Electrical Systems

DC Power Sources 271

DC Power Sources provides a foundational overview of direct current and the different power sources that produce it. A DC power source is any means used to convert various forms of energy into DC electricity. DC power sources include batteries, fuel cells, solar cells, and DC generators. Most DC power is supplied by batteries, but generators are used for some applications. Fuel cells and solar cells are not widely used but may be more popular in the future if they can be made less expensive and more efficient.

After taking this class, users will be familiar with different methods of producing DC power and other important concepts, such as magnetic induction. This is important for working with electrical systems because many electrical devices operate on direct current. In addition, DC may become more popular in the future if alternative sources such as fuel and solar cells become more cost-effective.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 18

  • Language English


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Course Outline
  • Direct Current
  • DC Power Sources
  • Batteries
  • Primary and Secondary Cells
  • Fuel Cells
  • Solar Cells
  • Compact DC Power Sources Review
  • DC Generators
  • Hand Rules for Generators
  • DC Generator Components
  • DC Generator Operation
  • DC Generator Operation Review
  • Field Excitation Methods
  • Types of DC Generators
  • Types of Armatures
  • Generator Efficiency Losses
  • Limitations of DC Power
  • Final Review
  • Describe direct current.
  • Distinguish between common DC power sources.
  • Describe batteries.
  • Distinguish between primary and secondary cells.
  • Describe fuel cells.
  • Describe solar cells.
  • Describe DC generators.
  • Describe the hand rules for generators.
  • Describe major components of DC generators.
  • Describe how DC generators operate.
  • Distinguish between self-excited and separately excited generators.
  • Describe the main types of DC generators.
  • Describe the main types of armatures used in DC generators.
  • Describe efficiency losses in DC generators.
  • Describe limitations of DC power.
Vocabulary Term


Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz, in the U.S.


A device that converts AC to DC. Adapters take the AC from a wall outlet and convert it to the DC that recharges many secondary cells.

alternating current

AC. Current that flows in opposite directions at different times. Alternating current switches direction 60 times per second, or 60 hertz, in the U.S.

arc welding

A joining process that uses heat generated by electricity to melt filler and base metals to form an airtight joint. Arc welding applications often use compound DC generators.


The rotating portion of a DC generator in which current is induced. The armature usually consists of a series of coils or groups of conductors surrounding a core of iron.

armature windings

The conducting coils wound around the armature in which voltage is induced in a generator. Armature windings can connect in series, parallel, or both.


A device that uses metallic elements to convert chemical energy into electrical energy. Batteries are a source of direct current.


A device that uses metals to convert chemical energy into electrical energy. Batteries are a source of direct current.


A friction-reducing device that allows one moving part to glide past another moving part. Bearings operate using a sliding or rolling mechanism.


Moving in two different directions. Bidirectional current is another way to describe alternating current.


Sliding electrical contacts used to provide a connection between the armature and the external circuit. Brushes contact the commutator to maintain direct current.


A material that occurs in addition to the primary product of a chemical reaction. Byproducts are sometimes harmful to the environment.


The amount of electricity that can safely flow through an electrical device or circuit without it overheating. Capacity is an extremely important electrical quantity.


The amount of energy a battery can provide. Capacity depends on battery materials and volume.


A single unit of a battery. A cell contains two different metals in an electrolyte solution.

charging cycle

The process of restoring the chemical reactivity of a secondary cell battery. The charging cycle involves forcing DC back through the system.


A controlled path for electricity. A circuit includes a source, path, load, and control.


A loop of conducting wire wrapped around the armature of a DC generator. Coils in an armature are on different axes to produce smoother output voltage.


The rotating switch that contacts the brushes of a DC generator. The commutator maintains DC when the rotation of the armature switches the polarity of the conductor.

compound generator

A DC generator that has field windings connected to the armature in series and in parallel. The compound generator combines the positive qualities of series and shunt generators.


A material that allows free movement of electrons and therefore allows easy flow of electricity. Conductors are typically metals.

conventional current theory

The belief that electricity flows out from a positive source to seek a negative conclusion. Conventional current theory is no longer privileged but still accepted by some fields.

copper loss

A power loss due to current flowing through wire. Copper loss dissipates power into heat.


Experiences a deterioration in useful properties due to oxidation. Corroded metals cause batteries to fail.


The flow of electricity. Current strength is called amperage and is measured in amperes (A).


Direct current. Electricity that travels in one direction. DC does not reverse the direction of flow.

direct current

DC. Electricity that travels in one direction. Direct current does not reverse the direction of flow.

directly proportional

A relationship in which one number increases or decreases at the same rate or ratio as another number. Directly proportional is the opposite of inversely proportional.

discharging cycle

The chemical reaction process of a battery. The discharging cycle is complete when a battery can no longer react to produce voltage.

eddy currents

Swirling currents induced in solid metals by changes in the magnetic environment. Eddy currents lead to magnetic efficiency loss in DC generators.

eddy-current loss

A power loss due to circulating eddy currents in a conductor. Eddy-current loss occurs when a rotating conductor intersects a magnetic field.


A measure of the energy output of a system versus the total energy supplied to it. Efficiency ensures that electrical systems do not lose large amounts of power.

efficiency losses

Natural effects that cause energy output to be less than energy input. Efficiency losses can be minimized, but no system is 100% efficient.


A device within a circuit that conducts electricity. Electrodes have a positive or negative charge.


A conductive solution containing an acid, an alkaline, or a salt. The electrolyte in a cell or battery connects the two electrodes.


A magnet that gains an attractive force only when current passes through it. Electromagnets are extremely powerful.

electromotive series

A list of metals in order of most reactive, or most likely to give up electrons, to least reactive. The electromotive series lists metals that are used in batteries.

electron theory

The belief that electricity flows out from a negative source to seek a positive conclusion. Electron theory is currently accepted.


A negatively charged particle that orbits the nucleus of an atom. Electrons flowing between atoms causes electricity.

field windings

The conducting wire connected to the armature that energizes the pole pieces. Field windings are connected in series or parallel.

fossil fuels

Naturally occurring organic fuels formed in the Earth's crust that can be burned to release stored energy. Fossil fuels include petroleum, coal, and natural gas.


A force that resists the movement of two objects sliding against each other. Friction can lead to mechanical efficiency losses.

frogleg-wound armature

A series-parallel armature winding. Frogleg-wound armatures are more common than lap- and wave-wound armatures.

fuel cell

A device that uses gases to convert chemical energy into electrical energy. Fuel cells are potentially very efficient power sources.


A device that converts mechanical energy into electrical energy. Generators operate on the principle of magnetic induction.


A device that converts mechanical energy into electrical energy. Generators operate on the principle of magnetic induction.


A colorless, odorless gas used in fuel cells. Hydrogen is the most abundant element on the planet.

hysteresis loss

A power loss in a conductive material caused by molecular friction. Hysteresis loss occurs when molecules rub against each other as an armature rotates.

lap-wound armature

A parallel armature winding. Lap-wound armatures are used for high-current, low-voltage loads.

left-hand flux rule

A method used to determine the direction of magnetic flux in relation to the direction of current flow in a conductor. The left-hand flux rule uses the thumb to show the direction of current flow and the fingers to show the direction of flux.

left-hand generator rule

A method used to determine the direction of current flow induced in a generator. The left-hand generator rule uses the index finger, thumb, and middle finger to show the direction of flux, conductor movement, and current flow, respectively.

lines of flux

Imaginary lines of force that surround a magnet. Lines of flux form a magnetic field.

magnetic field

The area in and around a magnet in which a magnetic force exists. Magnetic fields exhibit the powers of attraction and repulsion.

magnetic induction

The use of magnets to cause voltage in a conductor. Magnetic induction occurs whenever a conductor passes through magnetic lines of flux.


A small DC generator that uses permanent magnets instead of electromagnets to cause magnetic induction. The magneto is the only type of DC generator that does not use electromagnets for field excitation.


To release or remove electrons from an atom, an ion, or a molecule. Oxidization is used to rank the metals on the electromotive series.


An electrical route that has multiple loads and multiple paths. Parallel windings consist of many turns of thin wires.

permanent magnets

A magnet that retains its attractive force after it is removed from a magnetic field. Permanent magnets have high residual magnetism.


A material's tendency to become magnetized. Permeability is the opposite of reluctance.


Particles of light energy produced by the sun. Photons are converted into electricity by solar cells.

photovoltaic cell

A device that uses semiconductors to convert light energy into electrical energy. Photovoltaic cells are also known as solar cells.

PN diode

A semiconductor diode that has one positively charged side and one negatively charged side. The PN diode allows current to flow only from the negative side to the positive side.


A state of opposites. Polarity determines the north and south attractions of a magnet and the positive and negative charges in a circuit.

pole pieces

Devices mounted on the inside of a generator armature. Pole pieces form the electromagnets that create lines of flux when connected to the field windings.

power source

The device that provides electrical energy to a circuit. Power sources include batteries, generators, and other devices.

primary cell

A battery cell that cannot be recharged. Primary cell batteries are usually thrown away once their charge is spent.


Having a tendency to react chemically. Reactive metals are the negative electrodes in battery cells.

residual magnetism

The attractive force that exists in an object or substance after it has been removed from a magnetic field. Residual magnetism energizes the field in a self-excited generator.


Fluctuation of voltage. Ripple can be decreased by adding additional coils to the armature.

secondary cell

A battery cell that can be recharged. Secondary cell batteries are charged by running current back through the cell to restore its chemical potential.


Energized by residual magnetism of pole pieces. Self-excited fields use residual magnetism to produce the initial generator voltage that permits current to flow through the field.


A material that restricts electrons but still allows them to flow. Semiconductors, such as silicon, have more electrical conductivity than insulators but less than conductors.

separately excited

Energized by an external source of DC. Separately excited magnetic fields rely on this outside source of electricity to provide the initial generator voltage.


An electrical route that may have multiple loads but has only one path. Series windings consist of a few turns of thick wire.

series generator

A DC generator that has field windings connected in series with the armature. The series generator is not very efficient.


A compound connection that uses both series and parallel connections. Series-parallel connections are used in frogleg-wound armatures.

short circuits

The condition in which current takes a shorter, unintended path between two conductors. Short circuits cause excess current flow.


A low-resistance connection between two points in an electrical circuit. A shunt forms an alternative path for a portion of the current.

shunt generator

A DC generator that has field windings connected in parallel with the armature. The shunt generator is self-protected and provides high torque.


A lustrous gray semimetallic material. Silicon is a semiconductor.

sine wave

An AC waveform that consists of 360 electrical degrees. The sine wave is the most common type of AC wave and is produced by rotating machines.

solar cell

A device that uses semiconductors to convert light energy into electrical energy. Solar cells are also known as photovoltaic cells.

solar panel

A circuit of linked solar or photovoltaic cells. Solar panels are powered by the sun and often very large.

stepped down

Transformed to a lower voltage. AC can be stepped down, but DC cannot be transformed effectively.


A force that causes rotation. Torque determines how much work a motor or generator can accomplish.


To increase or decrease the voltage of electricity with a transformer. There is no effective way to transform DC.


A loop or wrap of conducting wire around one axis of the armature of a DC generator. Turns affect generator output voltage.


Moving in only one direction. Unidirectional current is another way to describe direct current.


A measure of electrical pressure or potential. Voltage is measured in volts (V).

voltage drop

A decrease in voltage along a conductor through which electricity flows. Voltage drop occurs when electricity passes through a load.

wave-wound armature

A series armature winding. Wave-wound armatures are used for high-voltage, low-current loads.


A force caused by friction between air and a rotating object. Windage causes power loss because the friction generates heat.