Extreme Running Conditions - Danfoss VLT 6000 HVAC 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 5-10 ms 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 100 ms 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 VLT frequency con-
verter, the motor and the installation.
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VLT 6000 HVAC
Leakage currents to earth
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 volt-
age 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 torque 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 VLT frequency converter cut out after
approx. 1.5 sec.
Operation within the torque limit can be limited in time
(0-60 s) in parameter 412 Trip delay overcurrent, I
MG.60.B1.02 - VLT is a registered Danfoss trade mark
has been
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