2.9.2 Dynamic Braking
Dynamic Brake established by:
•
Resistor brake: A brake IGBT keeps the
overvoltage under a certain threshold by
directing the brake energy from the motor to the
connected brake resistor (
2-10 Brake Function
=
[1] Resistor brake
).
The threshold can be adjusted by
2-14 Brake
voltage reduce
, with 70 V range
•
AC brake: The brake energy is distributed in the
motor by changing the loss conditions in the
motor. The AC brake function cannot be used in
applications with high cycling frequency as this
will overheat the motor (
2-10 Brake Function
=
[2]
AC brake
).
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy current brake
(
2-02 DC Braking Time
≠
0 s ).
2.9.3 Brake Resistor Selection
To handle higher demands by generatoric braking, a brake
resistor is necessary. Using a brake resistor ensures that the
heat is absorbed in the brake resistor and not in the
frequency converter. For more information, see the
Brake
Resistor Design Guide
.
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated on the basis of the cycle time and
braking time. The resistor intermittent duty cycle is an
indication of the duty cycle at which the resistor is active.
shows a typical braking cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle = t
b
/T
t
b
is the braking time in seconds
T = cycle time in seconds
T
ta
tc
tb
to
ta
tc
tb
to
ta
130BA167.10
Load
Time
Speed
Illustration 2.35 Typical Braking Cycle
Power range
0.37–75 kW
1)
380–480 V
Cycle time (s)
120
Braking duty cycle at 100% torque
Continuous
Braking duty cycle at overtorque
(150/160%)
40%
Table 2.13 Braking at High Overload Torque Level
1) For 30-75 kW frequency converters, an external brake resistor is
needed to meet the specification in Table 2.13.
Danfoss offers brake resistors with duty cycle of 10% and
40%. If a 10% duty cycle is applied, the brake resistors are
able to absorb brake power for 10% of the cycle time. The
remaining 90% of the cycle time is used for dissipating
excess heat.
NOTICE
Make sure the resistor is designed to handle the
required braking time.
The maximum permissible load on the brake resistor is
stated as a peak power at a given intermittent duty cycle
and can be calculated as:
Brake Resistance Calculation
Rbr Ω =
Udc,br
2
x 0.83
Ppeak
where
P
peak
= P
motor
x M
br
[%] x η
motor
x η
VLT
[W]
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
3x380–480 V
770 V
800 V
800 V
Product Overview
Design Guide
42
Danfoss A/S © 09/2014 All rights reserved.
MG06B402
2
2
