Emitter Base Collector FIGURE 8-233. Junction transistor connections.
power may be furnished by d.c. engine-driven generators, motor-generator sets, vacuum tube rectifiers, or dry-disk or solid-state rectifiers.
In aircraft with a.c. systems, a special d.c.
generator is not desirable since it would be necessary for the engine accessory section to drive an additional piece of equipment. Motor-generator sets, consisting of air-cooled a.c. motors that drive d.c. generators, eliminate this objection because they operate directly off the a.c. power system. Vacuum tube or various types of solidstate rectifiers provide a simple and efficient method of obtaining high voltage d.c. at low amperage. Dry-disk and solid-state rectifiers on the other hand are an excellent source of high amperage at low voltage.
A rectifier is a device which transforms alternating current into direct current by limiting or regulating the direction of current flow. The principal types of rectifiers are dry-disk, solid-state, and vacuum tube rectifiers. Solid-state, or semiconductor, rectifiers are rapidly replacing all other types; and, since dry-disk and vacuum tube rectifiers and motor-generators are largely limited to older model aircraft, the major part of the study of rectifiers will be devoted to solid-state devices used for rectification.
Motor-Generator A motor-generator is an a.c. motor and a d.c.
generator combined in one unit. This combination is often called a converter. Converters operate directly on either single-phase or three-phase voltage. Converters used on large aircraft are often operated by a three-phase, 208-volt, a.c. system and develop a direct current of 200 amperes at 30 volts with an approximate 28-ampere current drain on the a.c. system. Units similar to those used in airplanes with d.c. systems are provided for voltage regulation and paralleling of motorgenerator sets.
A motor-generator offers a number of advantages as a source of d.c. power in an airplane.
With a motor-generator, a momentary interruption of a.c. power does not cut off the d.c. power completely, since the inertia of the armature keeps it turning during the a.c. power interruption.
Extreme temperature changes affect a motorgenerator only slightly. Failure from overheating is negligible compared to that in a vacuum tube rectifier when it is operated above a safe temperature. In addition, a motor-generator can be operated at temperatures below those required by either dry-disk or vacuum tube rectifiers.
The greatest objection to a motor-generator is that, like all rotary devices, it requires considerable maintenance and creates a noise which is especially objectionable if the set is in the cabin of the airplane. For these reasons and, because of weight, space, and cost consideration, the motorgenerator set is rapidly being replaced by various solid-state power sources.
Dry-Disk Dry-disk rectifiers operate on the principle that electric current flows through a junction of two dissimilar conducting materials more readily in one direction than it does in the opposite direction.
This is true because the resistance to current flow in one direction is low, while in the other direction it is high. Depending on the materials used, several amperes may flow in the direction of low resistance but only a few milliamperes in the direction of high resistance.
Three types of dry-disk rectifiers may be found in aircraft: the copper-oxide rectifier, the selenium rectifier, and the magnesium copper-sulfide rectifier. The copper-oxide rectifier (figure 8-234) consists of a copper disk upon which a layer of copper oxide has been formed by heating. It may also consist of a chemical copper-oxide preparation spread evenly over the copper surface. Metal plates, usually lead plates, are pressed against the two opposite faces of the disk to form a good contact. Current flow is from the copper to the copper oxide.
The selenium rectifier consists of an iron disk, similar to a washer, with one side coated with selenium. Its operation is similar to that of the copper-oxide rectifier. Current flows from the selenium to the iron.
The magnesium copper-sulfide rectifier is made of washer-shaped magnesium disks coated with a 374
Copper-oxide P-type N-type Copper - semi-conductor semi-conductor Direction of circuit flow Insulating washer External lead External lead Insulating tube Pressure plate Lead FIGURE 8-234. Copper-oxide dry-disk rectifier.
layer of copper sulfide. The disks are arranged similarly to the other types. Current flows from the magnesium to the copper sulfide.
Solid-State Rectifiers In the study of transistors it was pointed out that the solid-state diode is manufactured from semiconductor material. It consists of N-type and P-type material joined in a single crystal. The point, or junction, where the two materials are in contact is called a P-N junction. This type of semiconductor, regardless of rating or size, is called a junction diode. The first type of semiconductor used was called the point-contact diode.
It utilized a single type of semiconductor material, against which a tungsten or phosphor-bronze wire, called a "cat whisker," was pressed or fused. The point-contact diode has been largely replaced by the junction diode because of its limited currentcarrying capabilities. The most common semiconductor materials are germanium and silicon.
A typical junction diode is shown in figure 8-235.
In figure 8-236, the positive terminal of the battery is connected to the P-type semiconductor material, and the negative terminal is connected to the N-type. This arrangement constitutes forward bias. The holes in the P-type material are repelled from the positive terminal and move toward the junction. The electrons in the N-type FIGURE 8-235. Junction diode.
material are repelled from the negative terminal and likewise move toward the junction. This decreases the space charge existing at the junction, and electron current flow is maintained through the external circuit. The current in the P-type material is in the form of holes, and in the N-type material it is in the form of electrons. If the forward bias is increased, current flow will increase.
If the forward bias is increased excessively, it will cause excessive current. The excessive current will increase thermal agitation and the crystal structure will break down. One important fact worth remembering is that all solid-state devices are sensitive to heat and will be destroyed if the heat becomes too intense.
If the battery connections shown in figure 8-236 are reversed, the junction diode is reverse-biased.
Now the holes are attracted toward the negative terminal and away from the junction. The electrons are attracted toward the positive terminal, also away from the junction. This widens the depletion region, increases the space charge, and reduces current to a minimum condition. It is possible to apply too high a reverse bias. When this happens, the crystal structure will break down.
The symbol for the semiconductor diode is shown in figure 8-237. Note that this is the same FIGURE 8-236. Forward bias on a junetion diode.
High current flow N-type P-type FIGURE 8-237. Semiconductor diode symbol.
symbol used for other types of diodes, such as the copper-oxide and selenium dry-disk rectifiers.
The forward-bias, or high-current, direction is always against the arrow of the symbol.
Figure 8-238 shows a typical characteristic curve for a junction diode. As forward bias is increased a small amount, current flow is increased a considerable amount. For this reason, solid-state devices are said to be current-operated devices, since it is easier to measure the relatively large changes in current flow as compared to the small changes in applied voltage.
With forward bias applied, the diode displays a low-resistance characteristic. On the other hand, with reverse bias applied, a high-resistance state exists. The most important characteristic of a diode is that it allows current to flow in one direction only. This permits solid-state devices to be used in rectifier circuits.
+600 +500 + 400 + 300 +200
- 150 -100 -50 + 100 5.75 2
- 0.5
- 1.0 Milliamperes — 1. .5 + 10V- - - - DC + 10V Input Rectifier OV circuit Output OV — 10V FIGURE 8-239. Rectification process.
Rectification Rectification is the process of changing alternating current to direct current. When a semiconductor rectifier, such as a junction diode, is connected to an a.c. voltage source, it is alternately biased forward and reverse, in step with the a.c.
voltage, as shown in figure 8-239.
In figure 8-240 a diode is placed in series with a source of a.c. power and a load resistor. This is called a half-wave rectifier circuit.
The transformer provides the a.c. input to the circuit; the diode provides the rectification of the
a.c.; and the load resistor serves two purposes:
(1) It limits the amount of current flow in the circuit to a safe level, and (2) it develops an output signal due to the current flow through it.
Assume, in figure 8-241, that the top of the transformer secondary is positive and the bottom negative. With this polarity, the diode is forwardbiased, resistance of the diode is very low, and current flows through the circuit in the direction of the arrows. The output (voltage drop) across the load resistor follows the waveshape of the positive half of the a.c. input. When the a.c.
input goes in a negative direction, the top of the transformer secondary becomes negative and the diode becomes reverse-biased.
With reverse bias applied to the diode, the resistance of the diode becomes very great, and current flow through the diode and load resistor becomes zero. (Remember that a very small current will flow through the diode.) The output, Transformer Diode Input Output Load resistor FIGURE 8-238. Typical junction diode characteristic curve. FIGURE 8-240. Half-wave rectifier circuit.
FIGURE 8-241. Output of a half-wave rectifier.
taken across the load resistor, will be zero. If the position of the diode were reversed, the output would be negative pulses.
In a half-wave rectifier, a half cycle of power is produced across the load resistor for each full cycle of input power. To increase the output power, a full-wave rectifier can be used. Figure 8-242 shows a full-wave rectifier, which is, in effect, two half-wave rectifiers combined into one circuit. In this circuit a load resistor is used to limit current flow and develop an output voltage, two diodes to provide rectification, and a transformer to provide an a.c. input to the circuit. The transformer, used in full-wave rectifier circuits, must be center tapped to complete the path for current flow through the load resistor.
Assuming the polarities shown on the transformer, diode Di will be forward-biased and current will flow from ground through the load resistor, through diode Di, to the top of the transformer.
When the a.c. input changes direction, the transformer secondary will assume an opposite polarity.
Transformer ( Center Diode D, Tapped) Load resistor Diode D, FIGURE 8-242. Full-wave rectifier.
Diode D2 is now forward-biased and current will flow in the opposite direction, from ground through the load resistor, through diode D2, to the bottom half of the transformer.
When one diode is forward-biased, the other is reverse-biased. No matter which diode is forward biased, current will flow through the load resistor in the same direction; so the output will be a series of pulses of the same polarity. By reversing both diodes, the output polarity will be reversed.
The voltage which is felt across a rectifier when reverse bias is being applied is often referred to as "the inverse peak voltage." By definition, this is the peak value of the instantaneous voltage across the rectifier during the half-cycle in which current does not flow or that reverse bias is applied. If an inverse voltage is applied that is too large, the rectifier will be destroyed. The term "breakdown voltage" is often used instead of the term "inverse peak voltage rating," but both terms have the same meaning. Breakdown voltage is the maximum voltage that the rectifier can stand while it is not conducting (reverse-biased); the inverse peak voltage is the voltage actually being applied to the rectifier. As long as the inverse peak voltage is lower than breakdown voltage, there will be no problem of rectifier destruction.
Diode Bridge Rectifier Circuit An advantageous modification of the full-wave diode rectifier is the bridge rectifier. The bridge rectifier differs from the full-wave rectifier in that a bridge rectifier does not require a center-tapped transformer, but does require two additional diodes.
To illustrate how a bridge rectifier performs, consider a sine wave input which is on its positive alternation as denoted on the schematic of figure
8-243. With the secondary of 11 functioning as the bridge rectifier's power supply, point A is the most positive point of the bridge, while B is the most negative. Current flow will be from B to A through the forward-biased diodes. As an aid in finding the path of electron flow, consider the redrawn bridge circuit in figure 8-214. The forward-biased diodes, CR and CR, are easily recognized. Voltage is dropped across each voltage loop as indicated. Thus, on the positive half-cycle input CRs and CR4 are both forward-biased and CRI and CR are reverse-biased.
As long as diode breakdown voltage is not exceeded, current flow will be from point B up and 377
CR, CRE' AC input Rectifier DC
CR CR, FIGURE 8-245. A capacitor used as a filter.
FIGURE 8-243. Diode bridge rectifier.
across CR, down and to the left across Rz. After current crosses RI, it will flow to point A through CR. Notice that current flow across Rz is from right to left, or in respect to polarity, a negative half-cycle output for positive half-cycle input.
Remember that, when tracing current flow for the negative half-cycle, electron flow through a diode is against the symbolic arrow and from negative to a less negative or positive point.
Therefore, no confusion should arise when tracing electron flow up to and away from the common point between CR and CRI. Although it may appear that CR, as well as CR is forward-biased, such is not the case. The collector of CR, is more negative than its emitter; therefore, it is reversebiased.
Since, on the negative half-cycle, CRI and CR are forward-biased, the output signal on the negative half-cycle is negative.
Since both half-cycles of the input signal result in negative output pulses, the bridge rectifier has accomplished the same goal as the full-wave diode rectifier.
50V pk CR FIGURE 8-244. Redrawn bridge rectifier circuit.
FILTERING That part of the rectification process which involves the converting of an a.c. voltage into pulses of d.c. voltage has been treated in the discussion of vacuum tube, dry-disk, and semiconductor diodes. To complete the rectification process so that the pulses of voltage are changed to an acceptable approximation of smooth d.c. involves a process called filtering.
Any reactance which opposes a change in voltage (or current) by storing energy and then releasing this energy back to the circuit may be used as a filter.
In the study of capacitors, it was demonstrated that a capacitance opposes a voltage change across its terminal by storing energy in its electrostatic field. Whenever the voltage tends to rise, the capacitor converts this voltage change to stored energy. When the voltage tends to fall, the capacitor converts this stored energy back to voltage. The use of a capacitor for filtering the output of a rectifier is illustrated in figure 8-245. The rectifier is shown as a block, and the capacitor Ci is connected in parallel with the load R1.
The capacitor C1 is chosen to offer very low impedance to the a.c. ripple frequency and very high impedance to the d.c. component. The ripple voltage is therefore bypassed to ground through the low-impedance path, while the d.c. voltage is applied unchanged to the load. The effect of the capacitor on the output of the rectifier can be seen in the waveshapes shown in figure 8-246.
Dotted lines show the rectifier output; solid lines show the effect of the capacitor. Full-wave rectifier outputs are shown. The capacitor C charges when the rectifier voltage output tends to increase and discharges when the voltage output tends to decrease. In this manner, the voltage across the load R, is kept fairly constant.
An inductance may be used as a filter, because it opposes a change in current through it by storing energy in its electromagnetic field when- 378
Voltage across C, Voltage across C, with large load circuit with small load circuit FIGURE 8-246. Half-wave and full-wave rectifier outputs using capacitor filter.
ever current tends to increase. When the current through the inductor tends to decrease, the inductor supplies the energy to maintain the flow of current. The use of an inductor for filtering the output of a rectifier is shown in figure 8-247.
Note that the inductor Li is in series with the The inductance L1 is chosen to offer high impedance to the a.c. ripple voltage and low impedance to the d.c. component. Therefore, for the a.c. ripple, a very large voltage drop occurs across the inductor and a very small voltage drop across the load Ri. For the d.c. component, however, a very small voltage drop occurs across the inductor and a very large voltage drop across the load. The effect of an inductor on the output of a full-wave rectifier in the output waveshape is shown in figure 8-248. Note that the ripple has been attenuated (reduced) in the output voltage.
Capacitors and inductors are combined in various ways to provide more satisfactory filtering than can be obtained with a single capacitor or inductor. These are referred to collectively as "LC filters." Several combinations are shown schematically in figure 8-249. Note that the L-, or inverted L-type, and the T-type filter sections resemble schematically the corresponding letters of the alphabet. The pi-type filter section resembles the Greek letter pi (7) schematically.
All the filter sections shown are similar in that AC input Load Rectifier FIGURE 8-247. An inductor used as a filter.
FIGURE 8-248. Output of an inductor filter rectifier.
the inductances are in series and the capacitances are in parallel with the load. The inductances must, therefore, offer a very high impedance and the capacitors a very low impedance to the ripple frequency. Since the ripple frequency is comparatively low, the inductances are iron-core coils having large values of inductance (several henries).
Because they offer such high impedance to the ripple frequency, these coils are called chokes.
The capacitors must also be large (several microfarads) to offer very little opposition to the ripple frequency. Because the voltage across the capacitor is d.c., electrolytic capacitors are frequently used as filter capacitors. The correct polarity in connecting electrolytic capacitors should always be observed.
Additional filter sections may be combined to improve the filtering action.
LC filters are also classified according to the position of the capacitor and inductor. A capacitor-input filter is one in which the capacitor is connected directly across the output terminals of the rectifier. A choke-input filter is one in which a choke precedes the filter capacitor.
If it is necessary to increase the applied voltage to more than a single rectifier can tolerate, the usual solution is to stack them. These rectifiers are similar to resistors added in series. Each resistor will drop a portion of the applied voltage rather than the total voltage. The same theory applies to rectifiers added in series, or stacked.
Series stacking increases the voltage rating. If, for example, a rectifier wili be destroyed with an applied voltage exceeding 50 volts, and it is to be used in a circuit with an applied voltage of 150 volts, stacking of diodes can be employed.
The result is shown in figure 8-250.
Identification of Semiconductor Diodes There are many types of semiconductor diodes in existence today and several methods are used to identify the emitter and collector. The following are the three most common methods used to identify the emitter and collector.
Load Load L-filter Inverted L-filter B Load Load T-filter 7 - filter D FIGURE 8-249. IC filters.
One method places a small dot near the emitter lead (A of figure 8-251). A second method stamps the rectifier symbol on the diode case (B of figure 8-251). The third method used quite frequently is the color code method (C of figure 8-251).
Frequently the color code used is the same as the color code used for resistors.
One very common diode is the 1N538. The "IN' indicates that there is only one PN junction, or that the device is a diode; the numbers that follow normally indicate manufacturing sequence; that is, a 1N537 was developed before the 1N538, which may be an improved model of the IN537 or may be an entirely different diode altogether.
A. C. MEASURING INSTRUMENTS A d.c. meter, such as an ammeter, connected in an a.c. circuit will indicate zero, because the moving ammeter coil that carries the current to be measured is located in a permanent magnet 50V E ~ • 150V 150V FIGURE 8-250. Stacking diodes in a circuit. FIGURE 8-251. Diode identification.
field. Since the field of a permanent magnet remains constant and in the same direction at all times, the moving coil follows the polarity of the current. The coil attempts to move in one direction during half of the a.c. cycle and in the reverse direction during the other half when the current reverses.
The current reverses direction too rapidly for average position. Since the current is equal and opposite during each half of the a.c. cycle, the direct current meter indicates zero, which is the average value. Thus, a meter with a permanent magnet cannot be used to measure alternating voltage and current. However, the permanent magnet D'Arsonval meter may be used to measure alternating current or voltage if the current that passes through the meter is first rectified- that is, changed from alternating current to direct current.
Rectifier A. C. Meters Copper-oxide rectifiers are generally used with D'Arsonval d.c. meter movements to measure alternating currents and voltages; however, there are many types of rectifiers which may be used, some of which are included in the discussion of alternator systems.
A copper-oxide rectifier allows current to flow through a meter in only one direction. As shown in figure 8-252, the copper-oxide rectifier consists of copper-oxide disks separated alternately by copper disks and fastened together as a single unit. Current flows more readily from copper to copper oxide than from copper oxide to copper.
When a.c. is applied, therefore, current flows in only one direction, yielding a pulsating d.c. output as shown by the output wave shapes in figure
8-253. This current can then be measured as it flows through the meter movement.
Copper Oxide Contact Direction of electron flow FIGURE 8-252. Copper-oxide rectifier.
In some a.c. meters, selenium or vacuum tube rectifiers are used in place of the copper-oxide rectifier. The principle of operation, however, is the same in all meters employing rectifiers.
Electrodynamometer Meter Movement The electrodynamometer meter can be used to measure alternating or direct voltage and current.
It operates on the same principles as the permanent magnet moving-coil meter, except that the permanent magnet is replaced by an air-core electromagnet. The field of the electrodynamometer meter is developed by the same current that flows through the moving coil (see figure In the electrodynamometer meter, two stationary field coils are connected in series with the movable coil. The movable coil is attached to the central shaft and rotates inside the two stationary field coils. The spiral springs provide the restraining force for the meter and also a means of introducing current to the movable coil.
When current flows through field coils A and B and movable coil C, coil C rotates in opposition to the springs and places itself parallel to the field coils. The more current flowing through the coils, the more the moving coil overcomes the opposition of the springs and the farther the pointer moves across the scale. If the scale is properly calibrated and the proper shunts or multipliers are used, the dynamometer movement will indicate current or voltage.
Although electrodynamometer meters are very accurate, they do not have the sensitivity of D'Arsonval meters and, for this reason, are not widely used outside the laboratory.
Electrodynamometer Ammeter In the electrodynamometer ammeter, low resistance coils produce only a small voltage drop in the circuit measured. An inductive shunt is connected in series with the field coils. This shunt, similar to the resistor shunt used in d.c.
ammeters, permits only part of the current being measured to flow through the coils. As in the d.c.
ammeter, most of the current in the circuit flows through the shunt; but the scale is calibrated accordingly, and the meter reads the total current.
An a.c. ammeter, like a d.c. ammeter, is connected in series with the circuit in which current is measured. Effective values are indicated by the 381
D'Arsonval meter Applied voltage Current in meter circuit Resistor Rectifier plate FIGURE 8-253. A half-wave rectifier circuit.
meter. A schematic diagram of an electrodynamometer ammeter circuit is shown in figure 8-255.
Electrodynamometer Voltmeter In the electrodynamometer voltmeter, field coils are wound with many turns of small wire. Approximately 0.01 ampere of current flow through both coils is required to operate the meter. Resistors of a noninductive material, connected in series with the coils, provide for different voltage ranges. Voltmeters are connected in parallel across the unit in which voltage is to be measured. The values of voltages indicated are effective values.
A schematic diagram of an electrodynamometer voltmeter is shown in figure 8-256.
the scale reading is zero. The amount of magnetization of the vanes depends on the strength of the field, which, in turn, depends on the amount of current flowing through the coil. The force of
• Fixed coils Moving Iron-Vane Meter The moving iron-vane meter is another basic type of meter. It can be used to measure either
a.c. or d.c. Unlike the D'Arsonval meter, which employs permanent magnets, it depends on induced magnetism for its operation. It utilizes the principle of repulsion between two concentric iron vanes, one fixed and one movable, placed inside a solenoid, as shown in figure 8-257. A pointer is attached to the movable vane.
When current flows through the coil, the two iron vanes become magnetized with north poles at their upper ends and south poles at their lower ends for one direction of current through the coil.
Because like poles repel, the unbalanced component of force, tangent to the movable element, causes it to turn against the force exerted by the springs.
The movable vane is rectangular in shape and the fixed vane is tapered. This design permits the use of a relatively uniform scale.
When no current flows through the coil, the movable vane is positioned so that it is opposite the larger portion of the tapered fixed vane, and Movable coil -- Shunt FIGURE 8-254. Simplified diagram of an electrodynamometer movement.
Line Load Pointer D Control spring Field coil (stationary) Moving coil Cylindrical Shunt coil coil Moving iron Stationary iron FIGURE 8-255. Electrodynamometer ammeter segment segment circuit. FICURE 8-257. Moving iron-vane meter.
repulsion is greater opposite the larger end of the fixed vane than it is nearer the smaller end.
Therefore, the movable vane moves toward the smaller end through an angle that is proportional to the magnitude of the coil current. The movement ceases when the force of repulsion is balanced by the restraining force of the spring.
Because the repulsion is always in the same direction (toward the smaller end of the fixed vane), regardless of the direction of current flow through the coil, the moving iron-vane instrument operates on either d.c. or a.c. circuits.
Mechanical damping in this type of instrument can be obtained by the use of an aluminum vane attached to the shaft so that, as the shaft moyes, the vane moves in a restricted air space.
When the moving iron-vane meter is designed to be used as an ammeter, the coil is wound with relatively few turns of large wire in order to carry the rated current.
When the moving iron-vane meter is designed to be used as a voltmeter, the solenoid is wound with many turns of small wire. Portable voltmeters are made with self-contained series resistance for ranges up to 750 volts. Higher ranges Line Load Series resistor Stationary coil Moving coil FIGURE 8-256. Electrodynamometer voltmeter circuit.
are obtained by the use of additional external multipliers.
The moving iron-vane instrument may be used to measure direct current but has an error due to residual magnetism in the vanes. The error may be minimized by reversing the meter connections and averaging the readings. When used on a.c.
circuits the instrument has an accuracy of 0.5 percent. Because of its simplicity, its relatively low cost, and the fact that no current is conducted to the moving element, this type of movement is used extensively to measure current and voltage in a.c. power circuits. However, because the reluctance of the magnetic circuit is high, the moving iron-vane meter requires much more power to produce full-scale deflection than is required by a D'Arsonval meter of the same range. Therefore, the moving iron-vane meter is seldom used in high-resistance low-power circuits.
Inclined-Coil Iron-Vane Meter The principle of the moving iron-vane mechanism is applied to the inclined-coil type of meter, which can be used to measure both a.c. and d.c.
The inclined-coil, iron-vane meter has a coil mounted at an angle to the shaft. Attached obliquely to the shaft, and located inside the coil, are two soft-iron vanes. When no current flows through the coil, a control spring holds the pointer at zero, and the iron vanes lie in planes parallel to the plane of the coil. When current flows through the coil, the vanes tend to line up with magnetic lines passing through the center of the coil at right angles to the plane of the coil.
Thus, the vanes rotate against the spring action to move the pointer over the scale.
The iron vanes tend to line up with the magnetic lines regardless of the direction of current 383
Copper Copper lead lead Thermocouple Heater (resistance wire) FIGURE 8-258. Simplified diagram of a thermocouple meter.
flow through the coil. Therefore, the inclined-coil, iron-vane meter can be used to measure either alternating current or direct current. The aluminum disk and the drag magnets provide electromagnetic damping.
Like the moving iron-vane meter, the inclinedcoil type requires a relatively large amount of current for full-scale deflection and is seldom used in high-resistance low-power circuits.
As in the moving iron-vane instruments, the inclined-coil instrument is wound with few turns of relatively large wire when used as an ammeter and with many turns of small wire when used as a voltmeter.
Thermocouple Meter If the ends of two dissimilar metals are welded together and this junction is heated, a d.c. voltage is developed across the two open ends. The voltage developed depends on the material of which the wires are made and on the difference in temperature between the heated junction and the open ends.
In one type of instrument, the junction is heated electrically by the flow of current through a heater element. It does not matter whether the current is alternating or direct because the heating effect is independent of current direction. The maximum current that can be measured depends on the current rating of the heater, the heat that the thermocouple can stand without being damaged, and on the current rating of the meter used with the thermocouple. Voltage can also be measured if a suitable resistor is placed in series with the heater. In meter applications, a D'Arsonval meter is used with a resistance wire heater, as shown in figure 8-258.
As current flows through the resistance wire, the heat developed is transferred to the contact point and develops an e.m.f. which causes current to flow through the meter. The coil rotates and causes the pointer to move over a calibrated scale.
The amount of coil movement is dependent on the amount of heat, which varies as the square of the current. Thermocouple meters are used extensively in a.c. measurements.
Varmeters Multiplying the volts by the amperes in an a.c.
circuit gives the apparent power: the combination of the true power which does the work and the reactive power which does no work and is returned to the line. Reactive power is measured in units of vars (volt-amperes reactive) or kilovars (kilovolt-amperes reactive, abbreviated KVAR). When properly connected, wattmeters measure the reactive power. As such, they are called varmeters.
The illustration in figure 8-259 shows a varmeter connected in an a.c. circuit.
Wattmeter Electric power is measured by means of a wattmeter. Because electric power is the product of current and voltage, a wattmeter must have two elements, one for current and the other for voltage, as indicated in figure 8-260. For this reason, wattmeters are usually of the electrodynamometer type.
The movable coil with a series resistance forms the voltage element, and the stationary coils constitute the current element. The strength of Var meter Load Voltage coil i Reactor мої • Current I coil FIGURE 8-259. A varmeter connected in an
a.c. circuit.
Potential coil Current coils Load FICURE 8-260. Simplified electrodynamometer wattmeter circuit.
the field around the potential coil depends on the amount of current that flows through it. The current, in turn, depends on the load voltage applied across the coil and the high resistance in series with it. The strength of the field around the current coils depends on the amount of current flowing through the load. Thus, the meter deflection is proportional to the product of the voltage across the potential coil and the current through the current coils. The effect is almost the same (if the scale is properly calibrated) as if the voltage applied across the load and the current through the load were multiplied together.
If the current in the line is reversed, the direction of current in both coils and the potential coil is reversed, the net result is that the pointer continues to read up-scale. Therefore, this type of wattmeter can be used to measure either a.c.
or d.c. power.
FREQUENCY METERS Alternating-current electrical equipment is designed to operate within a given frequency range.
In some instances the equipment is designed to operate at one particular frequency, as are electric clocks and time switches. For example, electric clocks are commonly designed to operate at 60
c.p.s. If the supply frequency is reduced to 59
c.p.s., the clock will lose one minute every hour.
Transformers and a.c. machinery are designed to operate at a specified frequency. If the supply frequency falls more than 10 percent from the rated value, the equipment may draw excessive current, and dangerous overheating will result.
It is, therefore, necessary to control the frequency of electric power systems. Frequency meters are employed to indicate the frequency so that corrective measures can be taken if the frequency varies beyond the prescribed limits.
Frequency meters are designed so that they will not be affected by changes in voltage. Because
a.c. systems are designed to operate normally at one particular frequency, the range of the frequency meter may be restricted to a few cycles on either side of the normal frequency. There are several types of frequency meters, including the vibrating-reed type, the fixed-coil and movingcoil type, the fixed-coil and moving-disk type, and the resonant-circuit type. Of these types, the vibrating-reed frequency meter is used most often in aircraft systems, and is discussed in some detail.
Vibrating-Reed Frequency Meter The vibrating-reed type of frequency meter is one of the simplest devices for indicating the frequency of an a.c. source. A simplified diagram of one type of vibrating-reed frequency meter is shown in figure 8-261.
The current whose frequency is to be measured flows through the coil and exerts maximum attraction on the soft-iron armature twice during each cycle (A of figure 8-261). The armature is attached to the bar, which is mounted on a flexible support. Reeds of suitable dimensions to have natural vibration frequencies of 110, 112, 114, and so forth up to 130 c.p.s. are mounted on the bar (B of figure 8-261). The reed having a frequency of 110 c.p.s. is marked "55" cycles; the one having a frequency of 130 c.p.s. is marked "65" c.p.s.; the one having a frequency of 120
c.p.s. is marked "60" c.p.s., and so forth.
In some instruments the reeds are the same lengths, but are weighted by different amounts at the top so that they will have different natural rates of vibration.
When the coil is energized with a current having a frequency between 55 and 65 c.p.s., all the reeds are vibrated slightly; but the reed having a 385