6. Electric current sensors, power measurements
Task
1. Study the properties and principles of operation of the current measurement sensors
(instruments) used in the task. Use them to measure a current flowing through a single-phase
load. There are following principles present:
(A1) Current measurement with a hand-held DMM with an internal current shunt
(A2) Current measurement with an external current shunt (Agilent 34330A) connected to
bench-top DMM Agilent (34401A)
(A3) Current measurement with a current transformer
(A4) Current measurement with Current clamps – hand-held ammeter and power-meter.
(A5) Current measurement with a current sensor based on magnetic field compensation
principle
(A6) Current measurement with the use of current clamps (Hall-probe based) and
oscilloscope
2. Determine in all cases (and justify), which value is measured in case of harmonic and nonharmonic currents, and in a case that there is a DC current superposed to the alternating
current.
3. Determine the internal resistance of the current measurement sensor (instrument) in all
cases.
Optional part:
1. Calculate the uncertainty of the measurements and check whether the tolerance regions are
overlapping.
2. Explain the term „Insertion impedance“ used for sensors working on magnetic principle.
3. Measure all available values with the electronic power meter “Hioki 3333 Power Hitester”
4. Measure all available parameters of the power network with the electronic energy-meter.
Compare the results with the values get during the previous measurements. While
comparing the values measured by the electronic power-meter and energy-meter, take into
account the possibility of influence by the self-power-consumption of the other sensors
placed in the circuit.
Circuit diagram
Fig.1 Circuit diagram for single-phase current measurement in three-phase power network +
demonstration of a three-phase energy meter
1/4
Basic theory concerning the ammeters used in the task
-> 1a) Hand-held DMM with built-in current shunt (also valid for low-cost bench-top DMMs)
The instruments usually have the mA ranges protected by fuse. Ayrton shunt is used to switch
between the ranges. The high current range (10-20A) is sometimes not protected by fuse (be
careful!!!). There is usually specific terminal for this high range, so the current does not flow
through the range selector switch. The frequency bandwidth of these instruments is typically very
limited (hundreds of Hz maximum, typically not even mentioned in the datasheet).
-> 1b) External current shunt and DMM
External current shunts are used typically up to hundreds of amperes. Common units have
the full-scale output voltage drop 50 – 100mV. There can be considerable power loss across
the shunt, e.g. 100A/100mV means 10W. The mechanical construction as well as the
temperature stability of the shunt must address this issue.
There is a large difference in construction of shunts designed for measurement of AC and
DC currents. There are no problems with leaking magnetic field and self inductance of the
shunt for DC current measurements. The shunts are connected exclusively using 4-terminal
technique. The shunt used in this task (34330A 1mV/1) is designed to be put directly into
the input terminals of the Agilent’s bench-top DMM. The manufacturer guarantees
maximum error of 0.3% in a frequency range DC – 1kHz and 5% within the range 1kHz –
5kHz. Coaxial current shunts are typically used for higher frequencies.
Special care must be taken if the current shunt is supposed to be used for power
measurement. The phase error introduced by the shunt can cause serious measurement
errors.
The current shunts have two main disadvantages: galvanic connection of the power and
measurement circuit and relatively high power loss
-> 1c) Current transformer (CT)
Current transformers are used mainly for measurement of high currents (up to thousands of
amperes) or in the cases where galvanic isolation of the measurement circuit is desired. The
current is transformed in the inverted ratio of number of turns. The current transformer is
designed to work “short-circuited” on the secondary side. The secondary current
compensates the magnetic voltage generated by primary winding, the magnetic flux in the
core is thus small, the core is not saturated and the losses are minimal (negligible). The
phase and amplitude error is minimal in this state. In real measurement there must be some
impedance on the secondary side in order that the secondary current could be measured (e.g.
a current shunt).
Beware: do not leave the secondary side of the CT unconnected. The core will over-saturate
and high voltage peaks can appear on the secondary transformer winding.
Note: It is not possible to use CT for measurement currents with a non-zero DC part. The
DC part can over-saturate the core, and this will cause high measurement errors also in the
measured AC part.
The current transformers are now extensively used in the household energy meters. In this
application there are no more used CTs with a high-permeability core. It is because of
possibility of cheating - load connected through diode will generate “DC” current which can
confuse the energy meter. So called “DC tolerant” current transformers are used. They have
2/4
core with a lower permeability and so it is harder to over-saturate them, higher phase error is
compensated in the circuit or by software.
Basic parameters of a CT which is used in the task:
“DC tolerant” CT type “T60404-E4624-X131” made by Vacuumschmelze. Current transfer
ratio is 1:2500, maximal measurement current 60 A, secondary side load resistance 10 Ω
(12.5 Ω is prescribed by manufacturer, lower resistance should decrease the errors). DC
tolerance (error less than 3%) is ensured up to 60A of a half-wave rectified current.
Sensitivity: 10mA (The energy meter can measure also the energy consumed by appliances
in “stand-by” mode).
-> 1d) Current clamps – hand-held ammeter, powermeter PK 430.1
Note: the power factor is measured only for currents > 40A (so it is not possible to use it in this
application– unless you increase the number of primary turns)
Attachment: technical parameters of the PK 430.1, made by Metra Blansko
Current measurement is based on Hall probes.
-> 1e) Current sensor Honeywell CSNE 151
The CSN Series of closed loop current sensors are based on the principles of the
Magnetoresistive or Hall effects, and the null balance or zero magnetic flux method
(feedback system). The magnetic flux in the sensor core is constantly controlled to zero. The
amount of current required to balance zero flux is the measure of the primary current
flowing through the conductor, multiplied by the ratio of the primary to secondary windings.
This closed loop current is the output from the device and presents an image of the primary
current reduced by the number of secondary turns at any time. This current can be changed
to voltage by passing it through a resistor.
Range: 5...25 A (5A – 5 turns on primary, 25A – one turn on primary => 25Ampere/turns
maximum), compensating current = I*n/1000 (n=primary turns). Secondary current sensing
resistor (shunt) R = 100 Ω.
-> 1f) Measurement with current clamps and oscilloscope
AC/DC current clamps Agilent 1146A, sensitivity 10mV/A or 100mV/A, Bandwidth: DC –
100 kHz.
Please TURN it OFF after the measurement has been finished.
3/4
Demonstration of a functionality of a three-phase energy meter EMU 30,
made by ZPA.cz Trutnov
With respect to Fig.1 there is an electronic energy meter in the measurement circuit. It allows to
measure wide range of electrical quantities. In common use, it usually shows only the total real
energy, which was used in up to four different tariffs. The units were donated free of charge by ZPA
cz Trutnov (www.zpa.cz). The unit has been modified for measurements in custom power network:
3x120/70V, 5A.
Note: energy meters with a current range of 5A are usually called “indirect” and are used together
with an external current sensor (current transformer). The secondary current of such a transformer is
then typically 5A and primary (input) current can range up to thousands of amperes
There is custom program in this unit which shows total real energy in all phases after start-up. By
pressing a button on the front panel it is possible to change the reading to following values:
total reactive energy (kVArh),
total apparent energy (kVAh),
actual real power in all phases (1,2,3) (kW),
actual real power in phase 1 (kW),
actual real power in phase 2 (kW),
actual real power in phase 3 (kW),
actual reactive power in all phases (1,2,3) (kVAr),
actual reactive power in phase 1 (kVAr),
actual reactive power in phase 2 (kVAr),
actual reactive power in phase 3 (kVAr),
actual apparent power in all phases (1,2,3) (kVA),
actual apparent power in phase 1 (kVA),
actual apparent power in phase 2 (kVA),
actual apparent power in phase 3 (kVA),
phase voltage, 1.phase (V),
phase voltage, 2.phase (V),
phase voltage, 3.phase (V),
phase to phase voltage 1-2 (V),
phase to phase voltage 2-3 (V),
phase to phase voltage 1-3 (V),
current in phase 1 (A),
current in phase 2 (A),
current in phase 3 (A),
cos ϕ in phase 1,
cos ϕ in phase 2,
cos ϕ in phase 3.
Insertion impedance is the impedance that is transformed from the current probe’s coil (the
secondary) into the current carrying conductor (the primary) that’s being measured. Typically, a
current probe’s reflected impedance values are in the range of milliOhms and present an
insignificant effect on circuits of 25Ω or more impedance. (ABCs of Probes, Tektronix)
4/4