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Questions and Answers
What is the primary objective of the load test on an alternator experiment?
What happens to the generated voltage in the armature when the excitation current is increased?
What is the formula to calculate the efficiency of the alternator?
What is the term for the voltage rise observed when the load is capacitive?
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What is the component of the alternator that is responsible for adjusting the A.C. voltage generated at the terminals?
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What is the assumption made about the mechanical loss in the alternator efficiency calculation?
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What is the purpose of using a rheostat to adjust the speed of the dc motor?
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What is the main purpose of the experiment described in the text?
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What is the function of the variable Y-connect resistive load in the experiment?
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What is the purpose of short-circuiting the terminals A1, A2, and A3?
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What is the rated voltage of the alternator?
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What is the purpose of using a single-phase variac in the experiment?
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Study Notes
Load Test on an Alternator
- The objective of the experiment is to perform a load test on an alternator to determine its regulation and efficiency.
Load Characteristics of an Alternator
- The AC voltage generated at the alternator terminals can be adjusted by varying the excitation current flowing through the field winding.
- The higher the excitation current, the higher the generated voltage in the armature.
- The open-circuit characteristic of the alternator does not commence from the origin, indicating a residual flux.
- The terminal voltage will be equal to the alternator's rated voltage when the excitation current is zero.
Efficiency of an Alternator
- Alternator efficiency is determined from input (mechanical power) and output (electrical power) powers.
- Efficiency (η) is calculated as (Pout / Pin) x 100%, where Pin is the input power and Pout is the output power.
- Pin is the sum of the DC driving motor's input voltage (Vdc) and current (Idc), minus the losses (Ploss).
- Ploss is the sum of the mechanical losses (Pmsc) and electrical losses (Pelect).
Experimental Procedure
- Connect a shunt-DC motor and adjust its speed to the rated value of the alternator (1500 rpm) using a rheostat.
- Connect the alternator field terminals to a variable DC voltage source.
- Connect the alternator stator terminals to a variable Y-connect resistive load with terminals short-circuited.
- Increase the field current of the alternator to reach the rated voltage (380V).
- Increase the load in steps, recording the values of load voltage, load current, DC voltage, and DC current of the DC motor at each step.
Results and Conclusion
- Draw the variation of terminal voltage with load current.
- Calculate the voltage regulation of the alternator at each load current value.
- Calculate the efficiency of the alternator for all data points.
- Plot the variation of efficiency against the armature current.
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Description
This experiment aims to perform a load test on an alternator and determine the regulation and efficiency of the alternator from the results. It covers the load characteristics of an alternator, including the effect of excitation current on AC voltage. Students will learn how to analyze the performance of an alternator under different load conditions.
