Theory of Current Clamps
INTRODUCTION
The AYA line of current measuring clamp-on transformers is the most extensive, with both AC and DC capabilities. Most models are available with either voltage or current output signals having full scales ranging from 1 Ampere to 7,500 Amperes. These current clamps are designed for use with analyzers, recorders, loggers and multimeters in applications requiring measurement of currents without interrupting the electric circuits. Such requirements are common in monitoring power quality and loads by utilities, electric equipment manufacturers and heavy equipment operators that must furnish their service personnel with a quick method of measuring currents in the power lines of rotating machinery, compressors, chillers, coolers and other electrical equipment. For high current applications, the open-frame current transducers measure up to 30,000 Amperes peak-to-peak and 15,000 Amperes DC.
CURRENT CLAMP THEORY OF OPERATION
Magnetic-core AC transformers are closely coupled electrical devices used for transmitting AC voltages and currents. They generally have two coils wound on a magnetic core as show on the right.
The VOLTAGE ratio of a transformer, ignoring losses, is defined by the equation: E2 = (N2/N1) x E1 (1)
Where E1 = Input Voltage E2 = Output Voltage
N1 = Number of turns of primary coil N2 = Number of turns of secondary coil
The CURRENT ratio of a transformer, ignoring losses, is defined by the equation: I2 = (N1/N2) x I1 (2)
Where I1 = Input Current I2 = Output Current
CURRENT TRANSFORMERS
An AC current transformer is a special transformer that does not have a primary coil. The conductor whose current is to be measured acts as the primary coil when it is placed inside the magnetic path of the core. A Current clamp is a special type of current transformer. It enables the user to "open" the magnetic path in order to snap the clamp around a conductor for performing accurate current measurements without having to interrupt the current flow in the conductor.
The signal obtained from an AC current transformer has the same waveshape as the current being measured. However, current transformers introduce measurement errors of amplitude, phase and waveshape due to frequency range limitations. The AYA clamps are designed to provide the lowest possible errors by using the highest quality magnetic core and by using multiple compensating coils for error cancellation.
A clamp-on current probe, shown in the following diagram, has a split core. It converts the primary current of the conductor to a current output whose value depends on N2. Using equation (2), the output current can be computed if N2 is known. If N2 is 1000 turns, the output current is 1/1000 of the primary current, which can be expressed as 1 Milliampere per Ampere. Such a clamp is referred to as having a ratio of 1000:1. The output of this current clamp can be read by any AC ammeter whose input impedance is compatible with the specifications of the current clamp. Current clamps that provide output currents are also used with power analyzers that are designed for receiving the current input signals in the form of a voltage.
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| Current Clamp with Current Output | Current Clamp with Voltage Output |
The output current of a current clamp can be converted to a voltage signal by a current-to-voltage converter. AYA offers current clamps with voltage output whose converters are incorporated in the handle of the clamp.
CURRENT CLAMPS WITH DC OUTPUT
Some current clamps incorporate a rectifier circuit whose output is a DC voltage that is proportional to the average current being measured. Such clamps facilitate the use of strip chart recorders for obtaining real time trends of current loads. To obtain a TRUE RMS output, AYA provides a DC-to-RMS converter as shown below
CURRENT CLAMPS for MEASURING AC and DC CURRENT
AC current transformers can only measure the changing magnetic field of an AC current. DC currents generate a non-changing magnetic field that can only be measured magnetically by a HALL-EFFECT sensor. The sensor generates an accurate signal that is proportional to either a DC or an AC current. The AYA AC/DC current clamps incorporate two HALL-EFFECT sensors located in the magnetic path. A battery-operated circuit is required to provide the excitation and amplification of the signal generated by the HALL-EFFECT sensor. The two matched sensors provide an output signal which is independent of the location of the current conductor in the clamp opening. The conductor does not have to be exactly at the center of the opening.
AYA offers the most extensive line of AC/DC HALL-EFFECT clamps for measuring currents ranging from 1 Amperes DC to 7,500 Amperes DC. They incorporate multiple HALL-EFFECT sensors with special circuitry that provides high linearity, low phase shift and very wide frequency response
APPLICATIONS
Clamp-on current probes enable the user to measure currents without disconnecting the conductor from the electric network. They are designed with jaws that can be snapped open, placed around the conductor and snapped closed to form a magnetic loop around the conductor. If a clamp is snapped around both wires of a single-phase electric network, the current flowing in one conductor would cancel the current flowing in the other conductor, and the resultant output of the clamp would be "zero". This fact can be used advantageously in detecting accidental leakage current to ground.
