PLC Motor Control Fundamentals and SafetyFeatures; A Case Study

¹Computer Science Department, Texas Southern University, Houston, TX.
²PLC Automation, Louisville, KY.

*Corresponding author: Khaled Kamel, Computer Science Department, Texas Southern University, Houston, TX, USA; E-mail:
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Received: October 20, 2018; Accepted: November 1, 2018; Published: November 26, 2018

Citation: Khaled K, Eman K (2018) PLC Motor Control Fundamentals and Safety Features; A Case Study. J Comp Sci Appl Inform Technol. 3(3): 1-8. DOI: 10.15226/2474-9257/3/3/00138

Abstract

Electric motors are the most used final control elements / actuators in process control and automation. This include DC, Synchronous, and ACinduction motors. Squirrel cage three phase induction motors are the most popular in industry as they only require one AC source, self-started,and exhibit a high degree of ruggedness. The motor speed is typically regulated using a PLC microprocessor based variable speed / frequencydrives, which greatly reduce energy consumption when adequately utilized in the control. All industrial motor starters must at least contain fuses,disconnect switch, and thermal overload protective relays. They can also include protection against over load / torque and other failures or upnormal conditions, which might damage the motor. PLC software must account for additional protections and adheres to all safety standards.This paper uses a case study extracted from an actual process control implemented project to highlight PLC motor control safety and fundamentals.Real time control used in a waste water treatment facility for the processing of high flow storm rain water using the Allen Bradley (AB) SLC 500Programmable Logic Controller (PLC). The rain water is channeled to two large wet wells, the east wet well and the west wet well. The water ispumped to the river from the two connected wells at constant rate using a predefined process sequence control. Two motor derived constant speedimmersed pumps are used, one in the east wet well and one in the west wet well. Each pump is equipped with an overload alarm switch, which isused to trigger any unusual conditions such as over temperature or over load. The motors provide an input discrete signal indicating if the motor isrunning or not. The motors can also start by activating the Push Button located on the local panel if the AUTO/MAN switch is in Manual.Three float switches are used to provide an accurate indication of the water level at three pre- specified critical east / west wet well. TheLow-Level Float switch triggers the stopping of the running pump. The High-level Float switch triggers the starting of the scheduled pump. If thescheduled pump fails to start within 5 seconds, the second pump is selected and started. An alarm must be issued to alert the operator of any motorfailure. The Very High-level float switch triggers the starting of both pumps. If either of the two pumps fails to start the corresponding alarm isactivated by the control. Failure of both pumps shuts down the system.Pumps are scheduled to run according to pre-defined calendar. This input is expected in hours of accumulated total pump run time. The twopumps must alternate while the water level is below the Very High Level and above the Low Level. The two pumps run at levels above the VeryHigh Level and cascaded timers are not altered during this condition. This paper shows the design and implementation of the process control usingan abbreviated version of the typical original system used in waste water facilities with secondary treatment and separate rain water processing.

Wet well water pumps are driven by a sophisticated inductionmotor. They are designed to safely operate in and aroundwater. Their secure sealing, rugged construction, and mountedprotective safety monitoring instrumentation provide pumpequipment long life and safe operation. In wet pit applications,submersible pump relies on the liquid in which it is submergedto dissipate the heat from the frame. If pump runs on dry or lowlevel well, the shaft and impeller spin at extremely fast rates. Withno water to transfer their rotational energy to, that energy isreleased as heat instead. Its moving parts will become extremelyhot, causing severe damage to the pump over time and greatlylimiting its service life. Also, if the pump pressure becomes toohigh, it can put excess stress on the sealed casing and pipes,potentially causing them to crack or even burst. Float switchesused to monitor wet well level can fail or get stuck on causingpump to run dry, which will burn out the pump prematurely.All potential failures are prevented by utilizing mounted pump/ motor safety features and the. redundant PLC implementedprecautions discussed next.
The first step in the design of a PLC control application is thetranslation of the process specification to actual input / outputresources. This is known as the PLC Input / Output (I/O) map.This important step lists all I/O tags, assigned PLC addresses, anddescription. Figure 1 lists the wet wells pump control systemdiscrete inputs and the corresponding PLC input tags. Figure 2shows the same process for the PLC symbol editor screen. Figure3 and Figure 4 repeat the same process for the control systemdiscrete outputs. Notice that none of the real analog inputs /outputs for this control process is listed. We only limited our casestudy to ON / OFF control based on water level simulated analogreal time measurements relative to user defined set point for thewet wells.

The PLC processor supports subroutines that allow an efficientprogram structure. File 2 (main file) define the structure of theprogram. Subroutine (SUB3) contains the code that correspondsFigure 4: Pump Station PLC System Outputs Tagsto specific tasks or combinations of parameters. Each subroutinef provides a set of input and Output parameters for sharing datawith the calling file. Figure 5 shows the subroutines designedand implemented for the wet well Pumping station control in theProject View. Figure 6 shows the same in the PLC Ladder View.

Figure 5: Pump Station System PLC Subroutines (Project View)

Figure 6: Pump Station PLC Subroutines (Ladder View)

C. Pumping Station Ladder Implementation

The Initialization Block “INITIATE” is shown in Figure 7. Aone-shot cause this rung to execute once when selector switchAUTO/MANUAL is in AUTO.

The Pump Alarm Subroutine will include, eight Rungs, (Figure8 through 10). One common alarm is dedicated for the east wetwell and the other for the West wet well. A common Alarm istriggered from east pump motor fail to start, west pump motorfail to start, or emergency shutdown.

This paper offers readers an insight to PLC programmingwith focus on real industrial process automation applicationsand immersible motors / pumps safety control features. RockwellAllen Bradley SLC-500 PLC hardware configuration and the ABRSLogix 500 software were used for this implementation andfinal commissioning. LogixPro 500 simulation software was alsoused initially to implement and test the required process controlbefore any need for the real PLC hardware and physical fieldinstrumentation. The case study selected for this paper can serveas a cap stone project encompassing most of the concepts coveredin PLC process control and industrial automation. The project ispart of larger waste water treatment control process, which wasimplemented by the authors several years ago and documented ina recently published text book. The abbreviated part included inthis paper deals with a common process control task in the WasteWater Treatment industry, which has to do with the pumpingstation control. The coverage in this paper is simplified to one siteand can be easily transformed to an equivalent implementationusing the Learning Pit LogixPro 500 simulation software.

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