Measurement Of Powers - Mitsubishi Electric FR-D720S SC EC Instruction Manual

Meters and measuring methods
7.7.1

Measurement of powers

Use digital power meters (for inverter) or an electro-dynamometer type meter, measure the pow-
er 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 measured value differences produced by different measuring meters are shown
below. An error will be produced by difference between measuring instruments, e.g. power cal-
culation 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.
Fig. 7-8: Differences when measuring power with different instruments
7 - 32
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%
3-wattmeter method (Electro-dynamometer type)
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced 3-phase load)
Clamp-on wattmeter
(Hall device power arithmetic type)
Example of measuring inverter input power
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%
3-wattmeter method (Electro-dynamometer type)
2-wattmeter method (Electro-dynamometer type)
Clip AC power meter
(For balanced 3-phase load)
Clamp-on wattmeter
(Hall device power arithmetic type)
Example of measuring inverter input power
Troubleshooting
I001301E, I001302E