VLT
®
2800 Series
All about VLT
2800
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Special conditions
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Galvanic Isolation (PELV)
PELV (Protective Extra Low Voltage) insulation is
achieved by inserting galvanic isolators between the
control circuits and circuits that are connected to
the mains potential. The VLT is designed to meet
the requirements for protective separation by means
of having the necessary creepage and clearance
distances. These requirements are described in
standard EN 50 178. It is also a requirement
that the installation is carried out as described in
local/national regulations regarding PELV.
All control terminals, terminals for serial communication
and relay terminals are safely separated from the
mains potential, i.e. they comply with the PELV
requirements. Circuits that are connected to control
terminals 12, 18, 19, 20, 27, 29, 33, 42, 46, 50,
53, 55 and 60 are galvanically connected to one
another. Serial communication connected to fieldbus
is galvanically insulated from the control terminals,
although this is only a functional insulation.
The relay contacts on terminals 1 - 3 are insulated
from the other control circuits with reinforced/double
isolation, i.e. PELV is observed for these, even though
there is mains potential at the relay terminals.
The circuit elements described below form the safe
electric separation. They fulfill the requirements
for reinforced/double insulation and associated
testing pursuant to EN 50 178.
1. Transformer and optical separation in
voltage supply.
2. Optical insulation between Basic Motor
Control and control card.
3. Insulation between the control card and
the power part.
4. Relay contacts and terminals relating to other
circuits on the control card.
PELV insulation of the control card is guaranteed
under the following conditions:
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TT network with maximum 300 Vrms
between phase and earth.
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TN network with maximum 300 Vrms
between phase and earth.
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IT network with maximum 400 Vrms
between phase and earth.
In order to maintain PELV all connections made to
the control terminals must be PELV, e.g. thermistor
must be reinforced/double insulated.
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Earth leakage current and RCD relays
Earth leakage current is primarily caused by the
parasitic capacitance between motor phases and
motor cable screen. When an RFI filter is used, this
contributes additional leakage current, as the filter
circuit is connected to earth through capacitors.
The size of the earth leakage current depends on
the following factors, in order of priority:
1. Length of the motor cable
2. Motor cable with/without screen
3. High switching frequency
4. RFI filter used or not
5. Motor earthed 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!:
Since the leakage current is > 3.5 mA,
reinforced earthing must be established, as this
is required to ensure compliance with EN 50178.
The cable cross section must be at least 10 mm
2
or
2 rated earth wires that are terminated seperately.
NB!:
Never use RCD relays (type A) that are
not suitable for DC fault currents from
3-phase rectifier loads.
If RCD relays are used, they must be:
-
Suitable for protecting equipment with a DC content
in the faulty current (3-phase bridge rectifier)
-
Suitable for a pulse-shaped, brief discharge
on power-up
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Suitable for a high leakage current (300 mA)
See the section entitled
Earth connection
for further information.
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Extreme operating conditions
Short circuit
The frequency converter is protected against short
circuits on motor terminals U, V, W (96, 97, 98).
A short circuit between two motor terminals would
lead to an overcurrent in the IGBT module, which
means that all transistors in the IGBT module
would independently cut out.
The inverter turns off after 5-10
µ
s and the frequency
converter displays a fault code, although this depends
on impedance and motor frequency.
MG.28.E9.02 - VLT is a registered Danfoss trademark
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