Hardware and Engineering DF6-340-... Frequency ... - Moeller.com.tr

01/02 AWB8230-1413GB PID controlThe example in the following figure shows a fan control system:wG1Figure 116: Example of a fan control systemG1:<strong>DF6</strong> frequency inverterw: Setpointx: Actual valueP1: Controlled variableB1: Measured value convertera FanP: Proportional <strong>com</strong>ponentThis <strong>com</strong>ponent ensures that the output frequency <strong>and</strong> the systemdeviation are proportional to each other. Using PNU A072, youcan define the proportional gain K p in %.The following figure illustrates the relationship between systemdeviation <strong>and</strong> output frequency. A large value of K p results in aquick response to a change of the system deviation. If, however,K p is too large, the system be<strong>com</strong>es unstable.f[%]100755025M3~xB10... +10 V H ; 4 – 20 mAKp = 2aP1Kp = 1Kp = 0.75Kp = 0.5Kp = 0.250.2 F Kp F 0.5The maximum output frequency in figure 117 is defined as 100 %.With PNU A072, you can set K p between 0.2 <strong>and</strong> 5.0.I: Integral <strong>com</strong>ponentThis <strong>com</strong>ponent results in a correction of the output frequency byintegration of the system deviation. With purely proportionalcontrol, a large control deviation results in an equally large changein the output frequency; If, therefore, the control deviation is verysmall, the change in the output frequency is also small. Theproblem is that the system deviation cannot be <strong>com</strong>pletelyeliminated. Hence the need for an integral <strong>com</strong>ponent.The integral <strong>com</strong>ponent causes a continuous adding up of thesystem deviation so that the deviation can be reduced to zero. Thereciprocal value of the integration gain is the integration timeT i =1/K i .For the <strong>DF6</strong> frequency inverters, set the integration time (T i ). Thevalue may be between 0.5 <strong>and</strong> 3600 s. To disable the integral<strong>com</strong>ponent, enter 0.0.D: Differential <strong>com</strong>ponentThis <strong>com</strong>ponent causes a differentiation of the system deviation.Because pure proportional control uses the current value of thesystem deviation <strong>and</strong> pure integral control the values fromprevious actions, a certain delay in the control process alwaysoccurs. The D <strong>com</strong>ponent <strong>com</strong>pensates for this behaviour.Differential control corrects the output frequency using the rate ofchange of the system deviation. The output frequency cantherefore be <strong>com</strong>pensated very quickly.You can set K d between 0 <strong>and</strong> 100 s.PID controlPID control <strong>com</strong>bines the P, I <strong>and</strong> D <strong>com</strong>ponents described in theprevious sections. In order to achieve the optimum controlcharacteristics, each of the three PID parameters must be set.Uniform control behaviour without large steps in the outputfrequency is guaranteed by the proportional <strong>com</strong>ponent; theintegral <strong>com</strong>ponent minimizes the existing system deviation thesteady-state <strong>and</strong> the differential <strong>com</strong>ponent ensures a quickresponse to a rapidly changing actual value signal.As differential control is based on the differentiation of the systemdeviation, it is very sensitive <strong>and</strong> also responds to unwantedsignals – such as interference – which can result in systeminstability. Differential control is normally not required for flow,pressure <strong>and</strong> temperature control.02550 75100x [%]Figure 117: Proportional gain K px: System deviation117