Derating For Running At Low Speed; Galvanic Isolation (Pelv) - Danfoss VLT FCD 303 Operating Instructions Manual

Derating for running at low speed

When a motor is connected to a frequency converter,
it is necessary to ensure adequate cooling of the
motor. At low rpm values, the motor fan is not able
to supply an adequate volume of cooling air. This
problem occurs when the load torque is constant (e.g.
with a conveyor belt) across the full regulating range.
The reduced amount of ventilation determines the
permissible torque in continuous operation. If the motor
is to run continuously at an rpm lower than half the
rated value, extra cooling air must be supplied to the
motor. Instead of providing extra cooling, it is possible
to reduce the motor load ratio. This can be done by
selecting a larger motor. However, the design of the
frequency converter puts limits on the size of motors
that can be connected to the frequency converter.
Derating for ambient temperature
The ambient temperature (T
temperature allowed. The average (T
measured over 24 hours, must be at least 5
°C lower. If the frequency converter operates
at temperatures above 40 °C, a derating of the
rated output current is necessary.
FCD 303-305 +10 °C
FCD 307 +5 °C
FCD 335 -5 °C

Galvanic isolation (PELV)

PELV (Protecti v e Extra Low Voltage) isolation is
achieved by inserting galvanic isolators between the
control circuits and circuits that are connected to
the mains potential. These separators are designed
to meet the requirements for increased separation
by means of having the necessary creepage and
air clearance. These requirements are described
in standard EN 50 178. It is also a requirement
MG.04.B7.02 - VLT is a registered Danfoss trademark
) is the maximum
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VLT FCD Series
that installation is carried out as described in
local/national PELV regulations.
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.
If switch S100 is opened the potentials of the group
18, 19, 20, 27, 29, 33 are separated from all other
input/output. In that case terminal 12 cannot be used
for supply for the digital inputs on these terminals.
Serial communication connected to terminals 67 -
70 is galvanically isolated from the control terminals,
although this is only a functional isolation.
The relay contacts on terminals 1 - 3 are isolated
from the other control circuits with increased isolation,
i.e. PELV is observed for these, even though there
is mains potential in the relay terminals.
The circuit elements described below form the safe
electric separation. They fulfill the requirements
for increased isolation and associated testing
pursuant to EN 50 178.
1. Transformer and optical separation in
voltage supply.
2. Optical isolation between Basic Motor
Control and control card
3. Isolation between the control card and
the power part.
4. Relay contacts and terminals relating to other
circuits on the control card.
PELV isolation of the control card is guaranteed
under the following condition:
- There may be max. 300 V between
phase and earth.
A motor thermistor connected to terminals 31a-31b
must be double isolated to obtain PELV. Danfoss
Bauer provides double isolated thermistors.
See also the section Diagram in the Design Guide.
107