dc separately excited

April 3, 2018 | Author: Rachelle Vi | Category: Electric Generator, Magnetic Field, Machines, Power (Physics), Electrical Equipment


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Electromechanical EnergyConversion Laboratory EE112L-A23 Experiment 1: The DC Separately Excited Shunt Generator Submitted by : Vinoya, Rachelle L. Program/Year : ECE - 5 Date Performed : 20 June 2016 Date Submitted : 3 July 2016 Instructor : Elvis C. Abellera I. OBJECTIVE The experiment aims the students to gain knowledge on the properties of a separately DC shunt generator under no-load and full-load conditions; and to determine the saturation curve of the generator and the armature voltage versus armature current load curve of the generator; II. CONCLUSION The characteristics of a separately-excited generator can be varied by varying the strength of the fixed magnetic field produced by the stator electromagnet. This process can be done by varying the current flowing through the stator electromagnet. A rheostat was connected in series with the electromagnet winding to vary the field current. The field current is used to produce a fixed magnetic field across the dc generator. The magnetic field can be increased until magnetic saturation occurs. Saturation of magnetic field occurs when no more magnetic flux can be produced even with an increase in field current. The magnetic polarity of the field poles is controlled by the direction of the field current. It can be noted that a DC generator acts as a motor in order to produce a back emf or voltage induced across the armature windings. In this experiment, the generator was tested out under no-load and full-load conditions. the data gathered were used to plot the graphs of the main operating characteristics of a separately-excited shunt generator. It can be observed that the back emf or the voltage induced across the armature windings is directly proportional to the shunt field current. The relationship between the output voltage and the field current shows the rotational speed of the motor, whether it is constantly increasing or not. Finally, changing the field current allows the output voltage to be changed. Therefore, a separately DC generator is actually equivalent to a DC power source that has a variable output voltage. III. ANSWER TO REVIEW QUESTIONS 1. State two ways by which the output polarity of a shunt DC generator can be changed. The ways where the output polarity of a shunt DC generator can be changed are by: (a) interchanging the positive and negative leads at the power supply and by (b) interchanging the positive and negative leads at the shunt terminal. 2. A. If a DC generator delivers 180 W to a load, what is the minimum mechanical power (in watts) needed to drive the generator (assume 80% efficiency)? P = P/efficiency = 180 W / 0.8 P = 225 Watts B. If a DC generator delivers 180 W to a load, what is the minimum hp needed to drive the generator (assume 100% efficiency) ? P = P/efficiency = 180 W / (1*746) P = 0.25 hp 3. Plot the Ea vs. IF curve for your DC shunt generator on the graph of Figure 7-4. Use the data from Table 7-1. Note that the curve “bends over” as the field current increases. Can you explain why this happens? Figure 1. Shows the Armature Voltage EA (V) vs. Field Current IF (mA) It can be observed that as the field current increases, the armature voltage induced also increases. The ‘bend over’ of the curve only shows that the rotational speed of the motor/generator is rapidly increasing. 4. Plot the EA vs. IA regulation curve on the graph of Figure 7-5. Use the data from Table 7-2. 5. Calculate the Figure 2. Shows the graph of the Armature Voltage EA (V) vs. Armature Current IA (mA) regulation from no-load to full-load (1 A dc). VNL−VFL 135−120 Voltage Regulation= ∗100= ∗100=12.5 VFL 120 ( ) ( ) REFERENCES: (2014). Lab-Volt Manual: Electricity and New Energy: AC/DC Motors and Generators. Quebec, Canada: Festo Didactic Ltd. (2014). Lab-Volt Manual: Conventional DC Machines and Universal Motor. Canada: Lab-Volt Ltd. (n.a). (2014). Characteristics of Separately-Excited DC Generator. Retrieved 3 July 2016 from http://www.studyelectrical.com/2014/08/characteristics-of-separately-exciteddc-generator.html Chapman, S. J. (2012). Electric Machinery Fundamentals. 5th Edition. NY: McGraw-Hill Education. The Separately-Excited DC Generator. Retrieved 3 July 2016 from http://www.industrial-electronics.com/elecy3_2.html Umans, S. (2013). Fitzgerald & Kingsley’s Electric Machinery. 7th Edition. McGraw-Hill Education.
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