5
Carlo Gavazzi Ltd.
RSBS
RSBS DS ENG
27/05/20
Mode of operation
The RSBS soft starter series works on a current limit algorithm. When control signal is applied via the On terminal, the
RSBS start the ramp-up sequence. The compressor start current is limited to 45 Arms. Once the compressor reaches full
speed, the RSBS automatically detects this condition and switches on the bypass relay to reduce heat dissipation within
the electrical panel. The RSBS will optimise the motor start time to around 600 ms (See Fig. 1).
If the motor does not reach full speed within a maximum of 1 second, the RSBS will trigger the excess ramp-up time alarm
(5 flashes) to protect the internal relay from switching a too high current that would reduce the relay lifetime. The alarm will
self-recover after 5 minutes.
Current limit algorithm
During the ramp-up sequence, the RSBS will check if the motor is rotating. If the RSBS finds that the motor is in locked
rotor condition, it will trigger the HP function after ~ 250 ms (See Fig. 2). During the HP sequence, the RSBS will gradually
increase the current limit setpoint up to a maximum of 80 Arms. This mode of operation is required when the starting pres-
sures are not balanced or where voltage supply is very weak.
Note: Starts under unbalanced pressure conditions can reduce the lifetime of the RSBS due to the higher current flowing
through the components. Allow enough time between starts to ensure that starting pressures difference is kept to a
minimum.
High pressure (HP) function
Current (AAC)
45
Time (ms)
≤ 600
Current (AAC)
HP func
�o
n
80
~ 250
< 1s
Time (ms)
45
End of ramp
Current (AAC)
HP func
�o
n
80
~ 250
< 1s
Time (ms)
45
Fig. 1
Fig. 2
The RSBS series of soft starters is designed for single phase compressors with a maximum start time of 1 second. Typical
applications include scroll and piston compressors.