Measurement Of Powers; Measurement Of Voltages And Use Of Pt - Mitsubishi Electric FR-CS82S-025 Instruction Manual

7.2.1

Measurement of powers

Use digital power meters (for inverter) both on the inverter's input and output sides. Alternatively, use electrodynamic type
single-phase wattmeters both on the inverter's input and output sides in the two-wattmeter or three-wattmeter method. As the
current is liable to be imbalanced especially on the input side, it is recommended to use the three-wattmeter method.
Examples of measured value differences produced by different measuring meters are shown in the following figure.
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.
[Measurement conditions]
Constant output of 60 Hz or more frequency with a constant-
torque (100%). The value obtained by the 3-wattmeter
method with a 4-pole 3.7 kW induction motor is assumed to
be 100%.
％
120
100
3-wattmeter method (Electro-dynamometer type)
80
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced three-phase load)
60
Clamp-on wattmeter
(Hall device power arithmetic type)
0 20 40 60
Example of measuring inverter input power
7.2.2

Measurement of voltages and use of PT

 Inverter input side
As the input voltage has a sine wave and it is extremely small in distortion, accurate measurement can be made with an
ordinary AC meter.
 Inverter output side
Since the output voltage has a PWM-controlled rectangular wave, always use a rectifier type voltmeter. A needle type tester
cannot be used to measure the output 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 displayed on the operation panel is the inverter-controlled voltage itself. Hence, that value is accurate and it
is recommended to monitor values using the operation panel.
 PT
No PT can be used on the output side of the inverter. Use a direct-reading meter. (A PT can be used on the input side of the
inverter.)
234
7. PRECAUTIONS FOR MAINTENANCE AND INSPECTION
7.2 Measurement of main circuit voltages, currents, and powers
80 100 120Hz
[Measurement conditions]
Constant output of 60 Hz or more frequency with a constant-
torque (100%). The value obtained by the 3-wattmeter
method with a 4-pole 3.7 kW induction motor is assumed to
be 100%.
％
120
100
・
3-wattmeter method (Electro-dynamometer type)
80
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced three-phase load)
60
Clamp-on wattmeter
(Hall device power arithmetic type)
0 20 40 60 80 100 120Hz
Example of measuring inverter output power