Sunday, January 11, 2015

AC Instruments and Meters : Synchroscopes, Frequency meters and Recording instruments .

SYNCHROSCOPES

The synchroscope is an instrument that shows the relative phase angle and the frequency difference between two alternating voltages. This instrument indicates when alternators are in phase. It also indicates whether the frequency of the incoming generator is higher or lower than that of the generator already connected to the line.

There are a number of versions of synchroscopes available. Two commonly used synchroscopes are the polarized-vane type and the moving iron type. These synchroscopes are

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designed for operation on single-phase circuits. Both devices show when the voltages of two single-phase ac generators are synchronized. They also can be used with three-phase genera- tors if the phase sequences of the generators are known.

Polarized-Vane Synchroscope

The polarized-vane synchroscope (Figure 11–24) uses a mechanism that is similar in physical structure to that of the polarized-vane power factor meter. The basic difference in this mechanism is that the polarizing coil is wound as a potential coil rather than as a current coil. The stator winding has a phase-splitting network and is connected across one phase of the incoming generator. The polarizing winding for the vanes is connected across the corresponding phase of the generator already on the line.

The stator winding of a polarized-vane synchroscope is arranged so that a two- phase field effect is obtained by a phase-splitting network, as shown in Figure 11–24.

In the stator network system, capacitor C causes current I to lead the voltage (V) by a large phase angle of 75° to 80°. Current I lags V because of the inductance of coil A. The amount of lag is about 10° to 15°. As a result, the angle between currents I is 90°.

Operation of the Polarized-Vane Synchroscope

Figure 11–25 shows the relationships between the field and vane fluxes for one cycle with the voltages of the two generators in synchronism. If the rotating field is 61 Hz and the field of the vane polarization is 60 Hz, then the rotating field is 1⁄60 th revolution ahead each time the vanes reach maximum magnetism in one polarity. The vanes line up with the position of the field at the instant of maximum vane magnetism. Thus, each complete cycle of vane polarization brings the vanes 1⁄60 th of a turn further around. For this case, the vanes make one complete revolution in one second. If the stator field rotates at 62 Hz, the vanes will rotate twice as fast. That is, they will make two revolutions in one second.

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Assume that a frequency of 59 Hz is applied to the stator windings and 60 Hz is applied in the polarizing coil. The vanes will then make one revolution per second in the opposite direction. In other words, the speed of the pointer is the difference in frequency of the incoming generator and the machine already connected to the line wires. The direction of rotation of the pointer shows whether the speed of the incoming generator is too fast or too slow. When the two generators are operating at the same frequency, the vanes do not rotate. The position of the pointer for this case indicates the phase relationship between the two voltages.

FREQUENCY METERS

Electric motors, transformers, and other types of machines require the correct volt- age for efficient operation. These machines are also designed to operate at a definite frequency. In the case of electric clocks, it is very important that the frequency be accurately indicated. A variation of a small fraction of a cycle, continued through a long period of time, can result in serious errors in time indication on an electric clock. Therefore, electric power systems must operate at the correct frequency. It is obvious that frequency- indicating instruments must be used to show the frequency of a system.

Standard practice requires that ac systems operate at a single frequency. Thus, frequency indications are required to cover only a narrow band of frequency values on either side of the normal frequency. This means that the accuracy of the instrument is improved over what it would be if the instrument covered a wide frequency range.

Resonant Circuit Frequency Meter

One type of commonly used frequency meter is the resonant circuit meter. The physical structure of such a frequency meter resembles that of the dynamometer.

A schematic diagram of a resonant circuit frequency meter is shown in Figure 11–26. The use of two series resonant circuits provides a deflecting torque that has a definite relationship to the applied voltage regardless of its magnitude. The two field coils are alike and are connected so that their fluxes oppose each other. Each field coil is

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connected in series with an inductor–capacitor combination. The constants of this combination permit series resonance below the normal operating frequency in one field coil and series resonance above the normal frequency in the other field coil. If a frequency meter is designed to operate on a normal frequency at 60 Hz, the field circuits are designed so that they are in resonance at 45 Hz and 75 Hz. The armature coil is connected through lead-in spirals. It has almost no countertorque effects and carries the total current of both field circuits.

Changing Circuit Impedance. The change in the circuit impedance of each field coil circuit with a change in frequency is shown in Figure 11–27. Figure 11–27B shows the currents in each of the field circuits and the total armature current. The magnitudes of these currents vary with frequency. Within the operating range of the frequency, the curves show that the impedance of the circuit resonating at 45 Hz is inductive. For the circuit resonating at 75 Hz, the impedance is capacitive. This means that the current in field coil F in circuit A always lags the terminal voltage. Also, the current in field coil F in circuit B always leads the voltage. The actual value of lag or lead and the magnitude of the current in each circuit all depend on the frequency. The current in the armature is the vector sum of the two field currents. Both the armature current and the armature flux lead or lag the terminal voltage, depending on which field current is greater. When the frequency is such that the leading and lagging currents are equal, the armature current is in phase with the terminal voltage.

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Field Flux

The fluxes of the field coils oppose each other. Thus, the resultant field flux is the vec- tor difference between the two fluxes. At 55 Hz, the resultant flux leads the armature flux by an angle that is slightly larger than 90°, as shown in Figure 11–28A.

The torque due to the resultant flux acting on the iron vane is proportional to the product of the armature flux and the in-phase component of the resultant field flux. The direction of the torque in Figure 11–28A causes the pointer to move downscale.

This deflection of the movement causes the iron vane to move out of alignment with the field flux. A countertorque is developed as the vane flux and the field flux align them- selves to obtain the shortest possible flux path. When the countertorque equals the armature coil torque, the pointer comes to rest.

Increasing Frequency. With a higher frequency, the leading current (I ) increases and its phase angle with the line voltage decreases. The lagging current (I ) decreases and its phase angle with the line voltage increases. As a result, the phase angle between the

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terminal voltage and the resultant field flux also decreases, as does the phase angle between the armature current and the applied voltage.

Figure 11–28B represents a condition in which the two field currents are equal. These currents have equal and opposite phase angles with the applied voltage. The armature cur- rent is in phase with the applied voltage. The resultant of the two field fluxes is 90° out of phase with the voltage. Therefore, the in-phase component of the field flux is zero and there is no deflecting torque. The field flux aligns the iron vane so that the pointer is held at midscale.

If the frequency increases even more, the leading current increases and its phase angle with the applied voltage decreases. The lagging current in the other circuit decreases and has a larger phase angle with the applied voltage. The armature current now lags the applied voltage, as shown in Figure 11–28C. The in-phase component of the resultant field flux is now in the opposite direction. The resulting torque causes the pointer to move upscale.

Resonant-type frequency meters are usually designed to operate on single-phase 115- or 120-V circuits. If the meter scale has a range of 55–60–65 Hz, the meter will indicate frequency values to an accuracy of 0.15 Hz.

RECORDING INSTRUMENTS

Many applications require that the conditions existing in an electrical circuit be monitored constantly. However, it is uneconomical to assign a person to record instrument readings repeatedly. To overcome this problem, recording instruments are used. Such instruments provide a graphical record of the actual circuit conditions at any time. Recording instruments may be grouped into two broad categories:

1. Instruments to record electrical values, such as volts, amperes, watts, power factor, and frequency.

2. Instruments to record nonelectrical quantities. For example, a temperature recorder uses a potentiometer system to record the output of a thermocouple.

Recording instruments are similar to indicating instruments in many ways. They use a permanent-magnet, moving-coil-type construction for dc circuits. For ac circuits, recording instruments may use either the moving iron or the dynamometer-type con- struction. Whereas the pointer of an indicating instrument just shows the measured quantity on a fixed scale, a recording instrument provides a permanent graphical record. The measured quantities are drawn on a scaled paper chart as it moves past a pen at a constant speed. Because of the friction between the pen and the chart, the indicating movement must have a higher torque than is required in an indicating instrument. As a result, recording instruments are larger and require more power to operate than is required by indicating instruments of the same scale range. Recording instruments are also more highly damped than are indicating instruments so that the pen does not overshoot the chart.

Strip-Chart Recorder

The strip-chart recorder is the most commonly used graphical recording instrument. The permanent record of measurements is made on a strip of paper 4 to 6 in. wide and up to 60 ft long. Figure 11–29 shows an ac recording voltmeter. This strip-chart-type voltmeter has several advantages over nonrecording voltmeters. The long charts allow the record to cover a considerable amount of time. This means that a minimum of operator attention is required. The chart can be operated at a relatively high speed to provide a detailed graphical record.

The principal parts of a strip-chart recording instrument are

1. the frame supporting the various parts of the instrument.

2. the system that moves according to variations in the quantity being measured.

3. the chart carriage, consisting of the chart, the clock mechanism, the timing gears and drum, the chart spool, and the reroll mechanism.

4. the fixed scale on which the value of the quantity being recording is indicated.

5. the recording system, consisting of a special pen-and-ink reservoir or an inkless marking system.

The same basic types of moving systems are used in both graphical recording instruments and indicating instruments. A permanent-magnet moving coil mechanism is shown in Figure 11–30. This mechanism is used with a dc recording instrument to overcome the friction between the pen and the chart. The pen in this movement is carefully counterbalanced. A repulsion-vane moving system for an ac recording ammeter is shown in Figure 11–31A. Figure 11–31B illustrates a two-element electrodynamometer mechanism for a three-phase recording wattmeter.

Chart and Drum. The graphical record is drawn on a chart that is graduated (scaled) in two directions. One of these scales corresponds to the scale range of the instrument. The second scale represents hours, minutes, or seconds, depending on the clock mechanism and the timing gears of the chart carriage. The timescale is uniformly spaced for a constant chart speed, which is provided by the drive mechanism. The drive for the timing drum may be a synchronous electric clock, a conventional hand-wound spring clock, or a spring-type clock mechanism wound by a small electric motor. The last drive listed provides the convenience of an electric drive. Also, it guarantees that a power failure will not stop the chart motion until the spring runs down.

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The friction between the chart and the pen requires that the pen drive mechanism have a greater torque than in a simple indicating instrument. The larger instrument movements are costly and take a great deal of power from the circuit being measured. The amount of power required can be reduced by using electronic amplifiers. Such an amplifier can drive the recorder element using less power from the circuit to be measured. Voltage and current signals can be amplified directly; the measurement of power and VARs requires special amplifiers.

Amplifiers. One type of amplifier used for power and VARs measurements is a photo- electric device. This device uses conventional wattmeter elements mounted on an auxiliary shaft with a reflecting mirror. An optical system with photocells and electronic amplifiers develops a dc current proportional to the quantity to be measured. This type of amplifier has sufficient power to drive the recorder pen.

Pen Positioning. Many recorders use a motor-and-gear system to position the pen. A regulating system compares the input voltage to the voltage of a precision slide-wire potentiometer (which is driven from the recorder output shaft). If the input voltage changes, the servoamplifier causes the motor to run until the pen and its attached potentiometer are positioned so that the slider voltage is equal to the signal voltage.

A recorder using this system requires very little input power from the circuit being measured. The output drive is so powerful that accessories can be added, such as auxiliary slide-wire potentiometers, limit switches, and devices to code the measured quantities into digital computer language. These accessories can operate alarms and remote devices and can supply information to digital computers.

Other accessories permit recorders to graph many different quantities on one chart. A stepping switch connects the measuring circuit to a different input at regular intervals. As soon as the pointer is positioned, the printer places a dot with a number beside it on the chart to indicate the input being measured. The stepping switch then moves to the next input, and the process is repeated. As a result, the chart contains a series of dots for each of the input signals.

Most recorders operate from a dc signal having a magnitude in millivolts. Such recorders require transducers to convert quantities such as pressure, flow, strain, ac volts, ac amperes, watts, and VARs to dc millivolt signals that can be recorded.