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7.3 Measurement of Electrical Amounts in Main Circuit
Because the voltage and current of the power supply (input) of the main circuit of the inverter and
the output (motor) include harmonic components, the indicated value deviates according to 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
Electric
Power
factor
=
3
×Voltage
Input (power supply) side
Voltage
Ammeter
Voltmeter
A
, A
, A
V
R
S
T
R
Rectifier or
Moving iron
moving iron
type
When the output voltage is measured by a rectifier type voltmeter, an error may be
included or the voltmeter might be burn out. To measure it with higher accuracy, use a
digital AC power meter.
power
(W)
×
(V)×Curren
t
(A)
Table 7.3 Meters for Measurement of Main Circuit
Current
Wattmeter
, V
, V
W
, W
S
T
R
T
Digital power
Digital power
meter
type
Figure 7.1 Connection of Meters
Single-phase input
100
%
Power
factor
=
Output (motor) side
Voltage
Current
Ammeter
Voltmeter
A
, A
, A
V
, V
, V
U
V
W
U
V
W
Digital power
meter
meter
7-6
Electric
power
(W)
×
Voltage
(V)×Curren
t
(A)
DC link circuit
voltage
(P (+)-N (-))
Wattmeter
DC voltmeter
W
, W
U
W
Digital power
Moving coil type
meter
100
%
V
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