Extreme Running Conditions - Danfoss VLT 5000 Series Instruction Manual

Extreme Running Conditions

Short circuit
The adjustable frequency drive is protected against
short circuits by means of current measurement in
each of the three motor phases. A short circuit be-
tween 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 cur-
rent exceeds the permitted value.
After 5-10 s the driver card turns off the inverter and
the adjustable frequency drive will display a fault code,
although depending on impedance and motor frequen-
cy.
Earth fault
The inverter cuts out within a few s in case of an earth
fault on a motor phase, although depending on impe-
dance and motor frequency.
Switching on the output
Switching on the output between the motor and the
adjustable frequency drive is fully permitted. It is not
possible to damage VLT 5000 Series 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 adjustable frequency
drive), ie. the load generates energy.
MG.51.C5.22 - VLT is a registered Danfoss trademark.
®
VLT 5000 Series
2. During deceleration ("ramp-down") if the mo-
ment 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 adjustable fre-
quency drive, the motor and the installation.
The control unit attempts to correct the ramp if possi-
ble.
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, the adjustable frequency
drive continues until the intermediate circuit voltage
drops below the minimum stop level, which is typically
15% below the adjustable frequency drive's lowest rat-
ed 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 the adjustable frequency drive is overloaded
(the torque limit in parameter 221/222 has been
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 adjustable frequency drive cut out after ap-
prox. 1.5 sec.
Operation within the torque limit can be limited in time
(0-60 s) in parameter 409.
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