5.1.3 Extreme Operating Conditions
Short circuit
The frequency converter is protected against short circuits
on motor terminals U, V, W (96, 97, 98). A short circuit-
between 2 motor terminals would lead to an overcurrent
in the IGBT module, which means that all transistors in the
IGBT module would independently cut out.
The inverter turns off after 5-10 s and the frequency
converter displays a fault code, although this depends on
impedance and motor frequency.
Ground fault
The IGBT module cuts out within 100 s if there is a ground
fault on one of the motor terminals U, V, W (96, 97, 98),
although depending on impedance and motor frequency.
Output connection
Motor terminals U, V, W (96, 97, 98) for the motor can be
connected/disconnected as often as required. The
frequency converter cannot be destroyed by connecting/
disconnecting the motor terminals. However, fault
messages may appear.
Motor-generated overvoltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator. To protect the frequency
converter, the IGBT module is disconnected when a
specific voltage level is reached.
Motor-generated overvoltage can occur in 2 instances:
1.
The load drives the motor, that is, the load
generates energy.
2.
During deceleration (ramp-down) if the inertia
moment is high, the load is low and the ramp-
down time too short for the energy to be
dissipated as a loss in the frequency converter,
the motor and the unit. The control unit attempts
to correct the ramp if possible.
The fault can be removed by connecting a brake resistor, if
the frequency converter has an integrated brake module. If
the frequency converter does not have an integrated brake
module, an AC brake can be used, see
parameter 400 Brake
function
.
See the section
for further
details.
Static overloading
hen the frequency converter is overloaded (current limit in
parameter 221 Current limit I
LIM
is reached), the control
reduces the output frequency in an attempt to reduce the
load. If the overloading is extreme, there might be an
output current that causes the frequency converter to trip
after approx. 1.5 s. See
parameter 409 Trip delay overcurrent,
I
LIM
.
An extreme overload may cause the switching frequency
to be derated to 3000 Hz.
5.1.4 dU/dt on Motor
When a transistor in the inverter is opened, the voltage
across the motor terminals rises by a voltage/time ratio
(dU/dt) determined by:
•
the motor cable (type, cross-section, induction,
capacity, length and screened/armoured/
unscreened/unarmoured).
•
the mains voltage.
Self-induction in the motor cable leads to an overswing
U
PEAK
of the output voltage each time a transistor in the
inverter is opened. After U
PEAK
, the output voltage
stabilises at a level determined by the voltage in the
intermediate circuit. U
PEAK
and dU/dt influence the lifetime
of the motor, especially motors without phase insulation
paper in the coils. If the motor cable is short (a few
metres), the overshoot U
PEAK
is low, while the dU/dt is
high. If the motor cable is long (>20 m), U
PEAK
is increased
to about twice the intermediate voltage, while dU/dt
decreases. When using small motors without phase
insulation paper or submerged water pumps, connect an
LC-filter after the frequency converter.
5.1.5 Switching on the Input
The waiting time between switching the mains voltage on
terminals 91, 92 and 93 must be minimum 30 s. Start up
time appr. 2.3 s.
5.1.6 Peak Voltage on Motor
When a transistor in the inverter is opened, the voltage
across the motor increases by a dU/dt ratio that depends
on:
•
the motor cable (type, cross-section, length
screened or unscreened).
•
inductance.
The natural induction causes an overshot U
PEAK
in the
motor voltage before it stabilises itself at a level which
depends on the voltage in the intermediate circuit. The
rise time and the peak voltage U
PEAK
affect the service life
of the motor. If the peak voltage is too high, motors
without phase coil insulation are primarily affected. If the
motor cable is short (a few metres), the rise time is higher
and the peak voltage is lower.
If the motor cable is long (100 m), the rise time decreases
and the peak voltage increases to about twice the
intermediate voltage.
If very small motors without phase insulation, paper or
submerged water pumps are used, fit an LC filter after the
frequency converter.
All about VLT 2800
Design Guide
128
Danfoss A/S © Rev. May/2014 All rights reserved.
MG27E402
5
5
