Du/Dt On Motor; Switching On The Input; Acoustic Noise - Danfoss VT2800 Series Design Manual

Earth fault
The IGBT module cuts out within 100 µs in case
of an earth 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.
There is no way that the frequency converter can 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
two instances:
1. The load dri v es the motor, 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 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 entitled Brake resistors.
Static overloading
When the frequency converter is overloaded (current
limit in parameter 221 Current limit I
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 sec. See
parameter 409 Trip delay overcurrent, I
An extreme overload will cause the switching
frequency to be derated to 3000 Hz.

dU/dt on motor

When a transistor in the inverter is opened, the
voltage across the motor terminals will rise by a
voltage/time ratio (dU/dt) determined by:
- the motor cable (type, cross-section, induction,
capacity, length and screened/armoured/un-
screened/unarmoured)
132
- the mains voltage
Self-induction in the motor cable leads to an overswing
U
in the inverter is opened. After U
voltage will stabilise at a level determined by the
voltage in the intermediate circuit. U
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
cable is long (>20 m), U
decreases. When using small motors without phase
insulation paper, it is recommended that an LC-filter
is connected after the frequency converter.

Switching on the input

The waiting time between switching the mains
voltage on terminals 91, 92 and 93 must be min.
30 sec. Start up time appr. 2.3 sec.

Acoustic noise

The acoustic noise from the frequency converter
comes from two sources:
1. DC intermediate circuit coils.
2. Integral fan.
Below are the typical values measured at a distance
of 1 m from the unit at full load:
VLT 2803-2815 1 x 220 V: 52 dB(A).
VLT 2803-2822 3 x 220 V: 52 dB(A).
VLT 2805-2840 3 x 400 V: 52 dB(A).
VLT 2855-2875 3 x 400 V: 54 dB(A).
VLT 2880-2882 3 x 400 V: 55 dB(A).
Derating for ambient temperature
is reached), the
LIM
The ambient temperature (T
temperature allowed. The average (T
measured over 24 hours, must be at least 5
°C lower. If the frequency converter operates
at temperatures above 45 °C, a derating of the
.
LIM
rated output current is necessary.
®
VLT 2800 Series
of the output voltage each time a transistor
PEAK
is low, while the dU/dt is high. If the motor
PEAK
AMB,MAX
MG.28.E9.02 - VLT is a registered Danfoss trademark
the output
PEAK
and dU/dt
PEAK
is increased, while dU/dt
) is the maximum
)
AMB,AVG