Extreme Running Conditions - Danfoss VLT 6000 series Operating Instructions Manual

Extreme running conditions

Short circuit
VLT 6000 HVAC is protected against short circuits
by means of current measurement in each of the
three motor phases. A short circuit between two
output phases will cause an overcurrent in the
inverter. However, each transistor of the inverter
will be turned off individually when the short circuit
current exceeds the permitted value.
After a few microseconds the driver card turns
off the inverter and the frequency converter will
display a fault code, although depending on
impedance and motor frequency.
Earth fault
The inverter cuts out within a few micorseconds in
case of an earth fault on a motor phase, although
depending on impedance and motor frequency.
Switching on the output
Switching on the output between the motor and the
frequency converter is fully permitted. It is not possible
to damage VLT 6000 HVAC in any way by switching on
the output. However, fault messages may appear.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. This occurs in two cases:
1. The load drives the motor (at constant output
frequency from the frequency converter), 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 is too short for the energy to be
dissipated as a loss in the frequency converter,
the motor and the installation.
MG.61.A5.02 - VLT is a registered Danfoss trademark
®
VLT 6000 HVAC Series
The control unit attempts to correct the ramp if possible.
The inverter turns off to protect the transistors
and the intermediate circuit capacitors when a
certain voltage level is reached.
Mains drop-out
During a mains drop-out, VLT 6000 HVAC continues
until the intermediate circuit voltage drops below the
minimum stop level, which is typically 15% below VLT
6000 HVAC's lowest rated supply voltage.
The time before the inverter stops depends on the mains
voltage before the drop-out and on the motor load.
Static overload
When VLT 6000 HVAC is overloaded (the current
limit in parameter 215 Current limit, I
reached), the controls will reduce the output frequency
in an attempt to reduce the load.
If the overload is excessive, a current may
occur that makes the frequency converter cut
out after approx. 1.5 sec.
Operation within the current limit can be limited in time
(0-60 s) in parameter 412 Trip delay overcurrent, I
has been
LIM
.
LIM
149