Specifications

T
ta
tc
tb
to ta
tc
tb
to ta
130BA167.10
Load
Time
Speed
Illustration 2.39 Intermittent Duty Cycle for the Resistor
Danfoss offers brake resistors with duty cycle of 5%, 10%
and 40% suitable for use with the VLT
®
HVAC Drive
frequency converter series. If a 10% duty cycle resistor is
applied, this is able of absorbing braking power upto 10%
of the cycle time with the remaining 90% being used to
dissipate heat from the resistor.
For further selection advice, contact Danfoss.
2.12.2
Brake Resistor Calculation
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[W]
Table 2.24 Brake Resistor Calculation
As can be seen, 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 [V] Brake active
[V]
Warning
before cut
out [V]
Cut out (trip)
[V]
3x200-240 390 (U
DC
) 405 410
3x380-480 778 810 820
3x525-600 943 965 975
3x525-690 1084 1109 1130
Table 2.25 Brake Function Settled in 3 Areas of Mains Supply
NOTICE
Check that the brake resistor can cope with a voltage of
410 V, 820 V or 975 V - unless Danfoss brake resistors
are used.
Danfoss recommends the brake resistance R
rec
, i.e. one that
guarantees that the 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 200 V, 480 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
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 > 7.5 kW shaft output
NOTICE
The brake resistor 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
while/after 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 enables reading out the momentary
power and the mean power for the latest 120 s. The brake
can also monitor the power energising and ensure that it
does not exceed the limit selected in 2-12 Brake Power
Limit (kW). In 2-13 Brake Power Monitoring, select the
Introduction to VLT® HVAC D...
Design Guide
48 Danfoss A/S © Rev. 06/2014 All rights reserved. MG11BC02
22