ac and dc motor

1.0 INTRODUCTION

An electric motor converts electrical energy into mechanical energy. Most electric motors

operate through interacting magnetic fields and current-carrying conductors to generate force,

although a few use electrostatic forces. The reverse process, producing electrical energy from

mechanical energy, is done by generators such as an alternator or a dynamo. Many types of

electric motors can be run as generators, and vice versa. For example a starter/generator for a gas

turbine, or traction motors used on vehicles, often perform both tasks.

Electric motors are found in applications as diverse as industrial fans, blowers and pumps,

machine tools, household appliances, power tools, and disk drives. They may be powered by

direct current (e.g., a battery powered portable device or motor vehicle), or by alternating current

from a central electrical distribution grid. The smallest motors may be found in electric

wristwatches. Medium-size motors of highly standardized dimensions and characteristics provide

convenient mechanical power for industrial uses. The very largest electric motors are used for

propulsion of large ships, and for such purposes as pipeline compressors, with ratings in the

millions of watts. Electric motors may be classified by the source of electric power, by their

internal construction, by their application, or by the type of motion they give.

The physical principle of production of mechanical force by the interactions of an electric current

and a magnetic field was known as early as 1821. Electric motors of increasing efficiency were

constructed throughout the 19th century, but commercial exploitation of electric motors on a

large scale required efficient electrical generators and electrical distribution networks.

Some devices, such as magnetic solenoids and loudspeakers, although they generate some

mechanical power, are not generally referred to as electric motors, and are usually termed

actuators and transducers, respectively.

2.0 OBJECTIVE

i. To identify AC and DC motor.

ii. To differentiate the components in AC and DC motor.

iii. To identify the advantages and disadvantages of both AC and DC motor.

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3.0 LITERATURE REVIEW

There are lots of motor that use in electrical system but the most commonly motor that used in

electrical system are alternating current or AC and direct current or DC. Basically, the reference

of these two motor are refer to the how the electrical current transferred through and from the

motor. Based on the name, it is known that these two motor have different function and uses. As

for DC motors, it is come in two general types which are brushes and brushless while AC motors

also come in two different types. They can be a synchronous motor or induction motor. Below

are the details information regarding AC and DC motors.

3.1 ALTERNATING CURRENT (AC) MOTORS

As mentioned above, AC motors come in two types which are synchronous motors and induction

motors. The AC motors are used differently based on what type of AC motor it is. There are two

types of AC motors, depending on the type of rotor used. The first is the synchronous motor,

which rotates exactly at the supply frequency or a submultiple of the supply frequency. The

magnetic field on the rotor is either generated by current delivered through slip rings or by a

permanent magnet. The second type is the induction motor, which runs slightly slower than the

supply frequency. The magnetic field on the rotor of this motor is created by an induced current.

The amount of power given off by an AC motor is determined by the amount of power needed to

operate the system.An AC motor has two parts. A stationary stator having coils supplied with

AC current to produce a rotating magnetic field, and a rotor attached to the output shaft that is

given a torque by the rotating field.

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An AC motor stator with preformed stator coils

3.1.1 INDUCTION MOTORS

One third of the world's electricity consumption is used for running induction motors driving

pumps, fans, compressors, elevators and machinery of various types. The AC induction motor is

a common form of asynchronous motor whose operation depends on three electromagnetic

phenomena:

Motor Action - When an iron rod (or other magnetic material) is suspended in a magnetic

field so that it is free to rotate, it will align itself with the field. If the magnetic field is

moving or rotating, the iron rod will move with the moving field so as to maintain alignment.

Rotating Field - A rotating magnetic field can be created from fixed stator poles by

driving each pole-pair from a different phase of the alternating current supply.

Transformer Action - The current in the rotor windings is induced from the current in the

stator windings, avoiding the need for a direct connection from the power source to the

rotating windings.

Induction motors have either wound rotors or squirrel cage rotors.

Wound Rotor. Wound rotors are constructed using the same principle as stator

construction.

Squirrel Cage Rotor. The SCIM rotor has conducting bars embedded in grooves that

are etched in the surface of the rotor along the direction of the rotor axis. The

conducting bars are placed around an iron core. To allow current flow in the bars, the

bars are shorted at either end of the rotor by large shorting rings. Squirrel cage rotor

construction is shown figure below. The rigid construction of this type of rotor

contributes significantly to the robustness of the SCIM.

Characteristics

One of the major advantages of the induction motor is that it does not need a commutator.

Induction motors are therefore simple, robust, reliable, maintenance free and relatively low

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Squirrel cage rotor construction

cost. They are normally constant speed devices whose speed is proportional to the mains

frequency. Variable speed motors are also possible by using motor controllers which provide

a variable frequency output.

Applications

Three phase induction motors are used wherever the application depends on AC power from

the national grid. Because they don't need commutators they are particularly suitable for high

power applications. They are also available with power handling capacities ranging from a

few Watts to more than 10 MegaWatts. They are mainly used for heavy industrial

applications and for machine tools. Other than that, the availability of solid state inverters in

recent years means that induction motors can now be run from a DC source. They are now

finding use in automotive applications for electric and hybrid electric vehicles. Nevertheless,

the induction motor is ill-suited for most automotive applications because of the difficulties

associated with extracting heat from the rotor, efficiency problems over wide speed and

power ranges, and a more expensive manufacturing process due to distributed

windings. Permanent magnet and reluctance motors offer better solutions for these

applications.

3.1.2 SYNCHRONOUS AC MOTORS

The synchronous motor is similar to the induction motor in that it is a polyphase machine

in which the stator produces a rotating field, however the rotor is constructed from either

permanent magnets or electromagnets energised by direct current supplied through slip rings.

Another way of saying this is that it has zero slip under usual operating conditions. Synchronous

motors are available in sub-fractional self-excited sizes to high-horsepower direct-current

excited industrial sizes. In the fractional horsepower range, most synchronous motors are used

where precise constant speed is required. The synchronous motor provides two important

functions. First, it is a highly efficient means of converting ac energy to work. Second, it can

operate at leading or unity power factor and thereby provide power-factor correction.

Synchronous motors have either wound rotors or permanent magnet rotors. Figure below

compares the two types.

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Field Wound Rotor. Field wound rotors are of the salient pole type. Salient pole rotors

are constructed of protruding pole assemblies bolted or dovetailed to a magnetic rotor

hub. The rotor poles are wound with magnetic wire to produce a rotor magnetic field.

This type of construction requires an external circuit for field excitation. The FWSM is

appropriate for large vessels such as icebreakers and auxiliary ships but its large size and

weight make it unacceptable for use in surface combatants.

Permanent Magnet Rotor. Permanent magnet rotors receive their field excitation from

permanent magnets mounted around the surface of the rotor instead of from field

windings. A major advantage of PM synchronous motors is that slip ring or brushless

exciter assemblies are not required. This eliminates excitation losses, which are a major

power loss component in field wound motors.

Characteristics

Synchronous motors show some interesting properties, which finds applications in power

factor correction. The synchronous motor can be run at lagging, unity or leading power

factor.

Applications

Synchronous motors find applications in all industrial applications where constant speed is

necessary. It is also use to Improving the power factor as Synchronous condensers. Beside

that, it is used in low power applications include positioning machines, where high precision

is required, and robot actuators. But majorly, mains synchronous motors are used for electric

clocks.

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Structure of synchronous motors: (a) permanent magnet rotor (two- pole); (b) salient-pole rotor (two-pole)

3.2 DIRECT CURRENT (DC) MOTORS

As stated earlier, the most common DC motor types are the brushed and brushless types,

which use internal and external commutation respectively to periodically reverse the current in

the rotor windings. A direct current (DC) motor is a fairly simple electric motor that uses

electricity and a magnetic field to produce torque, which turns the motor. At its most simple, a

DC motor requires two magnets of opposite polarity and an electric coil, which acts as

an electromagnet. The repellent and attractive electromagnetic forces of the magnets provide the

torque that causes the DC motor to turn. The attraction between opposite poles and the repulsion

of similar poles can easily be felt, even with relatively weak magnets. A DC motor uses these

properties to convert electricity into motion. As the magnets within the DC motor attract and

repel one another, the motor turns. A DC motor requires at least one electromagnet. This

electromagnet switches the current flow as the motor turns, changing its polarity to keep the

motor running. The other magnet or magnets can either be permanent magnets or

other electromagnets. Often, the electromagnet is located in the center of the motor and turns

within the permanent magnets, but this arrangement is not necessary. DC motors are generally

used for more precision and power than AC motors, as they tend to me more controllable.

3.2.1 BRUSHED DC MOTORS

Brushed DC Motors are the classic DC motors, which include a split ring commutator,

and can be powered by any kind of DC battery. These motors are often considered to be limited,

due to the need that brushes will always be in contact with the commutator ring, hence creating

friction. Brushes also scratch the surface of the ring, which eventually will lead to replacement of

the brushes and ring.

Although the brushes in these motors were originally made from copper wire (now obsolete),

they are now made from carbon, which is a longer-lasting material, gives less friction, and is

cheaper. The advantages of Brushed DC motors are that their initial cost is extremely low, and

that they have an extremely simple speed control system (Dynamo). However, it is the brushless

DC motor which is recommended by most.

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3.2.2 BRUSHLESS DC MOTORS

Brushless DC motors are extremely desireably as they completely eliminate the need for

brushes. This increases their life, survival without maintainance, power output and efficiency

dramatically.

Their basic working principle is to facilitate an external commutator, which will reverse the

direction of the current depending on the position of the rotor.

As there are no brushes, maintainence levels are lowered dramatically, and as there is no friction

caused by brushes, the efficiency of a brushless motor is typically between 85 and 90 percent (a

brushed motor's efficiency is usually about 75 to 80 %). This makes them ideal for heavy duty

use, and cost efficiency in the long term. They also run much cooler than AC and brushed

motors, which greatly increases the life of the motors in context.

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An exploded view of a DC motor. This diagram shows the relationship of all of the components

The four poles on the stator of a two-phase BLDC motor.

3.2.3 APPLICATIONS OF DC MOTORS

The DC Motors are used a lot in consumer electronics. BLDC motors fulfill functions

originally performed by brushed DC motors, but cost and control complexity prevents BLDC

motors from replacing brushed motors completely in lowest cost areas. The uses of DC such as

in computer hard drives and CD/DVD players. Small cooling fans in electronics equipment are

powered also by BLDC motors. Other than that, the DC motors also found in transport, for

example in electric vehicles and hybrid vehicles. These motors are essentially AC synchronous

motors with permanent magnet rotors. Beside that, the DC motors are currently the most popular

motor choice for aircraft model including helicopters. Nowadays, the DC motors also used in

electrical bicycles that are sometimes build into the wheel hub itself, with the stator fixed solidly

to the azle and the magnets attached to and rotating with the wheel. The bicycle wheel hub is the

motors. This type of bicycle also has a standard bicycle transmission.

3.3 ADVANTAGES & DISADVANTAGES OF DG & AC MOTORS

Advantages of AC motor Advantages of DC motor

1) They use conventional, low cost, 3-phase

AC induction motors for

most applications.

2) AC motors require virtually no

maintenance and are preferred for

applications where the motor is mounted

in an area not easily reached for servicing

or replacement.

3) AC motors are smaller, lighter, more

commonly available, and less expensive

4) AC motors are better suited for high

speed operation (over 2500 rpm) since

1) DC drives are less complex with a single

power conversion from AC to DC.

2) DC drives are normally less expensive for

most horsepower ratings.

3) Usually DC drives is use as adjustable

speed machines and a wide range of

options have evolved for this purpose

4) DC regenerative drives are available for

applications requiring continuous

regeneration for overhauling loads. AC

drives with this capability would be more

complex and expensive.

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there are no brushes, and commutation is

not a problem

5) It is desirable to use an existing constant

speed AC motor already mounted and

wired on a machine.

5) DC motors are capable of providing

starting and accelerating torques in excess

of 400% of rated.

6) Some AC drives may produce audible

motor noise which is undesirable in some

applications.

Disadvantages of AC motor Disadvantages of DC motor

1) Expensive speed control

-Speed control is expensive. The electronics

required to handle an AC inverter drive are

considerably more expensive than those

required to handle a DC motor.

-However, if performance requirements can

be met -- meaning that the required speed

range is over 1/3rd of base speed -- AC

inverters and AC motors are usually more

cost-effective than DC motors and DC drives

for applications larger than about 10

horsepower, because of cost savings in the

AC motor.

2) Inability to operate at low speeds

-We know that standard AC motors should

not be operated at speeds less than about

1/3rd of base speed. This is due to thermal

considerations. In fact a DC motor should be

considered for these applications.

1) less efficient and remain at the same

voltage and current, so we will lose a lot

of energy and therefore money from

energy lost to heat at high currents.

2) Although you can use a DC generator to

power smaller systems efficiently, the

wiring required to run a larger system can

become a fire hazard. This happens when

the correct wiring is not used to run the

current from the generator. Wiring a DC

generator can be quite a hassle for larger

jobs, and it is difficult to run the wire

according to code

3) Motor operation requires the purchase of

a complicated electronic motor driver.

4) The largest disadvantge of a direct current

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3) Poor positioning control

-Positioning control is also expensive and

crude. Even a vector drive is very crude when

controlling a standard AC motor. Servo

motors are more appropriate for these

applications.

machine is the care required to maintain

the mechanical interface used to get

current to the rotating field

5) The critical nature of the interface is due

in large part to the high currents required

by the DC machine.

6) Typically cost-effective because the

manufacturers of large heavy-duty DC

equipment have been building them for

several decades.

4.0 CONCLUSION

As a conclusion, there are many types of motor that used in electrical system but the most

common motor that used in electrical system are direct current or DC and alternating current or

AC motors. The reference of DC or AC refers to how the electrical current is transferred through

and from the motor. Both types of motors have different functions and uses. Dc motors come in

two general types. They can have brushes or be brushless. There are lots of motor that use in

electrical system but the most commonly motor that used in electrical system are alternating

current or AC and direct current or DC. Basically, the reference of these two motor are refer to

the how the electrical current transferred through and from the motor. Based on the name, it is

known that these two motor have different function and uses. As for DC motors, it is come in

two general types which are brushes and brushless while AC motors also come in two different

types. They can be a synchronous motor or induction motor. DC and AC motors are sometimes

subtle, but these differences are what make one types better for a certain use. Direct current or

DC electric motors work for situations where speed needs to be controlled. DC motors have a

stable and continuous current. DC motors were the first and earliest motors used. They were

found, however, to not be as good at producing power over long lengths. Electric companies

found using DC motors to generate electric did not work because the power was lost as the

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electric was transmitted. Brush DC motors use rings that conduct the current and form the

magnetic drive that powers the rotor. Brushless DC motors use a switch to produce the magnetic

drive that powers the rotor. Direct current motors are often found in appliances around the home.

Alternating current or AC electric motors are used differently based on what type of AC motor it

is. Single phase AC motors are known as general purpose motors. They work well in many

different situations. These AC motors work great for systems that are hard to start because they

need a lot of power up front. Three phase, also called polyphase, AC motors are usually found in

industrial settings. These motors also have high starting power build transmits lower levels of

overall power. AC power gets its name from the fact that it alternates in power. The amount of

power given off by an AC motor is determined by the amount of power needed to operate the

system. DC and AC electric motors are found everywhere from the home to the car to industrial

plants. Motors are important to everyday life. Dc motors were introduced and caused a great

revolution in the way many things are done. When AC motors came on the market the way

motors were looked at changed because of their amazing starting power potential. DC motors

and AC motors are different in many ways, but they still both are used to power the world.

4.1 RECOMMENDATIONS

As a recommendations, we know that AC and DC motors are commonly motors that used

in most of our electrical components, from our transport, fridge, clock, instrial machines, to

our household items. Therefore the technologies of these motors have to be improve and the

unused of its component have to be minimize to controls the pollutions of its components.

Placing the motor into overload conditions is one cause of over-temperature. High

ambient temperatures and dirty or clogged air filters on the machine or motor blowers also

contribute to over-temperature failures. High temperature inside the motor cause expansion

stress in the wire insulation, resulting in cracks, which in turn can cause contamination and

eventual wire failure. Therefore, it is recommended that the motor ambient conditions not to

exceed 40oC (104oF). Most motors are designed for continuous operation at this ambient

temperature. However, motors that will continuously be used in higher temperatures will

typically be designed with a lower temperature rise class of insulation. DC motor insulation

must have mechanical and dielectric strength. It must withstand the normal handling

necessary in the assembly of the motor, as well as operation thereafter

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5.0 REFERENCES

J.G. Ciezki and R.W. Ashton, “A Survey of AC Drive Propulsion Options,” presented at the 3rd

Naval Symposium on Electric Machines, December 4-7, 2000.

Stephen J. Chapman, Electric Machinery Fundamentals, pp. 359-373 and pp. 482-501, McGraw

Hill, New York, 1985.

Raymond Ramshaw and R.G. van Heeswijk, Energy Conversion: Electric Motors and

Generators, pp. 255-265, Saunders College Publishing, Philadelphia, 1990.

Clive Lewis, “The advanced induction motor,” Power Engineering Society Summer Meeting,

Vol. 1, pp. 250-253, IEEE, 2002.

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor, 21 January 2011 at 1.42 am.

http://highperformancehvac.com/hvac-ecm-blower-motors-hvac.html, 20 January 2011 at

10.32pm

http://www.wisegeek.com/what-is-a-dc-motor.htm, 19 January 2011 at 2.45pm.

http://www.globalspec.com/reference/10788/179909/chapter-3-ac-and-dc-motors-dc-motors-

over-temperature-conditions, 20 January at 2.04am.

http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA417341.

20 January 2011 at 10.30pm.

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6.0 APPENDICES

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Basic commutator for DC motor. AC & DC Gear Motor

Motor Construction synchronous motor diagram

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Tesla’s DC Motor Plan

AC motorTesla’s DC Motor Plan

ac and dc motor

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