Brake Resistor Calculation; Control With Brake Function - Danfoss VLT HVAC Drive FC 102 Design Manual

Introduction
cycle indicates the duty cycle when the resistor is active.
Illustration 2.32 shows a typical braking cycle.
2 2
The intermittent duty cycle for the resistor is calculated as:
Duty Cycle = t
T = cycle time in seconds
t = braking time in seconds (as part of the total cycle
b
time)
Load
Speed
Illustration 2.32 Typical Braking Cycle
Danfoss offers brake resistors with duty cycle of 10% and
40% suitable for use with the VLT
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

2
U dc
Rbr Ω =
Ppeak
P = P
peak motor
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
3 x 380-480 V
3 x 525-690 V
Table 2.14 Effect of Brake Function on Mains Power Supply
NOTICE
If not using Danfoss brake resistors, check that the brake
resistor can cope with a voltage of 820 V or 1130 V.
46
/T
b
ta tc tb to ta tc tb
T
®
HVAC Drive frequency
x M x η x η[W]
br motor
Brake active Warning
before cut
out
778 V 810 V
1084 V 1109 V
Danfoss A/S © Rev. 05/2014 All rights reserved.
Design Guide
Danfoss recommends the brake resistance R
guarantees that the frequency converter is able to brake at
the highest braking torque (M
can be written as:
R
Ω =
rec
P
η is typically at 0.90
motor
η is typically at 0.98
For 480 V and 600 V frequency converters, R
braking torque is written as:
690 V : Rrec =
NOTICE
For resistor selection information, refer to the Brake
Resistor Design Guide
to ta
NOTICE
The resistor brake circuit resistance selected should not
Time
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 the
frequency converter may cut 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/
Cut out (trip)
digital output can be used for protecting the brake resistor
against overloading in connection with a fault in the
frequency converter.
820 V
In addition, the brake makes it possible to read out the
1130 V
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).
) of 110%. The formula
br(%)
2
U dc x 100
x Mbr x x
motor
motor %
832664
Ω
Pmotor
, which
rec
at 160%
rec
MG16C202