NOTE
When the holding brake is included in a safety chain:
A frequency converter cannot provide a safe control of a
mechanical brake. A redundancy circuitry for the brake
control must be included in the total installation.
2.7.2 Dynamic Braking
Dynamic Brake established by:
•
Resistor brake: A brake IGBT keep the overvoltage
under a certain threshold by directing the brake
energy from the motor to the connected brake
resistor (
2-10 Brake Function
=[1]).
•
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 since
this will overheat the motor (par.
2-10 Brake
Function
=[2]).
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy current brake
(≠0 s ).
2.7.3 Selection of Brake Resistor
To handle higher demands by generatoric braking, a brake
resistor is necessary. Using a brake resistor ensures that the
energy is absorbed in the brake resistor and not in the
frequency converter. For more information see the
Brake
Resistor Design Guide, MG90O
.
If the amount of kinetic energy transferred to the resistor
in each braking period is not known, the average power
can be calculated based on the cycle time and braking
time also called intermittent duty cycle. The resistor
intermittent duty cycle is an indication of the duty cycle at
which the resistor is active.
shows a typical
braking cycle.
NOTE
Motor suppliers often use S5 when stating the permissible
load which is an expression of intermittent duty cycle.
The intermittent duty cycle for the resistor is calculated as
follows:
Duty cycle=t
b
/T
T=cycle time in s
t
b
is the braking time in s (of the cycle time)
ta
tc
tb
to
ta
tc
tb
to
ta
130BA167.10
Load
Time
Speed
Illustration 2.25 Dynamic Braking Cycle Time
Cycle time [s]
Braking duty cycle at 100%
torque
Braking duty cycle at over torque
(150/160%)
3x380-480 V
PK37-P75K
120
Continuous
40%
P90K-P160
600
Continuous
10%
P200-P800
600
40%
10%
Table 2.7 Braking at High overload torque level
Brake resistors have a duty cycle of 5%, 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 will be used on
dissipating excess heat.
NOTE
Ensure 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:
Rbr Ω =
Udc
2
Ppeak
where
P
peak
=P
motor
x M
br
[%]xη
motor
xη
VLT
[W]
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®
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26
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®
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