Tuesday, January 6, 2015

Dc generators : three types of self-excited generators (series, shunt, and compound), separately excited generators and generator calculations .

19–6 THREE TYPES OF SELF-EXCITED GENERATORS (SERIES, SHUNT, AND COMPOUND)

The Series Generator

Look back at Figure 19–19 and recall that the field of the series generator carries the entire load current to the external circuit; therefore, the greater the load current, the greater the magnetic field strength; see Figure 19–27. If the generator is started with the external circuit disconnected from the generator terminals, there is no buildup of field. The small voltage due to residual magnetism cannot produce any current at all in an open circuit. If a small load current is taken from the generator, its output voltage is low. If a reasonably high load current is taken, the output voltage is high. If an ordinary parallel- wired lighting circuit is used as the load and two or three lamps turned on, the lamps are dim. The more lamps that are turned on, the brighter each lamp becomes. However, a little

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of this voltage-increasing effect can be a good thing; it illustrates the effect of the series field of a cumulative compound generator; see Figure 19–27. The series generator, impractical for most jobs, does have one interesting application: It can be used in a simple motor-control system, driving a series motor at nearly constant speed with changing load on the motor (Chapter 21).

The Shunt Generator

We have seen, in Figure 19–20, that a shunt generator has its field winding connected parallel to the armature. Comparing this arrangement with that of the series generator, we find that the shunt field coil is constructed of many turns of relatively thin wire. Recall that magnetic field strength is proportional to the number of ampere-turns; thus, the shunt field coil requires relatively little current, generally amounting to less than 3% of the total current supplied by the armature.

With the field coil parallel to the armature, it is reasonable to expect a stable output voltage as long as the generator is driven at constant speed. This is true, indeed, except that the voltage is gradually reduced as the load on the generator is increased; see Figure 19–28.

In our discussion, we distinguish between the generated emf (when the generator is unloaded) and the terminal voltage (delivered to the load). The difference between these two values represents the drop in output voltage that accompanies the increase in load current. This reduction in voltage is due to two factors, as follows:

1. Armature reaction weakens the field, thereby decreasing the generated emf; see Figure 19–29 (drop A).

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2. The IR drop of the armature. There is only a little resistance in the armature winding but enough to cause a voltage drop (IR drop). This IR drop, which varies proportionally with the load current, is lost to the load and causes a further reduction in output voltage; see Figure 19–29 (drop B).

Both of these effects reduce the output voltage. Reduced output voltage causes reduced field current and therefore field flux. With reduced field flux, there is a further reduction in generated emf; see Figure 19–29 (drop C).

Assume that we have a shunt generator with armature resistance equal to 0.4 ohm, rated 10-ampere output current. When operating with no load (external circuit open), we find that the emf is 121 volts. When operated at its rated 10-ampere load, there is a 4-volt drop, or waste, in the armature. (E 5 IR 5 10 3 0.4 5 4 V. Four volts are used in pushing electrons through the generator itself.) This 10-ampere load in the armature also distorts and weakens the field so that there is a decrease in the generated emf of 3 volts. Because of all these voltage deficiencies, the field current and flux are reduced; thus, the generated emf is less by another 4 volts. Because of armature reaction and reduced field current, 121 2 7 5 114 volts are generated. After subtracting the 4 volts used in the armature, there is an output voltage at the terminal of 110 volts.

The characteristic of shunt generators dropping their voltage with increased load is not all bad. In extreme cases of overload, such as a short circuit, the generator will pro- tect itself from damage by reducing the field excitation and, correspondingly, the output voltage. For example, if the generator is overloaded to 15 amperes instead of 10 amperes, increased armature reaction and the resulting reduction in field current bring the generated emf down to 109 volts. Fifteen amperes through the 0.4-ohm armature use 6 volts, so the terminal voltage is down to 103 volts.

Voltage Regulation

As a measurable quantity, the term voltage regulation means the percentage change of voltage from rated-load to no-load conditions.

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The Compound Generator

The compound generator is designed to compensate for the voltage losses that characterize shunt generators under the influence of increased load. Recall that in a series generator, just the opposite phenomenon occurs: The voltage increases with increasing load. By adding a series winding to the shunt generator, we can combine the features of both windings to provide a more stable output voltage.

Let us examine the effect of adding series field coils to the shunt generator described previously. At a 10-ampere load with a series field coil present, we still have the field- weakening effect of armature reaction and the 4-volt IR drop on the armature, plus a possible 1-volt additional IR drop in the series field. All of these effects can be overcome by adding enough turns of the wire to the field. These turns are connected in series with the load; thus, the 10 amperes in these turns can add to the magnetizing effect of the shunt field coil, increasing the generated emf. The generated emf can readily be brought up to 126, which, less the 5-volt internal IR drops, makes the output voltage 121, the same as at no load. A generator with open-circuit voltage equal to rated load voltage is called flat compounded; see Figure 19–30.

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A generator can be overcompounded. Enough series ampere-turns can be provided to make the output voltage rise above no-load voltage as the output current increases. (Compare with series generators.) Usually, compound generators are built with enough series turns to accomplish overcompounding. The user can adjust the current in the series winding to suit specific operating conditions. Adjustment of the diverter rheostat allows some load current to bypass the series coils, as illustrated in Figure 19–31, and thereby to change the degree of compounding. For instance, if the effect of the series coil is severely limited, the shunt coil will dominate the output characteristics of the generator. Such a machine is said to be undercompounded.

The same circuit of Figure 19–31 is shown schematically in Figure 19–32 to illustrate two points.

1. The generator is connected as a short-shunt generator. Remember, compound generators can be connected either in a short-shunt or long-shunt configuration.

2. The drawing shows a rheostat in the shunt field circuit for the purpose of varying the output voltage. As explained earlier in this chapter, with this rheostat, the operator can weaken or strengthen the magnetic field, thereby changing the output voltage of the generator.

Before leaving the subject of compound generators, let us have another look at Figure 19–30, and note the term differentially compounded. Most generators are cumulatively compounded; that is, the two windings on the pole pieces (series and shunt) are wound in the same direction, so that their magnetic fields will reinforce each other; see Figure 19–31. In a differentially compounded generator, by contrast, the two magnetic fields are opposing each other. Not many generators are differentially compounded.

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19–7 SEPARATELY EXCITED GENERATORS

Two general ways of providing a magnetic field for a generator have been mentioned:

1. Permanent magnet (very limited application)

2. Self-excitation—shunt, series, or compound (the most widely used method)

A third possibility is a separate current source energizing the field coils of the generator, as illustrated in Figure 19–33. Normally, this method is used only as a part of a specialized motor-control circuit, such as Ward Leonard system, Rototrol system, or Amplidyne control system, to name a few. The purpose of these systems is to permit the operator to select any specific speed, after which the system holds the motor at that speed regardless of variations in the load on the motor. Separate excitation systems of this type are used in mine hoists, steel-mill rolling mills, paper machines, diesel-electric locomotives, and other similar devices.

19–8 GENERATOR CALCULATIONS

The following paragraphs are not intended to provide enough details to illustrate all of the factors that must be calculated by a generator designer. They are intended only to point out the basic principles of the energy conversion that goes on in a generator.

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Generator emf

As stated before, generator emf is proportional to field strength, number of wires on the armature, and rpm of the armature. When the field strength, armature windings, and rpm of the armature are known, the number of lines of force cut per second can be found. The cutting of 100,000,000 lines by wire each second makes 1 volt.

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EXAMPLE 19–1

Given: An armature as shown in Figure 19–34. (This is similar to the armature in Figures 19–13 and 19–14.) The armature has eight coils, each coil consisting of

40 turns, operating at 1,200 rpm, between two field poles, each pole having 15 square inches of face area and a flux density of 80,000 lines per square inch.

Find: The emf generated.

Solution

Figure 19–34 shows that the coils are in two parallel groups, with four coils in series in each group. The emf is produced by four coils in series, not eight. Therefore, we need to take into account four coils of 40 turns each 5 160 turns. Since the purpose of this calculation is to find the average emf, we need not be concerned about differences in instantaneous voltages in the coils.

Each turn of wire in the coil has two sides, both of which cut the entire field twice (once up and once down) during each revolution. The 160 turns, then, have to be multiplied by 4 to give the number of times that the entire field is cut by wire each revolution. The field is cut 640 times.

The total field flux is 80,000 lines per sq in. 3 15 sq in. 5 1,200,000 lines. During one rotation, 1,200,000 lines are cut 640 times: 640 3 1,200,000 5 768,000,000 total lines of force cut.

The coils rotate at 1,200 rpm, which is 20 revolutions per second. The total cutting of lines per second is 768,000,000 times 20. The emf generated is

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Planning a generator design to produce a given emf involves a sensible choice of turns, flux, rpm, and type of winding—based on both theory and experience—and takes into account any special demands on the generator in use.

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Emf vs.Terminal Voltage

The emf that we just calculated in the preceding problem is not the same as the voltage delivered to the terminals of the load. To understand why, consider the following: The armature has some resistance, say 0.4 ohm; and the brushes, riding on the commutator, have resistance of about 0.05 ohm. These resistances are shown as RA, lumped together outside the armature, in the schematic diagram of Figure 19–35.

Assuming that 10 amperes is flowing through the armature, there will be a voltage drop of 4.5 volts in the armature, which is lost to the load. Think of it as a series circuit, and you will see that only 149.1 volts will be available at the terminals of the load.

Kirchhoff’s voltage law is proven once again and is stated now like this:

Output voltage = Generated emf - IR drop in armature and brushes