GENERATOR CIRCUIT BREAKERS BASIC INFORMATION AND PARTS OF GENERATOR CIRCUIT BREAKERS TUTORIALS

What Are Generator Circuit Breakers?

Generator circuit breakers represent another class rated for very high continuous currents and short circuit currents, typically at generator voltages. Generator breakers are incorporated into generator bus ducts and can include other switchgear components for measuring current, detecting faults, and grounding.

Generator breakers are available up to 50 kA nominal current and up to 220 kA interrupting current. Two technologies are employed—air blast at the higher ratings and SF6 self blast at the lower and medium power levels (up to 120 kA). For nominal currents above 20 kA, the generator breaker is usually equipped with a forced cooling system, using water, for example. Generator breakers have been available since the 1960s.

Advantages of using generator breakers include the following:

Reduced station cost by eliminating station transformers and increasing station layout flexibility.

Simplification of operation, especially during commissioning and recommissioning; this is because the generator can be handled as a separate unit, isolated from the main and unit transformers.

Fault protection between the generator and transformer. Two zones of protection are created and generator faults are cleared by the opening of the generator breaker alone.

Unbalanced load protection of the generator.

Protection of the generator from transformer faults.

Reliability/availability increase.

Historically, generator circuit breakers have been of air-blast design with pneumatic operators. This is the technology still used today for large nuclear and fossil fuel power plants (up to 1500 MW), and large pumped storage installations.

The design has a tubular housing and is horizontal. Newer designs utilize SF6 self-blast technology and hydraulic operators. These are rated for application to smaller power plants (gas turbine/cogen, for example) from 60 to 400 MW and smaller pumped storage installations.

PARTS OF GENERATOR CIRCUIT BREAKERS

Outline and interrupter details of a generator air-blast circuit breaker-type DR, 36 Kv, up to 50 kA with forced cooling, 200 kA.

SHAFT CURRENT, BEARING INSULATION, AND PHASE SEQUENCE TEST OF SYNCHRONOUS GENERATORS

Shaft Current and Bearing Insulation

Irregularities in the SG magnetic circuit lead to a small axial flux that links the shaft. A parasitic current occurs in the shaft, bearings, and machine frame, unless the bearings are insulated from stator core or from rotor shaft.

The presence of pulse-width modulator (PWM) static converters in the stator (or rotor) of SG augments this phenomenon. The pertinent testing is performed with the machine at no load and rated voltage. The voltage between shaft ends is measured with a high impedance voltmeter.

The same current flows through the bearing radially to the stator frame. The presence of voltage across bearing oil film (in uninsulated bearings) is also an indication of the shaft voltage.

If insulated bearings are used, their effectiveness is checked by shorting the insulation and observing an increased shaft voltage. Shaft voltage above a few volts, with insulated bearings, is considered unacceptable due to bearing in-time damage.

Generally, grounded brushes in shaft ends are necessary to prevent it.

Phase Sequence

Phase sequencing is required for securing given rotation direction or for correct phasing of a generator prepared for power bus connection. As known, phase sequencing can be reversed by interchanging any two armature (stator) terminals.

There are a few procedures used to check phase sequence:

• With a phase-sequence indicator (or induction machine)

• With a neon-lamp phase-sequence indicator (Figure 8.1a and Figure 8.1b)

• With the lamp method (Figure 8.1b)

When the SG no-load voltage sequence is 1–2–3 (clockwise), the neon lamp 1 will glow, while for the 1–3–2 sequence, the neon lamp 2 will glow. The test switch is open during these checks.

The apparatus works correctly if, when the test switch is closed, both lamps glow with the same intensity (Figure 8.1a).

FIGURE 8.1 Phase-sequence indicators: (a) independent (1–2–3 or 1–3–2) and (b) relative to power grid.

With four voltage transformers and four lamps (Figure 8.1b), the relative sequence of SG phases to power grid is checked. For direct voltage sequence, all four lamps brighten and dim simultaneously. For the opposite sequence, the two groups of lamps brighten and dim one after the other.

COUPLING TO TEST GENERATORS BASIC TUTORIALS

Application of the test wave to the relay under test shall be done using coupling/isolating networks or capacitive coupling clamps as described following.

Coupling/isolating networks

This method shall be used when testing current, voltage, power supply, input circuit, or output external connection groups.

Capacitive coupling clamp

The capacitive coupling clamp is intended as a means for coupling test waveforms to shielded and other circuits where direct galvanic connection to the equipment under test (EUT) terminals may impair operation.

Typical use of the clamp for common mode tests is shown in Figure 7. The use of the capacitive clamp shall be limited.

The capacitive clamp consists of two conductive surfaces (made of galvanized steel, brass, copper, or aluminum) insulated from each other by means of high quality insulating supports.

The coupling effect is established by placing an insulated cable (feeding the signals to the device under test) through the V shaped duct on top of the clamp, and subsequently closing the clamp as much as possible in order to maximize the coupling capacitance between the clamp and the cable.

The lower conductive surface of the clamp shall be placed on (and solidly connected to) a ground plane with a minimum area of 1 m2. The reference ground plane shall extend beyond the clamp by at least 0.1 m on all sides.

The high voltage coaxial connector intended for connection to the test generator may, for convenience, be provided on both ends of the clamp. Regardless of the design however, during the test, the generator shall be connected to that end of the clamp that is nearest to the device under test.

Due to high voltages involved, the external insulation on the cable to which the transients are being coupled must be capable of withstanding a 5 kV 1.2/50 μs pulse.

VERIFICATION OF TEST GENERATOR CHARACTERISTICS BASIC INFORMATION

It is necessary to establish a simple test procedure intended to verify and document the SWC generator performance prior to its use for these tests. Two verification tests shall be defined:

a) SWC waveform validity tests

b) SWC test generator performance verification

Waveform validity tests

The waveform validity tests shall be performed before and after each SWC test session, with the results recorded and included with the SWC test report. The tests described are intended to verify that there is no major insulation breakdown, damage, or test system component failure that may have occurred during the physical test setup, test application, or prolonged instrument storage.

The waveform validity tests shall include

a) Measuring system feedthrough test

b) Oscillatory SWC test generator open circuit voltage waveform test

c) Fast transient SWC test generator open circuit voltage waveform test

Measurement methods different from those described in the Annex A are acceptable, as long as the same level of precision and the same parameters are used to document the results.

SWC test generator performance verification

A full set of SWC test generator characteristics shall be verified as per manufacturer’s recommendations or at least once every year, with the test results dated, recorded, and included with each SWC test report. These tests are intended to precisely document the test generator characteristics and are used to track the SWC generator’s compliance with this standard.

The characteristics to be verified are given in Table 1.

Table 1—Test generator characteristics to be verified

Fast transient Oscillatory waveform

Rise time Rise time of the first peak

Peak voltage level (no load) Peak voltage level (no load)

Output impedance Output impedance

Impulse duration Waveform envelope decay

n/a Oscillation frequency

Repetition rate during the burst Repetition rate

Burst duration n/a

Burst period n/a

Test duration Test duration

INCREASING ENERGY EFFICIENCY OF ELECTRIC MACHINERY BASIC TUTORIALS

With increasing concern for both the supply and cost of energy comes a corresponding concern for efficiency in its use. Although electric energy can be converted to mechanical energy with great efficiency, achieving maximum efficiency requires both careful design of the electric machinery and proper matching of machine and intended application.

Clearly, one means to maximize the efficiency of an electric machine is to minimize its internal losses. For example, the winding I2R losses can be reduced by increasing the slot area so that more copper can be used, thus increasing the cross-sectional area of the windings and reducing the resistance.

Core loss can be reduced by decreasing the magnetic flux density in the iron of the machine. This can be done by increasing the volume of iron, but although the loss goes down in terms of watts per pound, the total volume of material (and hence the mass) is increased; depending on how the machine design is changed, there may be a point beyond which the losses actually begin to increase.

Similarly, for a given flux density, eddy-current losses can be reduced by using thinner iron laminations.

One can see that there are trade-offs involved here; machines of more efficient design generally require more material and thus are bigger and more costly. Users will generally choose the "lowest-cost" solution to a particular requirement; if the increased capital cost of a high-efficiency motor can be expected to be offset by energy savings over the expected lifetime of the machine, they will probably select the high-efficiency machine.

If not, users are very unlikely to select this option in spite of the increased efficiency. Similarly, some types of electric machines are inherently more efficient than others. For example, single-phase capacitor-start induction motors are relatively inexpensive and highly reliable, finding use in all sorts of small appliances, e.g., refrigerators, air conditioners, and fans.

Yet they are inherently less efficient than their three-phase counterparts. Modifications such as a capacitor-run feature can lead to greater efficiency in the single-phase induction motor, but they are expensive and often not economically justifiable.

To optimize the efficiency of use of electric machinery the machine must be properly matched to the application, both in terms of size and performance. Since typical induction motors tend to draw nearly constant reactive power, independent of load, and since this causes resistive losses in the supply lines, it is wise to pick the smallest-rating induction motor which can properly satisfy the requirements of a specific application.

Alternatively, capacitative power-factor correction may be used. Proper application of modern solid state control technology can also play an important role in optimizing both performance and efficiency.

There are, of course, practical limitations which affect the selection of the motor for any particular application. Chief among them is that motors are generally available only in certain standard sizes. For example, a typical manufacturer might make fractional-horsepower ac motors rated at 1/8 , 1/6 , 1/4 , 1/3 , 1/2 , 3/4 , and 1 hp(NEMAs tandard ratings).

This discrete selection thus limits the ability to fine tune a particular application; if the need is 0.8 hp, the user will undoubtedly end up buying a 1-hp device and settling for a somewhat lower than optimum efficiency. A custom-designed and manufactured 0.8-hp motor can be economically justified only if it is needed in large quantities.

It should be pointed out that an extremely common source of inefficiency in electric motor applications is the mismatch of the motor to its application. Even the most efficient 50-kW motors will be somewhat inefficient when driving a 20-kW load.

Yet mismatches of this type often occur in practice, due in great extent to the difficulty in characterizing operating loads and a tendency on the part of application engineers to be conservative to make sure that the system in question is guaranteed to operate in the face of design uncertainties. More careful attention to this issue can go a long way toward increasing the efficiency of energy use in electric machine applications.

POWER FACTOR IN AC MACHINES BASIC AND TUTORIALS

The power factor at which ac machines operate is an economically important feature because of the cost of reactive kilovoltamperes. Low power factor adversely affects system operation in three principal ways.

(1) Generators, transformers, and transmission equipment are rated in terms of kVA rather than Kw because their losses and heating are very nearly determined by voltage and current regardless of power factor.

The physical size and cost of ac apparatus are roughly proportional to kVA rating. The investment in generators, transformers, and transmission equipment for supplying a given useful amount of active power therefore is roughly inversely proportional to the power factor.

(2) Low power factor means more current and greater 12 R losses in the generating and transmitting equipment.

(3) A further disadvantage is poor voltage regulation.

Factors influencing reactive-kVA requirements in motors can be visualized readily in terms of the relationship of these requirements to the establishment of magnetic flux. As in any electromagnetic device, the resultant flux necessary for motor operation must be established by a magnetizing component of current.

It makes no difference either in the magnetic circuit or in the fundamental energy conversion process whether this magnetizing current be carried by the rotor or stator winding, just as it makes no basic difference in a transformer which winding carries the exciting current. In some cases, part of it is supplied from each winding.

If all or part of the magnetizing current is supplied by an ac winding, the input to that winding must include lagging reactive kVA, because magnetizing current lags voltage drop by 90 °. In effect, the lagging reactive kVA set up flux in the motor.

The only possible source of excitation in an induction motor is the stator input. The induction motor therefore must operate at a lagging power factor. This power factor is very low at no load and increases to about 85 to 90 percent at full load, the improvement being caused by the increased real-power requirements with increasing load.

With a synchronous motor, there are two possible sources of excitation: alternating current in the armature or direct current in the field winding. If the field current is just sufficient to supply the necessary mmf, no magnetizing-current component or reactive kVA are needed in the armature and the motor operates at unity power factor.

If the field current is less, i.e., the motor is underexcited, the deficit in mmf must be made up by the armature and the motor operates at a lagging power factor. If the field current is greater, i.e., the motor is overexcited, the excess mmf must be counterbalanced in the armature and a leading component of current is present; the motor then operates at a leading power factor.

Because magnetizing current must be supplied to inductive loads such as transformers and induction motors, the ability of overexcited synchronous motors to supply lagging current is a highly desirable feature which may have considerable economic importance. In effect, overexcited synchronous motors act as generators of lagging reactive kilovoltamperes and thereby relieve the power source of the necessity for supplying this component.

They thus may perform the same function as a local capacitor installation. Sometimes unloaded synchronous machines are installed in power systems solely for power-factor correction or for control of reactive-kVA flow. Such machines, called synchronous condensers, may be more economical in the larger sizes than static capacitors.

Both synchronous and induction machines may become self-excited when a sufficiently heavy capacitive load is present in their stator circuits. The capacitive current then furnishes the excitation and may cause serious overvoltage or excessive transient torques.

Because of the inherent capacitance of transmission lines, the problem may arise when synchronous generators are energizing long unloaded or lightly loaded lines. The use of shunt reactors at the sending end of the line to compensate the capacitive current is sometimes necessary.

For induction motors, it is normal practice to avoid self-excitation by limiting the size of any parallel capacitor when the motor and capacitor are switched as a unit.

ROTOR DESIGN OF DIRECT CURRENT GENERATORS BASIC INFORMATION

Rotor Speeds. Standards list dc generator speeds as high as are reasonable to reduce their size and cost. The speeds may be limited by commutation, maximum volts per bar, or the peripheral speeds of the rotor or commutator.

Generator commutators seldom exceed 5000 ft/min, although motor commutators may exceed 7500 ft/min at high speeds. Generator rotors seldom exceed 9500 ft/min. If the prime mover requires lower speeds than these, generators can be designed for them but larger machines result.

Rotor Diameters.

Difficult commutating generators benefit from the use of large rotor diameters, but diameters are limited by the same factors as rotor speeds. The resultant armature length should be not less than 60% of the pole pitch, because such a small portion of the armature coil would be used to generate voltage.

Direct-current motor speeds must suit the application, and often the rotor diameter is selected to meet the inertia requirements of the application. Core lengths

may be as long as the diameter. Such motors are usually

force-ventilated.

Number of Poles and Other Rotor Design Factors.

The rotor diameter usually fixes the number of main poles. Typical pole pitches range from 17.5 to 20.5 in on medium and large machines. When a choice is possible, high-voltage generators use fewer poles to allow more voltage space on the commutator between the brush arms.

However, high-current generators need many poles to permit more current carrying brush arms and shorter commutators. Commutators for 1000 to 1250 A/(brush arm) (polarity) are costly, and lower values should be used where existing dies will permit.

The main-pole air-gap flux density Bg is limited by the density at the bottom of the rotor teeth. The reduced taper in the teeth of large rotors permits the higher gap densities.

Ampere conductors per inch of rotor circumference (q) is limited by rotor heating, commutation, and, at times, saturation of commutating poles. The commutator diameter is usually about 55% to 85% of the rotor diameter, depending on the sizes available to the designer, the peripheral speed, and the resulting single clearances.

Heating may also limit the choice. Brushes and brush holders are chosen from designs available to limit the brush current density to 60 to 70 A/in2 at full load, to obtain the needed single clearance, and to obtain acceptable commutator heating.

TEMPERATURE RISE CAUSES OF DIRECT CURRENT GENERATORS BASIC INFORMATION

What are the causes of heating of DC generators?

The losses in a dc machine cause the temperature of the parts to rise until the difference in temperature between their surfaces and the cooling air is great enough to dissipate the heat generated.

Permissible measured temperature rises of the parts are limited by the maximum “hot-spot” temperature that the insulation can withstand and still have reasonable life. The maximum surface temperatures are fixed by the temperature gradient through the insulation from the hot spot to the surface.

The IEEE Insulation Standards have established the limiting hot-spot temperatures for systems of insulation. The American National Standards Institute Standard C50.4 for dc machines gives typical gradients for those systems, listing acceptable surface and average copper temperature rises above specified ambient-air temperatures for various machine enclosures and duty cycles.

Typical values are 40°C for Class A systems, 60°C for Class B, and 80°C rise for Class F systems on armature coils.

Class H systems usually contain silicones and are seldom used on medium and large dc machines. Silicone vapors can cause greatly accelerated brush wear at the commutator and severe sparking, particularly on enclosed machines.

Heating of End Connections of Armature Windings. Small machines often have “solid” end windings banded down on insulated “shelf”-type coil supports. Larger machines are more heavily loaded per unit volume and usually have narrow coil supports, air spaces between the end turns, and ventilating air scouring both the top and bottom surfaces of the coil extensions.

With this construction, the approximate allowable product of ampere conductors per inch of outer circumference times the amperes per square inch for various rotor velocities is shown in Fig. 8-51 for a 40°C rise on the end turns.

BRUSH POTENTIAL CURVES OF DIRECT CURRENT GENERATORS BASIC INFORMATION

When a dc machine develops some commutation sparking, the user may suspect that the commutating-pole air gap is not set correctly.

“Brush potential curves” are often taken to prove or disprove such suspicions. These are taken by measuring the voltage drops between the brush and commutator surface at four points while the machine is operating at constant speed and load current (see Fig. 8-34).

The voltages at 1, 2, 3, and 4 are taken by touching the pointed lead of a wooden pencil to the commutator surface. The circuit is completed with leads and a low-reading voltmeter is shown.

The voltages are then plotted. A curve such as A of Fig. 8-34 may indicate undercompensation due to a too large commutating-pole gap. Curve C may indicate overcompensation with too much flux density in the commutating-pole air gap.

Curve B is typical of good compensation. Justification for such conclusions is based on the theory that best commutation (coil current reversal) will be linear while the coil passes under the brush.

This is possible only if there are no circulating currents. Undercompensation should cause circulating currents that would crowd the current to the leaving edge of the brush and cause a high voltage at point 4.

Overcompensation would reverse the current too soon and would actually reverse the voltage drop at point 4. Even to an expert, this test is only an indicator that more definitive tests, such as a buck-boost test, are needed.

Many other factors, including brush riding, commutator surface conditions, and sparking, influence the readings. Where machine changes may be required, the manufacturer should be consulted.

SPECIAL TYPES OF GENERATORS BASIC INFORMATION AND TUTORIALS

General. The adaptability of the dc generator for specific uses has led to the development of many special generators. These machines over the years made a significant contribution to industrial progress.

However, most of these special applications have disappeared or are now being met with other devices such as silicon controlled rectifiers or programmed control of field currents to the main dc generator.

Synchronous Converters. Of all the special generators, this was one of the earlier and most widely used. It was the principal dc power source for streetcars and interurban lines.

It was a most ingenious device, combining in a single armature and winding an ac motor taking its current from the lines through slip rings at the rear and a dc generator providing dc power from a commutator on the front end.

Because the flow of the currents was in opposition, the resulting rotor winding could be small in cross section. A single stator provided flux for both functions. With the decline of street railway systems, the synchronous converter disappeared.

Rotating Regulators. These dc machines had trade names like Rototrol, Regulex, and Amplidyne. They, too, have been replaced by solid-state devices. In addition to having fields for feedback intelligence, response was enhanced using self-excited shunt fields tuned to the air-gap line or by means of cross-magnetization from armature reaction.

Three-Wire Devices. Because three-wire dc circuits are no longer in use, balancer sets and threewire generators are relics in school labs or museums.

Homopolar or Acyclic DC Generators. The single-pole machine principle still fascinates electrical engineers and several research and development labs continue to study new arrangements of its basic parts.

Fundamentally, it consists of a single conductor moving through a uniform single direction flux with a collector at each end of the conductor. The output is a steady ripple-free pure dc current and no commutation.

Currents reaching 270,000 A at 8 V were provided by one commercial unit. Recent efforts have been mainly to use liquid metals to take the large currents from the rotating collectors and to obtain higher voltages by connecting units in series.

Some success has been possible, but restricting the sodium potassium to the collector area has proved difficult.

CAUSES OF SPARKING AND POOR COMMUTATION OF DC GENERATORS BASIC INFORMATION

Sparking and bar burning are usually due to one or more of the following causes:

1. Brushes not in the proper position.

2. Incorrect spacing of brushes. This may be checked by marking an adding-machine tape around the commutator.

3. Projecting-bar-edge mica. Mica between bars should be undercut about 0.063 in below the commutating surface, but occasionally slivers of mica are left inadvertently along the bar.

4. Rough or burned commutator. The commutator should be ground according to the manufacturer’s instruction book.

5. Grooved commutator. This may be prevented by properly staggering the brush sets so that the spaces between the brushes of an arm are covered by brushes of the same polarity of other arms.

6. Poor brush contact. This is due to improper fitting of the brushes to the commutator surface. To seat the brushes, sandpaper should be moved between the commutator and the brush face. Emery cloth should not be used because its abrasive is conducting.

7. Worn brushes replaced by others of wrong size or grade.

8. Sticking brushes. These brushes do not move freely in their holders so that they can follow the irregularities of the commutator.

9. Chattering of the brushes. This is usually due to operation at current densities below 35 A/in2 and must be corrected by lifting brushes to raise the density or by using a special grade of brush.

10. Vibration. This may be due to poor line up, inadequate foundations, or poor balance of the rotor.

11. Short-circuited turns on the commutating or compensating fields. These may be obvious on inspection but usually must be found by passing ac current through them for voltage-drop comparisons.

12. Open or very high resistance joints between the commutator neck and the coil leads. In this case, the bar at the bad joint will usually be burned.

13. An open armature coil. A broken coil conductor produces an effect similar to that produced by the poor joints described in the previous item. For emergency operation, the open coil may be opened at both ends, insulated from the circuit, and a jumper placed across the two affected necks. Since some sparking will probably result, operation should be limited.

14. Short-circuited main-field coils. With the resulting unbalanced air-gap fluxes under the poles, large circulating currents must be expected even with good armature cross connections. The offending coil may be found by comparing voltage drops across the individual coils.

15. Reversed main-field coil. This is an extreme case of the one described in the previous item.

16. Overloading.

PRE RUNNING AND RUNNING CHECKS OF GENERATORS BASIC INFORMATION

Prerunning Checks. The circumferential position of the brushes on the commutator is important for commutation and also to provide the voltage characteristics set at the factory. Brushes should be on the factory test setting.

The toes of the brushes should be aligned and should have no skew. The spacing between adjacent arms of brushes should be identical within 0.032 in.

The brushes should move freely in their holders and should have a pressure against the commutator of 2 to 3 lb/in2 on the basis of brush cross section. The faces of the brushes should accurately match the curvature of the commutator surface.

The polarity of the main fields may be checked by tracing the wiring around the frame or by lightly exciting the fields and using a compass around the frame behind the poles.

The oiling system for the bearings should be checked and the oil rings tested for freedom. The entire machine, particularly its air gaps, should be inspected for foreign material.

Running Checks. Note any unusual noise as the unit is brought up to speed. Bearing temperatures should level out at acceptable values within a few hours.

The voltage should be slowly raised at no load and commutation observed. If satisfactory, the voltage should be raised to 110% of rated and then reduced.

The generator may then be loaded gradually while commutation is observed, until rated current is reached. If commutation remains satisfactory until stable temperatures are achieved, the generator is ready for work.

DIRECT CURRENT GENERATORS PARALLEL OPERATIONS BASIC INFORMATION

Shunt-Wound Generators in Parallel. A and B of Fig. 8-59 are two similar generators feeding the same bus bars C and D. If A tends to take more than its share of the total load, its voltage falls and more load is automatically thrown on B.

Also, if the driver of one of the generators slows down to stop, the emf of the machine falls until the other generator starts to drive it as a motor. This continues until its driver takes over again.

The external characteristics of the two machines are shown in Fig. 8-60. At voltage E, the currents in the generators are Ia and Ib, and the line current is Ia # Ib. To make machine A take more of the load, its excitation must be increased to raise its characteristic curve.

If a 1000-kW generator and a 500-kW machine have the same regulation curves, the machines will divide the load according to their respective capacities, as shown in Fig. 8-61.

Compound-Wound Generators in Parallel. A and B of Fig. 8-62 are two compound-wound machines. If A tends to take more than its share of the load, the series excitation of A increases, its voltage rises, and it takes still more of the load.

Thus, the operation is unstable. If this continues until A takes all the load and the voltage of B drops to the point that A reverses the current in B, B will be driven as a motor. With the reversed current in the series field of B it becomes a differentially compounded motor, and the series weakens the flux to speed up the motor.

This may progress to a point at which the unit may be damaged mechanically and electrically. To prevent this, a bus bar of large section and of negligible resistance, called an equalizer bus, is connected from e to f (Fig. 8-62).

Points e and f are then practically at the same potential. Therefore, the current in each series coil is independent of the current in its particular generator, is inversely proportional to the resistance of the coils, and is always in the same direction.

When a single compound generator has too much compounding, a shunt in parallel with the series field coils will reduce the current in these coils and so reduce the compounding.

When compounded generators are operating in parallel using an equalizer bus, the current in the series field coils depends only on the resistance of the coils and a shunt connected across one of them is actually across all of them, reducing the compounding of all but not disturbing the relative compounding between the machines.

To reduce the compounding of a single machine, it is necessary to place a resistance in series with the coils. This may require a large resistor to handle the large load current it must carry.

ARMATURE SLOTS AND COIL DESIGN BASIC INFORMATION

The depth of an armature slot is limited by several factors, including the tooth density, eddy losses in the armature conductors, available core depths, and commutation. For reasonable frequencies (up to 50 Hz on medium and large dc machines), slots about 2 in deep can ordinarily be used.

Acceptable slot pitches range from 0.75 to 1.5 in. Small machines have shallower slots and a lower range of slot pitches. For medium and large machines, a reasonable tooth density usually results if the ratio of slot width to slot pitch is about 0.4.

Eddy losses in the conductors can be large compared with their load I 2R losses. Sometimes these must be reduced by making each armature conductor from several strands of insulated copper wire.

The number of strands and their size depend on the frequency and the total depth of the conductor. An approximate formula for reasonable eddy losses is

No. of strands = (0.168) (f ^0.83)(de^ 0.4)

where f is the frequency in hertz, (r/min # poles)/120, and dc is the total depth of a conductor.

The insulation space required depends on the type used. Typical conductor strands have about 0.018 in of glass strands and varnish total. Mica wrappers, binding tapes, and varnish and slot finish allowance (0.010 in) total about 0.085 in on the coil width.

If the space for the wedge and its retainer is included, the two coils depth wise total about 0.315 in.

Approximate Slot Design

Width [see text preceding and 0.563 in

following Eq. (8-17)] 0.4 # 1.407

Depth 2.0 in

Approx. total cond. depth 0.875 in

Frequency 42.8 Hz

No. of strands/conductor [Eq. (8-17)] 3

Slot width, in Depth, in

Approx.

Size 0.563 in 2.000 in

Insulation 0.139 (0.085 # 0.054) 0.423 (0.315 # 0.108)

Bare copper 0.424 in 1.577 in

Strand size 0.141 in 0.263 in

Use 3 (0.144 0.289) in strands/conductor

Use available slot 0.570 in 2.250 in

TWO CIRCUIT OR WAVE WINDINGS OF DC GENERATORS BASIC INFORMATION

Figure 8-16 shows a wave type of coil. Figure 8-17 gives a 6-pole wave winding. Study reveals that it has only two parallel paths between the positive and negative terminals. Thus, only two sets of brushes are needed

Each brush shorts p/2 coils in series. Because points a, b, and c are at the same potential (and, also, points d, e, and f ), brushes can be placed at each of these points to allow a commutator one-third as long.

The winding must progress or retrogress by one commutator bar each time it passes around the armature for it to be singly reentrant. Thus, the number of bars must equal (kp/2) # 1, where k is a whole number and p is the number of poles. The winding needs no equalizers because all conductors pass under all poles.

Although most wave windings are 2-circuit, they can be multicircuit, as 4 or 16 circuits on a 4-pole machine or 6, 12, or 24 circuits on a 12-pole machine. Multicircuit wave windings with the same number of circuits as poles can be made by using the same slot and bar combinations as on a lap winding.

For example, with an 8-pole machine with 100 slots and 200 commutator bars, the bar throw for a simplex lap winding would be from bar 1 to bar 2 and then from bar 2 to bar 3, etc. For an 8-circuit wave winding, the winding must fail to close by circuits/2 bars, or 4. Thus, the throw would be bar 1 to 50, to bar 99, to bar 148, etc.

The throw is (bars # circuits/2)(p/2), in this case, (200 # 4)/4 # 49. Theoretically such windings require no equalizers, but better results are obtained if they are used. Since both lap and multiple wave windings can be wound in the same slot and bar combination simultaneously, this is done by making each winding of half-size conductors.

This combination resembles a frog’s leg and is called by that name. It needs no equalizers but requires more insulation space in the slots and is seldom used.

Some wave windings require dead coils. For instance, a large 10-pole machine may have a circle of rotor punchings made of five segments to avoid variation in reluctance as the rotor passes under the five pairs of poles.

To avoid dissimilar slot arrangements in the segments, the total number of slots must be divisible by the number of segments, or 5 in this case. This requires the number of commutator bars to be also a multiple k of 5.

However, the bar throw for a simplex wave winding must be an integer and equal to (bars # 1)(p/2). Obviously (5k # 1)/5 cannot meet this requirement.

Consequently one coil, called a dead coil, will not be connected into the winding, and its ends will be taped up to insulate it completely. No bar will be provided for it, and thus the bar throw will be an integer. Dead coils should be avoided because they impair commutation.

MULTIPLE OR LAP WINDINGS OF DC GENERATORS BASIC INFORMATION

Figure 8-12 shows a lap-winding coil. The conductors shown on the left side lie in the top side of the rotor slot. Those on the right side lie in the bottom half of another slot approximately one pole pitch away. At any instant the sides are under adjacent poles, and voltages induced in the two sides are additive.

Other coil sides fill the remaining portions of the slots. The coil leads are connected to the commutator segments, and this also connects the coils to form the armature winding. This is shown in Fig. 8-13.

The pole faces are slightly shorter than the rotor core.

Almost all medium and large dc machines use simplex lap windings in which the number of parallel paths in the armature winding equals the number of main poles. This permits the current per path to be low enough to allow reasonable-sized conductors in the coils.

Windings. Representations of dc windings are necessarily complicated. Figure 8-14 shows the lap winding corresponding to the Gramme-ring winding. Unfortunately, the nonproductive end portions are emphasized in such diagrams, and the long, useful portions of the coils in the core slots are shown as radial lines.

Conductors in the upper layers are shown as full lines, and those in the lower layers as dotted lines. The inside end connections are those connected to the commutator bars. For convenience, the brushes are shown inside the commutator.

Note that both windings have the same number of useful conductors but that the Gramme-ring winding requires twice the number of actual conductors and twice the number of commutator bars. Figure 8-15 shows a 6-pole simplex lap winding. Study of this reveals the six parallel paths between the positive and negative terminals. The three positive brushes are connected outside the machine by a copper ring T# and the negative brushes by T#.

The two sides of a lap coil may be full pitch (exactly a pole pitch apart), but most machines use a short pitch (less than a pole pitch apart), with the coil throw one-half slot pitch less than a pole pitch. This is done to improve commutation.

Equalizers. As shown in Fig. 8-15, the parallel paths of the armature circuit lie under different poles, and any differences in flux from the poles cause different voltages to be generated in the various paths. Flux differences can be caused by unequal air gaps, by a different number of turns on the main-pole field coils, or by different reluctances in the iron circuits.

With different voltages in the paths paralleled by the brushes, currents will flow to equalize the  voltages. These currents must pass through the brushes and may cause sparking, additional losses, and heating. The variation in pole flux is minimized by careful manufacture but cannot be entirely avoided.

To reduce such currents to a minimum, copper connections are used to short-circuit points on the paralleled paths that are supposed to be at the same voltage. Such points would be exactly two pole pitches apart in a lap winding.

Thus in a 6-pole simplex lap winding, each point in the armature circuit will have two other points that should be at its exact potential. For these points to be accessible, the number of commutator bars and the number of slots must be a multiple of the number of poles divided by 2.

These short-circuited rings are called “equalizers.” Alternating currents flow through them instead of the brushes. The direction of flow is such that the weak poles are magnetized and the strong poles are weakened. Usually, one coil in about 30% of the slots is equalized. The crosssectional area of an equalizer is 20% to 40% that of the armature conductor.

Involute necks, or connections, to each commutator bar from conductors two pole pitches apart give 100% equalization but are troublesome because of inertia and creepage insulation problems. Figure 8-15 shows the equalizing connections behind the commutator connections. Normally they are located at the rear coil extensions, and so they are more accessible and less subject to carbon-brush dust problems.

DIRECT AND INDIRECT COOLING OF GENERATOR ROTORS BAIC INFORMATION

Direct cooling, the norm for rotor windings and widely used in stator windings, exposes the cooling medium directly to the conductors. Figure 7-24 shows hydrogen and water directly cooled conductors for both stator and rotor.

In a directly gas-cooled stator, relatively large passages are built into the conductor bar. The conductor strands are transposed around the gas passages. There is strand insulation between the conductor strands and gas passage (which is often made of stainless steel), but the gas is within the ground wall.

In a directly gas-cooled rotor the gas flow may be radial, axial, or diagonal, or some combination of all three. In a directly water-cooled stator winding, the water flow may be in direct contact with the conductors.

In some cases some or all of the conducting strands are made of hollow copper tubing. In others, stainless-steel tubes are used. Typically, water flows through the machine only one or two axial passes before being returned to the cooler.

If water cooling is used, then

1. The water is maintained at very high purity so that it has low conductivity.

2. Water is carried to the armature conductors through specially made hoses, since the conductor bars are at high potential and the water header is at ground.

3. Generally, hydrogen pressure in the machine is maintained above water pressure so that any leak will be of hydrogen into the water system, rather than water into the electrical insulation. 

Water-cooled field windings are relatively rare, although many have been in highly reliable service for decades in some of the world’s most powerful nuclear turbine generators.

BEARINGS OF ALTERNATING CURRENT GENERATORS BASIC INFORMATION

Although, antifriction bearings are occasionally used on alternators of smaller ratings, the great majority are furnished with oil-lubricated babbitted bearings. For horizontal shafts at small ratings ring oiled bearings are used, but at higher ratings recirculation of externally cooled oil is used.

INDUCTION GENERATOR BASIC INFORMATION AND TUTORIALS

If a machine of this type is connected to a supply, it accelerates as a motor up to a speed near its synchronous speed. If the machine is driven faster than the synchronous speed by an engine or other prime mover, the machine torque reverses and electrical power is delivered by the machine (now acting as a generator) into the connected circuit.

A simple form of wind turbine generator uses an induction machine driven by the wind turbine. The induction machine is first connected to the three-phase supply, and acting as a motor it accelerates the turbine up to near the synchronous speed. At this point, the torque delivered by the wind turbine is sufficient to accelerate the unit further, the speed exceeds the synchronous speed and the induction machine becomes a generator.

It is also possible to operate an induction machine as a generator where there is no separate mains supply available. It is necessary in this case to self-excite the machine, and this is done by connecting capacitors across the stator winding as shown in Fig. 5.20(a).

The leading current circulating through the capacitor and the winding produces a travelling wave of mmf acting on the magnetic circuit of the machine. This travelling wave induces currents in the rotor cage which in turn produces the travelling flux wave necessary to induce the stator voltage.

For this purpose, some machines have an excitation winding in the stator which is separate from the main stator output winding. Figure 5.20(b) shows a single-phase version of the capacitor excitation circuit.

In small sizes, the induction generator can provide a low-cost alternative to the synchronous generator, but it has a relatively poor performance when supplying a low power factor load.

Although induction generators have useful characteristics for use in combination with wind turbines, the magnetizing current must be supplied by other generators running in parallel, or capacitors connected across the stator windings.

Another problem is that the efficiency of an induction generator drops if its speed differs significantly from the synchronous speed, due to high rotor copper loss in the rotor cage.

This can be overcome by using a slipring-fed wound rotor combined with a power electronic converter connected between the stator and rotor windings. Such schemes are often referred to as slip energy recovery using a doubly fed induction generator.

The slip s, of an induction machine is the per unit difference between the rotor speed and the synchronous speed given by:

s = (Ns − Nr)/Ns (5.8)

where Ns is the synchronous speed and Nr is the rotor speed. It can be shown that if Tr is the mechanical torque supplied by a turbine to the rotor of the induction generator, the generated electrical stator power transferred across the air gap is given by TrNs.

Since the input mechanical power to the generator is TrNr, the difference TrNr − TrNs must be the power lost in the rotor, produced mainly by copper loss in the cage. By substitution from eqn 5.8: power transfer to stator = TrNs = rotor loss/s.

With a simple squirrel cage rotor therefore the slip must be low to avoid high rotor loss with a resultant low efficiency.

If the cage is replaced by a three-phase winding, and sliprings are fitted, the same power balance can be achieved by removing the generated rotor power via the sliprings. This power can then be returned to the stator of the generator via a frequency converter. The rotor generated frequency is given by the stator frequency times slip ( f × s).

Fig. 5.20 Self-excitation of an induction generator (a) three phase (b) single phase

CAPACITOR EXCITATION SYSTEM OF GENERATORS BASIC AND TUTORIALS

Although some generators are still produced with brushes and sliprings, most now have a brushless excitation system. One of the main techniques for synchronous generators is capacitor excitation and this is described in the following sections.

The use of this technique is usually restricted to single phase generators with a rated output less than 10 kW. A separate excitation winding in the stator has a capacitor connected directly across its output as shown in Fig. 5.19.

 Fig. 5.19 Capacitor excitation

The rotor is usually of salient-pole construction as described previously, but in this case the rotor winding is shorted through a diode. On starting, the residual flux in the rotor body induces a small voltage in the stator excitation winding and a current flows through the capacitor.

This current produces two waves of magnetic flux around the air gap of the generator. One wave travels in the same direction as the rotor, to create the armature reaction.

The second wave travels in a direction opposite to the rotor, and induces a voltage in the rotor windings at twice the output frequency. The current circulated in the rotor windings by this induced voltage is rectified by the diode to produce a dc current.

This dc current increases the magnetic flux in the machine, which in turn drives more current through the stator excitation winding, which in turn produces more rotor current. This self-excitation process continues until the flux reaches a point at which the magnetic circuit is saturated, and a stable voltage results.

The process also produces an inherent AVR action, since any load current in the output stator winding induces more rotor current to offset the armature reaction effect.