Specifications

T
ta
tc
tb
to ta
tc
tb
to ta
130BA167.10
Load
Time
Speed
Illustration 2.33 Typical Braking Cycle
Danfoss offers brake resistors with duty cycle of 10% and
40% suitable for use with the VLT
®
HVAC Drive frequency
converter series. If a 10% duty cycle resistor is applied, this
is capable of absorbing braking power up to 10% of the
cycle time with the remaining 90% being used to dissipate
heat from the resistor.
2.12.2 Brake Resistor Calculation
R
br
Ω =
U
dc
2
P
peak
P
peak
= P
motor
x M
br
x η
motor
x η[W]
The brake resistance depends on the intermediate circuit
voltage (U
DC
).
The brake function of the frequency converter is settled in
3 areas of mains power supply:
Size Brake active Warning
before cut
out
Cut out (trip)
3 x 380-480 V 778 V 810 V 820 V
3 x 525-690 V 1084 V 1109 V 1130 V
Table 2.19 Effect of Brake Function on Mains Power Supply
NOTICE
Check that the brake resistor can cope with a voltage of
820 V or 1130 V - unless Danfoss brake resistors are
used.
Danfoss recommends the brake resistance R
rec
, which
guarantees that the frequency converter is able to brake at
the highest braking torque (M
br(%)
) of 110%. The formula
can be written as:
R
rec
Ω =
U
dc
2
x 100
P
motor
x M
br
(%)
x x
motor
η
motor
is typically at 0.90
η is typically at 0.98
For 480 V and 600 V frequency converters, R
rec
at 160%
braking torque is written as:
690 V : R
rec
=
832664
P
motor
Ω
NOTICE
For resistor selection information, refer to
the Brake Resistor Design Guide
NOTICE
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
braking torque may not be achieved because there is a
risk that the frequency converter cuts out for safety
reasons.
NOTICE
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).
WARNING
Do not touch the brake resistor as it can get
very hot during braking.
2.12.3 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
B-12 Brake Power Limit (kW). In B-13 Braking Thermal
Overload, select the function to carry out when the power
transmitted to the brake resistor exceeds the limit set in
B-12 Brake Power Limit (kW).
NOTICE
Monitoring the brake power is not a safety function. A
thermal switch is required. The brake resistor circuit is
not earth leakage protected.
Over voltage control (OVC), for units without a brake
resistor, can be selected as an alternative brake function in
B-17 Over-voltage Control. This function is active for all
units. The function ensures that a trip can be avoided if
Introduction VLT® HVAC Drive FC 102 Design Guide
46 MG16C102 - Rev. 2013-08-20
2
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