Getting prepared for practical exams
Posted Date: 06-Jan-2012
Practical or lab term works are important part of the curriculum. Analysis of the practical depends on the final result obtained. To score well viva-voce and conduction of the experiment should be carried out successfully. This article highlights on the key factors for getting prepared for practical exams.
How well a subject can be understood? The answer is by performing an experiment related to it. Every subject begins with theory and every theory gets a shape of completeness when it is implemented practically. The analysis of every practical is not an easy task. Theoretical aspects of the subject are exposed effectively only when practical results are obtained. Practical shows the platform to understand the depth of the subject and makes it possible to visualize how stuffs really work. There are different procedures to conduct practical of different subjects. I would like to emphasize on the practical exams conducted in electrical engineering and will categorize it according to type of the lab term works conducted in the machines lab.
Basic machines used in this lab are DC machines, transformers, induction motors and synchronous machines. This lab can be again classified as:
• Transformers and induction motor lab.
• DC machines and synchronous machines lab.
Important terms to remember :
You should always take care that circuit diagram is perfectly correct. As you will be dealing with a live supply one can not take chance with wrong connections. Make sure that ammeters are always connected in series and voltmeters are always connected in parallel. Connections should be tightly made to avoid lose contacts between the equipments. If you are modifying any connections then turn off the supply then proceed. In some cases change over in connections are done when the supply is on. Ex: Watt meter connections are changed when the pointer moves beyond scale zero. These changes can be dealt carefully. It is always advisable to wear shoes while performing experiments in this lab. If you are wearing metallic bracelets then be careful when dealing with live wires and never play with them. These are some precautions and important factors you should always remember in machines lab.
Important theoretical aspects related to machines :
• Principal of operation of the machine
• Loading and unloading conditions of the machine
• Open circuit and short circuit characteristics
• Expected results when analyzed with theoretical calculations.
During the regular course students are guided by lab instructor/ faculty for every step. But in the exams they should perform individually with utmost care. Every machine has a different characteristics and it is necessary to understand the basics of it. Here are some important points to remember for machines mentioned individually while performing the experiments in the lab.
It is a static device which works on the principal of electromagnetic induction.
• Terminal voltage of a transformer reduces when load on the transformer secondary increases. Under loading conditions secondary voltage should never be more than the no load voltage.
• When secondary is short circuited it should be taken care that at primary of a transformer high voltage should never be applied. Because when secondary windings are short circuited high current will be circulating even after the application of small voltage at the primary.
This also works on the principle of electromagnetic induction.
• Machine has a low starting torque which needs to be connected with external resistance at starting till the rated speed is developed. This is incorporated using rheostats (which are also variable resistors) in series with the armature to limit the starting current.
• Before turning off the supply to the motor, reduce the load then turn off.
• Speed of the motor reduces with increase in load.
• Induction motor can be used as a prime mover for generators too.
DC machines( Generators and motors)
• Depending on the type of the winding DC machines are mainly classified as shunt, series and compound machines. There are some operating constraints need to be followed for these machines.
• Shunt machines are considered to be constant speed machines as with variation of load speed will not vary much.
• Series machines should never be started without load. As speed is inversely proportional to flux, lesser the load and higher will be the speed. Because of this reason load should be put on dc series machines when they are started.
These are also called as three phase alternators. When it is incorporated with other machines it is important to synchronize the frequency. Frequency and speed of these machines will normally remain constant.
Basic practical write up:
• Aim of the experiment
• Equipments required
• Circuit diagram
• Calculations and related graph
The proper record book should be maintained in which all the above points should be covered for every experiment briefly so that they can follow the procedure while preparing for the exam. To write in the examination, except related theory for the experiment they should follow all the above mentioned steps properly to score good marks. Marks will be distributed according to overall performance of the student.
This is an important part of the practical exam. This is nothing but student will be tested his/her knowledge about the subject apart from the curriculum. It is an oral exam which an examiner takes and allots marks according to how they answer confidently and correctly. To prepare for viva voce one should go through the basic points and should understand very well that why these experiments are being conducted. If a student understands idea behind these experiments then good performance can be expected from him/her.
Learning theoretical concepts is different from practically performing the experiments. Many students develop a fear to perform experiments individually in the exam thinking that it is very difficult. Referring your own written record book often, will make you understand and follow things easily. Practice circuit diagrams and do calculations repeatedly. You should better know the characteristics of the experiment which enables you to understand if you are going wrong. Practical exam is not your enemy. It is also a guide for your future. Learn it, understand it and perform it.
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