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Power measurement today is not only used for energy measurement
on the 50/60Hz power line. It more and more becomes a means
to improve the product to assure its quality, to reduce its
energy consumption, and to achieve marketing advantage. Precise
power measurement is not as simple as one might think. The
user is well advised to understand the fundamentals of analog
signal conditioning. Simple measurements of power on the 50/60Hz
power line may be up to 20 % in error if the Power Analyzer
is unable to pass DC components of current and voltage. For
example a one way rectified current and a sinusoidal voltage
requires a DC coupled current path in spite the fact that
power is transmitted at the line frequency.
Many electric sources are electronically controlled by predefined
criterions. These control criterions produce waveforms being
very different from the 50/60Hz power line waveforms. Using
pulsed voltage signals and little filtering almost any current
waveform can be generated. A good Power Analyzer copes with
these signals and is able to perform power measurement at
low current levels 1mA to 10mA.
Instantaneous power results by multiplying a voltage sample
and a current sample of the same instant in time. It can be
positive, indicating energy flow to the load, or negative,
indicating energy flow back to the source. If current and
voltage are in phase all values are positive. If current and
voltage are 90° out of phase, 50 % of the instantaneous power
values are positive and 50 % are negative.
Average power, or simply power, results by summing all instantaneous
power values over at least one signal period. Positive power
indicates energy flow to the load, negative indicates recuperation.
Be careful not to confuse matters. Negative power display
also results by reverse connecting current or voltage on the
power meter inputs, or when using ARON-connection in a three
phase system.
Power P of a sinusoidal voltage U and current I of the same
frequency and phase shift ? is determined by P = UI cosø.
How is power affected when several frequencies are present?
Only signals of the same frequency in voltage and current
contribute to power. Signals with less than 90° phase difference
add to power, signals with more than 90° phase difference
contribute a negative amount to average power. Different frequencies
in voltage and current do produce instantaneous power but
do not add to average power
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Fig. 1 shows the high precision Infratek 106A Power Analyzer
for single and three phase circuits. It is suitable for measurements
on frequency inverter drives and other electronically controlled
sources. It yields correct power for any waveform including
direct current. The three-phase instrument measures 1500 values
every measurement cycle. It’s excellent performance at very
low power factor down to 0.005 makes it an ideal tool for
transformer testing.
Three galvanically isolated 0.3V–1000V, DC-1MHz voltage inputs
and three galvanically isolated 1.5mA to 30A current inputs
give the user maximum freedom to connect his circuits. Every
phase contains 4 current inputs: 1.5mA–1A, 15mA–5A, 1A–30A
and 60mV–6V for external shunts, preferably current viewing
resistors, and other current sensors. The low current input
1.5mA-1A is suitable for standby measurements, which is becoming
more and more an issue. The low current input can also accept
currents from high current sensors. For example, using a 100A
current sensor with 0-50mA output connected to the Power Analyzer
1.5mA-1A input, results in current ranges 3A, 10A, 30A, and
100A. Enter a scaling factor 200 to obtain actual current-
and power readings. There are high precision current sensors
available to obtain excellent 0.1 % accuracy.
It is important that complex instruments such as Power Analyzers
are simple to operate. All Infratek instruments use 11 keys
and are simple to operate. You will intuitively do the right
thing in spite the fact that you can display any one of the
1500 values. The 106A Power Analyzer is available with IEEE-488,
RS232-, USB-, and Ethernet interface. Use the Infratek Operating
Software to control the instrument, read data, store data,
and make timer controlled data transfer over one of the interfaces.
A LabView driver as well as software for motor- and transformer
testing is available.
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Fig. 2 shows the Infratek 107A Power Analyzer. Its frequency
range is DC-300kHz; it is suitable for measurements on frequency
inverter drivers. It is fitted with special functions: IEC1000-3-2,
Logging, Dynamic Torque, Motor Testing, and Transformer Testing.
It comes with RS232 interface (USB, Ethernet) and Operating
Software under Windows. The motor version includes elaborate
software and hardware for induction motor testing.
Three 1V–1000V voltage inputs have one common terminal. The
current inputs are galvanically isolated from each other and
from the voltage inputs. The ranges are 100mA-3A, and1A-40A.
The clamp input for external current sensors can be scaled
for display of actual current and power. The 107A-3 measures
line-to-line voltages.
3.1 Special Functions
IEC1000-3-2: This function measures current harmonics
h01 to h63 on three phases simultaneously averaged over 16
signal periods. By means of the Operating Software the harmonics
are compared against standard limits and are marked if they
exceed limits. You can print results and modify the standard
limits.
LOGGING: The LOGGING-function is used to catch transient
processes, such as power versus time of a starting electric
motor. LOGGING also permits long-term monitoring up to more
than 220 days. Worldwide servers and computers are tested
according the SPECpower-BENCHMARK, it relates computing power
and energy consumption. The 107A Power Analyzer, using the
LOGGING-function, qualifies for these tests.
The Start-command initiates the measurement. The 107A sends
the selected quantities via interface (RS232, USB, Ethernet)
to the PC and simultaneously to nine analog outputs. The maximum
data transfer is 450 values per second. Only values of whole
signal periods are sent, these are: rms of voltage and current,
peak values, minima, maxima, power, apparent power, power
factor, torque, and speed. Off-line additional values can
be computed. The 107A-settings are simple: select LOGGING,
set in display fields 0, 1, and 2 those quantities you want
to transfer to the PC, select in the SETUP menu CYCLE=1 …
32767, which sets the averaging time in signal periods, and
finally select LOGGN=1 …. 32767, which specifies how many
times CYCLE is repeated. For example, for a transient measurement
at 50Hz power line frequency you would select CYCLE=1, 2,
3, 4, or 5, and LOGGN to 100 up to 500. This corresponds to
a total measurement time of 2 to 50 seconds. On the other
hand for long term data-logging you would select CYCLE=30’000.
This selection would generate one data set of 9 values every
10 minutes. With LOGGN set to 28800 a total number of 28800
data sets, corresponding to 200 days, will be transferred
to the PC and analog outputs.
All these parameters can be set via front panel or Windows
Operating Software. In EXCEL transient diagrams and trend
plots can be generated. Fig. 3 shows transient plots of current
and power of a small induction motor versus time (CYCLE).
DYNTORQ: This word construction indicates dynamic
torque measurement. It can also be used for other measurement
quantities, not just torque. Short term (transient) and long
term measurements are possible. The 107A-setting are the same
as for the LOGGING-function with one difference. The output
quantities to the PC are determined by the values in display
fields 0 and 1. For torque, power, and apparent power the
sums of three phases are sent to the PC, for voltage, current,
and power factor the average values of the 3 phases, and for
analog inputs (such as speed) a single value. These features
make dynamic torque measurement in the air gap of a motor
versus rotating speed possible. A programmable TTL-input for
motor speed measurement, and 9 analog inputs for torque, temperature,
and other quantities are in the 107A motor version included.
INDUCTION MOTOR TESTING: Two test methods are available:
The first uses no external torque and speed sensors, the second,
on the other hand, requires torque and speed sensors and a
mechanical load and is more amenable for laboratory type investigations.
Test method 1 is suitable for high speed testing and quality
control. The 107A Operating Software for test method 1 determines
complete motor characteristics from 2 measurements within
6 to 10 seconds. The characteristics of torque, power, efficiency,
current, and power factor, all versus slip are determined
and can be plotted. Fig. 4 shows torque and power versus slip
of a small induction motor.
Test method 1 delivers a considerable amount of quality information.
The plotted characteristics can be attached to the motor and
gives their user confidence that he is using good quality.
For large motors nonlinear current distribution can be entered
via Operating Software. To simplify the test, motors can be
supplied from frequency inverter drives.
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