1. GENERATING EQUIPMENT Prepared by: LENY A. ETCOY BSEE-5B
2. LEAKAGE REACTANCE AND ARMATURE - Two different result
produced in an alternator due to the armature windings carrying
current.
3. Indicates the conditions existing between the armature and
the field when the armature current is in phase with the generated
voltage.
4. Indicates conditions when the current in the armature lags
the generated voltage by 90 electrical degrees.
5. Indicates when the current in the armature leads the
generated voltage by 90 electrical degrees.
6. LEAKAGE REACTION The alternating current in the armature
windings will set up a certain magnetic flux, which will encircle
each conductor or group of conductors. ARMATURE LEAKAGE FLUX -the
flux that distinguish it from flux that crosses the air gap and is
available as useful flux to generate the alternator voltage. E e.i.
= -L(di/dt)
7. ARMATURE REACTION The resultant m.m.f. produced by a
three-phase alternator armature when carrying current is of
practically constant magnitude but revolves at synchronous speed;
hence it is fixed in position relative to the field m.m.f.
8. THREE CONDITIONS: 1. When the armature current is in phase
with the generated voltage, the armature m.m.f. is cross-
magnetizing 2. At zero percent power factor leading current the
armature m.m.f. is magnetizing 3. At zero percent power factor
lagging current the armature m.m.f. is demagnetizing in its effect
upon the field m.m.f.
9. It=Eg/( 3 Xt) Where: It=initial current (A) Eg= the
generated voltage per line, or normal no- load voltage Xt=
transient reactance in ohms, approximately equal to the leakage
reactance
10. M.m.f. net = m.m.f. f m.m.f. d Where: m.m.f. d =is the
portion of the total armature m.m.f. which is effective in
demagnetizing the field poles.
11. Ix=Ex/( 3 Xt) Where: Ix = current flowing into the short
circuit, at x sec. after the short circuit has been produced. Es =
generated voltage at x sec. after the short circuit has occurred.
Xt = transient reactance of the alternator The current flowing into
the short circuit will become steady at a value that will produced
sufficient armature- demagnetizing m.m.f. to limit the flux
crossing the air gap to a value just large enough to generate the
necessary to send the current through the armature-leakage
reactance.
12. Xs= Eg/( 3 Is) Where: Eg = no-load voltage Is = sustained
short-circuit current Xs = synchronous reactance, (ohms)
13. MAGNETIC-FLUX DISTRIBUTION IN THE AIR GAP OF ALTERNATORS AT
FULL LOAD TWO TYPES: A) Nonsalient-pole Machines B) Salient-pole
Machines
14. ALTERNATOR VECTOR DIAGRAM AT FULL LOAD A) Nonsalient pole
Machines Assume an alternator supplying a load whose power factor
is practically 80 percent lagging current. B) Salient-pole Machines
As stated in art. 41 it is not correct to represent the field or
the net m.m.f. acting across the air gap at full load by vectors in
the case of salient-pole machine; hence a salient-pole machine
cannot have a space vector diagram of m.m.fs.
15. ALTERNATOR CHARACTERISTICS CURVE A) No-load Saturation
Curve B) Full-load Saturation Curve 3 Eo = 3 I Zs And I Zs/ Eo = 1
or 100 percent
16. VOLTAGE REGULATION The voltage regulation of alternators is
found in the same manner as for direct-current generators (art.
25)
17. LOW SHORT-CIRCUIT CURRENT versus GOOD REGULATION In art 24
it was stated that direct-current generators used for lighting
should not have a voltage regulation higher than 2 percent. In
contrast to such value the regulation of an alternator at 8O
percent power factor may be high as 42 percent (art. 44)
18. PARALLEL OPERATION OF ALTERNATORS Before an alternator can
be connected in parallel with another machine that is supplying a
load, it is necessary that the incoming machine have the same
voltage and frequency, and also be in phase with the operating
machine; and before the switches are closed, it is necessary that
the polarities and phase sequence of the machine be identical.
19. I = E / AA + XB Where: I = the circulating current (A) E =
resultant voltage, causing current I to flow AA and XB =
synchronous reactance of the alternators
20. LOSSES IN ALTERNATING-CURRENT GENERATORS The losses of
alternating-current generators are essentially of the same nature
as those for direct- current generators given in art. 26.
21. VENTILATION OF ALTERNATORS The rating of any piece of
equipment is dependent upon the temperature rise of the different
parts of the equipment and, hence, upon the rate at which the
losses, which appear as heat, can be radiated from the different
surfaces of the equipment.
22. TWO METHODS: 1. Increasing the radiating surface 2.
Increasing the amount of ventilation
23. This method is the same as is used for water-wheel units,
except that, on amount of the much larger volume of air needed,
independent motor-driven fans are always supplied to maintain a
higher air velocity through the machine.
24. It differs from the simple radial system in that air from
the end bells is carried axially across the back of the core in
passages provided in the frame, passes radially into the frame
through radial vent ducts in parts of the core, and travels axially
through the gap and out to the back of the core through radial vent
ducts in other sections of the core.
25. Air delivered by the fans between and around the stator
coils into a chamber formed by the end housings.
26. HYDROGEN COOLING The used of hydrogen as the cooling medium
for electrical machinery has been introduced by manufacturers in
the last few years, and it has so far with considerable favor in
the operation of the certain types of machines.
27. ADVANTAGES OF HYDROGEN OVER AIR: 1. The density of hydrogen
is about one- fourteenth that of air 2. The heat conductivity
through loose types of insulation is about 25 percent better with
hydrogen 3. The heat conductivity across iron laminations is about
three times as good with hydrogen DISADVANTAGE: 1.When hydrogen and
air mixed in proper proportions will form an explosive gas;
however, if the hydrogen content is above 70 percent, there seems
to be little danger of an explosion.
28. INDUCTION GENERATOR Is obtain by driving an induction motor
at a speed above synchronous speed, in which case the machine can
be considered as receiving from the line the necessary exciting
current and supplying to the line the power or energy current.
29. THEORY- the torque produced by an induction motor depends
on the magnetic flux that crosses the air gap, the current, and the
power factor of the rotor.
30. MISCELLANEOUS GENERATING EQUIPMENT
31. MOTOR-GENERATOR SETS 1. Transforming from direct current to
direct current at different voltage. 2. Transforming from
alternating current to direct current or vice versa. 3.
Transforming from alternating current to alternating current at
different frequency.
32. INDUCTION MOTOR-GENERATOR SETS A. CONSTRUCTION- an
induction motor-generator set consists of an induction motor
direct-connected to a direct-current generator. B. APPLICATIONS-
induction motor generators are used to supply direct current for
lighting and general power up to medium capacities.
33. SYNCHRONOUS MOTOR- GENERATOR SETS A. CONSTRUCTION- as the
name implies, such a motor-generator set comprises a synchronous
motor connected to the same shaft of a direct-current generator. B.
AVANTAGES and DISADVANTAGES- the main advantage of the synchronous
motor-generator set is the power-factor corrective effect which can
be accomplish by properly adjusting the field current of the
synchronous motor.
34. C. APPLICATION 1. lighting service 2. industrial service 3.
electromechanical service 4. railway service 5. storage-battery
charging
35. D. STARTING OF SYNCHRONOUS MOTOR- GENERATOR SETS -the most
common method used in starting such units is from low-voltage taps
on transformers or autotransformers, by means of the torque
produced by squirrel-cage windings which are placed in the pole
shoes.
36. FREQUENCY CHANGERS A. CONSTRUCTION- frequency changers
consist of an alternating-current generator direct-connected to the
shaft of an alternating-current motor. B. APPLICATION 1. as a means
of interchanging energy between two systems of different
frequencies. 2. to supply certain types of loads at a frequency
different from the frequency of the system.
37. C. PARALLEL OPERATION D. ELECTRONIC FREQUENCY CHANGERS-
heavy power electronic frequency changers may soon replace the
rotating type changers. Alternating-current power of any frequency
is first converted to direct-current power through a typical
rectifier circuit and then converted back to alternating-current
power of any second frequency through an inverted rectifier circuit
or inverter.
38. ROTARY CONVERTERS A. CONNECTIONS- the armature winding is
an ordinary direct-current winding, which may be either series or
multiple (art. 20) B. VOLTAGE RATIOS- the theoretical voltage
ratios of rotary converters are definitely fixed by the number of
phases and the method by which the transformer terminals are
connected to the slip rings. C. CURRENT RATIOS- the current ratios
of a converter are not so definitely fixed as the voltage ratios,
but are different for different power factors.
39. D. HEATIGN AND CAPACITY- the effective current in each part
of a converter-armature winding is the difference between the
instantaneous values of alternating-current input and
direct-current output. F. VOLTAGE VARIATION- it is evident that the
ratio between the alternating and direct-current voltages of an
elementary converter is a nearly fixed quantity and remains
constant within a very few percent from no load to full load.
40. 1. SYNCHRONOUS-BOOSTER CONVERTER- is a rotary converter
with a mechanically connected alternating-current generator, the
armature winding of which is connected in series relation with the
armature winding of the rotary converter so that the voltage
generated in it, whenever its held is excited, either adds to or
subtracts from the voltage supplied to the converter and affects
the direct-current voltage accordingly. 2. REGULATING TRANSFORMER
OR POTENTIAL REGULATOR- consist of a transformer secondary with a
large number of taps and some switching device to change the
converter connections from tap to another.
41. 3. DIRECT-CURRENT BOOSTER- a dc generator can be connected
in series with the direct-current side of the converter, and by
varying the field of this generator the dc current can be varied.
4. AUTOMATIC COMPOUNDING- with this method of voltage variation, it
is necessary that there be a certain amount of reactance in the
supply lines to the converter, which may be reactance coils or
embodied in the supplying transformers.
42. G. THREE-WIRE SYNCHRONOUS CONVERTERS- in art. 28 it was
shown that, by connecting a high- reactance and low- resistance
coil between diametrically opposite points of a direct-current
generator armature, a neutral line could be obtained by bringing
out a tap from the middle point of the reactance coil, thereby
obtaining a three-wire generator. F. PARALLEL OPERATIONS-
converters can be operated in parallel, the division of load being
determined by the induced voltage of the individual. H. HUNTING-
hunting of synchronous converters may be caused by the periodic
variation of the supply frequency, by sudden changes of load, or by
excessive line drop.
43. J. STARTING OF SYNCHRONOUS CONVERTERS 1.
alternating-current self-starting method 2. alternating-current
motor-starting method 3. direct-current self-starting method