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Electric Motors

The computer you are using to view this page,the electric razor you shaved with, the car you recently rode in, the elevator you took, the pencil sharpener at your desk. They all share a unique and vital feature that their functioning is dependent on. Can you guess it? All of them have.....electric motors! Basically any appliance, in and outside of the home, that does work, has an electric motor. But what exactly is an electric motor, you ask?....

General DescritptionEdit

Electric motors are devices that convert electrical energy into mechanical energy. The electrical energy is the electric current that is coming from a particualr power source. These sources of energy can be chemical,solar,nuclear, water(hydrodynamic),wind, or any other generating medium. This energy is measured in Watts (Volts x Amperes). The mechanical energy, kinetic energy, is the motion that results from the flow of the electric current through the motor as it reacts with the natural forces of the magentic field. This kind of energy is measured in horsepower (foot-pounds/second). Essentially, a motor uses the forces of the magentic field to take an electrical current being fed into the motor, and convert it into mechanical motion to do work. The following picture shows the exterior of a normal electirc motor:

Motor2


This image is from:Howstuffworks.com

note!: Be careful not to confuse an electric motor with a generator. When placed side by side, they are seemingly identical in appearance. But in fact, their jobs are exact opposites. A generator creates electrical engery from mechancial energy, while a motor turns electrical energy into mechancial. The motor actaully depends on the generator as a source of electrical current.

HistoryEdit

In 1819, Hans Christian Oersteddiscovered that each time he brought a compass near a current carrying conductor,the compass would move to become perpendicular to the conductor. Michael Farady continued to research this idea, and he found that motion could be created using the magnetic field. Faraday created the first electric motor in 1821. However, while Faraday's invention had great potential, it didn't have enough power and thus usage for everyday life. Thomas Davenport went on to revise Farady's model of the motor, and in 1837 he recieved the first U.S. patent for one. However, during his lifetime, Davenport never recieved reocngintion. At the time, commercial find a better website :) generators had not been invented yet, so the motors lacked the powerful electicity that they needed. Thus, the motor had no real value for commericial appliances, and Davenport could not obtain success with his creation. After he had died, however, Davenport's motor was exhibited in Berlin in 1879 as the first motor to be used for industrial/commerical applications. Since then, our use of the motor has entered our homes, our work place and everywhere in between. Motors have become a crucial part of everyday living, although many of us have yet to realize it.

How Do They Operate?Edit

Electric Motor operation is based on the principles of electromagnetism. Motors depend on the itneraction of magnetic fields. Two electromagnets,one that is stationary on a stator, and one that can rotate on a rotor, are placed inside the motor's frame. A motor can use permanent magents (such as bar magnets), but it generally has electromagnets made of a coil of copper wire and an iron core that becomes magnetized when current flows through it. In the operation of a motor, electricity moves from a soruce (perhpas a battery or a power outlet), and through the wire of the particualr appliance. The current reaches the motor, and is fed to each of the magents. It flows from the wire to the rotary electromagnet via a brush (which is often a small piece of carbon that conducts the current, and "sweeps" it into the motor. At the same time it flows tothe staionary electromagnet A magnetic field is created around the magnets, and they become instantly repelled or attracted as the current flows through each of their copper bodies in opposite directions, creating opposite poles. The inner-most electromagent, on the rotor, spins, trying to get to, or get away from, the pole of the other magnet. This happenes because opposite poles of magnets attract, while like poles repel. The rotation is what alows the motor do work, and is the actaul mechanical enegry that has been produced. Here is what the inside of an electric motor often looks like:


Motor6

This image is from: http://electronics.howstuffworks.com/motor3.htm


Ths following diagram from, http://www.robotics.utexas.edu/rrg/images/learn_more/low_ed/actuator/motorexplode.jp, shows the individual parts of one motor when separetd.



Motorexplode


However, not all motors have the exact same configuration. Many motors have different parts, and depending on their purpose, are used for used for different tasks. Electric motors have varying efficincies, lifespans, sizes, shapes, costs, and power. Overall, however, motors can be categorized into three major types.


DC (Direct Current) MotorsEdit

Direct current means that the current from the external source (any generator) is flowing to the motor constantly. It flows through one main wire, (a single line of power),uninterupted. We often see see commutators on DC motors more often that on others. Commutaors ensure that the current flowing to the rotor, also known as the armature,( the house for the rotable electromagent) is constantly reversing so that the magnteic fields will never become aligned, and thus motion will remain constant. In the following model, the orange-colored ring around the armature is the commutator.

Electric Cycle 3


Image from: http://en.wikipedia.org/wiki/Image:Electric_motor_cycle_3.png#filelinks

Today, DC motors are uncommon both in home appliances and in the industrial world. They are used, but not as frequently as the other types mostly because they are not as efficent. DC was orginially good for large pieces of equipment, thus New York City subway cars still use DC motors in their engines, and some elevators still rely on them as well.

AC (Alternating Current) Single Phase MotorsEdit

Alternating current differes from direct current in that the flow of current is interrupted as it altenrates between different lines of wire. When referring to AC, these lines are called phases. An AC single phase motor is similar to a DC motor in that there is only one "path" for the electrical current. But these motors differ from DC in their electromagnet configuration. The "windings" of the copper wire, the conductor of the electromagnet, changes. In general, AC motors are more efficient for everyday applieances.

AC Polyphase MotorsEdit

In an AC polyphase motor, "poly" refers to the multiple phases. An AC Polyphase motor can have up to to two or three phases of electricity. The maximum amount of phases is three because after that the mechanical energy being produced does notjustify the amount of electicity being used (which is a lot..). Today, the larger motors are AC polyphase because many phases are often more efficient than one. Imagine have three people do the work instead of one; more phases means more magnetic fields which means more motion and power! AC polyphase is also more cost efficient because it uses less electricity but emits the same, or more, mechanical energy than other types of motors.

Power FactorEdit

The power factor is the ratio of the (horsepower) that is produced to the electrical energy (watts) that is put in. In calculations, however, electrical energy is converted into watts as well so that the units are the same. The equation to find the power factor is as follows:

 Power Factor = Horsepower(W)/ Electrical energy (W)

When there are three phases the equation used is:

 Power Factor= Horsepower (W)/(Electrical energy (W) X 1.73) 

When there is a poor power factor, the appliance is not as efficient.

EfficiencyEdit

The percent efficiency is closely related to the powerfactor. In fact, it is really just the powerfactor multiplied by 100 to get a percent.

Percent Efficiency = ((Horsepower X 746)/(Watts)) X 100

Percent efficiency can also be described as:

          (Input-Losses)/(Input))X 100

Efficiency if important for commercial purposes, because you want a motor that will produced as close to the same, if not more than the energy you put in. A high percent efficiency is most often the better motor.

ReferencesEdit

ResourcesEdit

  • Lazar, Miriam A. Barron's Review Course Series Let's Review: Physics: The Physical Setting. 3 ed. New York: Barron's Educational Series, 2005.
  • Shultz,Patrick George. Transformers and Motors. Wobrun, MA.:Butterworth-Heinemann,1989. pgs. 109-114
  • Taffel, Alezander,Ph.D. Physics: Its Methods and Meanings. New York, NY: Allyn an dBacon, Inc. 1986. pg. 501
  • Watt, John H. American Electricians' Handbook. 9th Ed.

U.S.A:McGraw-Hill, Inc.,1970. pgs. 7-7, 7-53

  • Zitzewitz, Paul W.,Ph.D. Physics: Problems and Principles. Colombus,OH: Glencoe/McGraw-Hill, 1999. pgs. 509-510, 571.

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