280
Measurement of main circuit voltages, currents and powers
6.2.1
Measurement of powers
Use digital power meters (for inverter) for the both of drive unit input and output side. Alternatively, measure using
electrodynamic type single-phase wattmeters for the both of drive unit input and output side in two-wattmeter or three-
wattmeter method. As the current is liable to be imbalanced especially in the input side, it is recommended to use the three-
wattmeter method.
Examples of process value differences produced by different measuring meters are shown below.
An error will be produced by difference between measuring instruments, e.g. power calculation type and two- or three-
wattmeter type three-phase wattmeter. When a CT is used in the current measuring side or when the meter contains a PT on
the voltage measurement side, an error will also be produced due to the frequency characteristics of the CT and PT.
6.2.2
Measurement of voltages and use of PT
(1) Drive unit input side
As the input side voltage has a sine wave and it is extremely small in distortion, accurate measurement can be made with an
ordinary AC meter.
(2) Drive unit output side
Since the output side voltage has a PWM-controlled rectangular wave, always use a rectifier type voltmeter. A needle type
tester cannot be used to measure the output side voltage as it indicates a value much greater than the actual value. A moving-
iron type meter indicates an effective value which includes harmonics and therefore the value is larger than that of the
fundamental wave. The value monitored on the operation panel is the drive unit-controlled voltage itself. Hence, that value is
accurate and it is recommended to monitor values using the operation panel.
(3) PT
No PT can be used in the output side of the drive unit. Use a direct-reading meter. (A PT can be used in the input side of the
drive unit.)
[Measurement conditions]
Constant output of 60Hz or higher with a constant
torque (100%). The value obtained by the 3-wattmeter
method with a 4-pole 3.7kW induction motor is assumed
to be 100%.
Example of
Measuring Drive Unit Input Power
[Measurement conditions]
Constant output of 60Hz or higher with a constant
torque (100%).
The value obtained by the 3-wattmeter
method with a 4-pole 3.7kW induction motor is assumed
to be 100%.
Example of
Measuring Drive Unit Output Power
3-wattmeter method (Electro-dynamometer type)
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced three-phase load)
Clamp-on wattmeter
(Hall device power arithmetic type)
0
20
40
60
80 100 120Hz
60
80
100
120
%
3-wattmeter method (Electro-dynamometer type)
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced three-phase load)
Clamp-on wattmeter
(Hall device power arithmetic type)
0
20
40
60
80 100 120Hz
60
80
100
120
%
Summary of Contents for FR-D720-0.2K-G
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