Cascade Control - Siemens SINAMICS G120 CU230P-2 HVAC Operating Instructions Manual

7.9.12

Cascade control

The cascade control function is used in applications that require between one and four
motors to be run at the same time depending on load, so that e.g. highly variable pressure
ratios or flow volumes can be corrected.
Cascade control consists of the speed-controlled main drive and up to three other drives that
are switched-on or switched-off via contactors or motor starters, either in a fixed
arrangement or dependent on the operating hours.
The PID deviation serves as the input signal for activating the other motors. The contactors
or motor starters are switched by the inverter's digital outputs.
Note
Technology controller as main setpoint
For cascade control, the main setpoint must be entered via the technology controller (p2251
= 0, p2200 = 1).
Operating principle
● Switching-in external motors
● Shutting down external motors
Frequency inverters with Control Units CU230P-2 HVAC, CU230P-2 DP, CU230P-2 CAN
Operating Instructions, 01/2011, FW 4.4, A5E02430659B AD
If the main drive is run at maximum speed and the deviation on the technology controller
input continues to increase, the control also switches the external motors on the line. At
the same time, the main drive is ramped down to the switch-on/switch-off speed (p2378)
to keep the total output power as constant as possible. The technology controller is
deactivated while ramping down to the switch-on/switch-off speed.
If the main drive is running at minimum speed and the deviation on the technology
controller input continues to decrease, the control switches external motors M1 to M3 off
the line. The main drive is simultaneously ramped-up to the switch-on/switch-off speed to
keep the total output power as constant as possible.
7.9 Application-specific functions
Functions
261