Extreme Running Conditions - Danfoss VLT 3000 Series Product Manual

Extreme running
conditions

Extreme running conditions

Short circuit
By means of current measurement in each
of the three motor phases the VLT
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).
®
MG.30.A7.02 - VLT is a registered Danfoss trademark
Motor-generated overvoltage
The voltage on the DC link can be increased
® 3000 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
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
loaded (the current limit I
control will reduce the output frequency f
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.
®
frequency
®
frequency converter is over-
is reached) the
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