Filter
Top Products
274
Measurement of main circuit voltages, currents and powers
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.
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.
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
Calculate using effective power and apparent power. A power-factor meter can not indicate an exact value.
6.2.6 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 270V to 300V (540V to 600V
for the 400V class) is output when no load is connected and voltage decreases when a load is connected.
When regenerative energy is returned from the motor during deceleration, for example, the converter output voltage rises to
nearly 400V to 450V (800V to 900V for the 400V class) maximum.
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
Example of measuring inverter input current
[Measurement conditions]
Value indicated by moving-iron type ammeter is 100%.
Example of measuring inverter output current
Total power factor of the inverter =
Effective power
Apparent power
=
3-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 measurement
Clamp meter
Clip AC power meter
120
100
80
60
0
60Hz40
20
%
Moving-iron type
Clip AC power mete
r
Clamp-on
wattmeter current
measurement
Clamp meter
3