Measurement Of Currents; Use Of Ct And Transducer; Measurement Of Inverter Input Power Factor; Measurement Of Converter Output Voltage (Across Terminals P-N) - Mitsubishi Electric FR-A700 Technical Manual

3.8.3

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
loss 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 (refer to page 507), 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 (parameter unit) is accurate if the
output frequency varies, and it is recommended to monitor
values (provide 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.
3.8.4

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.
3.8.5

Measurement of inverter input power factor

Use the effective power and apparent power to calculate
the inverter input power factor (refer to page 505). A
power-factor meter can not indicate an exact value.
3.8.6

Measurement of converter output voltage (across terminals P-N)

The output voltage of the converter is developed across
terminals P-N and can be measured with a moving-coil
type meter (tester). Varying with the power supply voltage,
the converter output voltage is approximately 270 to 300V
(approximately 540 to 600V for the 400V class) under no
load and lowers under load.
When regenerative energy is returned from the motor
during deceleration, for example, the converter output
3.8.7

Measurement of inverter output frequency

A pulse train proportional to the output frequency is output
across the frequency meter signal output terminal FM-SD
of the inverter. This pulse train output can be counted by a
frequency counter, or a meter (moving-coil type voltmeter)
can be used to read the mean value of the pulse train
[Measurement
120
conditions]
The reading of the
100
moving-iron type
ammeter is 100%.
[Measurement
120
conditions]
Value indicated by
100
moving-iron type
ammeter is 100%.
When using a transducer, use the effective value
calculation type which is immune to harmonics.
voltage rises to nearly 400V (800V for the 400V class)
maximum. (When the converter output voltage reaches
approximately 400V for the 200V class or approximately
800V for the 400V class, the inverter results in OVT shut-
off)
output voltage. When a meter is used to measure the
output frequency, approximately 5VDC is indicated at the
maximum frequency.
For detailed specifications of the frequency meter signal
output terminal FM, refer to page 78.
551
SELECTION
%
Clip AC
power meter
Moving-iron
type
80
60
Clamp-on wattmeter
Clamp meter
current measurement
0 20 40 60Hz
Example of measuring inverter
input current
%
Clip AC
power meter
Moving-iron type
80
Clamp meter
Clamp-on wattmeter
current measurement
60
40 60Hz
0 20
Example of measuring inverter
output current
3