Measurement Of Voltages And Use Of Pt; Measurement Of Currents - Mitsubishi Electric FR-E700 Instruction Manual

Meters and measuring methods
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 measurement
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 volt-
meter. A needle type tester can not be used to measure the output side voltage as it indicates a value
much greater than the actual value. A moving-iron type meter indicates an effective value which in-
cludes harmonics and therefore the value is larger than that of the fundamental wave. The value mon-
itored 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 car-
rier frequency exceeds 5 kHz, do not use that meter since an overcurrent losses produced in the in-
ternal metal parts of the meter will increase and the meter may burn out. In this case, use an approx-
imate-effective value type.
Since current on the inverter input side tends to be unbalanced, measurement of three phases is rec-
ommended. 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 val-
ue monitored on the operation panel is accurateif the output frequency varies, and it is recommend-
ed to monitor values (provide analog output) using the operation panel.
Examples of process value differences produced by different measuring meters are shown below.
Fig. 7-9:
7 - 32
Measurement conditions:
Value indicated by moving-iron type ammeter is 100%.
Clip AC power
meter
Moving-iron
type
Clamp-on wattmeter
Clamp meter
current measurement
Example of measuring Inverter Input Current
Differences when measuring currents with different instruments
Measurement conditions:
Value indicated by moving-iron type ammeter is 100%.
Clip AC power
meter
Moving-iron
type
Clamp meter
Clamp-on wattmeter
current measurement
Example of measuring Inverter Output Current
Troubleshooting
I001303E, I001304E,