Leroy-Somer FMV 2107, Руководство по установке и обслуживанию

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3.3 - Electrical and electromagnetic phenomena
associated with frequency inverters
3.3.1 - General
The power structure of frequency inverters leads to the
occurrence of two types of phenomenon :
- low frequency harmonic feedback on the power supply,
- emission of radio frequency signals which may interfere
with the operation of other equipment.
These are separate phenomena, which have different
consequences on the electrical environment.
3.3.2 - Low frequency harmonics
As the diode bridge at the head of the frequency inverter
rectifies the supply voltage it generates a non-sinusoidal
A.C. line current.
3.3.3 - Radio frequency interference
Frequency inverters use high-speed switches
(transistors, semi-conductors) for switching high voltages
(around 550V) and high frequency currents (several
kHz). This provides a high level of efficiency and a low
level of motor noise.
This results in the generation of radio frequency signals
which may disturb the operation of other equipment or
distort sensor measurements :
- due to high frequency leakage currents which escape
to earth via the inverter/motor cable leakage capacitance
This current carries harmonics 6n
±
1.
The amplitude of these harmonics decreases as their
frequency increases.
Harmonics 5, 7, 11, and 13 are 250 Hz, 350 Hz, 550 Hz,
650 Hz respectively for a supply frequency of 50 Hz, and
are the most significant.
Their amplitudes depend on the impedance of the mains
supply upstream of the rectifier bridge, and the structure
of the D.C. bus downstream of the rectifier bridge.
The more inductive the mains supply and the D.C. bus,
the more these harmonics are reduced.
They have virtually no effect on the level of electrical
energy consumption. Temperature rises associated with
these harmonics in transformers and motors connected
directly to the mains supply are negligible.
These low frequency harmonics never cause
interference on sensitive equipment.
They may affect the energy distributor due to the
fluctuating resonances which may be present in the
meshed system, and the additional losses in the supply
cables. However these are minor consequences. They
only have a significant effect on loads on frequency
inverters of several hundred kVA and where these
loads are more than a quarter of the total on-site
load.
and the leakage capacitance of the motor across the
metal structures which support the motor.
- by conduction or feedback of radio frequency signals
on the supply cable,
- by direct radiation close to the power supply cable or
the inverter/motor cable.
These phenomena have direct consequences for the
user.
The frequency range concerned (radio frequency) does
not cause interference for the energy distributor.
18
Frequency inverters
FMV 2107
FMV 2307
Mains supply line current drawn by a diode bridge.
I
E1
= leakage current fed back to the inverter caused by the cable and motor leakage capacitances.
I
E2
= leakage current escaping via metal structures.
Motor
I
E1
I
E1
I
E1
+ I
E2
Mains supply
transformer
Cable leakage
capacity
Winding
leakage capacity
I
E2
Interference radiated
by the motor cable,
effect limited to
50 cm around the cable
888
FMV
2107
Interference emission paths