In the conguration shown in Figure 11.6, the drive operates in open-loop mode. It receives input from either the LCP (hand-
on mode) or via a remote signal (auto-on mode). The signal (speed reference) is received and conditioned with the
following:
•
Programmed minimum and maximum motor speed limits (in RPM and Hz).
•
Ramp-up and ramp-down times.
•
Motor rotation direction.
The reference is then passed on to control the motor.
Figure 11.6 Block Diagram of an Open-loop Control Structure
11.2.6 Closed-loop Control Structure
In closed-loop mode, the drive uses 1 or more references (local or remote) and feedback sensors to control the motor. The
drive receives a feedback signal from a sensor in the system. It then compares this feedback to a setpoint reference value
and determines if there is any discrepancy between these 2 signals. The drive then adjusts the speed of the motor to
correct the discrepancy.
For example, consider a pump application in which the speed of the pump is controlled so that the static pressure in a pipe
is constant (see Figure 11.7). The drive receives a feedback signal from a sensor in the system. It compares this feedback to a
setpoint reference value and determines the discrepancy if any, between these 2 signals. It then adjusts the speed of the
motor to compensate for the discrepancy.
The static pressure setpoint is the reference signal to the drive. A static pressure sensor measures the actual static pressure
in the pipe and provides this information to the drive as a feedback signal. If the feedback signal exceeds the setpoint
reference, the drive ramps down to reduce the pressure. Similarly, if the pipe pressure is lower than the setpoint reference,
the drive ramps up to increase the pump pressure.
Basic Operating Principles ... Design Guide
MG22B222 Danfoss A/S © 01/2018 All rights reserved. 193
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