Basic Electrical Theory - Overview of DC Circuits, Batteries, DC Generators & Motors

A. Bhatia, B.E.

Course Outline

This electrical training course provides a basic introduction to DC theory, electrical circuits, batteries and DC machinery (motors and generators). This course will be extremely helpful to individuals who are just beginning a career in electrical work, or who require a basic knowledge of electrical principals and equipment to better their primary responsibilities. This course is also a prerequisite for the all other electrical training.

This 3-hr course material is based entirely on US Department of Energy training materials DOE-HDBK-1011/1-92, Fundamentals Handbook, Electrical Science, and Volume 2 of 4. The volumes 1, 3 and 4 of the handbook have been separately listed.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

• Describe how current flow, magnetic field, and stored energy in an inductor relate to one another;
• Understand how a capacitor stores energy and the construction of a capacitor;
• Describe the relationship between total battery voltage and cell voltage for a series-connected battery;
• State the advantage of connecting a battery in parallel with respect to current-carrying capability;
• Define terminal voltage, counter-electromotive force (CEMF) as it applies to DC generators;
• State the purpose of the components of a DC machine such as armature, rotor, stator, and field;
• Be able to determine the direction of the magnetic field, the motion of the conductor, or the direction of current induced into a conductor;
• Describe the differences in construction between a shunt-wound and a series-wound DC generator with respect to the relationship between the field and the armature;
• Describe the voltage-v/s-load current characteristics for a flat-compounded, over compounded and under-compounded DC generator;
• Describe the relationship between field current and magnetic field size in a DC motor;
• Describe the differences in construction between a shunt-wound and a series-wound DC motor with respect to the relationship between the field and the armature windings; and
• Describe torque-v/s-speed characteristics for a shunt-wound and a series-wound DC motor.

Intended Audience

This course is aimed at beginners, novice engineers, electricians, hobbyists, plant mechanics, service technicians, maintenance supervisors, plant engineers, contractors, energy auditors, layout professionals and general audience.

Course Introduction

Most students of electricity begin their study with what is known as direct current (DC), which is electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. In DC circuits, the polarity of the voltage source does not change over time. When a DC source is connected in a closed electrical circuit, current will flow in a direction determined by the polarity of the source. By convention, we show DC current flow as originating at the positive terminal of the source, traveling through the circuit and returning to the negative terminal. Common DC sources include batteries, photocells, fuel cells, rectifiers and the common DC machines are motors and generators.

In this course, you are required to study the following DOE-HDBK-1011/1-92, Fundamentals Handbook, Electrical Science, and Volume 2 of 4.

Course Content

This course is based entirely on US Department of Energy training materials DOE-HDBK-1011/1-92, Fundamentals Handbook, Electrical Science, Volume 2 of 4.

The link to the document is Basic Electrical Theory - Overview of DC Circuits, Batteries, DC Generators & Motors.

Course Summary

Remember these facts:

1) When an inductor has a DC current flowing through it, the inductor will store energy in the form of a magnetic field.
2) An inductor will oppose a change in current flow by the CEMF induced when the field collapses or expands.
3) Inductors in series are combined like resistors in series and inductors in parallel are combined like resistors in parallel.
4) A capacitor stores energy in the form of an electric field caused by the attraction of the positively-charged particles in one plate to the negatively charged particles in the other plate.
5) Capacitors in series are combined like resistors in parallel and capacitors in parallel are combined like resistors in series.
6) A voltaic cell is a combination of materials used to convert chemical energy into electrical energy. A battery is a group of two or more connected voltaic cells.
7) A voltaic cell develops a potential difference when electrodes of two different metals are immersed in an electrolyte. One electrode accumulates a positive charge. The potential difference is due to the difference in charge between the two electrodes.
8) The value of specific gravity at any given time is an approximate indication of the battery's state of charge.
9) The output voltage of a battery connected in series is equal to the sum of the cell voltages.
10) A battery that is connected in parallel has the advantage of a greater current carrying capability.
11) Secondary cells can be recharged; primary cells cannot be recharged.
12) Internal resistance in a battery will decrease the battery voltage when a load is placed on the battery.
13) Shelf life is a term that is used to measure the time that a battery may sit idle and not lose more than 10 percent of its charge.
14) The advantage of a carbon-zinc battery is that it is durable and very inexpensive to produce.
15) The alkaline cell has the advantage of an extended life over that of a carbon-zinc cell of the same size.
16) The nickel-cadmium battery has the advantage of being a dry cell that is a true storage battery with a reversible chemical reaction.
17) The edison cell has the advantage of being a lighter and more rugged secondary cell than a lead-acid storage battery.
18) The mercury cell has the advantage of maintaining a fairly constant output under varying load conditions.
19) Higher temperatures will give some additional capacity, but they will eventually reduce the life of the battery. Very high temperatures, 125°F and higher, can actually do damage to the battery and cause early failure. At low temperatures the capacity is reduced while operating life increases. Ideally the ambient temperatures shall be 75ºF ±2.
20) Terminal voltage, as applied to DC generators, is defined as the voltage that can be measured at the output of the generator. Counter-electromotive force (CEMF) is defined as the induced voltage that acts to counter the applied voltage in a DC motor or a DC generator. Applied voltage is defined as the voltage that is delivered across the load.
21) In a DC generator, commutation provides for the conversion of AC to a DC output that is generated in the armature windings.
22) The purpose of the armature is to provide the energy conversion in a DC machine.
23) In DC machines, the purpose of the stator is to provide the field.
24) The purpose of the field in a DC machine is to provide a magnetic field for producing either a voltage or a torque.
25) The left-hand rule states that if you point the index finger of the left hand in the direction of the magnetic field and point the thumb in the direction of motion of the conductor, the middle finger will point in the direction of current flow.
26) The terminal voltage of a DC generator is adjusted by varying the field strength.
27) The voltage rating of a DC generator is based on the insulation type and design of the machine.
28) The current rating of a DC generator is based on the size of the conductor and the amount of heat that can be dissipated in the generator.
29) The power rating of a DC generator is based on the mechanical limitation of the device that is used to turn the generator.
30) The upper speed rating of a DC generator is determined by the speed at which mechanical damage is done to the machine. The lower speed rating is based on the limit for field current.
31) There are four internal losses that contribute to lower efficiency of a DC generator.
- Copper losses
- Eddy-current losses
- Hysteresis losses
- Mechanical losses
32) A shunt-wound DC generator is constructed so that the field winding is in parallel with the armature winding. The voltage of a shunt-wound DC generator decreases with an increase in load current.
33) A series-wound DC generator is constructed so that the field winding is in series with the armature winding. The voltage of a series-wound DC generator increases sharply with an increase in load.
34) In a cumulatively-compounded DC generator, the series and shunt fields aid one another.
35) In a differentially-compounded DC generator, the series and shunt fields oppose one another.
36) The voltage of a flat-compounded DC generator changes less than 5 percent from no-load to full-load.
37) The voltage of an over-compounded DC generator gradually rises with an increasing load.
38) The right-hand rule for motors states that when the forefinger is pointed in the direction of the magnetic field lines, and the center finger is pointed in the direction of current flow, the thumb will point in the direction of motion.
39) Torque is developed in a DC motor by the armature (current-carrying conductor) being present in the motor field (magnetic field).
40) CEMF is developed in a DC motor by the armature (conductor) rotating (relative motion) in the field of the motor (magnetic field).
41) The function of the voltage that is developed in a DC motor (CEMF) opposes the applied voltage and results in the lowering of armature current.
42) The speed of a DC motor may be changed by using resistors to vary the field current and, therefore, the field strength.
43) Starting resistors are necessary for large DC motors to prevent damage due to high currents while starting the motor. When the motor reaches full speed, the starting resistors are cut out of the circuit.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.