Filter
Top Products
366
Measurement of main circuit voltages,
currents and powers
6.2.4 Measurement of currents
Use moving-iron type meters on both the input and output sides of the inverter. However, if the carrier frequency
exceeds 5kHz, 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.
As the inverter input side current is easily imbalanced, measurement of currents in all three phases is recommended.
Correct values cannot be measured in one or two phases. On the other hand, the phase imbalanced ratio of the output
side current must 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 monitored on the operation
panel is accurate if the output frequency varies, and it is recommended to monitor values (provide analog output) using
the operation panel.
An example of the measured value difference produced by different measuring meters is shown below.
6.2.5 Use of CT and transducer
A CT may be used in both the input and output sides of the inverter, but the one used should have 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.
6.2.6 Measurement of inverter input power factor
Use the effective power and apparent power to calculate the inverter input power factor. A power-factor meter cannot
indicate an exact value.
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
Example of measuring Inverter Input Current Example of measuring Inverter Output Current
Total power factor of the inverter =
Effective power
Apparent power
=
Three-phase input power found by 3-wattmeter method
× V (power supply voltage) × I (input current effective value)
120
100
80
60
0
60Hz40
20
%
Moving-iron
type
Clamp-on wattmeter
current measuremen
t
Clamp meter
Clip AC
power meter
120
100
80
60
0
60Hz40
20
%
Moving-iron type
Clip AC
power meter
Clamp-on wattmeter
current measurement
Clamp meter
3