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7.2.3
Measurement of currents
Use moving-iron type meters both on the inverter's input and output sides. However, if the carrier frequency exceeds 5 kHz,
do not use that meter since an overcurrent losses produced in the internal metal parts of the meter will increase and the meter
may burn out. In this case, use an approximate-effective value type.
Since the inverter input current tends to be unbalanced, measurement of three phases is recommended. The correct value
cannot be obtained by measuring only one or two phases. On the other hand, the unbalanced ratio of each phase of the output
current should be within 10%.
When a clamp ammeter is used, always use an effective value detection type. A mean value detection type produces a large
error and may indicate an extremely smaller value than the actual value. The value displayed on the operation panel is accurate
even if the output frequency varies. Hence, it is recommended to monitor values using the operation panel.
Examples of measured value differences produced by different measuring meters are as follows:
[Measurement conditions]
The value indicated on the moving-iron type ammeter is
100%.
％
Clip AC
120
p ower meter
Moving-iron
type
100
80
60
Clamp-on wattmeter
Clamp meter
current measurement
0
20
40
Example of measuring the inverter input current
7.2.4
Use of CT and transducer
A CT may be used both on the inverter's input and output sides. Use the one with the largest possible VA ability because an
error will increase if the frequency gets lower.
When using a transducer, use the effective value calculation type which is immune to harmonics.
7.2.5
Measurement of inverter input power factor
Calculate the factor from the effective power and the apparent power. A power-factor meter cannot indicate an exact value.
Total power factor of the inverter =
=
7.2.6
Measurement of converter output voltage (between
terminals P and N)
The output voltage of the converter can be measured with a moving-coil type meter (tester) between terminals P and N. The
voltage varies according to the power supply voltage. Approximately 270 to 300 V (540 to 600 V for the 400 V class) is output
when no load is connected. The voltage decreases when a load is applied.
When energy is regenerated from the motor during deceleration, for example, the converter output voltage rises to nearly 400
to 450 VDC (800 to 900 VDC for the 400 V class) maximum.
60Hz
Effective power
Apparent power
Three-phase input power found by the 3-wattmeter method
√
3×V (power supply voltage) × I (input current effective value)
[Measurement conditions]
The value indicated on the moving-iron type ammeter is
100%.
％
120
Clip AC
power meter
Moving-iron type
100
80
Clamp meter
Clamp-on wattmeter
current measurement
60
0
20
40
Example of measuring the inverter output current
7. PRECAUTIONS FOR MAINTENANCE AND INSPECTION
7.2 Measurement of main circuit voltages, currents, and powers
60Hz
2
2
3
4
5
6
7
8
9
10
235
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