7.3.12 Brake Resistors
Brake resistors are used to dissipate the excess energy
from the regenerative braking. The resistor is selected in
respect to its ohmic value, its power dissipation rate, and
its physical size. Danfoss offers a wide variety of different
resistors that are specially designed to our frequency
converters. For more information, see
chapter 13.2.1 Selection of Brake Resistor
. Also, see the
VLT
®
Brake Resistor MCE 101 Design Guide
.
7.3.13 Sine-wave Filters
When a frequency converter controls a motor, resonance
noise is heard from the motor. This noise, which results
from the motor design, occurs every time an inverter
switch in the frequency converter is activated. The
frequency of the resonance noise thus corresponds to the
switching frequency of the frequency converter.
For the frequency converter, Danfoss can supply a sine-
wave filter to dampen the acoustic motor noise. The filter
reduces the ramp-up time of the voltage, the peak load
voltage U
PEAK
, and the ripple current
Δ
I to the motor. The
filter results in the current and voltage becoming almost
sinusoidal, which reduces the acoustic motor noise.
The ripple current in the sine-wave filter coils also causes
some noise. This problem can be solved integrating the
filter in a cabinet or similar enclosure.
For specific sine-wave filter part numbers, see
7.3.14 dU/dt Filters
The combination of rapid voltage and an increase in
current stresses the motor insulation. These rapid energy
fluctuations can be reflected back to the DC-line in the
inverter, which can cause a shutdown. The dU/dt filter is
designed to reduce the voltage rise time and the rapid
energy change in the motor. This intervention avoids
premature aging and flashover in the motor insulation.
The dU/dt filters have a positive influence on the radiation
of magnetic noise in the cable that connects the frequency
converter to the motor. The voltage wave form is still pulse
shaped, but the dU/dt ratio is reduced in comparison to an
installation without a filter.
Ordering Information
Design Guide
MG37N102
Danfoss A/S © 6/2016 All rights reserved.
79
7
7
Summary of Contents for VLT 380-500 V
Page 2: ......
