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24 Planning the 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 using
the data given under
P
P
brmax
max
The P
values in the rating table are specified for the reference braking cycle (10
brmax
seconds of braking, 50 seconds of rest). If the actual duty cycle does not correspond
to the reference cycle, the maximum allowed braking power P
See section
Calculating the maximum braking
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 which
R
two standard resistors are connected in parallel, two in series. The E
four-resistor assembly is four times the value specified for the standard resistor.
Note:
•
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 ratings table. You can calculate the braking power capacity
of the resistor using this equation:
U
2
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 · 1.25 · 690 V DC when supply voltage is 525...690 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 or brake chopper combination.
The chopper and the drive are not able to handle the overcurrent by the low
resistance.
￭
Calculating the maximum braking power
The P
value for each standard brake chopper and brake resistor combination is given
brmax
in the rating table. The rated value is specified for a reference braking cycle (10 s of braking,
Ratings (page
73). This condition must be met:
must be used instead.
br
power.
.
R
).
max
value of the
R
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