Measurement Of Powers; Measurement Of Voltages And Use Of Pt; Meters And Measurement Methods - Mitsubishi Electric FR-A700 Technical Manual

3.8 Meters and measurement methods

Since voltages and currents in the primary and secondary
side of the inverter include harmonics, different meters
indicate different measurement values. When making
measurement with the meters designed for commercial
frequency, use the following measuring instruments and
circuits:
Inverter
Ar W R
11
Three
Vr
phase
As W S
12
power
Vs
supply
At W T
13
Vt
P
Moving-iron type
Electrodynamometer type
Examples of measuring points and instruments
3.8.1

Measurement of powers

Use a digital power meter (designed for inverter) on both the input and output sides of the inverter.
3.8.2

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 (parameter unit) is
the inverter-controlled voltage itself. Hence, that
value is accurate and it is recommended to monitor
values (or analog output) using the operation panel
(parameter unit). An example of the measurement
value difference produced by different measuring
meters is shown on the right.
U Au W
21
Vu
To the motor
V Av
Vv
W Aw W
22
Vw
N
V
Moving-coil type
Rectifier type
When installing meters etc. on the inverter output
side
When the inverter-to-motor wiring length is long,
especially in the 400V class, small-capacity models,
the meters and CTs may generate heat due to line-
to-line leakage current.
equipment which has enough allowance for the
current rating.
When measuring and indicating the output voltage
and output current of the inverter, it is recommended
to utilize the AM-5 and FM-SD terminal output
function of the inverter.
(3) PT
No PT can be used in the output side of the inverter.
Use a direct-reading meter. (A PT can be used on the
input side of the inverter.)
[Measurement conditions]
Motor 3.7kW 4-pole + inverter 3.7K
Value indicated by rectifier type voltmeter is 100%.
Example of measuring inverter output
550
SELECTION
Therefore, choose the
voltage