NOTE
Make sure the resistor is designed to handle the required
braking time.
The max. permissible load on the brake resistor is stated as
a peak power at a given intermittent duty cycle and can
be calculated as:
The brake resistance is calculated as shown:
R
br
Ω =
U
dc
2
P
peak
where
P
peak
= P
motor
x M
br
[%] x η
motor
x η
VLT
[W]
As can be seen, the brake resistance depends on the
intermediate circuit voltage (U
dc
).
The FC 301 and FC 302 brake function is settled in 4 areas
of mains.
Size Brake active Warning
before cut
out
Cut out
(trip)
FC 301/FC 302 3
x 200-240 V
390 V (UDC) 405 V 410 V
FC 301 3 x
380-480 V
778 V 810 V 820 V
FC 302 3 x
380-500 V*
810 V/795 V 840 V/828 V 850 V/855 V
FC 302 3 x
525-600 V
943 V 965 V 975 V
FC 302 3 x
525-690 V
1084 V 1109 V 1130 V
Table 3.18 Brake Limits
* Power size dependent
NOTE
Check that the brake resistor can cope with a voltage of
410 V, 820 V, 850 V, 975 V or 1130 V - unless Danfoss
brake resistors are used.
Danfoss recommends the brake resistance R
rec
, i.e. one that
guarantees that the frequency converter is able to brake at
the highest braking torque (M
br(%)
) of 160%. The formula
can be written as:
R
rec
Ω =
U
dc
2
x
100
P
motor
x
M
br
(%)
x
η
VLT
x
η
motor
η
motor
is typically at 0.90
η
VLT
is typically at 0.98
For 200 V, 480 V, 500 V and 600 V frequency converters,
R
rec
at 160% braking torque is written as:
200
V
:
R
rec
=
107780
P
motor
Ω
480
V
:
R
rec
=
375300
P
motor
Ω
1)
480
V
:
R
rec
=
428914
P
motor
Ω
2)
500
V
:
R
rec
=
464923
P
motor
Ω
600
V
:
R
rec
=
630137
P
motor
Ω
690
V
:
R
rec
=
832664
P
motor
Ω
1) For frequency converters ≤ 7.5 kW shaft output
2) For frequency converters 11-75 kW shaft output
NOTE
The resistor brake circuit resistance selected should not be
higher than that recommended by Danfoss. If a brake
resistor with a higher ohmic value is selected, the 160%
braking torque may not be achieved because there is a risk
that the frequency converter cuts out for safety reasons.
NOTE
If a short circuit in the brake transistor occurs, power
dissipation in the brake resistor is only prevented by using
a mains switch or contactor to disconnect the mains for
the frequency converter. (The contactor can be controlled
by the frequency converter).
NOTE
Do not touch the brake resistor as it can get very hot
while/after braking. The brake resistor must be placed in a
secure environment to avoid fire risk
CAUTION
D-F size frequency converters contain more than one brake
chopper. Consequently, use one brake resistor per brake
chopper for those frame sizes.
3.8.4 Brake Resistor Cabling
EMC (twisted cables/shielding)
To reduce the electrical noise from the wires between the
brake resistor and the frequency converter, the wires must
be twisted.
For enhanced EMC performance a metal screen can be
used.
Introduction
VLT
®
AutomationDrive FC 300 Design Guide, 0.25-75 kW
46 MG33BE02 - VLT
®
is a registered Danfoss trademark
3
3
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