Brake Resistors; Sine-Wave Filters; Du/Dt Filters; Common-Mode Filters - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration
VSP = Variable-speed pump (directly connected to the
adjustable frequency drive)
FSP = Fixed-speed pump (the motor could be connected via
contactor, soft starter or star/delta starter)
3 3

3.7.4 Brake Resistors

In applications where the motor is used as a brake, energy
is generated in the motor and sent back into the
adjustable frequency drive. If the energy cannot be
transported back to the motor, it increases the voltage in
the adjustable frequency drive DC line. In applications with
frequent braking and/or high inertia loads, this increase
may lead to an overvoltage trip in the adjustable
frequency drive and, finally, a shutdown. Brake resistors are
used to dissipate the excess energy resulting from the
regenerative braking. The resistor is selected based on its
ohmic value, its power dissipation rate and its physical size.
Danfoss offers a wide variety of different resistors that are
specially designed to Danfoss adjustable frequency drives.
See chapter 3.4.12 Brake Resistor Calculation for
dimensioning of brake resistors. For ordering numbers,
refer to chapter 6.2 Options, Accessories, and Spare Parts.

3.7.5 Sine-wave Filters

When a motor is controlled by an adjustable frequency
drive, resonance noise is heard from the motor. This noise,
which is the result of the motor design, arises every time
an inverter switch in the adjustable frequency drive is
activated. The frequency of the resonance noise thus
corresponds to the switching frequency of the adjustable
frequency drive.
Danfoss supplies 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
motor, which means that current and voltage become
almost sinusoidal. The acoustic motor noise is thus reduced
to a minimum.
The ripple current in the sine-wave filter coils also causes
some noise. Solve the problem by integrating the filter in a
cabinet or similar.
74
®
VLT
and the ripple current ΔI to the
PEAK
Danfoss A/S © 09/2014 All rights reserved.
AQUA Drive FC 202

3.7.6 dU/dt Filters

Danfoss supplies dU/dt filters which are differential-mode,
low-pass filters that reduce motor terminal phase-to-phase
peak voltages and reduce the rise time to a level that
lowers the stress on the insulation at the motor windings.
This is especially an issue with short motor cables.
Compared to sine-wave filters (see chapter 3.7.5 Sine-wave
Filters), the dU/dt filters have a cut off frequency above the
switching frequency.

3.7.7 Common-mode Filters

High-frequency common-mode cores (HF-CM cores) reduce
electromagnetic interference and eliminate bearing
damage by electrical discharge. They are special nanocrys-
talline magnetic cores, which have superior filtering
performance compared to regular ferrite cores. The HF-CM
core acts like a common-mode inductor between phases
and ground.
Installed around the three motor phases (U, V, W), the
common mode filters reduce high-frequency common-
mode currents. As a result, high frequency electromagnetic
interference from the motor cable is reduced.
The number of cores required is dependent on motor
cable length and adjustable frequency drive voltage. Each
kit consists of two cores. Refer to Table 3.19 to determine
the number of cores required.
Cable length 1)
[m (ft)]
50 (164)
100 (330)
150 (500)
300 (1000)
Table 3.19 Number of Cores
1) Where longer cables are required, stack additional HF-CM cores.
Enclosure size
A and B C
T2/T4 T7 T2/T4 T7
2 4 2 2
4 4 2 4
4 6 4 4
4 6 4 4
MG20N622
D
T7
4
4
4
6