Dynamic braking
With the VLT 2800 the dynamic braking quality in an
application can be improved in two ways, either with
the aid of brake resistors or AC braking.
Danfoss offers a complete range of brake resistors for
all VLT 2800 frequency converters.
It is the job of the brake resistor to apply a load to the
intermediate circuit during braking, thereby ensuring
that the brake power can be absorbed by the brake
resistor.
Without a brake resistor, the intermediate circuit volt-
age of the frequency converter would go on rising, until
cutting out for protection. The advantage of using a
brake resistor is that you can brake quickly with large
loads, e.g. on a conveyor belt.
Danfoss has chosen a solution in which the brake re-
sistor is not integrated into the frequency converter.
This gives the user the following advantages:
-
The resistor's cycle time can be selected as
required.
-
The heat generated during braking can be di-
verted outside the panel cabinet, where the
energy can possibly be utilised.
-
No overheating of the electronic compo-
nents, even if the brake resistor is overloa-
ded.
AC braking is an integrated function that is used for
applications in which there is a need for limited dy-
namic braking. The AC braking function makes it pos-
sible to reduce the brake power in the motor instead of
in a brake resistor. The function is intended for appli-
cations where the required braking torque is less than
50% of rated torque. AC braking is selected in par. 400
Brake function.
NB!
The AC brake cannot be used if the re-
quired braking torque is more than 50% of
rated braking torque. In such instances a
brake resistor must be used.
Brake Setup
The figure shows a brake Setup with a frequency con-
verter.
In the following paragraphs, expressions and acro-
nyms are used about brake Setups that can be seen
from the figure.
Calculation of brake resistance
The following example and formula only apply to VLT
2800 Series.
To ensure that the frequency converter does not cut
out for safety reasons when the motor brakes, the re-
sistance value is selected on the basis of the peak
braking effect and the intermediate circuit voltage:
R
br
=
U
DC
²
P
PEAK
Ω
It can be seen that the brake resistance depends on
the intermediate circuit voltage (UDC).
With frequency converters that have a mains voltage
of 3 x 380 - 480 Volt, the brake will be active at 770
Volt (UDC); if the frequency converter has a mains
voltage of 3 x 200 - 240 Volt, the brake will be active
at 385 Volt (UDC).
You can also choose to use the brake resistance rec-
ommended by Danfoss (R
REC
). This is a guarantee
that the frequency converter is able to brake at the
highest braking torque (M
BR
). The recommended
brake resistance can be seen from the ordering table
for brake resistors.
R
REC
calculated as:
R
REC
=
U
DC
²
× 100
P
motor
×
Mbr
(
%
)
× η
motor
× η
inv
Ω
NB!
Remember to check that the brake resist-
ance can manage a voltage of 850 Volt or
430 Volt, if Danfoss brake resistors are not
being used.
•
motor
is typically 0.90 and •
INV
is typically 0.98. For 400
Volt and 200 Volt frequency converters, respectively,
R
REC
at 160% braking torque can be written as:
MG.27.E2.02 - VLT is a registered Danfoss trademark 23
Introduction to VLT 2800
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