Measurement Of Voltages And Use Of Pt; Measurement Of Currents - Mitsubishi Electric F700 Instruction Manual

Meters and measuring methods
4.5.2

Measurement of voltages and use of PT

(1) 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.
(2) 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 indicates a value much greater than the
actual value. A moving-iron type meter indicates an effective value which includes harmonics and therefore the
value is larger than that of the fundamental 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.
(3) 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.)
4.5.3

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 losse 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 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.
An example of the measurement 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
242
Clip AC
power meter
Clamp-on wattmeter
current measurement
40 60Hz
[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
Clip AC
power meter
Clamp meter
40 60Hz