ABB ACS880-07, Hardware Manual

Page: 268 / 274

Custom resistor
Resistors other than those available as D151 can be used provided that
• the resistance is not lower than the value given in the ratings table
WARNING!
Never use a brake resistor with a resistance below the value specified for the
particular drive / brake chopper / resistor combination. The drive and the
chopper would not able to handle the overcurrent caused by the low resistance.
• the resistance of the custom resistor does not restrict the braking capacity needed, ie.
P
max
< U
DC
2
/R
where
Maximum power generated by the motor during braking P
max
Voltage over the resistor during braking. UDC equals
1.35 · 1.25 · 415 V DC (when supply voltage is 380 to 415 V AC)
1.35 · 1.25 · 500 V DC (when supply voltage is 440 to 500 V AC) or
1.35 · 1.25 · 690 V DC (when supply voltage is 525 to 690 V AC)
U
DC
Resistor resistance (ohm) R
• the heat dissipation capacity E
R
of the resistor is sufficient for the application (see step
3 above).
Calculating the maximum braking power for a custom duty cycle
These rules must be met during any braking cycle:
1. Braking energy transferred during any ten minute period must be less than or equal to
the energy transferred during the reference braking cycle (1/9 min).
2. The maximum braking power for a custom braking cycle ( P
br
) must not exceed the rated
maximum value P
brmax
.
The rules as equations:
1. n × P
br
× t
br
≤ P
brmax
× 60 s => P
br
≤ ( P
brmax
× 60 s)/(n × t
br
)
2. P
br
≤ P
brmax
Number of braking pulses during a 10-minute period n
Maximum braking power (kW) for a custom braking cycle P
br
Braking time (s) t
br
Maximum braking power for a reference braking cycle (1 minute of braking, 9 minutes of rest) P
brmax
Example 1
The duration of a braking cycle is 30 minutes. The braking time is 15 minutes.
Result: If the braking time exceeds 10 minutes, the braking is considered continuous. The
allowed continuous braking power is 10% of maximum braking power ( P
brmax
).
Example 2
The duration of a braking cycle ( T ) is three minutes. The braking time ( t
br
) is 40 seconds.
1. n × P
br
× t
br
≤ P
brmax
× 60 s => P
br
≤ ( P
brmax
× 60 s) / (4 × 40 s) = 0.375 × P
brmax
2. P
br
≤ P
brmax
<=> 0.375 × P
brmax
≤ P
brmax
OK
Result: The maximum braking power for the custom braking cycle is 37% of the rated value
given for the reference cycle.
268 Resistor braking