Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-65 are taken by the motor. What are the apparent power and the power factor? Solution: Apparent power = V olts × Amperes Apparent power = 220 × 50 = 11,000 watts or volt-amperes. (PF) = Watts (True Power) × 100 V A (Apparent Power) (PF) = 9,350 × 100 11,000 (PF) = 85, or 85%
Transformers
A transformer changes electrical energy of a given voltage into electrical energy at a different voltage level. It consists of two coils that are not electrically connected, but are arranged so that the magnetic field surrounding one coil cuts through the other coil. When an alternating voltage is applied to (across) one coil, the varying magnetic field set up around that coil creates an alternating voltage in the other coil by mutual induction. A transformer can also be used with pulsating DC, but a pure DC voltage cannot be used, since only a varying voltage creates the varying magnetic field that is the basis of the mutual induction process.
A transformer consists of three basic parts. [Figure 12-142] These are an iron core, which provides a circuit of low reluctance for magnetic lines of force; a primary winding, which receives the electrical energy from the source of applied voltage; and a secondary winding, which receives electrical energy by induction from the primary coil. The primary and secondary of this closed core transformer are wound on a closed core to obtain maximum inductive effect between the two coils. There are two classes of transformers: voltage transformers, used for stepping up or stepping down voltages; and current transformers used in instrument circuits. In voltage transformers, the primary coils are connected in parallel across the supply voltage. [Figure 12-143A] The primary windings of current transformers are connected in series in the primary circuit. [Figure 12-143B] Of the two types, the voltage transformer is the more common.
There are many types of voltage transformers. Most of these are either step-up or step-down transformers. The factor that determines whether a transformer is a step-up or step- down type is the “turns” ratio. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. For example, the turns ratio of the step-down transformer is 5 to 1, since there are five times as many turns in the primary as in the secondary. [Figure 12-144A] The step-up transformer has a 1 to 4 turns ratio. [Figure 12-144B] The ratio of the transformer input voltage to the output voltage is the same as the turns ratio if the transformer is 100 percent efficient. Thus, when 10 volts are applied to the primary of the transformer, two volts are induced in the secondary. [Figure 12-144A] If 10 volts are applied to the primary of the transformer, the output voltage across the terminals of the secondary is 40 volts. [Figure 12-144B] No transformer can be constructed that is 100 percent efficient, although iron core transformers can approach this figure. This is because all the magnetic lines of force set up in the primary do not cut across the turns of the secondary coil. A certain amount of the magnetic flux, called leakage flux, leaks out of the magnetic circuit. The measure of how well the flux of the primary is coupled into the secondary is called the “coefficient of coupling.” For example, if it is assumed that the primary of a transformer develops 10,000 lines of force and only 9,000 cut across the secondary, the coefficient of coupling would be 0.9. Stated another way, the transformer would be 90 percent efficient.
When an AC voltage is connected across the primary terminals of a transformer, an AC flows and self induces a voltage in the primary coil that is opposite and nearly equal to the applied voltage. The difference between these two voltages allows just enough current in the primary to magnetize its core. This is called the exciting, or magnetizing, current. The magnetic field caused by this exciting current cuts across the secondary coil and induces a voltage by mutual induction. If a load is connected across the secondary coil, the load current flowing through the secondary coil produces a magnetic field that tends to neutralize the magnetic field produced by the primary current. This reduces the self-induced (opposition) voltage in the primary coil and allows more primary current 12-66 Primary coil Secondary coil S P Iron core Reactive power Watts True power Apparent power volts x amperes to flow. The primary current increases as the secondary load current increases, and decreases as the secondary load current decreases. When the secondary load is removed, the primary current is again reduced to the small exciting current sufficient only to magnetize the iron core of the transformer.
If a transformer steps up the voltage, it steps down the current by the same ratio. This should be evident if the power formula is considered, for the power (I × E) of the output (secondary) electrical energy is the same as the input (primary) power minus that energy loss in the transforming process. Thus, if 10 volts and 4 amps (40 watts of power) are used in the primary to produce a magnetic field, there is 40 watts of power developed in the secondary (disregarding any loss). If the transformer has a step-up ratio of 4 to 1, the voltage across the secondary is 40 volts and the current is 1 amp. The voltage is 4 times greater and the current is one-fourth the primary circuit value, but the power (I × E value) is the same.
When the turns ratio and the input voltage are known, the output voltage can be determined as follows: E2 E1 = N2 N1 Where E is the voltage of the primary, E2 is the output voltage of the secondary, and N1 and N2 are the number of turns of the primary and secondary, respectively. Transposing the equation to find the output voltage gives: E2 = E1 N2 N1 The most commonly used types of voltage transformers are: 1. Power transformers are used to step up or step down voltages and current in many types of power supplies. They range in size from the small power transformer [Figure 12-145] used in a radio receiver to the large transformers used to step down high power line voltage to the 110–120 volt level used in homes.
iron core transformer. In this case, the secondary is made up of three separate windings. Each winding supplies a different circuit with a specific voltage, which saves the weight, space, and expense of three separate transformers. Each secondary has a midpoint connection called a “center tap,” which provides a selection of half the voltage across the whole winding. The leads from the various windings are color coded by the manufacturer. [Figure 12-146] This is a standard color code, but other codes or numbers may be used. 2. Audio transformers resemble power transformers. They have only one secondary and are designed to operate over the range of audio frequencies (20 to 20,000 cps).
3. RF transformers are designed to operate in equipment that functions in the radio range of frequencies. The symbol for the RF transformer is the same as for an RF choke coil. It has an air core as shown in Figure 12-147. 4. Autotransformers are normally used in power circuits; however, they may be designed for other uses. Two different symbols for autotransformers used in power or audio circuits are shown in Figure 12-148 . If used in an RF communication or navigation circuit [Figure 12-148B], it is the same, except there is no symbol for an iron core. The autotransformer uses part of a winding as a primary; and, depending on whether it is step up or step down, it uses all or part of the same winding as the secondary. For example, the autotransformer shown in Figure 12-148A could use the following possible choices for primary and secondary terminals.
12-67 AC power supply Load AC power supply Meter To load A B 10 turns primary 2 turns secondary 8 turns secondary 2 turns primary A B Current Transformers Current transformers are used in AC power supply systems to sense generator line current and to provide a current, proportional to the line current, for circuit protection and control devices. The current transformer is a ring-type transformer using a current carrying power lead as a primary (either the power lead or the ground lead of the AC generator). The current in the primary induces a current in the secondary by magnetic induction. The sides of all current transformers are marked “H1” and “H2” on the unit base. The transformers must be installed with the “H1” side toward the generator in the circuit in order to have proper polarity. The secondary of the transformer should never be left open while the system is being operated; to do so could cause dangerously high voltages and could overheat the transformer. Therefore, the transformer output connections should always be connected with a jumper when the transformer is not being used but is left in the system.
Transformer Losses In addition to the power loss caused by imperfect coupling, transformers are subject to “copper” and “iron” losses. The resistance of the conductor comprising the turns of the coil causes copper loss. The iron losses are of two types: hysteresis loss and eddy current loss. Hysteresis loss is the electrical energy required to magnetize the transformer core, first in one direction and then in the other, in step with the applied alternating voltage. Eddy current loss is caused by electric currents (eddy currents) induced in the transformer core by the varying magnetic fields. To reduce eddy current losses, cores are made of laminations coated with an insulation, which reduces the circulation of induced currents.
Power in Transformers Since a transformer does not add any electricity to the circuit but merely changes or transforms the electricity that already exists in the circuit from one voltage to another, the total amount of energy in a circuit must remain the same. If it were possible to construct a perfect transformer, there would be no loss of power in it; power would be transferred undiminished from one voltage to another. Since power is the product of volts times amperes, an increase in voltage by the transformer must result in a decrease in current and vice versa. There cannot be more power in the secondary side of a transformer than there is in the primary.
The product of amperes times volts remains the same. The transmission of power over long distances is accomplished by using transformers. At the power source, the voltage is stepped up in order to reduce the line loss during transmission. At the point of utilization, the voltage is stepped down, since it is not feasible to use high voltage to operate motors, lights, or other electrical appliances.
DC Measuring Instruments
Understanding the functional design and operation of electrical measuring instruments is very important, since they are used in repairing, maintaining, and troubleshooting electrical circuits. The best and most expensive measuring instrument is of no use unless the technician knows what is 12-68 Red Red - Yellow Yellow Black Iron core High-voltage winding 5-volt winding 6-volt winding Secondary windings Black Primary Yellow Green Green - Yellow Yellow - Blue 1 2 Input Output 3 Primary 1–2 1–3 2–3 used with “ ” Secondary 1–3 2–3 1–2 2–3 1–3 1–2 A B transformer. being measured and what each reading indicates. The purpose of the meter is to measure quantities existing in a circuit. For this reason, when a meter is connected to a circuit, it must not change the characteristics of that circuit.
Meters are either self-excited or externally excited. Those that are self-excited operate from a power source within the meter. Externally-excited meters get their power source from the circuit that they are connected to. The most common analog meters in use today are the voltmeter, ammeter, and ohmmeter. All of which operate on the principles of electromagnetism. The fundamental principle behind the operation of the meter is the interaction between magnetic fields created by a current gathered from the circuit in some manner. This interaction is between the magnetic fields of a permanent magnet and the coils of a rotating magnet. The greater the current through the coils of the rotating magnet, the stronger the magnetic field produced. A stronger field produces greater rotation of the coil.
While some meters can be used for both DC and AC circuit measurement, only those used as DC instruments are discussed in this section. The meters used for AC, or for both AC and DC, are discussed in the study of AC theory and circuitry. D’Arsonval Meter Movement This basic DC type of meter movement—first employed by the French scientist, d’Arsonval, in making electrical measurement—is a current measuring device, which is used in the ammeter, voltmeter, and ohmmeter. The pointer is deflected in proportion to the amount of current through the coil. Basically, both the ammeter and the voltmeter are current measuring instruments, the principal difference being the method in which they are connected in a circuit. While an ohmmeter is also basically a current measuring instrument, it differs from the ammeter and voltmeter in that it provides its own source (self-excited) of power and contains other auxiliary circuits.
Current Sensitivity and Resistance The current sensitivity of a meter movement is the amount of current required to drive the meter movement to a full-scale deflection. A simple example would be a meter movement that has 1 mA sensitivity. What this indicates is that meter movement requires 1 mA of current to move the needle to a full-scale indication. Likewise, a half-scale deflection requires only 0.5 mA of current. Additionally, what is called 12-69 movement resistance is the actual DC resistance of the wire used to construct the meter coil. In a standard d’Arsonval meter, movement may have a current sensitivity of 1 mA and a resistance of 50 Ω. If the meter is going to be used to measure more than 1 mA, then additional circuitry is required to accomplish the task. This additional circuitry is a simple shunt resistor. The purpose of the shunt resistor is to bypass current that exceeds the 1 mA limitation of the meter movement. To illustrate this, assume that the 1 mA meter in question is needed to measure 10 mA. The shunt resistor used should carry 9 mA while the remaining 1 mA is allowed to pass through the meter.
[Figure 12-149] To determine the proper shunt resistance for this situation: R SH = Shunt resistance R M = Meter resistance = 50 Ω Because the shunt resistance and the 50 Ω meter resistance are in parallel, the voltage drop across both of them is the same. E SH = EM Using Ohm’s Law, this relationship can be rewritten as: E SH = ISH × RSH E M = IM × RM I SH × RSH = IM × RM Simply solve for RSH RSH = IM × RM ISH Substituting the values RSH = = 5.56 ΩImA × 50 Ω 9 mA Damping To make meter readings quickly and accurately, it is desirable that the moving pointer overshoot its proper position only a small amount and come to rest after not more than one or two small oscillations. The term “damping” is applied to methods used to bring the pointer of an electrical meter to rest after it has been set in motion. Damping may be accomplished by electrical means, by mechanical means, or by a combination of both.
Electrical Damping A common method of damping by electrical means is to wind the moving coil on an aluminum frame. As the coil moves in the field of the permanent magnet, eddy currents are set up in the aluminum frame. The magnetic field produced by the eddy currents opposes the motion of the coil. The pointer therefore swings more slowly to its proper position and comes to rest quickly with very little oscillation. Mechanical Damping Air damping is a common method of damping by mechanical means. As shown in Figure 12-150, a vane is attached to the shaft of the moving element and enclosed in an air chamber.
The movement of the shaft is retarded because of the resistance that the air offers to the vane. Effective damping is achieved if the vane nearly touches the walls of the chamber. A Basic Multirange Ammeter Building upon the basic meter previously discussed is the more complex and useful multirange meter, which is more practical. The basic idea of a multirange ammeter is to make the meter usable over a wide range of voltages. In order to accomplish this, each range must utilize a different shunt resistance. The example given in this handbook is that of a two-range meter. However, once the basics of a two-range multirange ammeter are understood, the concepts can easily be transferred to the design of meters with many selectable ranges.
selectable ranges. This example builds upon the previous 10 mA range meter by adding a 100 mA range. With the switch selected to the 10 mA range, the meter indicates 10 mA when the needle is deflected to full-scale and likewise indicates 100 mA at full-scale when selected to 100 mA. The value of the 100 mA shunt resistor is determined the same way the 10 mA shunt resistor was determined. Recall that the meter movement can only carry 1 mA. This means that in a 100 mA range the remaining current of 99 mA must pass through the shunt resistor. RSH = IM × RM ISH Substituting the values: RSH = = 0.51 ΩImA × 50 Ω 99 mA Precautions The precautions to observe when using an ammeter are summarized as follows: 1. Always connect ammeter in series with the element 12-70 Meter movement [+] [−] RSH ISH IMM Meter movement Basic ammeter [+] [−] RSH ISH 1 mA 10 mA 9 mA through which the current flow is to be measured.
2. Never connect an ammeter across a source of voltage, such as a battery or generator. Remember that the resistance of an ammeter, particularly on the higher ranges, is extremely low and that any voltage, even a volt or so, can cause very high current to flow through the meter, causing damage to it. 3. Use a range large enough to keep the deflection less than full-scale. Before measuring a current, form some idea of its magnitude. Then switch to a large enough scale or start with the highest range and work down until the appropriate scale is reached. The most accurate readings are obtained at approximately half-scale deflection. Many milliammeters have been ruined by attempts to measure amperes. Therefore, be sure to read the lettering either on the dial or on the switch positions and choose proper scale before connecting the meter in the circuit.
4. Observe proper polarity in connecting the meter in the circuit. Current must flow through the coil in a definite direction in order to move the indicator needle up scale. Current reversal because of incorrect connection in the circuit results in a reversed meter deflection and frequently causes bending of the meter needle. Avoid improper meter connections by observing the polarity markings on the meter. The Voltmeter The voltmeter uses the same type of meter movement as the ammeter but employs a different circuit external to the meter movement. As shown before, the voltage drop across the meter coil is a function of current and the coil resistance. In another example, 50 μA × 1,000 Ω = 50 mV . In order for the meter to be used to measure voltages greater than 50 mV , there must be added a series resistance to drop any excess voltage greater than that which the meter movement requires for a full-scale deflection. The case of the voltmeter, this resistance is called multiplier resistance and is designated as RM. [Figure 12-152] The voltmeter only has one multiplier resistor for use in one range. In this example, the full-scale reading is 1 volt. RM is determined in the following way: The meter movement drops 50 mV at a full-scale deflection of 50 μA. The multiplying resistor RM must drop the remaining voltage of 1 V − 50 mV = 950 mV . Since RM is in series with the movement, it also carries 50 μA at full scale.
RM = = 19k Ω 950 mV 50 μA Therefore, for 1 volt full-scale deflection, the total resistance of the voltmeter is 20k Ω. That is, the multiplier resistance and the coil resistance. Voltmeter Sensitivity V oltmeter sensitivity is defined in terms of resistance per volt (Ω/V). The meter used in the previous example has a sensitivity of 20k Ω and a full-scale deflection of 1 volt. Multiple Range Voltmeters The simplified voltmeter in Figure 12-152 has only one range (1 volt), which means that it can measure voltages from 0 volts to 1 volt. In order for the meter to be more useful, additional multiplier resistors must be used. One resistor must be used for each desired range.
For a 50 μA movement, the total resistance required is 20k Ω for each volt of full-scale reading. In other words, the sensitivity for a 50 μA movement is always 20k Ω regardless of the selected range. The full-scale meter current is 50 μA at any range selection. To find the total meter resistance, multiply the sensitivity by the full-scale voltage for that particular range. For example for a 10 volt range, RT = (20k Ω/V) (10V) = 200k Ω. The total resistance for the 1 volt range is 20k Ω, so RM for a 10 V range is 200k Ω − 20k Ω = 180k Ω. [Figure 12-153] 12-71 Closed air chamber Light vane swinging in chamber with small clearance 1mA, 50 ohm movement [+] [−] 10mA 100mA 5.56Ω 5.51Ω Voltmeter Circuit Connections When voltmeters are used, they are connected in parallel with a circuit. If unsure about the voltage to be measured, take the first reading at the high value on the meter and then progressively move down through the range until a suitable read is obtained. Observe that the polarity is correct before connecting the meter to the circuit or damage occurs by driving the movement backwards.
Influence of the Voltmeter in the Circuit When a voltmeter is connected across two points in a circuit, current is shunted. If the voltmeter has low resistance, it draws off a significant amount of current. This lowers the effective resistance of the circuit and change the voltage readings. When making a voltage measurement, use a high resistance voltmeter to prevent shunting of the circuit. The Ohmmeter The meter movement used for the ammeter and the voltmeter can also be used for the ohmmeter. The function of the ohmmeter is to measure resistance. A simplified one-stage ohmmeter is illustrated in Figure 12-154, which shows that the basic ohmmeter contains a battery and a variable resistor in series with the meter movement. To measure resistance, the leads of the meter are connected across an external resistance, which is to be measured. By doing this, the ohmmeter circuit is completed. This connection allows the internal battery to produce a current through the movement coil, causing a deflection of the pointer proportional to the value of the external resistance being measured.
Zero Adjustment When the ohmmeter leads are open, the meter is at a full-scale deflection, indicating an infinite (∞) resistance or an open circuit. [Figure 12-155] When the leads are shorted as shown in figure “zero adjust,” the pointer is at the full right-hand position, indicating a short circuit or zero resistance. The purpose of the variable resistor in this figure is to adjust the current so that the pointer is at exactly zero when the leads are shorted. This is used to compensate for changes in the internal battery voltage due to aging. Ohmmeter Scale Between zero and infinity (∞), the scale is marked to indicate various resistor values. Because the values decrease from left to right, this scale is often called a back-off scale.
In the case of the example given, assume that a certain ohmmeter uses a 50 μA, 1,000 Ω meter movement and has an internal 1.5 volt battery. A current of 50 μA produces a full-scale deflection when the test leads are shorted. To have 50 μA, the total ohmmeter resistance is 1.5 V/50 μA = 30k Ω. Therefore, since the coil resistance is 1k Ω, the variable zero adjustment resistor must be set to 30k Ω – 1k Ω = 29k Ω. Now consider that a 120k Ω resistor is connected to the ohmmeter leads. Combined with the 30k Ω internal resistance, the total R is 150k Ω. The current is 1.5 V/150k Ω = 10 μA, which is 20 percent of the full-scale current and appears on the scale shown in Figure 12-156.
Now consider further that a 120k Ω resistor is connected to the ohmmeter leads. This results in a current of 1.5 V/75k Ω = 10 μA, which is 40 percent of the full-scale current and marked on the scale. Additional calculations of this type show that the scale is nonlinear. It is more compressed toward the left side than the right side. The center scale point corresponds to the internal meter resistance of 30k Ω. The reason is as follows: 12-72 1k ohm movement [+] [−] 10 V 1 V RM2 180k Ω 19k Ω RM1 0∞ Basic voltmeter 50A, 1k ohm movement [+] [−] RM With 30k Ω connected to the leads, the current is 1.5 V/60k Ω = 25 μA, which is half of the full-scale current of 50 μA.
The Multirange Ohmmeter A practical ohmmeter has several operational ranges. These typically are indicated by R × 1, R × 10, R × 100, R × 1k, R × 100k and R × 1M. These range selections are interpreted in a different manner than that of an ammeter or voltmeter. The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting. For example, if the pointer is set on the scale and the range switch is set at R × 100, the actual resistance measurement is 20 × 100 or 2k Ω. To measure small resistance values, the technician must use a higher ohmmeter current than is needed for measuring large resistance values. Shunt resistors are needed to provide multiple ranges on the ohmmeter to measure a range of resistance values from the very small to very large. For each range, a different value of shunt resistance is switched in.
The shunt resistance increases for higher ohm ranges and is always equal to the center scale reading on any selected range. In some meters, a higher battery voltage is used for the highest ohm range. [Figure 12-157] Megger (Megohmmeter) The megger, or megohmmeter, is a high range ohmmeter containing a hand-operated generator. It is used to measure insulation resistance and other high-resistance values. It is also used for ground, continuity, and short-circuit testing of electrical power systems. The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential, or “breakdown” voltage. This type of testing ensures that insulation or a dielectric material will not short or leak under potential electrical stress.
The megger consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: a hand-driven DC generator, G, which supplies the necessary current for making the measurement; and the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed coil type. Coils A and B are mounted on the movable member with a fixed angular relationship to each other and are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The coils are mounted on a light, movable frame that is pivoted in jewel bearings and free to move about axis 0. [Figure 12-158] Coil A is connected in series with R3 and the unknown resistance, RX, to be measured. The series combination of coil A, R3, and RX is connected between the + and − brushes of the DC generator. Coil B is connected in series with R2, and this combination is also connected across the generator.
There are no restraining springs on the movable member of the instrument portion of the megger. When the generator is not in operation, the pointer floats freely and may come to rest at any position on the scale. If the terminals are open circuited, no current flows in coil A, and the current in coil B alone controls the movement of the moving element. Coil B takes a position opposite the gap in the core (since the core cannot move and coil B can), and the pointer indicates infinity on the scale. When a resistance is connected between the terminals, current flows in coil A, tending to move the pointer clockwise. At the same time, coil B tends to move the pointer counterclockwise.
Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position at which the two forces are balanced. This position depends upon the value of the external resistance, which controls the relative magnitude of current of coil A. Because changes in voltage affect both coils A and B in the same proportion, the position of the moving element is independent of the voltage. If the terminals are short circuited, the pointer rests at zero because the current in A is relatively large. The instrument is not damaged under these circumstances because the current is limited by R3.
There are two types of hand-driven meggers: the variable type and the constant pressure type. The speed of the variable pressure megger is dependent on how fast the hand crank