VLT
®
FCD Series
Earth fault
The IGBT module cuts out within 100
s in case of
an earth fault on one of the motor terminals U, V, W
(96, 97, 98), although depending on impedance and
motor frequency.
Output connection
Motor terminals U, V, W (96, 97, 98) for the motor
can be connected/disconnected as often as required.
There is no way that the frequency converter can be
destroyed by connecting/disconnecting the motor ter-
minals. However, fault messages may appear.
Motor-generatedovervoltage
The voltage in the intermediate circuit is increased
when the motor acts as a generator. To protect the
frequency converter the IGBT module is discon-
nected when a specific voltage level is reached.
Motor-generated overvoltage can occur in two in-
stances:
1. The load drives the motor, i.e. the load generates
energy.
2. During deceleration (ramp-down) if the moment of
inertia is high, the load is low and the ramp-down
time too short for the energy to be dissipated as
a loss in the frequency converter, the motor and
the unit. The control unit attempts to correct the
ramp if possible.
The fault can be removed by connecting a brake re-
sistor, if the frequency converter has an integrated
brake module. If the frequency converter does not
have an integrated brake module an AC brake can
be used, see parameter 400
Brake function.
See the section entitled Brake resistors.
Static overloading
When the frequency converter is overloaded (current
limit in parameter 221 Current limit I
LIM
is reached),
the control reduces the output frequency in an
attempt to reduce the load. If the overloading is ex-
treme, there might be an output current that causes
the frequency converter to trip after approx. 1.5 sec.
See parameter 409 Trip delay overcurrent, I
LIM
.
An extreme overload will cause the switching fre-
quency to be derated to 3000 Hz.
■ dU/dt on motor
When a transistor in the inverter is opened, the volt-
age across the motor terminals will rise by a voltage/
time ratio (dU/dt) determined by:
- the motor cable (type, cross-section, induction,
capacity, length and screened/armoured/un-
screened/unarmoured)
- the mains voltage
Self-induction in the motor cable leads to an over-
swing U
PEAK
of the output voltage each time a
transistor in the inverter is opened. After U
PEAK
the
output voltage will stabilise at a level determined by
the voltage in the intermediate circuit. U
PEAK
and
dU/dt influence the lifetime of the motor, especially
motors without phase insulation paper in the coils. If
the motor cable is short (a few metres), the over-
shoot U
PEAK
is low, while the dU/dt is high. If the
motor cable length is increased, U
PEAK
is increased,
while dU/dt decreases.
■ Switching on the input
The waiting time between switching the mains volt-
age on terminals 91, 92 and 93 must be min. 30 sec.
■ Acoustic noise
The acoustic noise from the frequency converter
comes from two sources:
1. DC intermediate circuit coils.
2. Inverter.
Below are the typical values measured at a distance
of 1 m from the unit at full load:
FCD 303-330 3 x 400 V: 52 dB(A).
■ Derating for ambient temperature
The ambient temperature (T
AMB,MAX
) is the maxi-
mum temperature allowed. The average (T
AMB,AVG
)
measured over 24 hours, must be at least 5
C
lower. If the frequency converter operates at temper-
atures above 40
C, a derating of the rated output
current is necessary.
■ Temperature-dependentswitch frequency
This function ensures the highest possible switch fre-
quency without the frequency converter becoming
thermally overloaded. The internal temperature is
the actual expression of the degree to which the
MG.04.A1.02 - VLT is a registered Danfoss trade mark
119
All about FCD 300
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