Some Important Compatibility Problems And Their Elimination In The Application Of Induction Motor With Variable Frequency Drive

Transcription

1 Some Important Compatibility Problems And Their Elimination In The Application Of Induction Motor With Variable Frequency Drive S. Balakrishna Abstract It is very common to use Variable Frequency Drive (VFD) with Induction Motor (IM) in Industry due to its numerous advantages. The advance of power electronics technology has also facilitated mass utilization of this combination. However, the application of IM with VFD may affect the actual design, performance and reliability of IM. When small and medium size motors are improperly used with VFD s, windings and bearings can fail prematurely. The Corona Inception Voltage (CIV), the voltage at which corona degradation begins in an insulation system is discussed, and the major factors contributing to the CIV of a motor s insulation system are presented. The concept of shaft induced voltage (SIV) and its effect on bearings is also stated. The aim of this paper is to investigate the sources of the failures when motor and drive combination is set into practice and to discuss the possible remedies to eliminate them. In addition, harmonics, resonance and efficiency aspects related with VFD s are also reviewed. Keywords Variable Frequency Drive, Insulation stress, Bearing Failure, CIV, SIV 1 Introduction In general, industrial loads require precise adjustment of speed over the complete speed range. In the past, IMs were used mainly in the applications requiring constant speed due to expensive and inefficient conventional control methods. Hence, the DC motor drives were dominated AC drives in all variable speed applications even though they require frequent maintenance and regular replacement of commutator and brushes. On the other hand, AC motor drives especially squirrel cage IM s are cheaper, lighter, more efficient and require less maintenance [1]. With the advent of solid-state electronic switches technology and its rapid development, AC motor drives are now capable to meet all performance S. Balakrishna, Lecturer, Electrical Engineering Department, Faculty of Engineering, Bahir Dar University, Ethiopia, balaskrishna@yahoo.co.in criteria and now the trend is towards replacing all conventional DC motor drives with AC motor drives. The advantages of VFD with motor includes [5]. i) Stepless speed variation ii) Heavy Load Inertia starting iii) High starting torque iv) Low starting current v) High efficiency even at low speeds vi) High power factor However, to achieve all these merits, IM should be properly associated with VFD. This paper discusses the compatibility problems that could arise while connecting the combination and finally discusses the possible remedies. The inter-turn insulation system failures in induction motors fed by pulse width modulated (PWM) variable speed controllers have affected the industry especially after the introduction of IGBT based drives. These failures are mainly due to high voltage stresses imposed upon the motor by the drive [2]. Several papers have been written which conclude that the repeated dielectric stress imposed upon the insulation system of the motor by high voltage transients is the primary cause of premature winding failures. Due to the fact that these voltages are unevenly distributed throughout the coils, high levels of stress are present within the first several turns of particular coils in the winding. These stress levels are substantially below the dielectric strength of the insulation system, but they are often at a level, which causes steady degradation of the organic material in the enamel coating of the magnet wire and the varnish used by the motor manufacturer. The level at which this voltage stress becomes harmful is called the Corona Inception Voltage (CIV) [2]. Since around 1907, many technical papers have been published to describe that shaft voltages and bearing currents affect the life of the bearings. As per [4], air gap eccentricity and rotor shorted turns are the main reasons for generating irregularities in the magnetic circuit that leads to a small amount of flux to link the shaft resulting in a electromotive force between the shaft ends. This SIV generates an electrical current that flows through the bearings, deteriorating the

2 ICECE, October 29 th to November 1 st 2003 lubricant and electro-eroding the bearing raceways ending up in an eventual bearing failure. This paper is organized into 3 major parts. The first part discusses how VFD create high voltage pulses at motor which rises stress on motor insulation and the second part discusses the sources of bearing currents and how these currents cause baring failures. Finally the third part will overview other compatibility issues such as harmonics, resonance, efficiency aspectsetc. the rectangular pulse until it stops again at T. Current will now flow forward in the leads again. This cycle will continue until the energy originally stored in the leads inductor is dissipated by the resistance of the leads [3]. 2 Insulation Stress Generally, VFD s produce an output of rectangular voltage pulses. The height of each pulse is equal to the DC bus voltage as shown in Fig.1 [6]. The pulses are generated at the rate called carrier frequency usually 2 to 20K pulses/sec. The width of each pulse is continuously adjusted to create the mean voltage necessary to drive the motor. This technique is called PWM. Each edge of these rectangular pulses causes a voltage overshoot at the motor terminals as shown in Fig.2 [3]. The changing of carrier frequency affects the number of overshoots per second. The over shoot will settle down to the DC bus level after several ringing cycles. Figure 2: Inductor-Capacitor Model 2.2 Effect Of Lead Length The overshoot and ringing is mainly due to the energy stored in the leads during the initial build up. Longer leads will cause higher inductance. The longer leads cause increase the time required to charge the capacitance of the motor resulting in more energy stored in the leads and more overshoot. Generally, longer leads will cause more overshoot. With reference to [3] 20ft leads can create nearly 50% overshoot. 2.3 Effect Of Rise Time Figure 1: PWM Inverter Line Voltage Waveform 2.1 Cause of Voltage Over Shoot To understand the overshoot, consider the leads between motor and VFD as an inductor, and the motor as a capacitor [3]. A rectangular pulse from the VFD drives a current in the inductor (leads) and it charges the capacitor upto a voltage equal to the peak of the rectangular pulse. In Fig.2, at point R, the inductor has no voltage across it but still has current flowing in it. The energy stored in the inductor will force the current to continue and the capacitor voltage will increase until the current stops. At point S, the capacitor voltage is above the peak of the VFD s pulse. The voltage across the leads is reversed and current will flow from the motor back into the VFD re-energizing the inductance of the leads. The capacitor voltage now goes below the peak of The time required for the pulse to go from 10% to 90% of its final voltage is called the rise time. The rise time is controlled by the transistors and the gate circuit in the VFD and is not adjustable. The rise time allows the leads to store energy, generating the overshoot and also causes the voltage within the motor to be unevenly distributed. If the rise of the initial rectangular pulse is slow, the capacitor may charge as quickly as the pulse rises, resulting in little voltage applied across the leads and little energy stored. A faster rise time creates more stored energy resulting in more overshoot [3]. 2.4 Uneven Voltage Distribution As stated above, the rise time at the motor also affects the distribution of voltage within the motor. The first coil in the motor will have more voltage across it than the rest of the coils. The percentage of peak terminal voltage on the first coil will be greater if the rise time is faster. The first coil may have ten times as much voltage as when operating across the line. 90

3 International Conference on Electrical & Computer Engineering (ICECE 2003) 2.5 Determination Of Maximum Over Shoot It is important to know the peak voltage at the motor so that the motor can be designed to withstand against it. The peak voltage at the terminals of a motor under VFD operation depends on the input voltage to the VFD and the amount of overshoot at the motor [3]. The actual overshoot due to the leads is difficult to determine without details of rise time, lead inductance etc. As per [3], tests have shown that the maximum overshoot is 2.1 times the VFD output. The VFD s input is normally known but one must account for high line conditions, which is an additional 10%. Putting all these factors together for a 440V line we have, i) High line voltage = 484 Vrms ii) Rectified output = Vdc iii) Max. Overshoot = Vpk Thus a 440V VFD may produce V spikes at the motor. It shows that, maximum possible peak is nearly 3.27 times the nominal rms line voltage. This is the voltage that one should expect if designing a motor for general-purpose inverter applications. However, it is not always necessary to design to the maximum possible value [3]. The OEM (Original Equipment Manufacturers) applications may have short leads or slow VFD s and a motor could be designed to a lower level. However, this should be carried out only for a product under change control to avoid system changes that could result in motor damage. 2.6 Effect Of High Voltage And Corona When two sample conductors have potential between them, an electric field is distributed within, and outside, the insulation on the conductors (Fig. 3) [3]. When the field strength in the air is high enough, the air breaks down. This break down is referred as corona. The discharge has several detrimental affects. It causes oxygen to recombine into O 3 (ozone). The ozone is very reactive and quickly combines with chemicals in the insulation. The corona will start when the voltage reaches a certain level called the Corona Inception Voltage (CIV). The CIV is dependent on the spacing and type of insulation, temperature, surface features, and humidity. 2.7 Factors Affecting CIV The corona inception voltage of a motor insulation system is a value, which is determined substantially by the design and construction of the motor. There are several factors, which play an important role in determining CIV [2] Winding Type Figure 3: Location of CORONA In case of random wound or coil-inserted motor windings, there is a possibility of having close proximity between start and finish turns. As high voltage transients cause uneven voltage distribution in the coil, this close proximity will help to have very high potential between adjacent turns. This situation often results in an area where corona degradation begins. However, if the turns of a coil are placed in the stator slot sequentially (as in a form wound stator), there is a substantially decreased probability that start and finish turns to be close to one another within the winding [2] Varnish Coating The stator varnishing process also affects the value of CIV. The principal purpose of applying varnish is to increase the dielectric strength of the insulation system of stator. To have higher CIV to act against high voltage transients, number of varnish coatings has to be increased (through multiple dips and bakes) Phase Paper The advantage of adding phase paper between various phase windings is to avoid phase shorts. A motor without phase paper will have a lower CIV than an identical motor built using this type of added insulation protection because of the reduced amount of material between separate phases of the winding [2] Wire Size and Enamel Coating The size of the magnet wire used in the windings can also have effect on CIV, because larger diameter wires inherently have heavier insulation build than of wires with smaller diameter. For example, a #14 SWG heavy build wire may have a 40% higher CIV than a #18 SWG wire. The value 91

4 ICECE, October 29 th to November 1 st 2003 of CIV can also be increased by the utilization of traditional magnet wire with a thicker enamel coating Wire Handling and Coil Insertion During the wire handling and coil insertion processes, if a magnet wire is damaged (or even for slight scratch that may not be detected by surge testers), the affected part of the wire will become a corona inception region in future motor operation [2]. Hence, special care shall be taken at each step of construction and testing of an inverter duty motor Operating Temperature The temperature at which the motor operates will have definite impact on the value of CIV. For example, CIV of twisted pair of a motor with Class F rating operating at its maximum temperature (155 0 C) is considerably lower than the value of CIV of a same twisted pair at room temperature. Therefore, The objective in practical inverter-duty motor design is to design and build a motor insulation system with a CIV that is high enough at operating temperature to ensure that degradation causing partial discharges do not occur in the motor windings when it is subjected to the high voltage stress generated by a variable speed drive. If the insulation system design does not allow these partial discharges to exist, then the resultant degradation will not occur [2]. 2.8 Recommended Solutions Use Of Power Conditioning Devices And Reduced Cable Length Using the power conditioning equipment such as isolating transformers, filters and reactors as well as restricting the cable length between motor and VFD, limit the effect of voltage reflections. However, the power conditioning accessories needs added cost and, in addition, compromise on efficiency and reduction in torque is necessary as they simply redirect the voltage stress to other drive components [2]. As per [3], the critical cable length is given by L critical = ν cable (t r /2) (1) Where ν cable is peak wave propagation speed, usually 150m/µs t r is the voltage rise time supplied by the inverter manufacturer The cable length should be kept as minimum as possible depending on the value of rise time and cable impedance. But restricting the cable length may need relaxation on installation Use Of Inverter Duty Magnet Wires The insulating enamel of this recently introduced inverter duty magnet wires contain inorganic component in addition to organic component. The purpose of inorganic content is to help the wire not to degrade against high voltage transients i.e. the inorganic component aids the wire insulation system as a whole to withstand the repeated dielectric stress [2]. The organic component of wire, however, degrades the insulation system under voltage stress above CIV. Anyhow; the new wire can increase the life of motors compared to motors built using traditional wires under voltage stresses greater than CIV Designing A Motor For Corona Free Operation To solve the problem of insulation stress caused by VFDs, the motor itself can be designed to operate reliably with the voltage overshoot. Formed coils, layered insulation, semi conductive layers, mica tapes, etc. are part of the designer s methodology. It is necessary to design the motor to be corona free up to the expected peak voltage. It requires knowing the CIV of the wire itself and the distribution of the voltage in the motor. As per [3], the recommended design approach can be stated as, Select wire and winding layout such that the peak voltage expected between touching wires is less than the CIV of the wire itself, otherwise add insulation between them if it is not possible. 3 Sources Of Shaft Voltages And Bearing Currents As we discussed before, the Shaft Induced Voltage (SIV) generates an electrical current that flows through the bearings, deteriorating the lubricant and electro-eroding the bearing raceways ending up in an eventual bearing failure. The SIV and bearing currents can be generated by the following five methods [3]: Potential applied to the shaft Electrostatic voltages Shaft magnetization effect Dissymmetry effects Common mode voltages generated from PWM inverters 3.1 Potential Applied To The Shaft Incidental or accidental application of potential to the shaft drives a current to flow through the bearings and ground, 92

5 International Conference on Electrical & Computer Engineering (ICECE 2003) which in turn, leads to bearing failure. For example, if system accessories like frames, mounting pads...etc electrically coupled to the shaft. 3.2 Electrostatic Voltages The electrostatic voltages are induced by the charged particles that may be associated with shaft mounted fans or certain belt driven applications especially in low humidity environments. 3.3 Shaft Magnetization Effect Sometimes shaft is magnetized by unbalanced ampere-turns surrounding it. These turns create a flux that travels axially along the shaft, across the bearing, through the frame and returns through the other bearing (Fig. 4) [3]. Figure 4: Bearing current generation due to axial shaft flux caused by unbalanced ampere turns. By transformer action, shaft voltages are produced if the magnetic paths in the rotor and/or the stator are not balanced. This may be due to the asymmetries in the stator or in the rotor (keyways) (Fig. 5) [3]. The shaft, bearings, ends brackets, and housing form a one turn secondary around and through the motor s core. If bearings conduct, a large current will flow but this circulating current does not flow to ground or to the supply. The induced shaft voltage is very low in amplitude and does not easily break down the grease film of the bearing. However, at low speed the grease film is minimal and current may flow. 3.5 Common Mode Voltages The power electronic converters are generally designed for a particular type of energy conversion, such as AC to DC or AC to AC, DC to AC or DC to DC. In most of these converters, the desired output voltages or currents are produced as differential mode voltages. The primary cause of bearing currents is due to common mode voltages caused when PWM inverters excite the motor [3]. A capacitive coupling exists between the stator winding and the rotor surface (Fig. 6). High frequency common mode voltage, produced by the VFD, forces current through this coupling, the rotor, the shaft, and bearings to the grounded end bracket. The amplitude of the common mode current (CMC) depends on the common mode impedance. When motor and inverter are earthed this impedance will decrease hence the increase in CMC. However, both motor and inverter must be solidly earthed to prevent electric shock from external parts How Common Mode Current Flow? To understand the path of the common mode current in motor, examine the following steps with the help of Fig. 6: Figure 5: Dissymmetry Effects due to keyway in rotor. Figure 6: Common Mode Basic Circuit 3.4 Dissymmetry Effects 1. The output from VFD is connected to stator windings. The current will flow from the stator winding to the rotor surface through the parasitic coupling capacitance, C w existing between them. 93

6 ICECE, October 29 th to November 1 st The current charges the other capacitance, C r existing from the rotor to the ground. 3. The bearing acts like an ON/OFF switch across the capacitance C r and is competent enough to discharge it. If shaft voltage, SIV reaches an adequate level, the bearing will conduct and C r will discharge through the ball and races. Once the bearing is conducting, the common mode current flows through C w and the bearing only bypassing capacitance C r. 4. The VFD forces current through the capacitor divider formed by C w and C r. The result is a common mode voltage on the rotor and shaft, proportional to the source common mode voltage at the VFD and the ratio of C r to C w. 5. The capacitor C w is formed by the winding in the slot and the end turns as one surface and the rotor surface and end rings as the other. The air gap and top sticks in the slot are the dielectric (Fig. 7) [3]. The capacitor C r is formed between the rotor and the ground with the entire rotor surface acting as one conductive surface, and the stator assembly (stator laminations, end brackets, and fan shrouds) forming the other conductive surface. The air gap becomes the insulating material or dielectric. C w is normally much smaller than C r. 7. The bearing current thus has two modes, conduction mode when Cr is not charged and discharge mode when C r is discharging through it. Conduction mode bearing currents exhibit continuous current flow through the bearings. This form of bearing current does not result in premature bearing failure because current flows continuously without arcing. Discharge mode bearing currents occur at random when the grease film momentarily breaks down [3]. Although both types of currents are present at the same time, the discharge bearing current is the most critical one. The conductive bearing current is usually of less harmful to bearings, since it is a low amplitude current that flows continuously without arcing. However it increases bearing temperature accelerating grease deterioration decreasing bearing life. On the other hand, the high energy level of the discharge bearing current works like an electro-erosion machine resulting in bearing pits or flutes. The amplitude of bearing currents depends upon operating conditions such as speed, temperature, lubrication type, motor size, etc. From all these factors, motor size is probably the most significant, since the larger the motor, the larger is its parasitic capacitances (C = ɛa/d). In addition, motor design will have reasonable influence over bearing current amplitudes [4]. 3.6 Recommended Remedies Figure 7: Electric machine showing parasitic couplings 6. The rotor is supported by bearings with a grease film that is not conductive. At high speeds, an even distribution of the grease film exists, and the rotor is not in contact with the outer (grounded) bearing race. The rotor voltage can increase with respect to ground. When this voltage builds up to a level capable of breaking down the grease film, a spark occurs and discharge mode current flows through the bearing. At low speeds, the grease film is minimized; the balls often make contact with the race. The rotor voltage does not build. The common mode current flows through the bearing in a conductive mode. Some papers say that a common mode choke or filter inserted between inverter output and motor input can be used as a means of reducing bearing current amplitude. Some state that a brush short-circuiting rotor to stator will eliminate bearing current. Insulating the bearings from the motor frame certainly prevents the conduction mode bearing current from flowing through them, but special attention should be paid to the discharge mode bearing current. Care should be taken with the capacitance of the insulation system. With the technological advance in machinery and materials, vast improvements in the tolerances and quality of electric motors were achieved in minimizing flux irregularities that reduced the SIV amplitude to a safe value [4] Isolation of Shaft The isolation of the shaft may be achieved by placing an insulating material between the shaft and the bearings or between the bearings and the motor frame (Fig. 8) [3]. The bearing failure by discharge mode currents could be eliminated, as there is no path for current to flow. This solution works well for directly driven loads such as blowers but is not suitable when driving a load having a bearing with a grounded inner race (such as gear box) as when the motor s bearings are insulated from ground, then the potential for discharge through the load exists. 94

7 International Conference on Electrical & Computer Engineering (ICECE 2003) Shaft Grounding Figure 8: Bearing Insulation The shaft grounding is accomplished by grounding the shaft with the help of brush rigging (or slip rings) so that current can be diverted through low resistance path. The disadvantage of grounding the shaft is that an undesired path for circulating currents due to the existence of dissymmetry [3]. These circulating currents may flow through the shaft grounding device and the motor bearing or load bearings in the end opposite to the shaft-grounding device. The differential mode has low open circuit voltage and cannot normally break down the grease film. However, the differential current will flow however at low speeds when the balls are in contact with the race Using Conductive Grease By using conductive grease in the bearing will provide a continuous path for current to flow. The charging of the rotor to winding capacitance, C w could be eliminated. Also, the effect of the intermittent point contact of the balls to the race is avoided [3]. The conductive grease is effectively the same as using a shaft-grounding device; therefore, it suffers from the same possibility of differential current Covering with Faraday Shield A grounded conductive material, placed between the rotor and the stator (including end turns), will eliminate the current flow through the bearings (Fig. 9)[3]. This solution is attractive because it eliminates the coupling to the rotor Using Common Mode Choke and Capacitor Combination The common mode voltages are filtered by using the combination of common mode choke and capacitors. To achieve Figure 9: Faraday Shield optimum performance, this filter network has to be tuned according to the application [3]. 4 Other Drive Related Aspects 4.1 Noise Level And Vibration The noise level will be increased by the harmonics generated in the system. As per [4], the sound pressure level at A scale at motor rated speed is increase by anything between 2 and 15 dba with a PWM inverter. This value agrees with IEC and NEMA, MG 1-30 specification. The extra noise level produced depends mainly on the inverter switching frequency and harmonic content. The harmonics also excite motors in different frequencies resulting in an overall increase in their vibration level. Depending on the severity of the vibration, an irregular magnetic field might be formed generating a shaft voltage. Moreover, the mechanical life of the motor parts can be drastically reduced. Therefore, as most motors are operated below rated speed, their natural frequency should be above 50Hz [4]. 4.2 Resonance The inverters will produce torque ripples on the motor shaft during their operation [4]. Thus, the inverters shall be programmed not to operate close to the natural frequencies of a machine in order to avoid resonance. 4.3 Motor Efficiency The Copper and Iron losses of machine will be increased by the high frequency harmonics produced by inverters; hence there will be reduction in motor efficiency. The expected motor efficiency with reference to [4] can be calculated using 95

8 ICECE, October 29 th to November 1 st 2003 η i = (DF H) 2 /(1/η + (DF H 2 1)) (2) Where η is the motor efficiency fed by a sinusoidal supply η i is the motor efficiency fed by inverter and DFH is the btorque-derating factor as a function of the harmonic content (HVF) as per NEMA, MG As different materials used in machine, they behave differently even under the same operating conditions, for a given sinusoidal direct supply, hence, one motor is more efficient than other. However, when it is fed with inverter, the former might have higher drop in efficiency than later. As per [4], nearly 10% higher rms current will be drawn to supply the same output, hence an increase in motor operating temperature. On average, inverter fed motors will have a temperature increase of about 15 o C, at rated speed and load. As we know [6], the motor life is halved for every 10 degrees rise in above the temperature limit of its insulation class. Hence, in order to avoid this drastic decrease in the life of a motor, it is recommended to use higher temperature materials with better thermal dissipation capabilities in the inverter duty motors. 5 Conclusion When IM is used in association with VFD, there are two major issues to be considered namely Insulation Stress and Bearing Failure. The maximum voltage overshoot is mainly due to the energy stored in the leads. A faster rise time creates more stored energy that results in more overshoot and also affects the distribution of voltage within the motor. It is shown that, the maximum possible peak is nearly 3.27 times the nominal rms line voltage. Therefore, while designing insulation system for an inverter duty motor, designer should keep this point in mind. The CIV depends on type of winding, varnish coating, phase paper, wire size, enamel coating and operating temperature. It is better to design and build a motor with sufficiently high CIV at operating temperature to make sure that degradation causing partial discharges does not occur when it is subjected to the high insulating stress produced by VFD. Use of power conditioning devices, reduced cable length, use of new inverter duty magnet wire and designing motor for corona free operation are the solutions for solving the problem of insulation stress. However, as power-conditioning devices require compromise on both cost and performance, and as minimizing cable length requires some relaxation on installation; use of new inverter duty magnet wires and designing a motor for corona free operation are the two best choices. The SIV generates an electrical current that flows through the bearings, deteriorating the lubricant and electro-eroding the bearing raceways ending up in an eventual bearing failure. The SIV and bearing currents can be generated by potential applied to the shaft, electrostatic voltages, shaft magnetization effect, dissymmetry effects, and common mode voltages. Placing an insulating material between the shaft and the bearings or between the bearings and the motor frame, grounding the shaft, using conductive grease, covering with Faraday Shield and use of choke and capacitor combination are the remedial measures for shaft bearing failure. It is to be noted that, as per [4], problem of insulation stress is more crucial than bearing failure problem. It is recommended to adopt harmonic rejection filtering techniques so that noise level and vibration resulting from the application of induction motor with VFD could be controlled. As use of VFD increases the temperature rise in motor, it is recommended to use good quality high temperature materials having better thermal dissipation capabilities in the inverter duty motors to avoid the reduction in motor life. Acknowledgement The author is grateful to Dr Mesfin Belachew, Electrical Department, Bahir Dar University, Ethiopia and Mr.Yilma Taye, Head of the Dept, Electrical Engineering Dept, Bahir Dar University, Ethiopia for their help and encouragement to carry out this work and for providing computational facilities. The author is also indebted to his beloved wife Mrs. Vasavi Latha for her perpetual encouragement in bringing out this article. References [1] Gopal K.Dubey, Fundamentals Of Electric Drives, Narosa Publishing House, New Delhi, [2] Evaluating Inverter-Duty Motor Insulation Systems Using Corona Inception Voltage, Lincoln Motor Dvn, Motor brief TB100, October 1997, [3] Langhorst, P., Hancock, C., The Simple Truth About Motor-Drive Compatibility, MagneTek, Inc. [4] Alex Settimi, S., The Unseen Truth Behind Motors Fed By Inverters, WEG-New Zealand, [5] Ney E.T. Merheb, Application Of Induction Motors With Variable Frequency Drives, GE Motors, artigos/asdi.pdf [6] S.Balakrishna and Makkena Suresh, Optimization Techniques For Energy Efficiency And Control Of Induction Motors, EJTE&SD Journal of Bahir Dar Uinversity, Vol1, No.2, pp 20-25, June