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.
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.
