STARTING ANALYSIS OF INDUCTION MOTOR. A COMPUTER SIMULATION BY ETAP POWER STATION.

Transcription

1 STARTING ANALYSIS OF INDUCTION MOTOR. A COMPUTER SIMULATION BY ETAP POWER STATION. Piyush S. Patil, Prof. K. B. Porate, Abstract The basic requirement for studying starting analysis of Induction motor are the starting current of Induction motor and the voltage flicker during start up. This paper will summarize several common methods and provide application guidelines for proper selection of starting devices. Starting method reviewed will include different types of starters, comparative study of results obtained in laboratory & results obtained by simulation in ETAP (Electrical Transient Analyzer Program) for DOL starter & autotransformer starter, simulation results are also obtained for different cable length, with & without capacitor connected across motor itself. Finally it is concluded that starting current is less in case of autotransformer starter. Starting current & starting time can be further reduced and voltage profile can be improved with the use of capacitor at the motor terminal as a compensating device. Keywords autotransformer start, DOL start, ETAP, Induction motor, Starting methods, voltage dip. I I. INTRODUCTION nduction motors are widely used as an electric drives in various Industrial applications like industrial motion control systems & to drive various machines, such as pumps, fans, compressors, conveyors, spindles, to name just a few, as well as in main powered home appliances. Electric drives can be classified basically as DC drives & AC drives. The rectifier unit increases the cost of the unit used for DC drives. Simple and rugged design, low-cost, low maintenance and direct connection to an AC power source are the main advantages of using AC induction motors. For variable speed the industries use the asynchronous motor i.e. Induction motor. Induction motors are the most common motors used in industrial motion control systems & to drive various machines, such as pumps, fans, compressors, conveyors, spindles, to name just a few, as well as in main powered home appliances. Induction motors are widely used in industry. An induction motor at rest can be modeled as a transformer with the ondary terminals short circuited. Thus when voltage is applied, a heavy surge of current is drawn from the power system that in turn causes a dip in system voltage. The magnitude of this dip is proportional to the magnitude of the surge and the impedance of the system. Full voltage starting of large induction motors draws large current & can cause serious voltage flicker problems on a power system 1. Flicker can poses a problem to both the utility and the customer. During motor starting, the voltage level at the motor terminals should be maintained at approximately 80 percent of rated voltage 2. Piyush S. Patil, is final year M. Tech (IPS) student, at G. H. Raisoni College of Engineering.& is Lecturer at Yeshwantrao Chavan College of Engineering, Nagpur. b4upiyush@yahoo.com. Prof. K. B. Porate, is working as Assistant Professor, Department of Electrical Engineering, G. H. Raisoni College of Engineering, kporate@yahoo.com By industry standards, ac control devices are not required to pick-up at voltages below 85 percent of rated nameplate voltage 2. A 35 percent reduction in starting voltage results in a 57 percent reduction in starting torque 1. During start motor accelerates for a short interval. If the load torque requirements exceed the torque produced by the motor for any speed less than rated speed, the motor will "lock-in" at that speed and fail to accelerate further. The motor will continue to draw a large magnitude of current, overheat, and either trips a protective devices or burn up 1. The acceleration time can be calculated using a torque representing the difference (positive) between motor torque and load torque. This time must not exceed the time allowed for the current drawn on the motor thermal limit curve 3. During starting period Use of Capacitor at motor terminal not only improves the power factor of the system but it also reduces the starting current of the motor. Induction motors are also used in mines for different applications. As motors are located in the interior of mines power is to be transferred with the help of power cables. As the impedance of cable is proportional to its length and voltage drop is proportional to the product of current & impedance. During start, induction motor draws a large current; this large current can cause a large voltage drop, resulting in low voltage appearing across the motor. This low voltage may avoid the induction motor to start, resulting in locking of rotor and heating of machine winding. In this paper focus is made on the need of starting analysis of induction motor along with different types of starters that can be used for starting of Induction motor. Here starting analysis of induction motor is made with the help of DOL & autotransformer starter. During the start of Induction motor study is made for the recorded values results and results obtained by simulation of induction motor in ETAP environment. In ETAP environment a special case study is done in which cable length is changed with and without capacitor connected across the motor terminals, a comparative study of results obtained for above case study is also done. II. STARTING ANALYSIS OF INDUCTION MOTOR During the motor starting period, Induction motors, is at rest, & it appear just like a short circuited transformer and if connected to the full supply voltage, draw a very high current known as the Locked Rotor Current. the Locked Rotor Current (LRC) is a function of the terminal voltage of the motor and the motor design. In other words the starting motor appears to the system as small impedance connected to a bus. It draws a large current from the system, about six times the motor rated current, which therefore results in voltage dip in the system and poses disturbances to the normal operation of other system loads. The magnitude of this dip is proportional to the magnitude of the surge and the impedance of the system. Because of the highly inductive nature of the motor

2 circuit at rest, the power factor of the surge is quite low, usually on the order of 10 to 20 percent. As the motor accelerates to rated speed, the surge decays and the system voltage recovers. This dip is detrimental to a power system in two ways. First, if the magnitude of the dip is large enough, it can cause erratic operation of voltage sensitive devices such as computers and relays. Even the contactor serving the motor being started could drop back out due to low voltage. Second, it creates an annoying flicker in the lighting facilities being served by the power system. Both magnitude and frequency of flicker affect the customers on the system, and too much of either can cause complaints. In order to control such problems, most utilities have limitations regarding the magnitude and frequency of dips produced by the starting of large motors. During motor starting, the voltage level at the motor terminals should be maintained at approximately 80 percent of rated voltage or above for a standard National Electrical Manufacturers Association (NEMA) Type B motor. This value results from examination of the speed-torque characteristics of this type motor (150 percent starting torque at full voltage), and the desire to be able to successfully accelerate a fully loaded motor at the reduced voltage (i.e., since torque varies with the square of the voltage, T= 0.82 X 1 50 percent 100 percent). By industry standards, ac control devices are not required to pick-up at voltages below 85 percent of rated nameplate voltage, whereas dc control devices must operate dependably (i.e., pick-up) at voltages above 80 percent of their rating. Critical control operations may therefore encounter difficulty during motor starting periods where voltage dips are excessive. The actual dropout voltages for all contactors used in industrial applications commonly range between percent of rated voltage depending on the manufacturer. Voltages in the range of percent of rated voltage, therefore, may be appropriate and are sometimes used as the criteria for the lower limit that contactors can tolerate Since the motor acceleration torque is dependent on motor terminal voltage, in some cases the starting motor may not be able to reach its rated speed due to extremely low terminal voltage. The starting current of a motor with a fixed voltage will drop very slowly as the motor accelerates and will only begin to fall significantly when the motor has reached at least 80% of the full speed. The actual curves for the induction motors can vary considerably between designs but the general trend is for a high current until the motor has almost reached full speed. The LRC of a motor can range from 500% of Full- Load Current (FLC) to as high as 1400% of FLC. Typically, good motors fall in the range of 550% to 750% of FLC. This makes it necessary to perform a motor starting analysis. Since the voltage & current profile are improper the power quality is not good & also the harmonics are introduced in the system. The main purpose of performing a motor starting study is twofold: (i) to investigate whether the starting motor can be successfully started under the operating conditions (i.e. motor torque & thermal limits of motor etc.) (ii) to see if starting the motor will seriously affect the normal operation of other loads in the system. A sample torque, current, voltage vs speed characteristic is as shown in the fig. 1. Fig. 1. Typical induction motor torque speed current curve. The starting torque of an induction motor starting with a fixed voltage will drop a little to the minimum torque, known as the pull-up torque, as the motor accelerates and then rises to a maximum torque, known as the breakdown or pull-out torque, at almost full speed and then drop to zero at the synchronous speed. The curve of the start torque against the rotor speed is dependant on the terminal voltage and the rotor design. Finding the proper motor base speed and torque to meet the running load requirements is the first necessity. A processdefined torque and speed should be used to determine the best fitting motor base speed and torque (hp torque speed). In addition, the breakaway torque and accelerating torque will provide the total torque requirement from the motor. The motor torque output at zero speed must be capable of breaking away from a standstill and must then exceed the running load torque at every speed up to full speed to avoid stalling the motor. A stall condition will cause the motor to reach thermal limits very quickly. Additionally, the motor torque must exceed the load torque by a magnitude that allows acceleration to full speed while staying within the thermal limits of the motor and starting device. Motor torque will further depend on the voltage applied and the type of starting device. The motor speed torque and speed current profiles will determine the level of current drawn (affecting voltage drop on the system feeding the motor) and the amount of torque produced. The amount of voltage drop will, in turn, affect flux and motor torque levels. To avoid oversizing motors for high starting torque or large load inertias, motors with higher torque profiles may be capable of the required torque in a lower motor horsepower package.. The motor speed torque profile and the motor thermal limit curve (usually provided in an inverse time current curve) is important information provided by the motor manufacturer. Motor torque produced at reduced voltage levels is approximately proportional to the square of the voltage applied to the motor multiplied by the torque produced at full voltage. The acceleration time can be calculated using a torque representing the difference (positive) between motor torque and load torque. This time must not exceed the time allowed for the current drawn on the motor thermal limit curve Many software packages exist that will calculate acceleration time considering the total load torque, motor torque, and load inertia, as well as applied voltage.

3 A. Starting methods of Induction motor Since the Induction motor draws a large starting current. The Induction motor has to be started by using certain starting devices called as starters. On the basis of technology used starters are basically classified as i) Conventional Starter or Traditional Starter As these starters are used by industries from a long time, they are called Conventional Starter or Traditional Starter. Most of the industries prefer these starter only. These starters allow a low voltage to appear across the motor. During starting period some voltage is dropped across some devices which are in series with the stator winding or rotor. Resistance of reactance is inserted in series with the rotor winding (only for slip ring induction motor). When the speed reaches upto 80% to 100% of rated speed the starting devices i.e. starters are removed form the system & motor is allowed to run on full voltage directly from supply. Depending on the location & type of voltage limiting element the different types of traditional starters are shown in table. 1. Table 1. Different types of IM traditional starters Type Description None No starting device Auto-transformer Auto-transformer Stator Resistor Series Resistor to the stator Stator Reactor Series Reactor to the stator Capacitor, Bus Shunt capacitor connected to the motor bus Capacitor, Terminal Shunt capacitor connected to the motor terminal Rotor Resistor Series Resistor to the rotor Rotor Reactor Series Reactor to the rotor Star Delta Stator Terminals are first connected in Y & then in Delta However this traditional starters has got some disadvantages such as a. Torque pulsations. b. High Inrush Current. c. Heating of machine windings during starting period. d. With low load, efficiency is less. e. Drop in motor speed is more. ii) Solid state or Soft Starter The other type of starter is a soft starter;.as the name implies these starters don t have any moving or rotating parts. This is a very recently electronic method & it has been frequently used in industry.. it consists of applying a voltage to the motor, which is gradually increased in a ramp wise manner, thus enabling the motor to start. In order to do this three phase AC controller power electronic devices is used. This equipment consists of two thyrister per phase in anitiparallel connection, where the input is connected to the respective phases of the mains supply and the output of each motor phase. In case of soft starters the motor parameters such as voltage, current & current are controlled by means of thyrister valve which are connected anti-parallel in each arm as shown in fig. 2 Fig. 2. Soft Starter The soft starters are classified on the basis of its working principle. i.e. whether they control the current, voltage or torque or they limit the current of the motor. The different types of soft starters are shown in table. 2. Table 2. Different types of Induction motor soft starters Type Description (controlling Parameter) Current Limit Current is not allowed to increase beyond a certain limit Current Control Current is controlled Voltage Control Voltage is controlled Torque Control Torque is controlled The advantages of using soft starters are a. Starting current can be controlled. b. Starting torque can be improved. c. Energy Saving is possible. d. Input power factor is increased. e. Minimizes transient during running conditions. Limitations of soft starters a. Efficiency reduces with increase in loads. b. Soft starters distorts the currents drawn from utility grid, c. Fifth harmonics are more pronounced. Apart from these soft starters are costly for low power applications & energy saving is very less as compared to the investment. So industries preferred conventional starters. III. CASE STUDY For study purpose experimental setup was made in the laboratory, for this a three phase 3 HP, 4 poles, 440 V, 4.5 A, 50 Hz induction motor was connected to the mains with the help of various meters and appropriate starters. To determine the motor parameters following test were carried out. o No Load test o S.C. test o On Load test (Lamp Loading) (75 % & 100% loading) {to determine Efficiency & slip of motor} Values were recorded with the help of various meters & results are obtained are shown in table 3.

4 Table 3. Induction motor parameters recorded & std values IM Parameters Std. values Recorded values Rs (stator resistance) 4.37 Ω 10.5Ω Xs ( stator reactance) 4.32 Ω 39 Ω Xm (mutual reactance) Ω Ω power factor (75%L) power factor (100%L) Efficiency (75%L) 77.96% 55% Efficiency (100%L) 80.91% 59% Starters which have been used for the starting of Induction motor are A. DOL starter In case of DOL starter, motors rated full voltage is directly applied across the motor terminals. DOL starter uses a main contactor to apply the motor stator windings directly across the main system voltage. This method of starting provides the lowest cost, a basic and simple design of controller, resulting in low maintenance, simple training requirements, and the highest starting torque possible without the use of a drive. DOL starters can be used for lower rating of motor where the driven load can withstand the shock of instantaneously applying full voltage to the motor and where line disturbances can be tolerated. Voltage, current during the start and starting time are recorded both for experimental setup as well computer simulation in ETAP. B. Auto-transformer starter In this starter a reduced voltage is applied across the motor terminals, and when the speed reaches to a certain value a full rated voltage is applied across the motor terminal. To achieve this a auto-transformer and a change over switch is required. The tapings available on the auto-transformer varies from 0 % to 100 % of rated voltage, from which 60% to 85% range is usually used. When speed reaches to 80% of rated speed the switch is changed from the auto-transformer tapings to the mains i.e. to the full rated voltage. For both experimental as well as in ETAP simulation a 65 % setting is made and when the speed reaches up to 80% of rated speed a full rated voltage is applied. Here also results are recorded both for experimental setup as well computer simulation. SIMULATION Simulation of the laboratory was carried out on computer. Various software s are available for simulation. Among which simulation was carried out in ETAP (Electrical Transient Analyzer Program) environment. Benefits of using ETAP simulation software are It is a fully graphical electrical transient analyzer program & supported by Microsoft Windows. It provides library for almost all components of the power system. More Accurate result with less simulation time. A single line diagram is created in ETAP which is connected to the mains though a cable of negligible resistance & length. In ETAP starting device is inbuilt. ETAP provides thirteen types of starting devices, one can choose the required one. The simulation diagram is shown in fig. 3. Fig. 3. Single line diagram used for simulation ETAP provides two types of motor starting calculations: Dynamic Motor Acceleration and Static Motor Starting. In the Dynamic Motor Acceleration calculation, the starting motors are represented by dynamic models and the Motor Acceleration module simulates the entire process of motor acceleration. This method is used to determine if a motor can be started and how much time is needed for the motor to reach its rated speed, as well as to determine the effect of voltage dips on the system. In Static Motor Starting, the starting motors are modeled by the locked-rotor impedance during acceleration time, simulating the worst impact on normal operating loads. This method is suitable for checking the effect of motor starting on the system when the dynamic model is not available for starting motors. For analysis purpose Dynamic Motor Acceleration calculations is used. Simulation of Induction motor is carried out in ETAP environment for different conditions (cases). A. CASE 1: Induction motor is simulated with help of DOL & autotransformer starter for negligible cable length at three different loading conditions i.e. 50% of full load, 75% of full load and 100% of full load. The results obtained are for simulation starting current is 6.6 times the full rated current i.e A for DOL starter, while for auto-transformer starter starting current is 277% of full rated current i.e A. however in case of auto-transformer starter the peak current is 3.5 times the rated current i.e A. For DOL starter as per recorded values starting currents are 11.2 A &12 A for 75% and 100% loading respectively. In case of auto-transformer starter the recorded values are 8 A & 8.8 A fro 75% and 100% loading respectively. Whereas settling time or the starting time depends on the loading condition. More the load more will be the settling time. In case of DOL starter the starting time for 100% and 75% loading for simulation are 4.2, 3.9 while recoded values are 2 & 2.2 respectively. & 3.7 for 50% load in case of simulation results. While in case of Auto transformer starter the starting time to reach up to 80% of rated speed for 100% & 75% loading for simulation are 16.7 & 10.2 while recorded time are 2.7 & 2.1 respectively, & 8.2 for 50% load in case of simulation. However the voltage during starting, in case of simulation is same irrespective of load, however it depends on the starter used for DOL starter the voltage at the start is % of rated voltage i.e V for simulation & 428 V is the recorded value. The results obtained in laboratory from the experimental setup & results obtained by simulation for DOL and autotransformer starter are shown in Table 4. & Table 5. resp.

5 Para meter Ist Table 4 Summary for DOL starter Tst Vst Ist Vst tst Load Simulation Recorded 75 % % A 6.6 Ifl v 98.25% A 2.48 Ifl 12 A 2.67 Ifl 428v 97.27% Para mete r Table 5 Summary for Auto-transformer starter tst Vst Ist (A) Ist Vst tst Load Simulation Recorded 75 % v 8 A I FL 2.77 I FL 100 % V rtd A 1.96I FL Vrtd 2.7 Fig. 5. Current in case of Auto-transformer starter for different loading conditions Simulation results for voltage obtained for different loading conditions and are shown in fig. 5. & fig. 6. B. CASE II This case is applicable to induction motor used in mines where cable length is considerable. The cable length is increased to 1 km with standard parameters and the load is kept constant to full load. Motor is started with the help of DOL & auto transformer starter. Starting current is 6.5 times & 2.74 times the rated current for DOL starter &autotransformer starter respectively.while the starting time is for DOL & auto-transformer starter respectively. C. CASE III Compensation is provided with the help of capacitor connected across the motor terminal used in case II, for the same cable length. Again Motor is started with the help of DOL & auto transformer.. Starting current is 6.49 times & 2.75 times the rated current for DOL starter &auto-transformer starter respectively.while the starting time is for DOL & auto-transformer starter respectively. RESULTS & CONCLUSSION Fig. 6. Voltage in case of DOL starter for different loading conditions Fig. 7. Voltage in case of Auto-transformer starter for different loading conditions Comparison of the results obtained by simulation for different case studies (case I, case II & case III) for 100% loading Simulation results for current are obtained for different loading conditions and are shown in fig. 5. & fig. 6. Fig. 4. Current in case of DOL starter for different loading conditions Fig. 8. Current in case of DOL starter for different case studies

6 DISCUSSION & FUTURE SCOPE Fig. 9. Current in case of Autotransformer starter for different case studies It is seen in the result that the observed & simulated values are not same; this may be because the machine parameters measured which are measured are very much deviated from the standard values. Auto transformer requires more time to settle. This is due to the fact that as less voltage is applied, more time is required to reach the rated speed. However the starting current is reduced to a very large extent by the use of auto-transformer starter. Using capacitor not only reduces the current but also improves the voltage profile. DOL starter can be used for a low power application. In case of autotransformer starter if the tap setting is made at 57%, it can be used as a star delta starter. From simulation the parameter scan also be studied. ETAP provides the scope for measurement of other parameters such as power factor, torque etc. Also the software provides the scope for calculation for protective devices REFERENCES Fig.10. Voltage in case of DOL starter for different case studies Fig.11. Voltage in case of Autotransformer starter for different case studies Results of simulation illustrates that the starting current of Induction motor is some what same for all the case studies, irrespective of the starter used, for DOL starter the starting time or settling time has changed for different case studies. Starting current is 6.6, 6.54 & 6.5 times the rated current for case 1, case 2 & case 3 respectively & the observed value is 2.7 times the rated current. For auto-transformer starter the starting current is almost same 2.7 times for different case studies & 2 times the rated current is the observed values. In case of DOL starter starting time varies from 3.9 to 4.3 from case 3 to case 1 & the observed value is 2.5. For auto transformer starter the starting time varies from 15.9 to 17 fro case 3 to case 1 & the observed value is 2.7 so that the motor reaches its 80% speed. In case of voltage profile, capacitor has improved the voltage at the load bus up to 99% for both the starters. [1] John H. Stout Capacitor Starting Of Large Motors IEEE Transactions On Industry Applications, Vol. Ia-14, No. 3, May/June 1978, pp [2] A. Jack Williams & M. Shan Griffith, Evaluating The Effects Of Motor Starting On Industrial And Commercial Power Systems IEEE Transactions On Industry Applications, Vol. Ia-14, No. 4, July/August 1978, pp [3] John A. Kay, Richard H. Paes, J. George Seggewiss, Robert G. Ellis Methods For The Control Of Large Medium-Voltage Motors: Application Considerations And Guidelines IEEE Transactions On Industry Applications, Vol. 36, No. 6, November/December 2000, pp [4] Solveson M.G. Soft-Started Induction Motor Modeling IEEE Transactions On Industry Applications Vol.42 No.4, July/August 2006, pp [5] Rezek A. J., Energy Conservation With Use Of Soft Starter IEEE 2000, pp [6] Blaabjerg F. Can Soft Starters Help Save Energy, IEEE Industry Applications Magazine Sep/Oct 1997 Pp [7] Blaabjerg F. Comparative Study Of Energy Saving Benefits In Soft Starters For 3ph Im, IEEE Industry Applications Magazine Sep/Oct 1995 pp [8] AC Induction Motor Fundamentals by Rakesh Parekh, Microchip Technology Inc [9] Induction motor - protection and starting by viv cohen - Circuit Breaker Industries, P.O. Box 881,Johannesburg 2000, South Africa. [10] Induction motors Parameters extraction by Sinisa Jurkovie [11] Performance chart for IM from Kirloskar Electric [12] Performance chart for IM ABB Group- Automation & Power Technologies [13] Consultancy with Mr. Rao, Nagpur motors, MIDC Hingna [14] Textbook of Electrical Engg. Vol-II B.L.Theraja [15] Basic Electrical Machines V.K.Mehta [16] Electrical Machine- P.S.Bimbhra