Energy Saving Mechanism Using Variable Frequency Drives Neetha John 1, Mohandas R 2, Suja C Rajappan 3 1,3 PG Scholar, Sri Shakthi Institute of Engineering & Technology, L&T Bypass Road,Coimbatore-62, Affiliated to Anna University of Chennai. 2 Assistant Professor/Department of EEE, Sri Shakthi Institute of Engineering & Technology, L&T Bypass Road, Coimbatore-62, Affiliated to Anna University of Chennai. Abstract Many industrial applications require variable flow control of fluid (air, chemical gases, water and liquid chemicals). The traditional method of such flow control is to use an induction motor at constant speed with 50-Hz power supply and then control the flow by means of a throttle. Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable-speed, by means of VFD. Such energy cost savings are especially used in variable-torque centrifugal fan and pump applications, where the load s torque and power vary with the square and cube, respectively of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. Also by Using a variable frequency drive to control the fluid flow with a fully open throttle saves a considerable amount of power. As most of the drives operate at part load most of the time, the accumulated energy saving or the corresponding financial benefit, may be substantial over a prolonged period of time. Because this type of fluid flow control is common in industry, widespread application of variable-frequency drives with power electronics area can help in large energy conservation. The main aim of this paper is to reduce the energy consumption by the implementation of VFD and hence the proper control of fluid flows. Keywords Variable frequency drive, Adjustable speed drive, Variable voltage variable frequency drive, Energy conservation, Affinity law I. INTRODUCTION Energy in its different form is the basic input for life. It is equally essential for the improvement quality of life. Energy crisis has a bearing on all socioeconomic development of a country and its sovereignty. There has been an enormous increase in the global demand for energy in recent years as a result of industrial development and population growth. Since, our conventional sources of energy or fossil fuels are running short; it is now the cry of the day to work harder for the development, improvement and up gradation of renewable sources of energy with protection, conservation and existing conventional sources. The reduction in the amount of energy consumed in a process or system, or by an organization or society through economy and elimination of wastage is called as energy conservation. 784 Energy conservation is necessary because with the ever increasing demand, need for electrical power can only be meet by conserving electrical power in addition to installation of new generating units. A major proportion of electrical power in a plant is consumed by electrical derives. Significant amount of electrical energy can be saved by the use of efficient and rigid type of electrical drives. Variable frequency drive is one of the many wellknown energy efficient drives. Within the industry, a variable frequency drive is commonly referred to as inverter. The speed controller is also known by other names such as, Variable Speed Drive (VSD), Adjustable Speed Drive (ASD), and Variable Voltage Variable Frequency Drive (VVVF). The growing popularity of variable frequency drives is due to its ability to control the speed of induction motors, which are the most commonly, used motors in industries. Traditionally, an induction motor is used for constant speed and constant torque applications and when variable speed or torque is required, a DC motor or wound ac motor is used. But now AC induction motors with Variable Frequency Drives are used for variable speed applications. Such drives reduce the energy consumption of motors and increase the energy efficiency of plants.. Energy crisis has a bearing on all socioeconomic development of a country and its sovereignty. Energy conservation is necessary because with the ever increasing demand, need for electrical power can only be meet by conserving electrical power in addition to installation of new generating units. Variable frequency drive is applicable for the air flow control to boiler of thermal power plants. At present, the air flow to boiler is controlled by a control vane mechanism associated with Forced Draft fan and induction motor. The replacement of control vane mechanism by means of a variable frequency drive or variable speed drive reduces the energy consumption of motor. A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. It is a specific type of adjustable-speed drive. Variable-frequency drives are also known as AC drives or inverter drives.
Variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. In variable speed applications, power required varies roughly with the cube of the speed. This is referred to as the Affinity laws, which define the relationships between speed and power. The implementation of Variable Frequency Drive helps the captive power plant to save about 23% of electrical energy consumption. Industrial processes are characterized by the necessity for variation and optimization of the process to achieve satisfactory products and to achieve the most efficient and economic method of production. This calls for the need to Fig 1: Comparison between power and flow for different fan control control the flow rates of materials throughout the plant so types that the most satisfactory condition can be achieved against the many plant variables. A variable frequency drive (VFD) is a type of adjustable speed drive used to control the rotational speed of an alternating current electric motor by adjusting the frequency and voltage applied to the motor. Electric motors drive many types of equipment, including fans, pumps, and air compressors. Although equipment can generally operate at velocities less than the maximum design speed, motors typically drive equipment at a constant rate. Flow and pressure are regulated through the use of a throttling device, such as a valve, damper, or bypass. A variable frequency drive provides a more efficient way to control varying flow rates and pressures. II. BENEFITS OF VFD The use of variable frequency drive control offers several advantages. The most significant benefit is its potential to reduce electrical energy consumption and demand from motor-driven processes. Figure 1 below compares the relative power requirements of a fan at different flow rates, using three types of throttling control: outlet damper control, variable inlet vane control, and VFD control. Although VFDs save far more energy than throttling, the technology has not yet achieved widespread adoption. According to the Bonneville Power Administration, throttling continues as one of the most common and inefficient methods to control a fan or pump. Variable frequency drives also have the potential to reduce system maintenance and related costs. Control with a VFD affords the capability to soft start a motor, which means the motor, can be brought up to its running speed slowly rather than abruptly starting and stopping. Similarly, running the motor at lower speeds extends the lifetime of other equipment components, including shafts and bearings. In addition to enabling precise speed control of applications such as conveyors or winders, other parameters such as pressure, flow and even temperature may be accurately controlled. The efficiency of the electrical supply is increased and more of the electrical current drawn is used to drive the load. Hence the implementation of VFD improves the power factor of the system. In addition to this VFD provides good dynamic response. This can be achieved by rapid adjustment of speed, torque and power and hence gives better control in high speed applications. In some applications it is also possible to operate motors at higher speeds than their nominal speeds. The other advantage of VFD is that it is possible to interface VFDs to wider process control systems such as supervisory control, data acquisition (SCADA) systems and building management systems (BMS). Hence VFD is able to compute intelligence and communication systems. III. VFDS OPERATION AND ENERGY SAVING PRINCIPLES A VFD can reduce energy consumption of a motor by as much as 60%. This is due to the fact that they control the speed of the motor by altering the frequency and therefore the power supplied to it. 785
Even a small reduction in the rotational speed can give At 50% speed for 20% of time; significant saving in the energy consumed by the motor. In Power @50% = power @100% Speed @ 50% ³ order to do so we take a closer look to the so called affinity Speed @100% laws which are used in hydraulics to express relationships = 20* (50/100) ³ between the variables involved in the operation and = 2.5 hp performance of rotary machines such as pumps and fans. Cost @50% = 2.5 * 0.746 * (20% *8760 hr) * 5/kWh Most HVAC equipment is designed to perform during peak = Rs.16337/- loads. These loads occur rarely during the operating year. At 80% speed for 50% of time; To control flow during off-max load conditions, flow Power @80% = power @100% Speed @ 80% ³ control devices such as dampers, valves, inlet guide vanes Speed @100% and bypass systems are used.these throttling devices are effective, but not energy efficient. Using variable frequency = 20* (80/100) ³ drives (VFD) varies the speed of fans and pumps, referred = 10.24 hp to as the affinity laws, allows the equipment to meet the Cost @80% = 10.24 * 0.746 * (50% *8760 hr)* 5/kWh partial load requirement and save energy. Affinity Laws are = Rs. 167295/- used in hydraulic and HVAC system to express the relation At 100% speed for 30% of time; between several variables involved in pump and fan Power @100% = 20 hp performance such as (such as, shaft speed, and power). Cost @100% = 20 * 0.746 * (30%*8760 hr)*5/kwh They apply to pumps, fans, and hydraulic turbines. = Rs.196048/- Flow is directly proportional to speed; Q2/Q1 = Annual cost savings from installing a VFD on this N2/N1 motor is, Torque required is proportional to speed squared; = 653496 - (16337 + 167295 + 196048) T2/T1 = (N2/N1)² = Rs. 273816/ Power is proportional to the cube of the shaft speed; = 42% of the cost P2/P1 = (N2/N1)³ IV. HOW MUCH ENERGY CAN SAVE? The potential energy savings from installing a VFD is illustrated in the following example. Consider a 20- horsepower motor that drives a centrifugal pump. The pump operates at full speed for 365 days annually, 24 hours each day. The operational cost is calculated with the following formula: Cost = Power (kw) * Running Time *Cost/kwh So, when constantly running at 100% speed (and assuming Rs.5/kWh), the cost is: Cost @ 100% = 20 hp *.746 kw/hp *365 *24 *5/kWh = Rs. 653496/- Since this particular pump accommodates a varying load, the pump does not need to be run at full speed throughout the day and therefore, a variable frequency drive can be employed to reduce the pump motor speed. The pump load schedule is: 20% of the time at 50% full speed; 50% of the time at 80% full speed; and 30% of the time at 100% full speed. The savings from the installation of a VFD to control a motor are estimated using the pump affinity laws, which estimate that the power required by a motor is proportional to the cube of the speed. V. HOW DOES A VFD WORK? As we know, the induction motors are the workhorse of industry, which will rotate at a fixed speed that is determined by the frequency of the supply voltage. Alternating current applied to the stator windings produces a magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line frequency by the number of magnetic pole pairs in the motor winding. A four-pole motor, for example, has two pole pairs, and therefore the magnetic field will rotate 60 Hz / 2 = 30 revolutions per second, or 1800 rpm. The rotor of an induction motor will attempt to follow this rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field. This small slip speed generates an induced current, and the resulting magnetic field in the rotor produces torque. Since an induction motor rotates near synchronous speed, the most effective and energy-efficient way to change the motor speed is to change the frequency of the applied voltage. VFDs convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed. A VFD converts 50 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process incorporates three functions: 786
Rectifier stage: A full-wave, solid-state rectifier convert TABLE I three- phase 50 Hz power from a standard 208, 460, 575 or Air Flow Control Using VFD higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher Time Air Speed Power supply voltages are used. Inverter stage: Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter. Control system: An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions. Converting DC to variable frequency AC is accomplished using an inverter. Most currently available inverters use pulse width modulation (PWM) because the output current waveform closely approximates a sine wave. Fig 2: Circuit Diagram of VFD Steam load (ton/hr) (ton/hr) (rpm) (kw) Energy (kwh) 0 45 70 1147 75 75 1 44 69 1131 72 72 2 43 68 1114 69 69 3 42 67 1098 66 66 4 45 70 1147 75 75 5 39 63 1033 55 55 6 46 71 1164 79 79 7 44 79 1131 72 72 8 45 70 1147 75 75 9 47 74 1213 89 89 10 46 71 1164 79 79 11 43 68 1114 69 69 12 42 67 1098 66 66 13 43 68 1114 69 69 14 45 70 1147 75 75 15 44 69 1131 72 72 16 45 70 1147 75 75 17 39 63 1033 55 55 18 46 71 1164 79 79 19 47 74 1213 89 89 20 46 71 1164 79 79 21 45 70 1147 75 75 22 47 74 1213 89 89 23 48 75 1229 93 93 787
VI. CALCULATIONS Average energy consumed per day with VFD (E1) = 1791 kwh Average energy consumed per day without VFD (E2) = power * time = 3*3.3*1000*24*0.89*24 = 2930 kwh Energy savings per day with VFD = E2-E1 = 2930-1791 = 1139 kwh Energy savings per year with VFD =365*(E2-E1) = 416000 kwh Profit = Energy savings per year with VFD* Unit cost of Energy Unit cost of Energy can be calculated based on the cost of the captive power. Let us take the unit cost of captive power =Rs.12.49/- Profit =Energy savings per year * Unit cost of Energy =416000*12.49 = 52 lakhs The table I describes the air flow control using variable frequency drive. Without using variable frequency drive, the air flow can be controlled by some control vane mechanism. But the energy consumption will be very high, since the motor is running at running at fixed speed. Hence by using variable frequency drive, we can control the speed of motor and hence the flow can be fully regulated. According to affinity law we can say that reducing the speed of motor reduces the power consumption also. VII. SIMULATION RESULTS The computer simulation result of the variable frequency drive fed induction motor is done by Matlab/simulink and the results are presented. The induction motor is fed by a current-controlled PWM inverter. The speed control loop uses a proportional-integral controller to produce the quadrature-axis current reference which controls the motor torque. Here the controlled switch used is IGBT. The speed control can be done by using v/f control. Fig 3: Open Loop speed control of induction motor using VFD The circuit diagram for Open loop speed control of Induction Motor using variable frequency drive is shown in Fig. 3. The specifications used for simulation are phasephase rms voltage = 311V, input line frequency=50hz, the induction motor machine parameter is about 215 hp (160kw), 400 V, 50 Hz, 1487 rpm and 50 Nm The gating signals are shown in fig 4.Fig 5 shows the simulation result for the applied frequency is about 120 Hz. Here 120 degree mode of conduction is applied. In this type of control, each switch conducts for 120. Only two switches remain on at any instant of time. The conduction sequence of switches is 61, 12, 23, 34, 45, 56, & 61. For 120 mode of operation, the duration of pulse width will be 33.33% (120/360). 788
Fig 4: Gating Signals for 120 Mode of Conduction Fig 5b : Power Waveform The above fig 5a& 5b represents the speed and the power waveform of the three phase induction motor respectively. The frequency given to the motor is about 120 Hz. Hence the time period will be T=1/120 = 8.33ms. When supplying this frequency, in accordance with the motor specifications, the motor is assumed to be held constant at 1373 rpm and the power consumption is nearly about 161 kw. Fig 5a : Speed Waveform 789 Fig 6: Closed Loop speed control of induction motor using VFD
The circuit diagram for closed loop speed control of Apart from speed control and energy savings, the uses of Induction Motor using variable frequency drive is shown in Variable Frequency Drives provide soft start, reduction in Fig. 6. In closed loop method, the motor running speed and starting current, and also reduce tear and wear. Also a VFD the reference speed are compared and it is given to a fed induction motor is modeled using open loop and closed controller circuit. So that the motor can always run at the loop and simulated using Matlab. reference speed. Here the reference speed given is 1400 rpm and the waveform shows that the motor running speed REFERENCES is also 1400 rpm. [1] W. Leonhard, Control of electrical drives, 2-nd Ed, Springer, 1996. [2] Sergelen B., Electric drive with two parallel synchronous motors Supplied by one current type frequency converter, Prague, 2000. [3] F.A.Toliyat, S.G.Campbell, DSP-based electromechanical Motion control, CRC Press, 2004. [4] Andreev B.F., Sabinin A., Electric drive. Leningrad, 1983 [5] Dombrovski B.B., Zaichik B.M Asynchronous machine, Leningrad 1990. [6] Ilinskii N.F, Fundamental Electric drive, MEI, Moskva, 2007. Fig 7: Speed Waveform VIII. CONCLUSIONS In this study it is found that, the speed control of induction motor using variable frequency drive can save energy according to affinity law. According to this assumption a small reduction in speed can save a large amount of energy. [7] A. de Almeida, P. Fonseca, F. Ferreira, F. Guisse, A. Diop, A. Previ S. Russo, H. Falkner, J. Reichert, and K. Malmose, Improving the penetration of energy-efficient motors and drives. [8] A. de Almeida, F. Ferreira, and D. Both, Technical and economical considerations in the application of variable-speed drives with electric motor systems, IEEE Trans. Ind. Appl., vol. 41, no. 1, pp. 18. [9] Andreev B.F., Sabinin A., Electric drive., Leningrad, 1983. [10] Handbook on Energy Audits and Management,Tata Energy Res. Inst (TERI) New Delhi 2000. [11] B.Wu, High Power Converters and AC Drives. Piscataway NJ:IEEE Press 2006. [12] F. J. Shinskey, Energy Conservation Through Control. New York: Academic. 1987. [13] R. J. Kenney, Fans and blowers, Machine Design, Mar. 14, 1968. [14] M. K. Langfeldt, Economic considerations of variable speed drives, presented at ASME Energy Technology Conference and Exhibition, Feb. 4, 1980. [15] DOE, AMCA: Improving Fan System performance, A SourceBook for Industry, CML Northern Blower Inc. 1989 790