Up to eight preset references can be programmed in theadjustable frequency drive. The active preset reference canbe selected using digital inputs or the serial communi-cations bus. The reference can also be supplied externally,most commonly from an analog input. This external sourceis selected by one of the three reference sourceparameters (3-15 Reference 1 Source, 3-16 Reference 2 Sourceand 3-17 Reference 3 Source).Digipot is a digital potentiometer. This is also commonlycalled a Speed up/Slow control or a floating point control.To set it up, program one digital input to increase thereference while another digital input is programmed todecrease the reference. A third digital input can be used toreset the digipot reference. All reference resources and thebus reference are added to produce the total externalreference. The external reference, the preset reference orthe sum of the two can be selected to be the activereference. Finally, this reference can be scaled by using3-14 Preset Relative Reference.The scaled reference is calculated as follows:Reference =X +X ×Y100Where X is the external reference, the preset reference orthe sum of these and Y is 3-14 Preset Relative Reference in[%].If Y, 3-14 Preset Relative Reference is set to 0%, thereference will not be affected by the scaling.2.8.9 Example of Closed-loop PID ControlFigure 2.25 Closed-loop Control for a Ventilation SystemIn a ventilation system, maintain the temperature at aconstant value. The desired temperature is set between23°F and 95°F [-5–+35°C] using a 0–10 volt potentiometer.Because this is a cooling application, if the temperature isabove the setpoint value, increase the speed of the fan toprovide more cooling air flow. The temperature sensor hasa range of 14–104°F [-10–+40°C] and uses a two-wiretransmitter to provide a 4–20 mA signal. The outputfrequency range of the adjustable frequency drive is 10 to50 Hz.1. Start/Stop via the switch connected betweenterminals 12 (+24 V) and 18.2. Temperature reference via a potentiometer (23–95°F [-5–+35 °C], 0–10 V) connected to thefollowing terminals:• 50 (+10 V)• 53 (input)• 55 (common)3. Temperature feedback via transmitter (14–104°F[-10–40 °C], 4–20 mA) connected to terminal 54.Switch S202 behind the LCP set to ON (currentinput).2.8.10 Programming OrderNOTICE!This example assumes an induction motor is used, so1-10 Motor Construction = [0] Asynchron.Introduction Design GuideMG16C122 Danfoss A/S © Rev. 2014-01-13 All rights reserved. 412 2
