Meters And Measuring Methods; Measurement Of Powers - Mitsubishi Electric F700 Instruction Manual

4.5 Meters and measuring 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:
When installing meters etc. on the inverter output side
When the inverter-to-motor wiring length is large, especially in the 400V class, small-capacity models, the meters
and CTs may generate heat due to line-to-line leakage current. Therefore, choose the 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 CA-5 terminal output function of the inverter.
3-phase
power
supply
Instrument
types
4.5.1

Measurement of powers

Using an electro-dynamometer type meter, measure the power in both the input and output sides of the inverter
using the two- or three-wattmeter method. As the current is liable to be imbalanced especially in the input side, it is
recommended to use the three-wattmeter method.
Examples of process value differences produced by different measuring meters are shown below.
An error will be produced by difference between measuring instruments, e.g. power calculation type and two- or
three-wattmeter type three-phase wattmeter. When a CT is used in the current measuring side or when the meter
contains a PT on the voltage measurement side, an error will also be produced due to the frequency characteristics
of the CT and PT.
[Measurement conditions]
Constant-torque (100%) load, constant-output at 60Hz
or more.
3.7kW, 4-pole motor, value indicated in 3-wattmeter
method is 100%.
%
120
100
3-wattmeter method (Electro-dynamometer type)
80
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced 3-phase load)
60
Clamp-on wattmeter
(Hall device power arithmetic type)
0 20 40 60 80 100 120Hz
Example of measuring inverter input power
Inverter
Ar R/L1 U
W11
Vr
As V
W12 S/L2
Vs
At T/L3 W
W13
Vt
P/+ N/-
V
+ -

Meters and measuring methods

Au
W21
Vu
To the motor
Av
Vv
Aw
W22
Vw
: Moving-iron type
: Electrodynamometer type
: Moving-coil type
: Rectifier type
[Measurement conditions]
Constant-torque (100%) load, constant-output at 60Hz
or more.
3.7kW, 4-pole motor, value indicated in 3-wattmeter
method is 100%.
%
120
100
3-wattmeter method (Electro-dynamometer type)
80
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced 3-phase load)
60
Clamp-on wattmeter
(Hall device power arithmetic type)
0 20 40 60 80 100 120Hz
Example of measuring inverter output power
4
241