The brake resistance depends on the intermediate circuit
voltage (U
dc
).
The brake function is settled in four areas of mains.
Size
Brake active
Warning
before cut
out
Cut out (trip)
FCD 302
3x380-480 V
778 V
810 V
820 V
Table 2.8 Brake Limit Values
NOTE
Check that the brake resistor can cope with a voltage of
410 V, 820 V, 850 V, 975 V or 1130 V - unless 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 100
Pmotor x Mbr (%) xηVLT x ηmotor
η
motor
is typically at 0.90
η
VLT
is typically at 0.98
For 200 V and 480 V frequency converters, R
rec
at 160%
braking torque is written as:
200
V : Rrec =
107780
Pmotor
Ω
480
V : Rrec =
375300
Pmotor
Ω 1)
480
V : Rrec =
428914
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.
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
D-F size frequency converters contain more than one brake
chopper. Consequently, use one brake resistor per brake
chopper for those frame sizes.
2.7.4 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 makes it possible to read out the
momentary power and the mean power for the latest 120
seconds. 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 to avoid unnecessary tripping of the
frequency converter, for example when the ramp-down
time is too short. In this situation the ramp-down time is
extended.
OVC can not be activated when running a PM motor
(when
1-10 Motor Construction
is set to
[1] PM non salient
SPM
).
Product Overview
VLT
®
Decentral Drive FCD 302
MG04H102 - VLT
®
is a registered Danfoss trademark
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