7-6
7.3 Measurement of Electrical Amounts in Main Circuit
Because the voltage and current of the power supply (input, primary circuit) of the main circuit of the
inverter and those of the motor (output, secondary circuit) include harmonic components, the
readings may vary with the type of the meter. Use meters indicated in Table 7.3 when measuring with
meters for commercial frequencies.
The power factor cannot be measured by a commercially available power-factor meter that
measures the phase difference between the voltage and current. To obtain the power factor,
measure the power, voltage and current on each of the input and output sides and calculate in the
following formula.
Three-phase input Single-phase input
%100×
(A)t(V)×Curren×Voltage3
(W)powerElectric
=factorPower
%100×
(A)t(V)×CurrenVoltage
(W)powerElectric
=factorPower
Table 7.3 Meters for Measurement of Main Circuit
Item
Input (primary) side Output (secondary) side
DC link bus
voltage
(P (+)-N (-))
Waveform
Voltage Current Voltage Current
Name of
meter
Ammeter
A
R, AS, AT
Voltmeter
VR, VS, VT
Wattmeter
W
R, WT
Ammeter
AU, AV, AW
Voltmeter
V
U, VV, VW
Wattmeter
W
U, WW
DC voltmeter
V
Type of
meter
Moving iron
type
Rectifier or
moving iron
type
Digital AC
power meter
Digital AC
power meter
Digital AC
power meter
Digital AC
power meter
Moving coil type
Symbol
of meter
⎯ ⎯ ⎯ ⎯
It is not recommended that meters other than a digital AC power meter be used fo
measuring the output voltage or output current since they may cause larger measurement
errors or, in the worst case, they may be damaged.
Figure 7.1 Connection of Meters
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