design of a plc-based variable load, speed control ...

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

DESIGN OF A PLC-BASED VARIABLE LOAD, SPEED CONTROL SYSTEM FOR A THREE-PHASE INDUCTION MOTOR B. Coetzer* and R. Gouws* *North-West University, School of Electrical, Electronic and Computer Engineering, Potchefstroom, Email: [email protected]

Abstract: Industries make use of PLCs to control bulk electronic devices and it is found that combining them with motor drives such as VFDs, the efficiency of induction motors are rapidly improved. Eskom in association with suppliers has spent much time and resources on this. To address this problem, a PLCbased control system was designed for a three-phase induction motor. The system is designed using a PLC for intelligent control on a three-phase induction motor and DC generator. The motor is tested at four different control speeds (rpms), of which each of these rpm reference values are tested on three different load condition provided by the generator. The intelligent control system for the three-phase induction motor in combination with the assembled product turned out to be very effective. The theoretical and practical results were compared and the system in whole showed positive efficiency results. Eskom with their resources may implement such a system and will definitely see energy efficient results in the long term.

1 INTRODUCTION Industries have rapidly changed in the last years, and make use of programmable logic controllers (PLCs) to control bulk electronic devices such as motors, trains, etc. [1]. A PLC is used to connect with computers to monitor load and electricity consuming devices. Combining the control of a PLC with the use of motor drives give efficient results, which is the main objective in the industry. South Africa needs more generation capacity just like any other country in the world, of which the national electricity supplier, Eskom, can provide the country with. This supplier mentioned uses drive systems in many applications because of the energy efficiency benefit; they are used for process control and energy conservation. Drive systems are quickly becoming the most used method for speed control in the industry. They control the speed of alternating current (AC) induction motors and are considered as the most energy efficient way of controlling the speed of a motor. They have the ability to vary voltage and frequency simultaneously, thus the frequency of the induction motor can be

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controlled from 0 to 400 Hz which is 0 to 24000 rpm [1]. This concept is important because in the end the more efficient the motor drive system, the more efficient the conveyor system that it controls [2]. The use of conveyor systems in the industry are considered as material handling and is an important sector of the industry, but it consumes a considerable proportion of the total power supply in South Africa, up to 10% of the total demand. Although their electricity usage is considered as a ridicules fact, their high efficiency of transportation is the main attraction. As mentioned above, energy efficiency is important and keeping this in mind the main goal is to better the energy consumption or energy cost of the material handling sector. This can only be done if the energy efficiency of belt conveyors is improved, as they are the main energy consuming components of material handling systems [2]. The energy efficiency of belt conveyors can be improved through proper control of motor drive systems to better the efficiency vs load curve through different load conditions that may occur in the mining industry with conveyor systems. After this brief background it is clear that the problem to be addressed is to improve the energy efficiency of induction motors which the supplier and so many people has spent much time and resources researching techniques on [3]. Therefore this paper focuses on designing a PLC-based variable load, speed control system for a three-phase induction motor.

2 LITERATURE REVIEW In this section the main components used in the design phase of the project will be discussed. These components are the controller and motor drive. The other components discussed in the original literature study are the induction motor, speed sensors, current transducers, dc generator and possible solutions for the project. In the literature study a variety of components available in the industry were discussed; however in this literature review on the

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

main components chosen for the design will be the discussion. 2.1. PLC Programmable logic controllers (PLCs) are microprocessor-based control systems designed for automation processes used in industrial environments. It has memory to that is programmable for the internal storage of user-orientated instructions such as arithmetic, counting, logic, sequencing, and timing. PLC’s can be programmed to sense, activate, and control industrial equipment. It therefore has a number of I/O points which allow electrical signals to be interfaced. Input- and output devices are that are part of the process are connected to the PLC and the control program is entered into the PLC memory. In the task at hand it controls through analog and digital inputs and outputs the varying load-constant speed operation of an induction motor. A PLC keeps monitoring the inputs and activating the outputs as they are needed. This PLC is composed of hardware building blocks called modules as shown in figure 1. These modules plug directly in a proprietary bus which is consumed of a central processing unit (CPU), a power supply unit, input-output modules (I/O), and a program terminal [4], [5], [6]. PLC Program

Input Module(s)

System Sensors

Central Control

Output Module(s)

System Actuators

2.2. VFD A variable frequency drive (VFD) is one of a few types of motor controllers. This type drives an electric motor by varying the frequency supplied to the electric motor. The frequency supplied is directly related to the motor’s speed in revolutions per minute (rpm), this means that if the frequency is increased, the speed of the motor increases simultaneously. If the motor gets power directly from the three-phase power supply it will run at rated rpms, but if this wants to be avoided the VFD is used to ramp down the frequency of the motor to meet the requirements of the motor’s load [7]. VFDs can be used in various areas in the practice, but in this project the sole area of focus is on a conveyor system [1]:     

Saving energy by slowing down the production line from 100% to 80%. Using a small PLC, the conveyors is stopped or slowed down when there are no products on them. When the products start to move, the conveyor could be started up slowly adjusting the acceleration time. Because of the gentle start up, no damages are caused to the products on the conveyor belts. If the VFD senses that it is in idle mode, it can go into a sleep mode for a small amount of time until it needs to start up again.

VFDs are supplied by an AC voltage which they convert to DC voltage and then back to an AC voltage. This all happens in the power conversion step 1, with the use of isolated-gate bipolar transistors (IGBTs) or siliconcontrolled rectifiers (SCRs). The IGBT switches on and off to create an AC voltage waveform using PWM switching. In the power conversion step 2, electrical energy is set mechanical power and control over the motor is done via frequency or voltage changes. This process is shown in figure 2 [8].

Figure 1: Main Unit and Expansion Modules

In table 1 a brief overview of the PLC used in the project is given [6]. When looking at the inputs, there are 8 available of which inputs 1, 2, 7 and 8 can be used as analog inputs. Table 1: PLC Specifications Product Specifications PLC LOGO! 0BA8 Maximum Temperature +70 oC Minimum Temperature -40 oC 8x Inputs and 4x Outputs Output Current 10 A Voltage 24 V

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Figure 2: VFD System

2.3. REMAINING COMPONENTS The remaining components used in the project were all briefly discussed in the literature study. All components used in the project were chosen using a process called a trade-off study, where a specified criteria was set out for

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

each component. The component choices are listed in table 2. Table 2: Components Used Component Controller Motor Drive Current Sensor Speed Sensor Load Possible Solutions

Chosen Product PLC VFD Split-Core Current Transducer Inductive Proximity Switch DC Generator PLC-Based Control

3 METHODOLOGY In order to successfully complete the project, the engineering design process was followed. This was done by getting a background on the project, identifying the problem, confirming the specifications with the client, doing thorough research on the components to be used that will fit the needs of the project perfectly, designing and simulating the product, testing each sub system followed by the system in whole, and processing the end results. By following this process step by step, the results in the following section were obtained. This section however will focus on the methods used to obtain those results. When the literature study was completed and the components in table 2 were chosen, the design of the final product was initiated. This could only be done if the right components were chosen for sensing and turning the induction motor with the PLC and so forth. The VFD used is a WEG CFW 08 Vector Inverter which was perfect for the induction motor whose specifications are given in table 3. The speed sensor used is an inductive proximity switch as mentioned in table 2. The switches diameter allows it to be attached 4 mm from the shaft of the motor for nominal sensing. The bigger the diameter of these switches, the further they can sense [9]. The control program of the system is designed to produce setpoint reference values representing curtain rpm values. The proximity switch is used to count pulses as the shaft of the motor turns and in turn help get the motor speed up to the pre-set set-point value. Table 3: Induction Motor Specifications Name Function Connection Type Y Input Voltage 380 V Input Current 6.63 A Input Frequency 50 Hz Rated Power 3.0 kW Number of Poles 4 Rated Speed 1410 rpm

The control system referred to is called a closed-loop control system and is shown in figure 3. In using this setup a constant speed operation configured with speed feedback, load current feedback as well as stator current

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feedback can be obtained. The induction motor drives a variable load represented by the DC generator, is fed by an inverter (VFD), while the PLC controls the inverter V/f output [4]. The three-phase power supply is connected to a rectifier pulse-width modulation (PWM) inverter representing the VFD. This in turn is connected to the induction motor on the high voltage side, giving it the relevant power to operate, and is then connected to the PLC on the other through low voltage inputs enabling control of the motor. As mentioned and seen in figure 3, a speed sensor is attached to the shaft of the motor to regulate speed, along with two current sensors of which one is attached to the stator of the motor and one to the load.

R Load

Figure 3: Electrical Diagram of Experimental System

The current sensors are merely a part of the system for data feedback to calculate the efficiency of the motor. These current sensors are split-core current transducers and are mainly used for automation and supervision of distributed PLCs or remote control systems [10]. The flowchart of the control system is given in figure 4 and indicates all relevant information sent and received from the sensors as well as the PLC and VFD.

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

No response

Start

NO

NO

Search increment Function

Is there a Problem?

Is increment Selected?

YES YES YES

YES

NO Is measure Okay?

Manual Is right load Selected?

Reset

Auto NO

Generator

NO

No response

Measure Speed

No response

Is increment Selected?

NO Is load Right?

YES YES

Measure Current

Start Motor Measure Current

Figure 4: Flow Chart of Control Program

𝑓 = 47 𝐻𝑧

4 RESULTS 4.1. CALCULATIONS In this section the experimental results of two different control systems will be compared. These system are a PLC with VFD control system and VFD only control system. The practical results started off by first checking what the voltage generated by the DC generator was before applying any load to the system. This was done to see that when eventually applying the load, that the same voltage is still generated, which in turn means that the motor is running at a constant speed. Because of this being a DC load system, the normal ohm rule was used to calculate the current drawn by the system. All results obtain data on PLC and VFD control and only VFD control. The rated speed of the motor as discussed in table 3 is 1 410 rpms and by using this information, combined with the knowledge that only four different rpm values will be tested, the following calculation was made. 120𝑓 𝑁𝑚 = (1) 𝑝

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Working from 50 Hz and dividing it by four concluded in using 12.50 Hz intervals. These intervals were most accurately chosen according to what the reference values sent from the control program to the VFD allowed. In table 4 these values with their frequencies are listed. Table 4: Reference Values and Frequencies Reference Value Frequencies (Hz) 265 12.5 515 25.3 761 37.5 900 47

The reference values were chosen from 0 – 1000 and were guided by the proximity switch with a 4 – 20 mA signal sent to the VFD to reach the set point values. This in turn allows 16 mA to still be used as 4 mA leaves the motor at standstill. Using this allowable 16 mA, the speed is calculated. 1 410 𝑁𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 = ( ) × 4 = 352 𝑟𝑝𝑚 16

(2)

If this rpm value were to be used, it would have been according to the 47 Hz calculated in equation 1. This was

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

not the case as the 380 V, 6.63 A, Y-connected the motor was started on uses a supply of 50 Hz as stated in table 3. This is why intervals of 12.5 Hz more or less were used, thus the rpm value started on was. 𝑁𝑢𝑠𝑒𝑑 =

120 × 12.5 = 375 𝑟𝑝𝑚 4

(3)

varying loads. This results in not generating the same voltage as with PLC control on the system, where a constant speed was held. The surprising fact was that on higher rpm values with PLC control, the motor still kept a constant speed operation on any of the three load conditions tested. This however narrowed down on lower speeds and the difference between the two control methods was very small.

Starting with the generator, it requires 330 VDC on the field of the motor and 310 VDC on the armature. This was not needed on the armature, as voltage was generator and a load in the form of resistors was connected. The load at first would have been 64 Ω, 128 Ω and 256 Ω, but with tolerance kept in mind the load in the end was 63.8 Ω, 127 Ω and 222 Ω. These are the load condition mentioned. Also working on a 50 Hz supply, the synchronous speed of the induction motor calculated was 1 500 rpm. Now there were four rpm values measured at each load condition, which are considered as full-load rpm values each on their own. These rpm values are seen in table 5.

1 − (𝑠 ×

𝑅1 ) 𝑅2

(5)

Where R1 and R2 are the stator and rotor resistance respectively. 4.2. CHARACTERISTICS The range in which the speed vs. torque characteristics was taken can be seen in figure 5 and was the topic of discussion in table 5. However, the results show that the configuration where only the VFD controls the motor that the speed is not kept on a constant rpm value with the

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PLC 1423 rpm

VFD 1389 rpm

VFD 1106 rpm

VFD 731 rpm

VFD 367 rpm

1400 1200

RPM

(4)

1−𝑠

PLC 1120 rpm

800

400

The efficiency of each system was then calculated using their related slip value. 𝜂=

PLC 745 rpm

600

The set out rpm values measured at the load conditions are also the reference values for the system. With the PLC and VFD control system, this set out rpm values were sustained perfectly doesn’t matter what the load conditions were. With only the VFD control system, this unfortunately could not be done. The motor slip for both of these control systems were then calculated using equation 4. 𝑁𝑠 − 𝑁𝑚 𝑁𝑠

PLC 375 rpm

1000

Table 5: Frequencies with Related RPM Frequencies (Hz) RPM 12.5 375 25.3 745 37.5 1 120 47 1 423

𝑠=

RPM VS. TORGUE

200 0

2

4

6

8

10

TORQUE (N.M) Figure 5: RPM vs. Torque Characteristics

Using the previous torque data of both systems and comparing it with the efficiency of both configuration systems, interesting results were obtained. First looking at the configuration system where only VFD control was done on the motor. It’s seen in figure 6 that as the torque grew constantly, the efficiency grew less constant on higher rpm values and different loads. This leaves the system questionable as a motor in practice runs at synchronous speed or even sometimes 80 % higher. Whereas looking at the PLC control configuration, there is a constant efficiency vs. torque grow. This system also showed higher efficiency values in all cases than that of the VFD configuration. With the three load conditions tested, it’s seen in figure 7 that on the lower rpm side the efficiency bettered with about 1 %. This happened for the first two rpm values, whereas with the second last rpm value it changed to 2 % and finale to 3 % on the final rpm value. What makes the results with the PLC and VFD control system so satisfactory, is that each individual efficiency is maintained no matter what the load conditions were. This is mainly because the rpm values were kept constant by using the proximity switch to permanently count pulses,

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

remain on a speed for testing purposes. The OR gate following input of the pushbutton, uses the input and the edge triggered wiping relay for activation to start progress on the up/down counters B003-B007 and B014 which is the reset.

EFFICIENCY VS. TORQUE PLC 375 rpm

PLC 745 rpm

PLC 1120 rpm

PLC 1423 rpm

VFD 1389 rpm

VFD 1106 rpm

VFD 731 rpm

VFD 367 rpm

EFFICIENCY VS. RPM

100

PLC and VFD Control

90 70 60 50 40 30 20 10 0 0

2

4

6

8

EFFICIENCY (%)

EFFICIENCY (%)

80

TORQUE (N.M) Figure 6: Efficiency vs. Torque Characteristics

and not allowing the motor to slow down on either of the three load conditions tested. This was not the case with just the VFD control system, as the frequency set out did not correspond with the rpm values measured and was lower than this expected value. This concludes that the VFD control system is not an as effective system relating with the PLC and VFD control system. The efficiency vs. load characteristics curve in figure 8 show that the efficiency improved even more with higher load conditions.

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100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 0

300

600

900

1200

1500

1800

RPM Figure 7: Efficiency vs. RPM Characteristics

EFFICIENCY VS. LOAD PLC and VFD Control

VFD Control

80

4.3. CONTROL PROGRAM

70 60

EFFICIENCY (%)

In this section just a fraction of the control program is given, just to illustrate how it was done. The program in figure 9 starts the motor using input 1 running through a set/reset function block to take control over output 1. The reset uses the stop and overload (O/L) inputs through an OR gate to put the system to a stop when trouble occurs. The stop and O/L inputs are wired normally close (N/C) according to rules set by law. In this part of the program auto incrementation of speed (rpms) on the motor is activated using a pushbutton connected to the system. Auto incrementation means that the program will automatically run through each speed set out for the motor and will stay on that speed for a set time using an edge triggered wiping relay which is function block B009. In this section of the program the same edge triggered wiping relay is used to count time for the speed of the motor, but instead of speed being adjusted automatically one of the pushbuttons is used to activate manual incrementation of the set out speeds. This means that the student can choose how long he would like to

VFD Control

50 40 30 20 10 0

0

50

100

150

200

LOAD (Ω) Figure 8: Efficiency vs. Load Characteristics

250

24th Southern African Universities Power Engineering Conference, 26 - 28 January 2016, Vereeniging, South Africa.

Figure 9: Start, Stop and Auto Increment

Following each counter is memory bit flag which represents a value for each reference speed set out. In figure 10 the discussed program is illustrated.

Figure 11: Experimental Project

doors for improvement in the efficiency. They have more inputs and outputs to handle multiple systems like discussed in this article. With the experimental results completed and discussed in section 4.1 and 4.2, the PLC and VFD control system is found to be more efficient than just the VFD control system. On the first two rpm values that tests were done on, there was a system efficiency increase of 1 %, whereas with the third and fourth rpm values the system efficiency increase went up to 2 % and 3 % respectively. As three-phase induction motors are constantly used at their synchronous speeds in the mining industry, an increase in efficiency of 91 % to 94 % is expected.

6 REFERENCES ESKOM, “Variable Speed Drives (VSDs) – saving energy in the industrial sector,” Sensible flow saves, Sep. 2010.E. H. Miller, "A note on reflector arrays," IEEE Trans. Antennas Propagat., to be published. [2] S. Zhang, X. Xia. “Modeling and energy efficient optimization of belt conveyors,” Applied Energy, pp. 1, 2011. [3] ESKOM, ‘The Energy Efficient series: Towards an energy efficient mining sector,” Feb. 2010. [4] M.G. Ionnides. “Design and Implementation of PLC-based Monitoring Control for Induction Motor,” Academia.edu, p. 8, Sep. 2004. [5] H.N.Y. Birber. “Design and Implementation of PLC-Based Monitoring Control System for Three-Phase Induction Motor Fed by PWM Inverter,” International Journal of Systems Applications, Engineering & Development, vol. 2, no. 3, 2008. [6] M. Snyhed. (2014, Oct.). “Siemens LOGO! 8 logikcontroller is now in stock at RS Components,” [Online]. http://www.electronicsupply.dk/announcement/view/40512/siemens_logo_8_logikkon trollere_er_nu_pa_lager_hos_rs_components#.VPVtScSmiSo, [August. 5, 2015]. [7] C. Hartman. “What is a Variable Frequency Drive?”, 20, March. 2014. [Online]. http://www.vfds.com/blog/what-is-a-vfd, [August. 5, 2015]. [8] S. Prabakaran. “LT/Medium/HT Variable Speed Drives in the Industry,” 2014. [Online]. http://electrical-engineeringportal.com/lt-medium-ht-vfd-part-2, [August. 5, 2015]. [9] Switches International. “Inductive Extended,” [Online]. http://www.switches.co.za/products/proximityswitches/inductive-extended/, [September. 4, 2015]. [10] RS Components. “Split Core Current Transducer, 20 A rms, 30 V dc,” [Online]. http://za.rs-online.com/web/p/currenttransducers/0198778/, [September. 9, 2015]. [1]

Figure 10: Manual Increment

5 CONCLUSION The results show that this project can be interpreted in a real system as intended. The project, which is shown in figure 11 is in practice an example of a conveyor system in the mining industry. It’s seen that the motor operates in a more efficient state on or close to the synchronous speed. This causes the motor slip to be less with control on the system and at a constant value. When using this system in the mining industry, more intelligent and faster PLCs will be used like Siemens S7-300, S7-400 PLCs, etc. This opens more

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