Republic of the Philippines
Batangas State University Campus II
College of Engineering, Architectural and Fine Arts
MECHANICAL AND PETROLEUM ENGINEERING DEPARTMENT
DC MACHINES
Assessment No. #1
Marasigan, Alexis A. – ME-2208
I.
INTRODUCTION
A DC machine is a type of electromechanical device that converts electrical energy into mechanical energy
or the other way around. The DC motor, which converts electrical energy into mechanical energy, and the DC
generator, which converts mechanical energy into electrical energy, are two types of DC machines. The same
machine can function as both a motor and a generator. The construction of a DC motor and a DC generator is the
same.
II.
OBJECTIVES
1. To define and differentiate DC Machines, namely DC Generator and DC Motors
2. To identify and elaborate the fundamental working principle operation of each DC Machine
3. To illustrate the construction and winding of each DC Machines
4. To define the basic parts in the construction of each DC Machines
5. To identify the characteristics of each DC Machines
6. To list some applications of each DC Machines
III.
BODY
A DC machine is an electromechanical energy alteration device. The working principle of a DC machine is
when electric current flows through a coil within a magnetic field, and then the magnetic force generates a torque
that rotates the dc motor. The DC machines are classified into two types such as DC generator as well as DC motor.
The main function of the DC generator is to convert mechanical power to DC electrical power, whereas a
DC motor converts DC power to mechanical power. The AC motor is frequently used in industrial applications for
altering electrical energy to mechanical energy. However, a DC motor is applicable where good speed regulation &
an ample range of speeds are necessary like in electric-transaction systems. AC and DC motors both create
mechanical energy in the form of a rotating motor shaft, there are some key differences.
AC motors operate from an input electrical signal that is an alternating current and voltage which changes
in amplitude and direction as the input AC waveform completes a cycle. AC Motors can be operated either from a
single-phase power source, of a polyphase source featuring multiple voltage inputs that operate at a phase angle
difference from each other. DC motors are powered from a unidirectional current (one that does not change
direction with time) supplied from a DC power source. The general prominence of AC power means there may be
a need for conversion to DC power when using a DC motor, such as using an AC-DC converter or DC power
supply.
AY/Date: 2021-2022 / February 19, 2022
1|Page
In polyphase AC motors, as the stator coils are supplied with an alternating current, a rotating magnetic
field, or RMF, is produced which, through Faraday’s law of induction, generates an EMF in the rotor coils. That
EMF results in a current in the rotor and a net torque to be applied, causing it to rotate, and which also generates a
rotating magnetic field. Induction motors exhibit a phenomenon known as slip, wherein the speed of the rotor is
less than synchronous speed of the rotating field of the stator. In a DC motor, a permanent magnet or a set of field
coils produce a magnetic field that does not rotate. Current is supplied to the coils of the armature, which results in
the armature’s rotation.
With an AC motor, energizing the stator coils through a direct connection to a polyphase AC power source
is all that is needed to produce rotation of the rotor. The principle of electromagnetic induction generates the
current in the rotor without the need for a direct electrical connection.
With a DC motor, current needs to be supplied to both the stationary field as well as to the armature. To
accomplish this, brush-type DC motors make use of a set of spring-loaded carbon brushes which press against a
commutator ring that carries the current to the armature coils and to the field coils as the armature rotates.
Depending on whether the field coil connection is done in parallel with the armature coil (shunt motor) or in series
with the armature coil, the resulting DC motor configuration will exhibit different performance characteristics.
In an AC motor, the speed of the motor is controlled by the input frequency of the alternating current
supplied to the stator coils and is directly proportional. As the frequency increases, the speed of the motor
increases. Variable frequency drive controllers are used to adjust the input frequency as desired to produce the
desired motor rpm. For DC motors, the speed of the device is controlled by varying the voltage and current that is
applied to the armature coils or windings, or by adjusting the current that flows to the field coils (hence impacting
the strength of the magnetic field for the field coil). The speed-current relationship is again a proportional one.
Polyphase AC motors are designated as self-starting, requiring no additional electronics beyond the variable
frequency control for speed. Single-phase AC motors, as well as DC motors, both require a start-up mechanism for
controlling start-up conditions. As an example, in large DC motors, the back EMF generated in the armature is
proportional to the speed of the armature and is therefore small at start-up. This condition can cause a large current
flow to the armature, potentially causing burnout. Thus, controlling the input voltage ramp-up at start-up is needed
for these motors.
AC motors are often used for their high-speed and variable torque, but typically torque will exhibit a drop
as the motor speed increases. DC motors can produce high torque and are valuable where speed control is needed.
DC motors can provide a more constant torque over the speed range, and generally provide faster response to load
changes that AC motors. Depending on the configuration of the coil connection (series versus parallel), different
performance across load value for DC motors can be obtained. Series motors exhibit higher starting torque but
have a steeper drop-off in speed as the load increases. Parallel or shunt DC motors provide lower starting torque
but have a flatter speed vs. load relationship and can, therefore, provide a constant speed almost independent of the
load applied.
AC motors suffer from efficiency issues because of the induction current loss and the slip mentioned
earlier. DC motors that use permanent magnets can be some 30% more efficient as they do not have to consume
power to create an electromagnet, but there is some loss in efficiency due to the energy loss from the friction of
brushes. Brushless DC motors are more efficient than those with brushes, but the efficiency gains are primarily at
the low-load or no-load areas of the motor performance curve.
For a given amount of mechanical work output, AC motors are usually larger than DC motors, with
brushless DC designs being the smallest. AC motors have a long service life while DC motors require more
maintenance for those designs that use brushes and commutators which feature mechanical wear. Electronically
Commutated Motors (ECMs) are a form of brushless DC motor that eliminates mechanical commutation and
brushes in favor of electronic commutation and control, therefore improving useful life, reducing power
consumption, running cooler, and providing better performance.
The construction of the DC machine can be done using some of the essential parts like Yoke, Pole core &
pole shoes, Pole coil & field coil, Armature core, Armature winding otherwise conductor, commutator, brushes &
bearings. Another name of a yoke is the frame. The main function of the yoke in the machine is to offer mechanical
support intended for poles and protects the entire machine from moisture, dust, etc. The materials used in the yoke
are designed with cast iron, cast steel otherwise rolled steel. The pole of the DC machine is an electromagnet and the
AY/Date: 2021-2022 / February 19, 2022
2|Page
field winding is winding among pole. Whenever field winding is energized then the pole gives magnetic flux. The
materials used for this are cast steel, cast iron otherwise pole core. It can be built with the annealed steel laminations
for reducing the power drop because of the eddy currents.
Pole shoe in the DC machine is an extensive part as well as to enlarge the region of the pole. Because of this
region, flux can be spread out within the air-gap as well as extra flux can be passed through the air space toward
armature. The materials used to build pole shoe is cast iron otherwise cast steed, and also used annealed steel
lamination to reduce the loss of power because of eddy currents. Windings are wounded in the region of pole core &
named as field coil. Whenever current is supplied through field winding than it electromagnetics the poles which
generate required flux. The material used for field windings is copper. An Armature core includes a huge number of
slots within its edge. The armature conductor is located in these slots. It provides the low-reluctance path toward the
flux generated with field winding. The materials used in this core are permeability low-reluctance materials like iron
otherwise cast. The lamination is used to decrease the loss because of the eddy current. The armature winding can
be formed by interconnecting the armature conductor. Whenever an armature winding is turned with the help of
prime mover then the voltage, as well as magnetic flux, gets induced within it. This winding is allied to an exterior
circuit. The materials used for this winding are conducting material like copper.
AY/Date: 2021-2022 / February 19, 2022
3|Page
The main function of the commutator in the DC machine is to collect the current from the armature conductor
as well as supplies the current to the load using brushes. And also provides uni-directional torque for DC-motor. The
commutator can be built with a huge number of segments in the edge form of hard drawn copper. The Segments in
the commutator are protected from the thin mica layer. Brushes in the DC machine gather the current from the
commutator and supply it to the exterior load. Brushes wear with time to inspect frequently. The materials used in
brushes are graphite otherwise carbon which is in rectangular form.
The excitation of the DC machine is classified into two types namely separate excitation, as well as selfexcitation. In a separate excitation type of dc machine, the field coils are activated with a separate DC source. In the
self-excitation type of dc machine, the flow of current throughout the field-winding is supplied with the machine.
The principal kinds of DC machines are classified into four types.
In Shunt wound DC Machines, the field coils are allied in parallel through the armature. As the shunt field
gets the complete o/p voltage of a generator otherwise a motor supply voltage, it is normally made of a huge number
of twists of fine wire with a small field current carrying. In series-wound D.C. Machines, the field coils are allied in
series through the armature. As series field winding gets the armature current, as well as the armature current is huge,
due to this the series field winding includes few twists of wire of big cross-sectional region. A compound machine
includes both the series as well as shunt fields. The two windings are carried-out with every machine pole. The series
winding of the machine includes few twists of a huge cross-sectional region, as well as the shunt windings, include
several fine wire twists. The connection of the compound machine can be done in two ways. If the shunt-field is
allied in parallel by the armature only, then the machine can be named as the ‘short shunt compound machine’ & if
the shunt-field is allied in parallel by both the armature as well as series field, then the machine is named as the ‘long
shunt compound machine’.
At present, the generation of electrical energy can be done in bulk in the form of AC (an alternating current).
Therefore, the utilization of DC machines like motors and generators DC generators are extremely limited because
they are utilized mainly for providing excitation of tiny & middle range of alternators. In industries, DC machines
are used for different processes like welding, electrolytic, etc.
AY/Date: 2021-2022 / February 19, 2022
4|Page
Generally, the AC is generated and after that, it is changed into DC with the help of rectifiers. Therefore, DC
generator is suppressed through an AC supply which is rectified to use in several applications. DC motors are
frequently used like variable speed drives & where changes in the severe torque occur. The application of DC
machine as a motor is used by dividing into three types like Series, Shunt &Compound whereas the application of
dc machine as a generator is classified into separately excited, series, and shunt-wound generators.
Thus, this is all about DC machines. From the above information, finally, we can conclude that DC machines
are dc generator & dc motor. The DC generator is mainly useful for supplying DC sources toward the DC machine
in power stations. Whereas DC motor drives some devices like lathes, fans, centrifugal pumps, printing presses,
electric locomotives, hoists, cranes, conveyors, rolling mills, auto-rickshaw, ice machines, etc.
IV.
REFERENCES
*, N. (2020, October 31). DC machine - construction, working, types, EMF Equation & Applications. ElProCus.
Retrieved February 25, 2022, from https://www.elprocus.com/dc-machine-types-and-their-applications/
Admin, Says:, G. M., says:, B., & Says:, Y. M. (2021, May 23). What is difference between AC Motor and DC
Motor? Ox Science. Retrieved February 25, 2022, from https://oxscience.com/ac-motor-and-dc-motor/
Brushless DC motor vs. AC motor vs. Brushed Motor. Oriental Motor U.S.A. Corp. (n.d.). Retrieved February 25,
2022, from https://www.orientalmotor.com/brushless-dc-motors-gear-motors/technology/AC-brushlessbrushed-motors.html
Difference between AC and DC Motors | Precision Electric ... (n.d.). Retrieved February 25, 2022, from
https://www.precision-elec.com/difference-between-ac-and-dc-motors/
Difference between DC Motor and DC Generator. Electrical Academia. (2017, November 20). Retrieved February
25, 2022, from https://electricalacademia.com/electrical-comparisons/difference-between-dc-motor-and-dcgenerator/#:~:text=The%20most%20significant%20difference%20between,energy%20into%20an%20elect
rical%20energy.
What is the difference between AC and DC Motors. What is the Difference Between AC Motor and DC Motor.
(n.d.). Retrieved February 25, 2022, from https://www.veichi.org/solutions/related-articles/what-is-thedifference-between-ac-and-dc-motors.html
What is the difference between AC motors and DC Motors? Power Electric. (2019, June 20). Retrieved February
25, 2022, from https://www.powerelectric.com/motor-resources/motors101/ac-motors-vs-dc-motors
AY/Date: 2021-2022 / February 19, 2022
5|Page
What is the difference between an AC motor and a DC motor? Ohio Electric Motors. (2015, July 21). Retrieved
February 25, 2022, from http://www.ohioelectricmotors.com/2015/07/what-is-the-difference-between-anac-motor-and-a-dc-motor/
What is the difference between an AC motor and a DC motor? Ohio Electric Motors. (2015, July 21). Retrieved
February 25, 2022, from https://www.ohioelectricmotors.com/2015/07/what-is-the-difference-between-anac-motor-and-a-dc-motor/
What’s the difference between AC, DC, and EC Motors ... (n.d.). Retrieved February 25, 2022, from
https://www.machinedesign.com/mechanical-motion-systems/article/21835874/whats-the-differencebetween-ac-dc-and-ec-motors
AY/Date: 2021-2022 / February 19, 2022
6|Page