(2) Short the leads together and set the meter to read zero ohms by setting the zero adjustment. If the scale is changed, readjust to zero ohms.
(3) Connect the unknown resistance between the test leads and read its resistance from the scale. Never attempt to measure resistance in a circuit while it is connected to a source of voltage. Disconnect at least one end of the element being measured to avoid reading the resistance of parallel paths.
FIGURE 8-139. Shunt-type ohmmeter circuit.
movement and the limiting resistor. Therefore, when using an ohmmeter with a low-ohm scale, do not leave the switch in low-ohm position.
Use of the Ohmmeter The ohmmeter is not as accurate a measuring device as the ammeter or the voltmeter because of the associated circuitry. Thus, resistance values cannot be read with greater than 5 to 10 percent accuracy. While there are instruments which read the resistance of an element with very great accuracy, they usually are more complicated to use.
In addition to measuring the resistance, the ohmmeter is a very useful instrument for checking continuity in a circuit. Often, when troubleshooting electronic circuits or wiring a circuit, visual inspections of all parts of the current path cannot be readily accomplished. Therefore, it is not always apparent whether a circuit is complete or whether current might be flowing in the wrong part of the circuit because of contact with adjacent circuits.
The best method of checking a circuit under these conditions is to send a current through the circuit.
The ohmmeter is the ideal instrument for checking circuits in this manner. It provides the power and the meter to indicate whether the current is flowing.
Observe the following precautions when using an ohmmeter:
(1) Choose a scale which will contain the resistance of the element to be measured. In general, use a scale in which the reading will fall in the upper half of the scale (near full-scale deflection).
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 (figure 8-140) consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: (1) A hand-driven d.c. generator, G, which supplies the necessary current for making the measurement and (2) 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 Coil A is connected in series with R3 and the R23 xis O I Hand generator Lero Ohmmeter FIGURE 8-140. Simplified megger circuit.
unknown resistance, Ra, to be measured. The series combination of coil A, R3, and Rz is connected between the + and - brushes of the d.c. 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 coil 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 is turned.
The constant-pressure megger utilizes a centrifugal governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant.
BASIC CIRCUIT ANALYSIS AND TROUBLE-
SHOOTING Troubleshooting is the process of locating causes for malfunctions or trouble in a circuit. The following definitions serve as a guide in the troubleshooting discussion:
(1) Short circuit-a low resistance path. It can be across the power source or between the sides of a circuit. It usually creates high current flow which will burn out or cause damage to the circuit conductor or components.
(2) Open circuit-a circuit that is not complete or continuous.
(3) Continuity-the state of being continuous, or connected together; said of a circuit that is not broken or does not have an open.
(4) Discontinuity-the opposite of continuity, indicating that a circuit is broken or not continuous.
Figure 8-141 includes some of the most common sources of open circuits (commonly called "opens" or "an open"). A loose connection or no connection is a frequent cause of an open circuit. In A of figure 8-141, the end of a conductor has separated from the battery terminal. This type of malfunction opens a circuit and stops the flow of current.
Another type of malfunction that will cause an open circuit is a burned-out resistor, shown in B of figure 8-141. When a resistor overheats, its resistance value changes; and, if the current flow through it is great enough, it can burn and open the circuit. In C, D, and E of figure 8-141, three more likely causes of open circuits are shown.
The opens shown can often be located by visual inspection; however, many circuit opens cannot be seen. In such cases, a meter must be used.
The circuit shown in figure 8-142 is designed to cause current to flow through a lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter can be used.
If a voltmeter is connected across the lamp, as shown in figure 8-143, the voltmeter will read zero.
Since no current can flow in the circuit because of the open resistor, there is no voltage drop across the lamp. This illustrates a troubleshooting rule that should be remembered: When a voltmeter is connected across a good (not defective) component in an open circuit, the voltmeter will read zero.
Next, the voltmeter is connected across the open resistor, as shown in figure 8-144. The voltmeter has closed the circuit by shunting (paralleling) the burned-out resistor, allowing current to flow.
Current will flow from the negative terminal of the battery, through the switch, through the voltmeter and the lamp, back to the positive terminal of the battery. However, the resistance of the voltmeter is so high that only a very small current flows in the circuit. The current is too small to light the lamp, but the voltmeter will read the battery voltage. Another troubleshooting point worth 332
Burned out Burned out Broken lamp bulb fuse wire D FIGURE 8-141. Common causes of open circuits.
remembering is: When a voltmeter is placed across an open component in a series circuit, it will read the battery, or applied voltage.
This type of open circuit malfunction can also be traced by using an ohmmeter. When an ohmmeter is used, the circuit component to be tested must be isolated and the power source removed from the circuit. In this case, as shown in figure 8-145, these requirements can be met by opening the circuit switch. The ohmmeter is zeroed and placed across (in parallel with) the lamp. In this circuit, some value of resistance is read. This illustrates another important troubleshooting point: when an ohmmeter is properly connected across a circuit component and a resistance reading is obtained, the component has continuity and is not open.
When the ohmmeter is connected across the open resistor, as shown in figure 8-146, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has been located with both a voltmeter and an ohmmeter.
An open in a series circuit will cause the current flow to stop. A short circuit, or "short," will cause the opposite effect. A short across a series circuit produces a greater than normal current flow. Some examples of shorts, as shown in figure 8-141, are two bare wires in a circuit that are touching each other, two terminals of a resistor connected together, etc. Thus, a short can be described as a Break Break FIGURE 8-143. Voltmeter across a lamp in an FIGURE 8-142. An open circuit. open circuit.
Break Break- FIGURE 8-144. Voltmeter across a resistor in an open circuit.
connection of two conductors of a circuit through a very low resistance.
In figure 8-148, a circuit is designed to light a lamp. A resistor is connected in the circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the current flow will increase and the lamp will become brighter. If the applied voltage were high enough, the lamp would burn out, but in this case the fuse would protect the lamp by opening first.
Usually a short circuit will produce an open circuit by either blowing (opening) the fuse or burning out a circuit component. But in some circuits, such as that illustrated in figure 8-149, there may be additional resistors which will not allow one shorted resistor to increase the current flow enough to blow the fuse or burn out a component. Thus, with one resistor shorted out, the circuit will still function since the power dissipated by the other resistors does not exceed the rating of the fuse.
To locate the shorted resistor while the circuit is functioning, a voltmeter could be used. When it is connected across any of the unshorted resistors, a portion of the applied voltage will be indicated on Break FIGURE 8-146. Using an ohmmeter to locate an open in a circuit component.
the voltmeter scale. When it is connected across the shorted resistor, the voltmeter will read zero.
The shorted resistor shown in figure 8-150 can be located with an ohmmeter. First the switch is opened to isolate the circuit components. In figure 8-150, this circuit is shown with an ohmmeter connected across each of the resistors. Only the ohmmeter connected across the shorted resistor shows a zero reading, indicating that this resistor is shorted.
The procedures used in troubleshooting a parallel circuit are sometimes different from those used in a series circuit. Unlike a series circuit, a parallel circuit has more than one path in which current flows. A voltmeter cannot be used, since, when it is placed across an open resistor, it will read the voltage drop in a parallel branch. But an ammeter or the modified use of an ohmmeter can be employed to detect an open branch in a parallel circuit.
If the open resistor shown in figure 8-151 was not visually apparent, the circuit would appear to be functioning properly, since current would continue to flow in the other two branches of the circuit. To determine that the circuit is not operating properly, the total resistance, total current, and the branch currents of the circuit should be calculated as if there were no open in the circuit:
RI = R N
- 3o = 10 Q total resistance.
FIGURE 8-145. Using an ohmmeter to check a Since the voltage applied to the branches is the circuit component. same and the value of each branch resistance is 334
Battery leads Resistor terminals Bare wires touching Improper wiring FIGURE 8-147. Common causes of short circuits.
known, An ammeter placed in the circuit to read total current would show 2 amperes instead of the calculated 3 amperes. Since 1 ampere of current should be flowing through each branch, it is obvious that one branch is open. If the ammeter is 30V 30V connected into the branches, one after another, = 30 8 30 g the open branch will be located by a zero ammeter reading.
= 1 ampere. = l ampere. A modified use of the ohmmeter can also locate this type of open. If the ohmmeter is connected Із - Е across the open resistor, as shown in figure 8-152, an erroneous reading of continuity would be obtained. Even though the circuit switch is open, 30V 30V the open resistor is still in parallel with R, and R2, = 30 ° = 10 S and the ohmmeter would indicate the open = 1 ampere. = 3 amperes (total current).
FIGURE 8-149. A short that does not open the FIGURE 8-148. A shorted resistor. circuit.
40 g 00 100 v 10 g 50g FIGURE 8-150. Using an ohmmeter to locate a shorted resistor.
resistor had a resistance of 15 ohms, the equivalent resistance of the parallel combination of Ri and R2.
Thus, it is necessary to open the circuit as shown in figure 8-153 in order to check the resistance of Rz- In this way the resistor is not shunted (paralleled) and the reading on the ohmmeter will indicate infinite resistance. On the other hand, if an open should occur in this circuit (figure 8-153) between the battery and point A, or between the battery and point B, current would not flow in the circuit.
As in a series circuit, a short in a parallel circuit will usually cause an open circuit by blowing the fuse. But, unlike a series circuit, one shorted component in a parallel circuit will stop current flow by causing the fuse to open. This can be seen by. referring to the circuit in figure 8-154.
If resistor R, is shorted, a path of almost zero resistance will be offered the current, and all the circuit current will flow through the branch containing the shorted resistor. Since this is practically the same as connecting a wire between the terminals of the battery, the current will rise Rz 302 300 308 3 Break FIGURE 8-152. A misleading ohmmeter indication.
to an excessive value, and the fuse will open. Since the fuse opens almost as soon as a resistor shorts out, there is no time to perform a current or voltage check. Thus, troubleshooting a parallel d.c.
circuit for a shorted component should be accomplished with an ohmmeter. But, as in the case of checking for an open resistor in a parallel circuit, a shorted resistor can be detected with an ohmmeter only if one end of the shorted resistor is disconnected.
Troubleshooting a series-parallel resistive circuit involves locating malfunctions similar to those found in a series or a parallel circuit.
In the circuit shown in figure 8-155, an open has occurred in the series portion of the circuit. When an open occurs anywhere in the series portion of a series-parallel circuit, current flow in the entire circuit will stop. In this case, the circuit will not function, and the lamp, Li, will not be lit.
If an open occurs in the parallel portion of a series-parallel circuit, as shown in figure 8-156, part of the circuit will continue to function. In this case, the lamp will continue to burn, but its brightness will diminish, since the total resistance Open 30 v Rz 300 Break 3025 300 3 2 a Break FIGuRE 8-153. Opening a branch circuit to FIGURE 8-151. Finding an open branch in a obtain an accurate ohmmeter parallel circuit. reading.
30 v 300 300 300 FIGURE 8-154. A shorted component causes the fuse to open.
of the circuit has increased and the total current has decreased.
If a break occurs in the branch containing the lamp, as shown in figure 8-157, the circuit will continue to function with increased resistance and decreased current, but the lamp will not burn.
To explain how the voltmeter and ohmmeter can be used to troubleshoot series-parallel circuits, the circuit shown in figure 8-158 has been labeled at various points. By connecting a voltmeter between points A and D, the battery and switch can be checked for opens. By connecting the voltmeter between points A and B, the voltage drop across Ri can be checked. This voltage drop is a portion of the applied voltage. Also, if R, is open, the reading between B and D will be zero. The conductor between the positive terminal of the battery and point E, as well as the fuse, can be checked for continuity by connecting the voltmeter between points A and E. If the conductor or fuse is open, the voltmeter will read zero.
If the lamp is burning, it is obvious that no open exists in the branch containing the lamp, and the voltmeter could be used to detect an open in the branch containing R2 by removing lamp, L1, from the circuit.
Troubleshooting the series portion of a seriesparallel circuit presents no difficulties, but in the Break Break FIGURE 8-156. An open in the parallel portion of a series-parallel circuit.
parallel portion of the circuit misleading readings can be obtained.
An ohmmeter can be used to troubleshoot this same circuit. With the switch open, the series portion of the circuit can be checked by placing the ohmmeter leads between points A and B. If R1 or the conductor is open, the ohmmeter will read infinity; if not, the value of the resistor will be indicated on the ohmmeter. Between points D and E the fuse and conductor can be checked for continuity, but in the parallel portion of the circuit, care must be exercised, since misleading ohmmeter indications can be obtained. To check between points B and E, the branch must be disconnected at one of these points, and while one of these points and the switch are open, the branch containing the lamp can be checked with the ohmmeter.
A short in the series part of a series-parallel circuit will cause a decrease in total resistance, which will cause total current to increase. In the circuit shown in figure 8-159, the total resistance is 100 ohms and the total current is 2 amperes. If RI became shorted, total resistance would hecome 50 ohms, and the total current would double to 4 amperes. In the circuit shown, this would cause the 3-amp fuse to blow, but with a 5-amp fuse the circuit would continue to function. The result would be the same if Ra or R3 were to become shorted. The total resistance in either case would m Break FIGURE 8-155. An open in the series portion of a FIcURE 8-157. An open lamp in a series-parallel series-parallel circuit. circuit.
E F FIGURE 8-158. Using the voltmeter to troubleshoot a series-parallel circuit.
drop to 50 ohms. From this, it can be stated that when a short occurs in a series-parallel circuit, the total resistance will decrease and the total current will increase. A short will normally cause an open circuit by either blowing the fuse or burning out a circuit component. And, as in the case of an open, a short in a series-parallel circuit can be detected with either an ohmmeter or a voltmeter.
ALTERNATING CURRENT AND VOLTAGE Alternating current has largely replaced direct current in commercial power systems for a number of reasons. It can be transmitted over long distances more readily and more economically than direct current, since a.c. voltages can be increased or decreased by means of transformers.
Because more and more units are being operated electrically in airplanes, the power requirements are such that a number of advantages can be realized by using a.c. Space and weight can be saved, since a.c. devices, especially motors, are smaller and simpler than d.c. devices. In most a.c.
motors no brushes are required, and commutation trouble at high altitude is eliminated. Circuit breakers will operate satisfactorily under load at high altitudes in an a.c. system, whereas arcing is so excessive on d.c. systems that circuit breakers must be replaced frequently. Finally, most airplanes using a 24-volt d.c. system have special equipment which requires a certain amount of 400-cycle a.c. current.
A. C. and D. C. Compared Many of the principles, characteristics, and effects of alternating current are similar to those of direct current. Similarly, there are a number of differences which are explained later. Direct current flows constantly in only one direction with a constant polarity. It changes magnitude only when the circuit is opened or closed, as shown in the d.c. wave form in figure 8-160. Alternating current changes direction at regular intervals, increases in value at a definite rate from zero to a maximum positive strength, and decreases back to zero; then it flows in the opposite direction, similarly increasing to a maximum negative value, and again decreasing to zero. D.C. and a.c. wave forms are compared in figure 8-160.
Since alternating current constantly changes direction and intensity, two effects take place in
a.c. circuits that do not occur in d.c. circuits. They are inductive reactance and capacitive reactance.
Both are discussed later in this chapter.
Generator Principles After the discovery that an electric current flowing through a conductor creates a magnetic field around the conductor, there was considerable scientific speculation about whether a magnetic field could create a current flow in a conductor. In 1831, the English scientist, Michael Faraday, demonstrated this could be accomplished. This discovery is the basis for the operation of the generator, which signalled the beginning of the electrical age.
To show how an electric current can be created by a magnetic field, a demonstration similar to that illustrated in figure 8-161 can be used.
Several turns of a conductor are wrapped around a cylindrical form, and the ends of the conductor are connected together to form a complete circuit which includes a galvanometer. If a simple bar magnet is plunged into the cylinder, the galvanometer can be observed to deflect in one direction from its zero (center) position (4 of figure 8-161).
When the magnet is at rest inside the cylinder, the 500 Rz 200 v 10050 1, = 2 Amps FIGURE 8-159. Finding a short in a series-parallel circuit.
Wave form for DC Wave form for AC VOLTS VOLTS 180° 270° 360° 0° 90° Operation of circuit Closed ' Open Switch Switchi Time - Time FIGURE 8-160. D.C. and a.c. voltage curves.
galvanometer shows a reading of zero, indicating that no current is flowing (B of figure 8-161).
In C of figure 8-161, the galvanometer indicates a current flow in the opposite direction when the magnet is pulled from the cylinder.
The same results may be obtained by holding the magnet stationary and moving the cylinder over the magnet, indicating that a current flows when there is relative motion between the wire coil and the magnetic field. These results obey a law first stated by the German scientist, Heinrich Lenz. Lenz's law states that the induced current caused by the relative motion of a conductor and a magnetic field always flows in such a direction that its magnetic field opposes the motion.
When a conductor is moved through a magnetic field, as shown in figure 8-162, an electromotive force (e.m.f.) is induced in the conductor. The direction (polarity) of the induced e.m.f. is determined by the magnetic lines of force and the direction the conductor is moved through the magnetic field. The generator left-hand rule (not to be confused with the left-hand rules used with a coil) can be used to determine the direction of the induced e.m.f., as shown in figure 8-163. The first finger of the left hand is pointed in the direction of the magnetic lines of force (north to south), the thumb is pointed in the direction of movement of the conductor through the magnetic field, and the second finger points in the direction of the induced Motion of magnet Galvanometer
- Coil Motion of magnet Magnet at Rest B FIGURE 8-161. Inducing a current flow.
Motion of conductor FIGURE 8-162. Inducing an e.m.f. in a conductor.
e.m.f. When two of these three factors are known, the third may be determined by the use of this rule.
When a loop conductor is rotated in a magnetic field (see figure 8-164), a voltage is induced in each side of the loop. The two sides cut the magnetic field in opposite directions, and although the current flow is continuous, it moves in opposite directions with respect to the two sides of the loop.
If sides A and B and the loop are rotated half a turn and the sides of the conductor have exchanged positions, the induced e.m.f. in each wire reverses its direction, since the wire formerly cutting the lines of force in an upward direction is now moving downward.
The value of an induced e.m.f. depends on three factors:
(1) The number of wires moving through the magnetic field.
(2) The strength of the magnetic field.
(3) The speed of rotation.
Generators of Alternating Current Generators used to produce an alternating current are called a.c. generators or alternators.
Conductor Moved up Flux forward Inducted E.M.F.
Left hand Z Inducted E.M.F.
FIGURE 8-163. An application of the generator left-hand rule.
FIGURE 8-164. Voltage induced in a loop.
The simple generator shown in figure 8-165 constitutes one method of generating an alternating voltage. It consists of a rotating loop, marked A and B, placed between two magnetic poles, N and S. The ends of the loop are connected to two metal slip rings (collector rings), C1 and C2. Current is taken from the collector rings by brushes. If the loop is considered as separate wires A and B, and the left-hand rule for generators (not to be confused with the left-hand rule for coils) is applied, then it can be observed that as wire A moves up across the field, a voltage is induced which causes the current to flow inward. As wire B moves down across the field, a voltage is induced which causes the current to flow outward. When the wires are formed into a loop, the voltages induced in the two sides of the loop are combined. Therefore, for explanatory purposes, the action of either conductor, A or B, while rotating in the magnetic field is similar to the action of the loop.
Figure 8-166 illustrates the generation of alternating current with a simple loop conductor rotating in a magnetic field. As it is rotated in a counterclockwise direction, varying values of voltages are induced in it. At position 1, conductor A moves parallel to the lines of force. Since it cuts no lines of force, the induced voltage is zero. As the conductor advances from position 1 to position 2, the voltage induced gradually increases. At 2, the conductor moves perpendicular to the flux and cuts a maximum number of lines of force; therefore, a maximum voltage is induced. As the conductor moves beyond 2, it cuts a decreasing amount of flux at each instant, and the induced voltage decreases. At 3, the conductor has made one half of a revoltion and again moves parallel to the lines of force, and no voltage is induced in the conductor.
As the A conductor passes position 3, the direction of induced voltage reverses since the A conductor 340
Brushes In current Out current FIGURE 8-165. Simple generator.
now moves downward, cutting flux in the opposite direction. As the A conductor moves across the south pole, the induced voltage gradually increases in a negative direction, until at position 4 the conductor again moves perpendicular to the flux and generates a maximum negative voltage. From position 4 to 5, the induced voltage gradually decreases until the voltage is zero, and the conductor and wave are ready to start another cycle.
The curve shown at position 5 is called a sine wave. It represents the polarity and the magnitude of the instantaneous values of the voltages generated. The horizontal base line is divided into degrees, or time, and the vertical distance above or below the base line represents the value of voltage at each particular point in the rotation of Cycle and Frequency alternations in a complete cycle, the positive alternation and the negative. Each is half a cycle.
The number of times each cycle occurs in a period of time is called the frequency. The frequency of an electric current or voltage indicates the number of times a cycle recurs in 1 second.
In a generator, the voltage and current pass through a complete cycle of values each time a coil or conductor passes under a north and south pole of the magnet. The number of cycles for each revolution of the coil or conductor is equal to the number of pairs of poles. The frequency, then, is equal to the number of cycles in one revolution multiplied by the number of revolutions per second. Expressed in equation form, F - Number of poles x t.p.m.
where 2 is the number of pairs of poles, and ".p.m. P Whenever a voltage or current passes through a series of changes, returns to the starting point, and then again starts the same series of changes, the series is called a cycle. The cycle is represented by the symbol. In the cycle of voltage shown in figure 8-167, the voltage increases from zero to a maximum positive value, decreases to zero; then increases to a maximum negative value, and again decreases to zero. At this point it is ready to go through the same series of changes. There are two the number of revolutions per second. If in a 2-pole generator, the conductor is turning at 3,600 r.p.m., the revolutions per second are
r.p.s. = 3600 = 60 revolutions per second.
Since there are 2 poles, = is 1, and the frequency is 60 c.p.s. In a 4-pole generator with an armature 341
Magnetic field 0° 90° 180°, 270°1 360° 0° 90°, 180° 270 360° Zero voltage Position 1 Rotating conductors moving parallel to magnetic field, cutting minimum lines of force.
Maximum positive voltage Position 2 Quarter turn completed Conductors cutting directly across the magnetic field as conductor A passes across the North magnetic pole and B passes across the S pole.
270° 360° 0° 90 180* 270 360° 10° 90 180 Voltage drops to zero Maximum negative voltage Position 3 Position 4 One-half turn completed Three quarter tum completed Conductors again moving parallel to magnetic Conductors again moving directly across magnetic field, cutting minimum lines of force. field 'At passes across South magnetic pole and "B" across N magnetic pole.
360° 0° 90 180° Conductor A has made one complete cycle and is in same position as in position A. The generator has generated one complete cycle of altemating voltage Zero voltage or current.
Position 5 Full turn completed FIGURE 8-166. Generation of a sine wave.