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28 Planning the electrical installation
Selecting the brake circuit components
1. Calculate the maximum power generated by the motor during braking (P
2. Select a suitable brake chopper and brake resistor combination for the application
from the rating table on page 57. This condition must be met:
P
> P
brmax
max
The P
values in the rating table are specified for the reference braking cycle (10 s
brmax
of braking, 50 s of rest). If the actual duty cycle does not correspond to the reference
cycle, the maximum allowed braking power P
Calculating the maximum braking power
3. Check the resistor selection. The energy generated by the motor during a 400-second
period must not exceed the resistor heat dissipation capacity E
Note: If the E
value is not sufficient, it is possible to use a four-resistor assembly in
R
which two standard resistors are connected in parallel, two in series. The E
the four-resistor assembly is four times the value specified for the standard resistor.
Notes:
•
The resistor assembly must be equipped with a thermal switch (all SAFUR resistors
include a thermal switch). Make sure that the switch is properly isolated (over 2.5 kV)
and shrouded against contact.
•
A custom resistor can be selected within the limits imposed by the brake chopper.
That is, the resistance value of the custom resistor is at least equal to the resistance
value of the resistor given in the rating table on page 57. You can calculate the braking
power capacity of the resistor using this equation:
2
U
DC
P
<
max
R
where:
P
Maximum power generated by the motor during braking
max
U
Voltage over the resistor during braking:
DC
1.35 · 1.2 · 415 V DC when supply voltage is 380...415 V AC
1.35 · 1.2 · 500 V DC when supply voltage is 440...500 V AC
R
Resistor resistance value (ohm)
WARNING! Never use a brake resistor with a resistance value below the
minimum allowed resistance value specified for that particular drive/brake
chopper combination. The chopper and the drive are not able to handle the
overcurrent caused by the low resistance.
must be used instead. See section
br
on page 29.
).
max
.
R
value of
R
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