VLT
®
FCM Series
All about FCM
300
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Galvanic isolation (PELV)
PELV offers protection by way of extra low voltage.
Protection against electric shock is considered to
be ensured when all connected devices are of the
PELV type and the installation is made as described
in local/national regulations on PELV supplies.
In FCM 300 Series all control terminals are supplied
from or in connection with extra low voltage (PELV).
Galvanic (ensured) isolation is obtained by fulfilling
requirements concerning higher isolation and
by providing the relevant creapage/clearance
distances. These requirements are described
in the EN 50178 standard.
The components that make up the electrical
isolation, as described below, also comply with the
requirements concerning higher isolation and the
relevant test as described in EN 50178.
The galvanic isolation can be shown in three
locations (see drawing below), namely:
1. Power supply (SMPS) incl. signal isolation of U
DC
,
indicating the intermediate current voltage.
2. Gate drive that runs the IGBTs (optocouplers).
3. Current transducers (opto-couplers).
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Earth leakage current
Earth leakage current is primarily caused by the
capacitance between motor phases and the
motor frame. The RFI filter contributes additional
leakage current, as the filter circuit is connected
to earth through capacitors.
The size of the leakage current to the ground depends
on the following factors, in order of priority:
1. Switching frequency
2. Motor grounded on site or not
The leakage current is of importance to safety during
handling/operation of the frequency converter if (by
mistake) the frequency converter has not been earthed.
NB!:
FCM 305-375 all have leakage currents > 3.5
mA, approx 4 to 20 mA. Varies with switching
frequencies within the given interval.
This means reinforced earthing mus be established
(see Quick Guide MG.03.A1.02), if EN50178
is to be complied with.
Never use ELCB (Earth Leakage Circuit Breaker) relays
also called RCD(Residual Current Device) that are
not suitable for DC fault currents (type A).
If a RCD is used it must be:
-
Suitable for protecting equipment with a DC current
content in the fault current (3-phase rectifier)
-
Suitable for power-up with short charging
current to earth
-
Suitable for a high leakage current.
This means it is possible to operate the FCM
300 on RCD type B:
Residual Current Devices (RCD) type B has a tolerance
on trip level. It is therefore recommended to use a
RCD where the max leakage current for the FCM (see
above, 20 mA) is less than 1/3 of the trip level for the
RCD. This means the trip level for the RCD will have
to be 60 mA or higher, i.e. a RCD type B with a trip
level 100 mA can be used for protection.
Galvanic isolation
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Extreme running conditions
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in two cases:
MG.03.H3.02 - VLT is a registered Danfoss trademark
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