voltage on armature coils TO DC voltage
2) convert applied dc voltage TO ac voltage on the armature coils
What is commutation?
Process of reversing direction of current in armature coil
At what point does the armature coil cut the fewest lines of flux?
0° or “neutral plane”
How do brushes operate differently between AC and DC machines?
DC- they contact the commutator
AC - they contact the slip rings
DC- they form electrical connection between armature windings and external cct
AC - they form electrical connection between field windings and external cct
What term defines the production of a voltage in a conductor due to relative motion between a conductor a d magnetic field?
Electromagnetic induction
What changes with the direction of relative movement between a conductor and magnetic field?
Polarity of induced emf and resulting current.
When can we apply flemings left hand rule?
When we know direction of flux and direction of relative motion of conductor.
How can we determine polarity of an induced emf?
Flemmings left-hand rule
Flemmings:
- Pointer finger
- Thumb
- Middle finger
- north to south (points south)
- direction of conductor
- indicates direction of polarity of induced emf
Using flemmings left hand rule, which finger points in the direction of the main field of flux?
Index finger
How would the left hand rule change with a stationary conductor and moving field?
It wouldn’t, thumb indicates RELATIVE direction of motion of conductor to the feild.
Magnitude of INSTANTANEOUS VOLTAGE induced into a single straight conductor moving in a straight line through a magnetic feild is
PROPORTIONAL to what?
Rate of change of LINES OF FORCE passing through that conductor. (Rate of change of FLUX)
What is the rate of change of FLUX dependent upon?
Flux density
Effective length of conductor within the feild
Velocity of the conductor
How is the INSTANTANEOUS INDUCED VOLTAGE expressed mathematically? Define each component.
e = Blv
B = flux density in teslas L= length of conductor in meters V= velocity of conductor in meters per second.
Where is the armature when it is in the neutral plan position?
At 0°
When the armature loop is at 0°, what do we expect from the following properties:
1) Physical movement of top and bottom of loop with respect to magnetic field
2) electromotive force (voltage) induced
3) load current
1) moving parallel
2) no voltage
3) no current
When the armature loop is moving from 0° to 90° what do we expect from the following properties:
1) Physical movement of top and bottom of loop with respect to magnetic field
2) electromotive force (voltage) induced
3) direction current
1) top moves down through lines of force while bottom moves up
2) series-adding (sum of each induced voltage from each half of armature loop)
3) current in both halves moving opposite direction (SAME DIRECTION THROUGH LOOP)
What is the current to the load at 180°?
0A
At what degrees is load current at a max?
270 and 90
Magnitude of INSTANTANEOUS voltage induced into a conductor is determined by:
1) flux density (magnetic feild intensity)
2) relative speed of conductor to the field
3) length (number of turns) of conductor
4) angle at which the conductor cuts the lines of force.
How do you control or vary the amount of generated voltage from an alternator?
Control feild intensity
Amount of voltage generated in armature conductors of an alternator is a function of:
Feild strength (B)
Rotational frequency / speed of rotation of armature (v)
Amount of armature winding effectively exposed to the field. (l)
In a dc generator, what kind of voltage do we see at the armature?
Same Ac voltage.
At what degrees is current at the load 0?
0°, 180°, 360°(back at 0)
What is a DC waveform with wide swings of voltage called?
Pulsating waveform
What happens to a pulsating DC waveform when we increase the number of armature coils and commutator bars?
Generator will produce smoother output waveform.
How would you connect additional coils to change a pulsating DC waveform to a ripple waveform? (Series or parallel)?
Coils connected in series with one another
What two functions does icreasing the number of series connected coils in a DC generator have?
1) decreases magnitude of ripple
2) increases avg. value of voltage at brushes
Why does adding more coils in series in a DC generator increase the average value of voltage at the brushes?
Because the brushes are always in contact with the commutator segment connected to the series of coils that has the greatest induced emf. So the generated voltage is closer to peak.
What are the TWO ways of controlling the amount of generated voltage in a generator?
Control feild intensity or speed.
Why do we usually just use feild intensity to control output voltage on generators (and not speed)
Most generators are designed to operate at a relatively constant speed (rated speed on nameplate)
How do we calculate generated voltage from a generator?
E(g) = K × phi × n
K- constant
Phi - flux per pole
N - rotational frequency (r/min)
How do we usually vary the field intensity? (To controll generated voltage)
Change amount of current in field winding using a field rheostat.
How is the feild rheostat connected with regards to the field winding?
In series with it.
What would be the effect of increasing the resistance of the field rheostat on the feild intensity?
Decrease feild intensity.
What is the generated voltage of a generator when there is no feild current? Why?
Just above 0. Because of the residual magnetism due to retentivity of the core material.
What is the relationship between generated voltage and field current?
Almost linear until the “knee”
What happens to the magnetic circuit material after the “knee”?
Reaches saturation
What is the danger when feild current has been increased to the point of saturating the magnetic circuit material?
Overheating the feild winding with excessive current
True or false: generated voltage of the generator is always directly proportional to speed and flux density.
False; no longer directly proportional once saturation of the magnetic circuit occurs.
How are generators rated?
Output kilowatt or megawatt capacity at their rated speed and terminal voltage
How is motor shaft output expressed?
Horsepower (NEMA) or
Kilowatts (IEC rated)
At rated rotational frequency with rated current and voltage.
Why is temperature rise and ambient temperature important to the rating of generators?
Actual temp of winding determines limit of life expectancy of the insulation.
What is the allowable temp rise for the machine based on?
Max ambient temperature
What is ambient temperature?
Temperature of the cooling medium. (Oil, gas, or air)
If ambient temp exceeds a specified value, what must happen to the rated allowable temperature rise? (Increase or decrease)
Decrease allowable temp rise
How is rotational frequency measured?
Revolutions per minute
What happens if a machine is operated at less than rated speed?
It will not deliver rated power output
What are standard rotational frequencies for dc generators?
1800, 1200, 900, 720 r/min
What are standard dc motor base speed ratings?
2500, 1750, 1150, 850, 500 r/min
What are typical voltage ratings for dc generators?
125, 250, 500V(dc)
What are typical voltage ratings for DC motors?
90, 120, 180, 240, 500, 550 V(dc)
What are typical voltage ratings of field windings?
50, 100, 150, 200, 240, 300
What are typical ac voltage ratings for single phase alternators?
120V and 240V
What are the typical voltage ratings for three-phase alternators?
From 208V to 13800V
Define FLC. What does it specify?
Full load current; value of current that occurs at rated voltage and speed while operating with rated load.
Specifies max armature current beyond which windings will overheat.
What is FLC nameplate information used for?
Select overload protection
Is the rated current of the field winding on the nameplate in addition to FLC?
Yes
What other info may be found on the nameplate of a DC machine?
Model Serial number Duty cycle Enclosure type Speed classification Frame size
What is X’SAT?
X prime d saturation: ability of generator to absorb sudden loads w/o having the voltage drop to unacceptable levels
What is X”d SAT?
X double prime d saturation: ractance value that indicates the ability of the generator to provide shirt circuit current.
What is duty rating based on?
Temp rise rating
What would a rotation rating of CCWFDE mean?
Counter clockwise facing the drive end
What does a sequence rating represent?
Phase rotation
What would the 4P in a code one nameplate indicate?
4 pole machine
What would the second number in a code on a nameplate indicate?
Frame size
What does a “wire” rating on a nameplate indicate?
Number of leads accessible to the electrician.
How do Hertz ratings on a generator nameplate differ from rpm ratings?
Hz = rated frequency of output RPM = rated speed unit operates at
