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Measurement of main circuit voltages, currents and powers
7.2.3
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.
Since current on the inverter input side tends to be unbalanced, measurement of three phases is recommended. Correct value
can not be obtained by measuring only one or two phases. On the other hand, the unbalanced ratio of each phase of the output
side 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 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.
Examples of process value differences produced by different measuring meters are shown below.
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
%
Clip AC power meter
120
Moving-iron
type
100
80
60
Clamp-on
Clamp meter
wattmeter current measurement
0
20
40
Example of measuring inverter input current
7.2.4
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.
7.2.5
Measurement of inverter input power factor
Calculate using effective power and apparent power. A power-factor meter can not indicate an exact value.
Total power factor of the inverter
7.2.6
Measurement of converter output voltage (across terminals P/+ and N/-)
The output voltage of the converter is developed across terminals P/+ and N/- and can be measured with a moving-coil type
meter (tester). Although the voltage varies according to the power supply voltage, approximately 270VDC to 300VDC
(540VDC to 600VDC for the 400V class) is output when no load is connected and voltage decreases during driving load
operation.
When energy is regenerated from the motor during deceleration, for example, the converter output voltage rises to nearly
400VDC to 450VDC (800VDC to 900VDC for the 400V class) maximum.
262
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
60Hz
Effective power
=
Apparent power
3-phase input power found by 3-wattmeter method
=
× V (power supply voltage) × I (input current effective value)
3
%
120
Moving-iron type
100
80
Clamp meter
Clamp-on
wattmeter current
60
measurement
0
20
Example of measuring inverter output current
Clip AC power meter
40
60Hz
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