All About Fcm; Galvanic Isolation (Pelv); Earth Leakage Current - Danfoss VLT Brook Crompton Motor FCM 300 Design Manual

All about FCM 300

4 All about FCM 300
4.1 Special Conditions

4.1.1 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
Illustration 4.1), namely:
•
Power supply (SMPS) incl. signal isolation of U
indicating the intermediate current voltage.
•
Gate drive that runs the IGBTs (optocouplers).
•
Current transducers (opto-couplers).

4.1.2 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.
®
VLT DriveMotor FCM Series
The size of the leakage current to the ground depends on
the following factors, in order of priority:
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.
NOTICE
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 must be established, 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 an RCD is used it must be:
,
DC
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 an 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. an RCD type B with a trip level 100 mA can be
used for protection.
®
MG03BA02 - VLT is a registered Danfoss trademark
1. Switching frequency
2. Motor grounded on site or not
- 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.
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