69/126
Siemens Building Technologies
SED2 variable speed drives
CM1U5192en
HVAC Products
Programming
01.2002
If MV runs at maximum frequency and the PID feedback shows that a higher speed is
demanded in accordance with the staging, the VSD switches on one of the relay-
controlled motors M1 or M2 (staging). To keep the controlled variable as constant as
possible, and to compensate for the difference in output, the VSD must be decreased
to minimum frequency. See the illustration below. During the staging process, PID
control is suppressed.
If MV runs in parallel to M1 and M2 at a minimum frequency, and if the PID feedback
demands an even lower speed, the VSD switches off one of the relay-controlled motors
M1 or M2 (destaging). In this case, the VSD must increase the ramp from the minimum
to the maximum frequency. In this phase, PID control is suppressed.
Motor 1
Motor 1
Motor 1
Motor 2
Motor 3
Motor 2
kW
Demand or volume flow
51
92
D
02
en
100%
Total power
Motor MV, controlled by SED2
Motor M1, switched by relay 1
Motor M1, switched by relay 2
100%
100%
Power
Power
Power
Switching sequence MV-M1
Switching sequence MV-M1-M2
t
t
t
t
51
92
D
05
en
Motor staging on output
demand
Diagram from motor
staging
