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Second International Conference on Emerging Trends in Engineering and Technology, ICETET-09

STARTING ANALYSIS OF INDUCTION MOTOR. A COMPUTER SIMULATION BY ETAP POWER STATION. Mr. Piyush S. Patil, final year M. Tech (IPS), G. H. Raisoni College of Engineering, [email protected]. Mr. K. B. Porate, Assistant Professor, Department of Electrical Engineering, G. H. Raisoni College of Engineering, Nagpur, M.S., India. [email protected] 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.

"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. 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.

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. At start an induction motor draws 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. 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

978-0-7695-3884-6/09 $26.00 © 2009 IEEE

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. The current is 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 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

494

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). 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 FullLoad 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.

Depending on where lighting buses are located, with respect to large starting motors, this may be a factor requiring a motor starting study. A. Weak Source Generation A motor starting study may be of use in analyzing the performance of small systems. Smaller power systems are usually served by limited capacity sources that are subject to severe voltage drop problems on motor starting, especially when large motors are involved. Electric utilities will often require that voltage dips on motor starting be held within specified limits. A motor starting study can be employed during the design phase of the power system when it is still economically feasible to make design revisions to verify that the system will perform within the limits imposed by the utility. Frequently, small systems are served by limited capacity onsite generation either exclusively or in parallel with a limited capacity utility system. Whenever local generation is involved, the relatively high transient impedances of the generators that become evident during the motor starting interval can aggravate the voltage drop that is ultimately realized. Also, the type of voltage regulator system applied with the generators can dramatically influence the effects of motor starting. Many digital computer programs can accurately model generator transient behavior and exciter/regulator response during motor starting. Such programs are invaluable for studying systems where an accurate representation of generator and exciter/regulator action is necessary in order for study results and conclusions to be meaningful. B. Special Torque Requirements High inertia loads may cause motor starting times to become excessive, and the heating in the motor due to the high currents drawn during starting may, therefore, be intolerable. A computer-aided motor starting study can provide accurate values of motor accelerating current and time. It also makes it possible to determine if the thermal limits of standard motors will be exceeded for longer than normal starting intervals.

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

495

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. Acceleration time or torque required can be calculated using the following formulas: 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 voltage applied, starters can be classified as

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.

i) Full-Voltage Starting Direct across the line starting can be used whenever the driven load can withstand the shock of instantaneously applying full voltage to the motor and where line disturbances can be tolerated. Full-voltage starting uses a main contactor to apply the motor stator windings directly across the main system voltage. This type of starting method 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. ii) Reduced-Voltage Starting Reduced-voltage starting may be considered if full-voltage starting creates objectionable line disturbances on the distribution system or where reduction of mechanical stress to a gear box or belt drive system is required. It must be noted that when the voltage is reduced from nominal, a decrease in inrush current will occur. The motor torque will also decrease at a rate roughly proportional to the square of the decrease in voltage. This phenomenon also occurs in the opposite manner when the voltage is increased. A common-sense rule of thumb is that if the load cannot be accelerated to full speed using full voltage, it cannot be accelerated to full speed using reduced voltage. This type can be further classified on the basis of devices used to reduce the voltage level.

Table 1. Different types of IM traditional starters Description Type None No starting device Auto-transformer Auto-transformer Stator Resistor Series Resistor to the stator Stator Reactor Series Reactor to the stator Shunt capacitor connected to the Capacitor, Bus motor bus Capacitor, Shunt capacitor connected to the Terminal motor terminal Rotor Resistor Series Resistor to the rotor Rotor Reactor Series Reactor to the rotor Stator Terminals are first connected Star Delta 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

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

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.

496

Table 2. Different types of Induction motor soft starters Type Description (controlling Parameter) Current is not allowed to increase Current Limit beyond a certain limit Current Control Current is controlled Voltage Control Voltage is controlled Torque Control Torque is controlled

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.

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. CASE STUDY III. 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. Table 3. Induction motor parameters recorded & standard values Std. values Recorded values IM Parameters Rs (stator resistance) 4.37 Ω 10.5Ω power factor (75%L) 0.84 0.39 power factor (100%L) 0.84 0.71 Efficiency (75%L) 77.96% 55% Efficiency (100%L) 80.91% 59% 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.

497

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, 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. 27.72 A for DOL starter, while for auto-transformer starter starting current is 277% of full rated current i.e. 12.47 A. however in case of auto-transformer starter the peak current is 3.5 times the rated current i.e. 14.7 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 1.1 sec, 1.2 sec while recoded values are 2 sec & 2.2 sec respectively. 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 2.26 sec & 2.92 sec while recorded time are 2.7 sec & 2.1 sec respectively However the voltage during starting, in case of simulation is same irrespective of load, however it depends on the starter

700

used for DOL starter the voltage at the start is 98.25 % of rated voltage i.e. 432.3 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 auto-transformer starter are shown in Table 4. & Table 5. respectively.

600

%Current

500 400

100% Load

300

75% Load

200 100

1401

1301

1201

1101

901

801

701

601

501

401

301

201

1001

Where Ist - Starting current of induction motor Vst – Voltage at the start across induction motor Tst – Time to reach the rated speed Ifl – full rated current

101

1

0

Time in mSec

Fig. 4. Current in case of DOL starter for different loading conditions

450 400

4801

4401

4001

3601

3201

2801

2401

2001

99.95

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 4.32 sec 16.7sec for DOL & auto-transformer starter respectively.

RESULTS & CONCLUSSION Simulation results for current are obtained for different loading conditions and are shown in fig. 4. & fig. 5.

498

99.85 99.8

1401

1301

1201

1101

1001

901

801

701

1

99.75

Time in mSec

Fig. 6. Voltage in case of DOL starter for different loading conditions 110 100 % Voltage

100% Load

90

75% Load

80 70 60

4801

4401

4001

3601

3201

2801

2401

50 1

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 4.3 sec 16.5sec for DOL & auto-transformer starter respectively.

100% Load 75% Load

99.9

601

2.7

2001

1.96IFL

100

2.1

501

2.92

97.27 Vrtd

1601

1.7IFL

401

100%

12.47 2.77 IFL

Recorded 2.26

301

432.3 v 98.25 Vrtd

1201

Simulation

Simulation results for voltage obtained for different loading conditions and are shown in fig. 6. & fig. 7.

%Voltage

Load

1601

Time in mSec

Fig. 5. Current in case of Auto-transformer starter for different loading conditions

Table 5 Summary for Auto-transformer starter tst Para Ist Vst Ist Vst tst sec meter (A) sec 75 %

1201

2.5

801

2.67 IFL

201

1.20

50

801

98.25%

97.27%

100

2

101

6.6 IFL

432.3 v

200

401

27..72 A

2.48 IFL

250

150

Recorded 1.10

75% Load

401

Simulation

tst sec

300

1

Load 75 % 100%

100% Load

350 %Current

Para meter

Table 4 Summary for DOL starter Tst Ist Vst Ist Vst sec

Time in mSec

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

studies & 2 times the rated current is the observed values. In case of DOL starter starting time varies from 1.1sec to 1.2 sec from case 3 to case 1 & the observed value is 2.5 sec. For auto transformer starter the starting time varies from 2.9 sec to 3.03 sec fro case 3 to case 1 & the observed value is 2.7 sec 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.

700 600 % Current

500 400 Case 3

300

Case 2

200

Case 1

100

1351

1201

1051

901

751

601

451

301

151

1

0

T im e in m S ec

Fig. 8. Current in case of DOL starter for different case studies Case 3

415 365

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

Case

Case 1

315 % Current

DISCUSSION & FUTURE SCOPE

265 215 165 115

3301

3001

2701

2401

2101

1801

1501

1201

901

601

301

1

65

T ime in m S ec

Fig. 9. Current in case of Autotransformer starter for different case studies 101 100.5 C as e 1

% Voltage

100 99.5 99

C as e 3 C as e 2

98.5

REFERENCES

98 97.5

[1]

1351

1201

1051

901

751

601

451

301

151

1

97

T im e in mS ec

Fig.10. Voltage in case of DOL starter for different case

[2]

105

[3]

C as e 1

% Voltage

96

C as e 3 C as e 2

87 78

[4]

69

3601

3301

3001

2701

2401

2101

1801

1501

901

1201

601

[5] 301

1

60

[6]

T ime in mS ec

Fig.11. Voltage in case of Autotransformer starter for different case studies

[7] [8]

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

499

[9] [10] [11] [12] [13] [14] [15] [16]

John H. Stout ” Capacitor Starting Of Large Motors ” IEEE Transactions On Industry Applications, Vol. Ia-14, No. 3, May/June 1978, pp. 209212 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. 292-305 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. 1688- 1696. Solveson M.G. “Soft-Started Induction Motor Modeling” IEEE Transactions On Industry Applications Vol.42 No.4, July/August 2006, pp 973-982 Rezek A. J., “Energy Conservation With Use Of Soft Starter” IEEE 2000, pp 354-359 Blaabjerg F. “Can Soft Starters Help Save Energy”, IEEE Industry Applications Magazine Sep/Oct 1997 Pp 56-66 Blaabjerg F. “Comparative Study Of Energy Saving Benefits In Soft Starters For 3ph Im”, IEEE Industry Applications Magazine Sep/Oct 1995 pp 367-373 AC Induction Motor Fundamentals by Rakesh Parekh, Microchip Technology Inc “Induction motor - protection and starting by viv cohen” - Circuit Breaker Industries, P.O. Box 881,Johannesburg 2000, South Africa. Induction motors Parameters extraction by Sinisa Jurkovie Performance chart for IM from Kirloskar Electric Performance chart for IM ABB Group- Automation & Power Technologies Consultancy with Mr. Rao, Nagpur motors, MIDC Hingna Textbook of Electrical Engg. Vol-II B.L.Theraja Basic Electrical Machines – V.K.Mehta Electrical Machine- P.S.Bimbhra