Transducers are finding an increasing application for ac circuit measurement used in configuration with conventional moving-coil or digital instruments or recorders. Basically these transducers are complete, special circuits designed for the following measurements:
current (sensing mean or rms current), voltage (sensing mean or rms voltage), power (watts and VAR), phase angle, frequency, sup pressed zero voltage, and linear inverse voltage.
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Such transducers can be provided with a current output or a voltage output. A true current output is normally preferred because this compensates automatically for variations in total loop resistance, such as temperature changes affecting the resistance of pilot wires, or changes in receiving equipment. A voltage-output transducer is normally only used when the indicating instrument or receiving device requires a true voltage input and , in consequence, current is consumed.
Transducers can be connected into the circuit to be measured in a similar manner to that of any other measuring instrument, either directly or coupled in by a transformer, depending on the magnitude of the quantity being measured. They then provide a dc analog output signal that is proportional to the input parameter being measured (see ill. 21-1). Alternatively, for working a digital instrument, the output signal can be rendered in digital form.
ill. 21-1. Ideally, an electrical transducer produces an output
that is directly (linearly) proportional to the input. This is graphically
shown by straight-line output-vs.-input functions.
ANALOG TRANSDUCERS
The design and construction of (ac) electrical measuring transducers is specific to individual manufacturers. The following descriptions are based on the GEC “1 state” range, representing typical state-of-the art development in static circuit design.
Current Transducer
A block diagram of a measure-sensing current transducer is shown in ill. 21-2. it's self-powered (requires no auxiliary power supply), with input isolated by a small internal transformer. Out put from the transformer is rectified and smoothed, with voltage limitation and protection against transients provided by zener diodes.
ill. 21-2. A transducer for current measurement.
Voltage Transducer
Here the circuitry is basically the same; however, because a voltage transformer has a low-impedance output, it's necessary to provide amplification to produce a high-impedance current output (see block diagram of ill. 21-3).
ill. 21-3. A transducer for voltage measurement. An amplifier is
incorporated, providing a low-impedance output.
Rms Current or Voltage Transducers (ill. 21-4)
ill. 21-4. A transducer for measuring root-mean-square (rms) values.
Within stated limits of crest factor these transducers provide an accurate dc analog current from the applied input voltage or cur rent. In both current and voltage versions, the input quantity is converted into an ac voltage, which is full-wave rectified, and applied to a square-law circuit. Although this circuit is nonlinear in opera tion, it provides a dc voltage output that is a linear function of the rms value of the applied input. The output from the square-law circuit is converted to a current signal in the amplifier, which in this case is powered from a separate supply.
Rms voltage transducers also require a separate power supply. Alternatively, power can be taken from the measured voltage source when the rated measurement is within ± 20 percent.
Power Transducer
A block diagram of a power transducer is shown in ill. 21-5. The circuit comprises an oscillator, a modulator, an amplifier, and an integrator. The current and voltage components of the input power are used to produce a train of rectangular pulses; each has height proportional to the instantaneous voltage and width proportional to the instantaneous current. The integral of this signal is proportional to the level of the power being measured. Protection against transient and overload conditions is provided. Links can be included for coarse adjustment and potentiometers for fine adjustment of the conversion ratio and calibration to cover a range 70 percent to 200 percent of the nominal input.
ill. 21-5. A transducer for measuring power.
Power transducers require an auxiliary power supply, but because the burden is relatively low, this supply can be taken from the measuring voltage transformer if necessary. A single circuit can be used for measuring single phase or for balanced-load three- phase power applications. Two circuits mounted in one housing can be used for unbalanced-load three-phase power applications.
Phase-Angle Transducers
ill. 21-6. A phase-angle transducer.
A block diagram of a phase-angle transducer is shown in ill. 21-6. The circuit has internal transformers that feed the current and voltage inputs into a bistable element. Consequently, the out put changes state when the inputs pass through zero. The signal from the bistable element is integrated, and the resultant dc volt age is fed to the output amplifier. Transducers for monitoring phase angles in the range ± 60 deg. and 0 deg. ± 180 deg. are available.
Phase-angle transducers require an auxiliary power supply.
Frequency Transducer
ill. 21-7. A transducer for measuring frequency.
A block diagram of a frequency transducer is shown in ill. 21-7. The circuit is based on a monostable circuit triggered by zero crossings of the input supply voltage, followed by an integrated circuit and a current-feedback amplifier. This transducer is self- powered in the sense that it measures the frequency of the input to the power supply.
Suppressed-Zero Voltage Transducer
A block diagram of a suppressed-zero voltage transducer is shown in ill. 21-8. Used with a negatively biased amplifier, the transducers give a range of suppression rates and a powerful high- impedance output. The accuracy is expressed as a percentage of the output span.
After rectification and smoothing, the measured voltage input is held in a negative state by the bias stage until it reaches a value equal to the bias level. Further increases of measured input result in a positive input to the amplifier. This provides a true current output that is proportional to the measured input voltage.
ill. 21-8. A suppressed-zero voltage transducer.
Linear Inverse-Voltage Transducer
A linear inverse-voltage transducer provides maximum out put at zero input and zero output at full scale input. it's used where the frequency and voltage of a generator output must be synchronized with the supply levels already in operation before final connection is made. The transducer provides 100 percent output only when the running and incoming voltages are equal both in magnitude and in phase. The inverse output is a safety feature that prevents wrong synchronization if a fault develops in the measuring equipment.
A power supply stage fed from the running input energizes the bias stage, and the output is achieved by the bias stage when the running input is zero. A difference in level between the running and incoming voltages is rectified and presented as a voltage of opposite polarity to the bias-stage voltage. Consequently, as the input voltage increases, the output current decreases.
TRANSDUCERS FOR DIGITAL INSTRUMENTS
A large number of digital instruments are limited to the measurement of dc current and voltage. it's necessary to use special transducers with these instruments to ensure that a smooth dc out put is connected to the instrument to mean ac current, voltage, power, phase angle, or frequency. These are sometimes known as transducer/converters.
Current and Voltage Transducer/Converters
The circuit is similar for both current and voltage transducers, the difference being in the transformer and tapping arrangements. Input is an ac voltage that is rectified, smoothed, and integrated to give a smooth dc output (ill. 21-9).
ill. 21-9. A transducer/converter for measurement of current or
voltage.
Rms Current and Voltage Transducers / Converters
Again current and voltage transducer circuits are identical, except for the transformer and tappings. The output from the transformer is full-wave rectified and passed to a squaring circuit. This produces a signal that is proportional to the rms value of the input. The signal is then smoothed, tapped down, and linearized before being fed to the output terminals (ill. 21-10).
ill. 21-10. A transducer/converter for determination of rms current
or voltage.
Power Transducer/Converter
The circuit is calibrated for two voltage tappings. It requires an auxiliary supply and has a zero adjuster and a phase-angle adjuster. The current and voltage inputs are used to produce a train of rectangular pulses; each has height proportional to the instantaneous voltage and width proportional to the instantaneous cur rent. The area under the pulses is then ∫ vi dt, which is a true measurement of watts.
Phase-Angle Transducer/Converter
The transducer consists of two switching circuits, each fed from a resistor connected across the secondary of the input transformer. These circuits are adjusted to switch at near-zero input voltage. Thus, the output changes state each time the input quantity passes through zero. The resulting pulses are differentiated and switch a pair of gates connected as a bi-stable element.
Both outputs from the bistable element are integrated, and the resultant dc voltage is fed to the output terminals in such a manner that it's a function of the difference between the integrated voltages.
Frequency Transducer/Converter
The circuit consists of a transistor pump circuit that delivers a train of pulses to the output. Integration of the signal produces a dc voltage proportional to the frequency of the supply.
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