Mitsubishi Electric FR-D720S SC EC, Instruction Manual

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Troubleshooting Meters and measuring methods
FR-D700 SC EC 7 - 33
7.7.2 Measurement of voltages and use of PT
Inverter input side
As the input side voltage has a sine wave and it is extremely small in distortion, accurate meas-
urement can be made with an ordinary AC meter.
Inverter output side
Since the output side voltage has a PWM-controlled rectangular wave, always use a rectifier
type voltmeter. A needle type tester can not be used to measure the output side voltage as it in-
dicates a value much greater than the actual value. A moving-iron type meter indicates an ef-
fective value which includes harmonics and therefore the value is larger than that of the fun-
damental wave. The value monitored on the operation panel is the inverter-controlled voltage
itself. Hence, that value is accurate and it is recommended to monitor values (provide analog
output) using the operation panel.
PT
No PT can be used in the output side of the inverter. Use a direct-reading meter. (A PT can be
used in the input side of the inverter.)
7.7.3 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 themeter 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 de-
tection type produces a large error and may indicate an extremely smaller value than the actual
value. The value monitored on the operation panel is accurateif 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 be-
low.
I001303E, I001304E
Fig. 7-9: Differences when measuring currents with different instruments
Moving-iron
type
Clip AC power
meter
Measurement conditions:
Value indicated by moving-iron type ammeter is 100%.
Example of measuring Inverter Input Current Example of measuring Inverter Output Current
Clamp-on wattmeter
current measurement
Clamp meter
Measurement conditions:
Value indicated by moving-iron type ammeter is 100%.
Moving-iron
type
Clip AC power
meter
Clamp-on wattmeter
current measurement
Clamp meter