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:
Rbr Ω =
Udc,br
2
x 0.83
Ppeak
where
P
peak
= P
motor
x M
br
[%] x η
motor
x η
VLT
[W]
Table 2.15 Brake Resistance Calculation
As can be seen, the brake resistance depends on the
intermediate circuit voltage (U
dc
).
Size
Brake active
U
dc,br
Warning
before cut
out
Cut out
(trip)
FC 360
3 x 380-480 V
770 V
800 V
800 V
Table 2.16 Areas of Mains
NOTE
Check that the brake resistor can cope with a voltage of
410 V or 820 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:
Rrec Ω =
Udc
2 x 100x 0.83
Pmotor x Mbr (%) xηVLT x ηmotor
η
motor
is typically at 0.80 (
≤
75. kW); 0.85 (11-22 kW)
η
VLT
is typically at 0.97
For FC 360 R
rec
at 160% braking torque is written as:
480
V : Rrec =
396349
Pmotor
Ω 1)
480
V : Rrec =
397903
Pmotor
Ω 2)
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.
The resistance should be bigger than R
min
.
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.
2.9.4 Control with Brake Function
The brake is protected against short-circuiting of the brake
resistor, and the brake transistor is monitored to ensure
that short-circuiting of the transistor is detected. A relay/
digital output can be used for protecting the brake resistor
against overloading in connection with a fault in the
frequency converter.
In addition, the brake enables readout of the momentary
power and the mean power for the latest 120 s. The brake
can also monitor the power energizing and make sure it
does not exceed a limit selected in
2-12 Brake Power Limit
(kW)
. In
2-13 Brake Power Monitoring
, select the function to
carry out when the power transmitted to the brake resistor
exceeds the limit set in
2-12 Brake Power Limit (kW)
.
NOTE
Monitoring the brake power is not a safety function; a
thermal switch is required for that purpose. The brake
resistor circuit is not earth leakage protected.
Over voltage control (OVC)
(exclusive brake resistor) can be
selected as an alternative brake function in
2-17 Over-
voltage Control
. This function is active for all units. The
function ensures that a trip can be avoided if the DC link
voltage increases. This is done by increasing the output
frequency to limit the voltage from the DC link. It is a very
useful function, e.g. if the ramp-down time is too short
since tripping of the frequency converter is avoided. In this
situation the ramp-down time is extended.
Product Overview
VLT
®
AutomationDrive FC 360 Design Guide
38
MG06B202 - VLT
®
is a registered Danfoss trademark
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2