Our Beloved friend Piumal Gunawardana has passed away today morning (31st August 2007). He was affected by a serious cancer in his throat. He is a final year student of faculty of engineering,
The author of this post is a friend of him, within last eight years.
http://samitharansara.blogspot.com/2007/06/piyumal-gunawardana-current-situation.html
The hybrid stepper motor consists of two pieces of soft iron, as well as an axially magnetized, round rotor. The term hybrid is derived from the fact that the motor is operated under the combined principles of the permanent magnet and variable-reluctance stepper motors.
The stator core structure of a hybrid motor is essentially the same as its VR counterpart. The main difference is that in the VR motor, only one of the two coils of one phase is wound on one pole, while a typical hybrid motor will have coils of two different phases wound on the same pole. The two coils at a pole are wound in a configuration known as a bifilar connection.
The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the air gap.
This type provides better performance with respect to step resolution, torque and speed. The hybrid stepper motor combines the best features of both the PM and VR type stepper motors. These facts further increase the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types.
The rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.
A stepper motor is a brush less, synchronous electric motor that can divide a full rotation into a large number of steps, for example, 200 steps. Thus the motor can be turned to a precise angle. There are three major types of stepper motors available in practice.
1. Variable Reluctance Stepper Motors
2. Permanent Magnet Stepper Motors
3. Hybrid Stepper Motors
The stepper motor required a special driving circuit. This is basically done by using an array of power transistors, or switches.
The theory behind stepper motor operation is the magnetic attraction. The stepper rotor will be non coiled rotor. The stator consists with a set of coils and those coils are connected as a set manner. When one set of coils energized the rotor pole will be subject to attract to particular pole and because of that the rotor axis will subject to rotate.
A fault in a power system is defined as a condition which adds a disturbance to the proper operation and the stability of the system. In simple words, a fault is referred to any abnormal condition, which has occurred in a power system, has properly operated prior to the abnormal condition.
A fault may occur due to various reasons such as environmental conditions, such as lighting, storms, etc., improper maintenance of the power system; insulation failures etc. They can cause the system instability. Therefore it is very important to study the faults and get necessary actions to overcome such situations.
It is essential to ensure the protection of the power system because a fault may cause severe damages to the expensive equipments that connected to the system, even life hazards. When in design phase of a power system, the designer should pay his attention to the protection scheme of the system, should implement the system as a system which has high degree of protection. The method of fault analyze by the aid of sequence components, is a great tool to the designer, to calculate the fault level at any point of the system prior to the implementation as well as after. The results of the fault study will be helping the designer to select the correct ratings for the protective devices such as breakers and relays.
The faults in power systems can mainly be classified as Series faults and Parallel faults. This classification is done in a broader sense. Series faults are referred as the faults that occur along the transmission line serially such as conductor aging, breaking etc. Here we consider only the Parallel faults. Parallel faults may subdivide in to two categories as Symmetrical faults and Asymmetrical faults, by the appearance of them to the system.
Assumptions made in the experiment
In order to reduce the complexity and for a trouble-free implementation in the DC network analyzer the following assumptions were made.
The pre-fault bus voltage is 1 p.u.
Fault currents are very much higher than the load currents and therefore the load currents were neglected
Line resistance is smaller compared to the reactance therefore it can be neglected.
The bus voltages kept in nominal values, therefore it is considered as the pre fault bus voltage is 1 in per unit basis. It is a valid assumption to neglect load currents compared with fault currents because fault currents are normally hundreds of times than the load currents. Normally the lines are made of good conductor materials to minimize the losses and voltage drops. So there resistance compared with reactance is very much smaller. Hence the third assumption is also a valid one and it also eases the analysis.
Generally, the majority of faults in power systems are asymmetrical in nature. Single Line to Ground, Line to Line, Double lines to Ground are the possible combinations of faults that can occur within the system. When this type of fault occurs, it gives rise to unsymmetrical currents. Unsymmetrical currents have different magnitudes in the three lines with unequal phase displacement.
The asymmetrical faults can be analyzed using symmetrical component method. This method resolves the unbalanced three phase system in to three systems, which have same phase sequence as the power system, opposite sequence to the power system, and independent in sequence to the power system, which known as positive sequence, negative sequence and zero sequence respectively. (Actually this is a theorem which can apply for any n number of unbalanced vector systems, which states that they can resolve in to n-1 number of balanced systems and an independent system to the source).
By using this theorem, we can derive the sequence networks to the unbalanced system. Then we can change the interconnections between the sequence networks to represent any type of fault that occur within the system.
Facts, which should consider when deriving, sequence networks
The positive and negative sequence impedances of a line are the same and it is the normal impedance of the line. However the zero sequence impedance is much grater than the positive or negative sequence impedance.
The positive sequence impedance of a synchronous generator is equal to the synchronous impedance of the machine. The negative sequence impedance is much less than the positive sequence impedance. The zero sequence impedance is a variable.
Transformers
The positive and negative sequence impedances of a transformer are equal and it is the impedance of the transformer. But the zero sequence impedance depends on the earth connection.
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