Mitsubishi Electric FR-A700 Series, Техническое руководство

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SELECTION
S
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LECTION
3
3.4 Power supply of the inverter
3.4.1 Inverter input current and power factor
The converter circuit of the inverter consists of three-phase
bridged diodes and capacitor-input smoothing circuit as
shown below.
The capacitance of the DC smoothing capacitor is very
large and the input current I from the power supply flows
only when the voltage V of the power supply is higher than
the terminal voltage Ed of the smoothing capacitor. Hence,
the conduction width of the converter circuit is very narrow
and the peak (crest) value of the input current is large. The
input power factors given in the table on the right assume
that the DC voltage does not vary. As shown in the table
on the right, if the DC voltage Ed is greater than 1.35 times
of the input voltage V under a light load, two pulse currents
flow in a half cycle and the power factor falls to 58.7% or
less.
Ordinarily, the power factor is represented by a phase
difference between voltage and current. When a current
having a distorted waveform flows in the inverter input, the
power factor is calculated from an apparent power and a
three-phase input power found through the three-wattmeter
method. The power factor thus calculated is called an
overall power factor.
AC power supply current and power factor converter
circuit in inverter
(The power factor value measured through three-phase
power factor meter which indicates a phase difference
between voltage and current is different from the above
overall power factor because the current flowing in the
inverter is not a sine-wave current.)
Since the input current has a distorted waveform and the
form factor is high (the form factor of a sine wave is 1.11)
as described above, the overall input power factor of the
inverter may become extremely low depending on the
power supply voltage and load factor. The overall input
power factor is usually about 75 to 80% for inverters.
Accordingly, the input current (effective value) increases.
I
V
R
C Ed
Motor
IM
Inverter
circuit
Power
supply
Converter circuit configuration
Ed
Power
Factor Form
Factor
Crest
(peak)
Factor
Ed 1.35V
58.7% or less 1.99 or more 2.16 or more
Ed=1.35V 58.7% 1.99 2.16
1.35V
Ed
1.225V
58.7 to 83.5% 1.99 to 1.27 2.16 to 1.71
Ed=1.225V 83.5% 1.27 1.71
1.225V
Ed 83.5 to 95.3% 1.27 to 1.23 1.71 to 1.28
Input current of inverter I =
Power supply
system capacity [kVA]
*
[A]
V
* Refer to page 2.
Overall power factor of the inverter =
Effective power
Apparent power
=
3-phase input power found by 3-wattmeter method
V (power supply voltage) I (input current effective value)