508
SELECTION
3.5 Inverter-generated harmonics, EMI and leakage
current
3.5.1
Differences between EMI and harmonics
Sometimes inverter affects power supply or other
peripherals. Confusion of EMI and harmonics may cause a
malfunction of the peripheral devices. Countermeasures
against EMI and harmonics are different, so different
countermeasures for each causes are required. For
example, electromagnetic interference produced by a
personal computer are distinctly different in their sources,
adverse effects etc. from harmonics in a power circuit, and
their countermeasures differ greatly.
Their differences will be described below.
Harmonics are defined to have a frequency that is an
integral multiple of the fundamental wave and is different
from high frequencies. The composition of a single
fundamental wave and several harmonics is called a
distorted wave.
A distorted wave generally includes harmonics in a high-
frequency wave (KHz to MHz order). However, a distorted
wave handled as harmonics in a power distribution system
is usually of up to about 40th to 50th degrees (to several
kHz), and a distorted wave above that value generally
assumes an irregular form and must therefore be handled
as EMI.
3.5.2
Power harmonics and their reduction techniques
Constituted by a power rectifier, the converter circuit of the
inverter generates harmonics, distorting the voltage and
current waveforms of the input power supply. It is
necessary to have a correct understanding of harmonics
and take appropriate measures against them when using
equipment having a converter circuit in the power supply
circuit, e.g. a large-capacity inverter, many inverters,
thyristor leonard, thyristor motor or CVCF.
The Japanese harmonic reduction guidelines issued by the
Ministry of Economy, Trade and Industry (formerly Ministry
of International Trade and Industry) in September 1994
require that the following two main points should be
considered as harmonic reduction techniques:
1) Actions taken for faults due to power harmonics
(overheating of power capacitors, non-utility
generators, etc.)
2) Comply to the Japanese harmonic suppression
guidelines
The actions taken to achieve the two points on the left are
not always the same, so specific actions must be taken
after clarifying their purposes.
(1) Influence on and actions taken for the
power supply line
The power supply line to which the inverter is
connected with parallel loads such as a power
capacitor and a generator. And a harmonic current
generated in the inverter is divided into the power
supply line and parallel loads according to their
impedances. The influence of the harmonic current
on the electrical devices (parallel loads) and actions
taken against that current will be described below.
1) Power capacitor
For the maximum working voltages and maximum
working currents of power capacitors, JIS-C4902
Differences between EMI and harmonics of an inverter and leakage current
Item
EMI
Harmonics
Leakage Currents
Frequency band
High frequency (several 10kHz to
1GHz order)
Normally 40th to 50th degrees or
less (up to 3kHz or less)
(several Hz to MHz order)
Source
Inverter circuit
Converter circuit
Inverter circuit
Cause
Transistor switching
Rectifying circuit commutation
Transistor switching
Generated amount
Depends on voltage variation ratio
and switching frequency
Depends on current capacity
Depends on switching frequency and
voltage
Propagation path
Electric channel, space, induction
Electrical channel
Insulating material
Transmission amount
Distance, wiring route
Line impedance
Capacitance
Affected equipment
and influence
Sensor, etc: Mis-detection
Radio, wireless equipment : Acoustic
noise
Power capacitor : Heat generation
Non-utility generator
: Heat generation
Earth leakage circuit breaker
: Unnecessary operation
Thermal relay
: Unnecessary operation
Output side devices
(e.g. CT, meter) : Heat generation
Main countermeasure
examples
Change the wiring route
Install a EMI filter
Install a reactor
Change detection sensitivity
Change switching frequency
Summary of Contents for FR-A700 Series
Page 245: ...279 2 PARAMETER PARAMETER MEMO ...
Page 440: ...474 PARAMETER MEMO ...
Page 522: ...556 SELECTION MEMO ...
