and ramp-down times, and the direction of motor rotation.The reference is then passed on to control the motor.130BB153.10100%0%-100%100%P 3-13ReferencesiteLocalreferencescaled toRPM or HzAuto modeHand modeLCP Hand on,off and autoon keysLinked to hand/autoLocalRemoteReference RampP 4-10Motor speeddirectionTo motorcontrolReferencehandlingRemotereferenceP 4-13Motor speedhigh limit [RPM]P 4-14Motor speedhigh limit [Hz]P 4-11Motor speedlow limit [RPM]P 4-12Motor speedlow limit [Hz]P 3-4* Ramp 1P 3-5* Ramp 2Illustration 2.11 Open-loop Mode Block Diagram2.4.2 Control Structure Closed LoopIn closed-loop mode, an internal PID controller allows thefrequency converter to process system reference andfeedback signals to act as an independent control unit. Theconverter can provide status and alarm messages, alongwith many other programmable options, for externalsystem monitoring while operating independently inclosed loop.P 20-81PID Normal/InverseControlPIDRef.HandlingFeedbackHandlingScale tospeedP 4-10Motor speeddirectionTo motorcontrol(Illustra-tion)(Illustra-tion)130BA359.12100%0%-100%100%*[-1]_+Illustration 2.12 Block Diagram of Closed-loop ControllerFor example, consider a pump application in which thespeed of a pump is controlled so that the static pressure ina pipe is constant (see Illustration 2.12). The frequencyconverter receives a feedback signal from a sensor in thesystem. It compares this feedback to a setpoint referencevalue and determines the error, if any, between these 2signals. It then adjusts the speed of the motor to correctthis error.The desired static pressure setpoint is the reference signalto the frequency converter. A static pressure sensormeasures the actual static pressure in the pipe andprovides this information to the frequency converter as afeedback signal. If the feedback signal is greater than thesetpoint reference, the frequency converter slows toreduce the pressure. In a similar way, if the pipe pressure islower than the setpoint reference, the frequency converterspeeds up to increase the pressure provided by the pump.While the default values for the frequency converter inclosed loop often provide satisfactory performance, systemcontrol can often be optimised by tuning the PIDparameters. Auto tuning is provided for this optimisation.Product Overview Design GuideMG20N602 Danfoss A/S © 09/2014 All rights reserved. 212 2
