Monday, December 8, 2014

Single-phase alternators, three-phase alternator and frequency

ANSWERS TO QUESTIONS Q1. THROUGH Q21.


A1. A conductor and a magnetic field.

A2. Armature.

A3. Rotating armature and rotating field.

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

A5. To provide dc current for the rotating field.

A6. Kilovolt-amperes (volt amperes).

A7. Steam turbine.

A8. Internal combustion engines, water force and electric motors.

A9. One voltage (one output).

A10. In series.

A11. Placement of armature coils.

A12. Three.

A13. C is 1.414 times greater than A or B.

A14. Each phase is displaced 120° from the other two.

A15. Wye and Delta.

A16. Three single-phase, delta-delta, step-down transformers.

A17. Speed of rotation and number of poles.

A18. 120 Hz.

A19. Voltage regulation. As a percentage.

A20. By varying the voltage applied to the field windings.

A21. Output voltage, frequency, and phase relationships.


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SINGLE-PHASE ALTERNATORS


A generator that produces a single, continuously alternating voltage is known as a SINGLE-PHASE alternator. All of the alternators that have been discussed so far fit this definition. The stator (armature) windings are connected in series. The individual voltages, therefore, add to produce a single-phase ac voltage. Figure 3-5 shows a basic alternator with its single-phase output voltage.
Figure 3-5. - Single-phase alternator.

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The definition of phase as you learned it in studying ac circuits may not help too much right here. Remember, "out of phase" meant "out of time." Now, it may be easier to think of the word phase as meaning voltage as in single voltage. The need for a modified definition of phase in this usage will be easier to see as we go along.
Single-phase alternators are found in many applications. They are most often used when the loads being driven are relatively light. The reason for this will be more apparent as we get into multiphase alternators (also called polyphase).
Power that is used in homes, shops, and ships to operate portable tools and small appliances is single-phase power. Single-phase power alternators always generate single-phase power. However, all single-phase power does not come from single-phase alternators. This will sound more reasonable to you as we get into the next subjects.

Q.9 What does the term single phase indicate?

Q.10 In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?


TWO-PHASE ALTERNATORS


Two phase implies two voltages if we apply our new definition of phase. And, it's that simple. A two-phase alternator is designed to produce two completely separate voltages. Each voltage, by itself, may be considered as a single-phase voltage. Each is generated completely independent of the other. Certain advantages are gained. These and the mechanics of generation will be covered in the following paragraphs.
Generation of Two-Phase Power Figure 3-6 shows a simplified two-pole, two-phase alternator. Note that the windings of the two phases are physically at right angles (90°) to each other. You would expect the outputs of each phase to be 90° apart, which they are. The graph shows the two phases to be 90° apart, with A leading B. Note that by using our original definition of phase (from previous modules), we could say that A and B are 90° out of phase. There will always be 90° between the phases of a two-phase alternator. This is by design.
Figure 3-6. - Two-phase alternator.

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Now, let's go back and see the similarities and differences between our original (single-  phase) alternators and this new one (two-phase). Note that the principles applied are not new. This alternator works the same as the others we have discussed.


The stator in figure 3-6 consists of two single-phase windings completely separated from each other. Each winding is made up of two windings that are connected in series so that their voltages add. The rotor is identical to that used in the single-phase alternator. In the left-hand schematic, the rotor poles are opposite all the windings of phase A. Therefore, the voltage induced in phase A is maximum, and the voltage induced in phase B is zero. As the rotor continues rotating counterclockwise, it moves away from the A windings and approaches the B windings. As a result, the voltage induced in phase A decreases from its maximum value, and the voltage induced in phase B increases from zero. In the right-hand schematic, the rotor poles are opposite the windings of phase B. Now the voltage induced in phase B is maximum, whereas the voltage induced in phase A has dropped to zero.

Notice that a 90-degree rotation of the rotor corresponds to one-quarter of a cycle, or 90 electrical degrees. The waveform picture shows the voltages induced in phase A and B for one cycle. The two voltages are 90° out of phase. Notice that the two outputs, A and B, are independent of each other.


Each output is a single-phase voltage, just as if the other did not exist.
The obvious advantage, so far, is that we have two separate output voltages. There is some saving in having one set of bearings, one rotor, one housing, and so on, to do the work of two. There is the disadvantage of having twice as many stator coils, which require a larger and more complex stator.
The large schematic in figure 3-7 shows four separate wires brought out from the A and B stator windings. This is the same as in figure 3-6. Notice, however, that the dotted wire now connects one end of B1 to one end of A2. The effect of making this connection is to provide a new output voltage. This sine-wave voltage, C in the picture, is larger than either A or B. It is the result of adding the instantaneous values of phase A and phase B. For this reason it appears exactly half way between A and B. Therefore, C must lag A by 45° and lead B by 45°, as shown in the small vector diagram.
Figure 3-7. - Connections of a two-phase, three-wire alternator output.

 

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Now, look at the smaller schematic diagram in figure 3-7. Only three connections have been brought out from the stator. Electrically, this is the same as the large diagram above it. Instead of being connected at the output terminals, the B1-A2 connection was made internally when the stator was wired. A two-phase alternator connected in this manner is called a two-phase, three-wire alternator.


The three-wire connection makes possible three different load connections: A and B (across each phase), and C (across both phases). The output at C is always 1.414 times the voltage of either phase. These multiple outputs are additional advantages of the two-phase alternator over the single-phase type.


Now, you can understand why single-phase power doesn't always come from single-phase alternators. It can be generated by two-phase alternators as well as other multiphase (polyphase) alternators, as you will soon see.


The two-phase alternator discussed in the preceding paragraphs is seldom seen in actual use. However, the operation of polyphase alternators is more easily explained using two phases than three phases. The three-phase alternator, which will be covered next, is by far the most common of all alternators in use today, both in military and civilian applications.

Q.11 What determines the phase relationship between the voltages in a two-phase ac generator?

Q.12 How many voltage outputs are available from a two-phase three-wire alternator? Q.13 What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?


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THREE-PHASE ALTERNATOR


The three-phase alternator, as the name implies, has three single-phase windings spaced such that the voltage induced in any one phase is displaced by 120° from the other two. A schematic diagram of a three-phase stator showing all the coils becomes complex, and it is difficult to see what is actually happening. The simplified schematic of figure 3-8, view A, shows all the windings of each phase lumped together as one winding. The rotor is omitted for simplicity. The voltage waveforms generated across each phase are drawn on a graph, phase-displaced 120° from each other. The three-phase alternator as shown in this schematic is made up of three single-phase alternators whose generated voltages are out of phase by 120°. The three phases are independent of each other.
Figure 3-8. - Three-phase alternator connections.

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Rather than having six leads coming out of the three-phase alternator, the same leads from each phase may be connected together to form a wye (Y) connection, as shown in figure 3-8, view B. It is called a wye connection because, without the neutral, the windings appear as the letter Y, in this case sideways or upside down.


The neutral connection is brought out to a terminal when a single-phase load must be supplied. Single-phase voltage is available from neutral to A, neutral to B, and neutral to C.


In a three-phase, Y-connected alternator, the total voltage, or line voltage, across any two of the three line leads is the vector sum of the individual phase voltages. Each line voltage is 1.73 times one of the phase voltages. Because the windings form only one path for current flow between phases, the line and phase currents are the same (equal).


A three-phase stator can also be connected so that the phases are connected end-to-end; it is now delta connected (fig. 3-8, view C). (Delta because it looks like the Greek letter delta, Δ.) In the delta connection, line voltages are equal to phase voltages, but each line current is equal to 1.73 times the phase current. Both the wye and the delta connections are used in alternators.


The majority of all alternators in use in the Navy today are three-phase machines. They are much more efficient than either two-phase or single-phase alternators.


Three-Phase Connections

  • The stator coils of three-phase alternators may be joined together in either wye or delta connections, as shown in figure 3-9.
  • With these connections only three wires come out of the alternator. This allows convenient connection to three-phase motors or power distribution transformers.
  • It is necessary to use three-phase transformers or their electrical equivalent with this type of system.


Figure 3-9. - Three-phase alternator or transformer connections.

 

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A three-phase transformer may be made up of three, single-phase transformers connected in delta, wye, or a combination of both. If both the primary and secondary are connected in wye, the transformer is called a wye-wye. If both windings are connected in delta, the transformer is called a delta-delta.
Figure 3-10 shows single-phase transformers connected delta-delta for operation in a three-phase system. You will note that the transformer windings are not angled to illustrate the typical delta (Δ) as has been done with alternator windings. Physically, each transformer in the diagram stands alone. There is no angular relationship between the windings of the individual transformers. However, if you follow the connections, you will see that they form an electrical delta. The primary windings, for example, are connected to each other to form a closed loop. Each of these junctions is fed with a phase voltage from a three-phase alternator. The alternator may be connected either delta or wye depending on load and voltage requirements, and the design of the system.
Figure 3-10. - Three single-phase transformers connected delta-delta.

 

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Figure 3-11 shows three single-phase transformers connected wye-wye. Again, note that the transformer windings are not angled.
Electrically, a Y is formed by the connections. The lower connections of each winding are shorted together. These form the common point of the wye. The opposite end of each winding is isolated. These ends form the arms of the wye.
Figure 3-11. - Three single-phase transformers connected wye-wye.

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The ac power on most ships is distributed by a three-phase, three-wire, 450-volt system. The single-phase transformers step the voltage down to 117 volts. These transformers are connected delta-delta as in figure 3-10. With a delta-delta configuration, the load may be a three-phase device connected to all phases; or, it may be a single-phase device connected to only one phase.
At this point, it is important to remember that such a distribution system includes everything between the alternator and the load. Because of the many choices that three-phase systems provide, care must be taken to ensure that any change of connections does not provide the load with the wrong voltage or the wrong phase.


Q.14 In a three-phase alternator, what is the phase relationship between the individual output voltages?

Q.15 What are the two methods of connecting the outputs from a three-phase alternator to the load?

Q.16 Ships' generators produce 450-volt, three-phase, ac power; however, most equipment uses 117-volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?


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FREQUENCY


The output frequency of alternator voltage depends upon the speed of rotation of the rotor and the number of poles. The faster the speed, the higher the frequency. The lower the speed, the lower the frequency. The more poles there are on the rotor, the higher the frequency is for a given speed.
When a rotor has rotated through an angle such that two adjacent rotor poles
(a north and a south pole) have passed one winding, the voltage induced in that winding will have varied through one complete cycle. For a given frequency, the more pairs of poles there are, the lower the speed of rotation. This principle is illustrated in figure 3-12; a two-pole generator must rotate at four times the speed of an eight-pole generator to produce the same frequency of generated voltage. The frequency of any ac generator in hertz (Hz), which is the number of cycles per second, is related to the number of poles and the speed of rotation, as expressed by the equation

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where P is the number of poles, N is the speed of rotation in revolutions per minute (rpm), and 120 is a constant to allow for the conversion of minutes to seconds and from poles to pairs of poles. For example, a 2-pole, 3600-rpm alternator has a frequency of 60 Hz; determined as follows:

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A 4-pole, 1800-rpm generator also has a frequency of 60 Hz. A 6-pole, 500-rpm generator has a frequency ofimage
A 12-pole, 4000-rpm generator has a frequency of

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Q.17 What two factors determine the frequency of the output voltage of an alternator? Q.18 What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?
Figure 3-12. - Frequency regulation.image


VOLTAGE REGULATION
As we have seen before, when the load on a generator is changed, the terminal voltage varies. The amount of variation depends on the design of the generator.
The voltage regulation of an alternator is the change of voltage from full load to no

load, expressed as a percentage of full-load volts, when the speed and dc field current are held constant.

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Assume the no-load voltage of an alternator is 250 volts and the full-load voltage is 220 volts. The percent of regulation is

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Remember, the lower the percent of regulation, the better it is in most applications.


Q.19 The variation in output voltage as the load changes is referred to as what? How is it expressed?

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