Measurement Of Currents; Use Of Ct And Transducer; Measurement Of Inverter Input Power Factor - Mitsubishi Electric FR-A760 Series Instruction Manual

6.2.3

Measurement of currents

Use a moving-iron type meter 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 can not 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 using a clamp ammeter, 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.
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
%
120
Moving-iron
type
100
80
60
Clamp meter
0 20
Example of measuring inverter input current
6.2.4

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.
6.2.5

Measurement of inverter input power factor

Use the effective power and apparent power to calculate the inverter input power factor. A power-factor meter can
not indicate an exact value.
Total power factor of the inverter
Clip AC
power meter
Clamp-on wattmeter
current measurement
40 60Hz
Effective power
=
Apparent power
Three-phase input power found by 3-wattmeter method
=
× V (power supply voltage) × I (input current effective value)
3
Measurement of main circuit voltages,
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
%
120
Moving-iron type
100
80
Clamp-on wattmeter
current measurement
60
0 20
Example of measuring inverter output current
currents and powers
Clip AC
power meter
Clamp meter
40 60Hz
421
6