129
MG.30.A7.02 - VLT
®
is a registered Danfoss trademark
Extreme running
conditions
Short circuit
By means of current measurement in each
of the three motor phases the VLT
®
3000
Series is protected against short circuits.
A short circuit between two output phases
will cause overcurrent in the inverter.
However, all transistors in the inverter are
turned off individually when the short-circuit
current exceeds the permitted value.
After 5-10 seconds the driver card turns off
the inverter and the frequency converter will
display a fault code.
Earth fault
In case of an earth fault in a motor phase
the inverter is turned off within 5-10 ms.
Switching on the output
The frequency converter output to the motor
can be cut in/out without any limit.
It is not possible to damage the VLT
®
frequency converter in any way by cutting
in/out on the output, although fault mes-
sages can occur.
Catching a spinning motor (flying start)
The frequency converter is able to control
a spinning load without stopping it or trip-
ping first, e.g. after a mains failure or when
a contactor is used on the output with a
large inverter load.
It performs this with a speed search when
cutting in on a spinning motor (see param-
eter 305).
Motor-generated overvoltage
The voltage on the DC link can be increased
when the motor acts as a generator. This
takes place in two cases:
1. The load runs the motor (at constant
output frequency from the frequency
converter), i.e. energy is supplied from
the load.
2. During deceleration ("ramp-down"),
if the moment of inertia is high, the
friction load is low and/or the ramp-
down time is short.
The control unit attempts to correct the ramp
if possible.
The inverter turns off to protect the transis-
tors and the d.c. bus capacitors when a
certain d.c. voltage level is reached.
Mains drop-out
During a mains drop-out the VLT
®
frequency
converter will continue to operate until the
DC link voltage drops below the minimum
stop level, which is typically 15% below the
VLT
®
frequency converter’s lowest rated
supply voltage.
The time before inverter stop depends on
mains voltage before drop-out, and the
load of the motor.
Ride-through and/or flying start can be
programmed.
Static overload
When the VLT
®
frequency converter is over-
loaded (the current limit I
LIM
is reached) the
control will reduce the output frequency f
M
in an attempt to reduce the load. If the re-
duction of output frequency does not re-
duce the load then the control unit’s final trip
is released when the output frequency has
reached below 0.5 Hz.
Running-in current limit can be limited
(0 - 60 sec.) by parameter 310.
Extreme running conditions
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