Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 1 of 8 MECHANISMS. ELECTROMAGNETISM. THE ELECTRIC MOTOR.
MACHINE ELEMENTS
A machine element is a component of a device which has a function and acts on other elements. It can be simple or complex. The more complex the machine is the more components it has.
CLASSIFICATION OF MACHINE ELEMENTS
TYPE DESCRIPTION EXAMPLES
Structural Devices that support loads and stresses.
Beams, pillars, columns, tie rods, arches, ...
Mechanical (mechanisms)
Devices that transmit and convert forces and motions.
Wheels, pulleys, levers, gears ...
Electrical Devices that use electric current. Batteries, bulbs, electric motors, ...
Pneumatic Devices that use air and other gases. Cylinders, compressors, tires, pistons ...
Hydraulic Devices that use liquids such as oil or water. Pumps, valves, pistons, ...
ROTARY MOTION MECHANISMS
These systems transmit motion and force in a circular way from the input to the output. Rotary motion mechanisms include friction drives, pulleys and gears.
GEAR MECHANISMS AND COGWHEELS
Gears or Cogwheels are sets of wheels that have teeth called cogs. A gear has teeth along its perimeter, which allows it to fit together with another gear into the teeth of another so that one wheel moves the other one. They transmit a rotary motion between the two connected axles.
GEAR FRICTION TRAINS OR DRIVE TRAINS
This is a system of two or more gears, connected together as shown in the picture. The rotary motion of the first wheel drives the second. The wheel that produces movement is called the driver wheel or powered wheel, while the one that receives the movement is called the driven wheel.
In order to transmit the movement from the driver gear wheel to the driven gear wheel, both wheels must be fitted, or meshed. The size of the teeth must be the same on both wheels.
Gear trains are used to multiply or reduce the speed of a device.
SPEED REDUCER
In this system a small driver gear wheel moves a big driven gear wheel.
The result is a reduction of the speed. The biggest gear spins at a slower speed than the smallest one, but the traction force is bigger.
Most of electric motors have high-speed rotation rates, but they are weak, with little force.
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 2 of 8 ELECTROMAGNETIC OPERATORS OF THE ELECTRIC MOTOR
PERMANENT MAGNETS
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets. A magnet has two poles, north (N) and south (S).
Like magnetic poles repel each other; unlike magnetic poles attract each other.
The magnet is a mechanical device because it produces movement.
TEMPORARY MAGNETS:THE ELECTROMAGNET
The electromagnet is a magnet that works with electricity.
An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops.
An electromagnet is an electric operator made of a coil of copper wire conductor wrapped around a core of "soft" ferromagnetic material (steel or iron). The core enhances greatly the magnetic field produced by the coil.
The wire is connected to a power source. Only when we switch on the power source the circuit behaves like a magnet. That’s why it’s so called temporary magnet. When disconnected from the power source it loses its magnetic properties and fails to attract ferromagnetic metal objects.
The north pole and south pole of the magnet change when the polarity of the battery is reversed (sign change in the battery terminals) and, therefore, the direction of the electric current changes too.
§ Practical applications: the electromagnet, the alarm bell and the electric motor.
Oersted experiment (Hans Christian Ørsted): If we put a compass in the middle of an electrical circuit we’ll observe how electric current produces a magnetic field that attracts the needles (magnets) of the compass. If we do not have a compass, we can use a simple magnet and see how the magnetic field of the circuit pulls the magnet in one direction.
Conclusion: The electric current produces magnetism.
EXPERIMENT
BUILD AN ELECTROMAGNET
1. Take a piece of iron, for instance a large nail, and spin a length of insulated copper wire around it several times.
3. Run some direct electric current from a battery through the wire.
4. You will notice that the iron nail becomes magnetized.
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 3 of 8
BRUSHED DC (DIRECT CURRENT) ELECTRIC MOTOR
ELECTRIC MOTOR: It’s a compound operator that transforms electrical energy into movement
(kinetic energy or motion).
Electric motors are found in almost every machine, tool, vehicle and electrical appliance.
Effect Operator Symbol Image
Transforms electric energy
into movement (or motion) Motor
COMPONENTS OF THE ELECTRIC MOTOR
§ Stator: This is the fixed part of the motor onto which magnets are installed. These are generally permanent magnets, but in cases where high-powered motors are needed electromagnets are used.
§ Rotor: This is the part of the motor that spins and onto which the coil is mounted. It spins around a shaft or axle.
§ Coil: An electromagnetic coil is a series of loops formed when a conductor (usually an insulated solid copper wire) is wound around a core to create an inductor and mounted onto the rotor.
§ Commutator: Part where terminals (ends) of the coil are connected. These are very thin metal sheets shaped like half-rings, usually made of copper. The brushes make contact with the commutator.
§ Brushes: these are the contact points which transmit electrical current from the power source to the coils mounted onto the rotor.
HOW AN ELECTRIC MOTOR WORKS
It works on the principles of attraction and repulsion, established by a magnet (stator) and a wire (coil) through which the electric current flows.
When electric current is run through the coil, this generates a magnetic field that puts the motor into motion.
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 4 of 8 rotate. When the rotor becomes horizontally aligned, the commutator reverses the direction of current through the coil, thus reversing the magnetic field. The process then repeats.
Functions of the commutator:
"Direct current" commutators make the current reverse in direction every half a cycle (in a two-pole motor) thus causing the motor to continue to rotate in the same direction.
One of the sheets of the commutator is connected to the positive pole of the battery and the other sheet is connected to the negative pole. The coil will rotate until the south pole of the magnet is facing the magnet’s north pole stator and vice versa. However, the coil and the rotor will rotate only half a cycle.
Without the commutator the movement would stop, but at the same time the commutator rotates. After half turn, each commutator sheet becomes connected to the opposite pole so that the polarity changes and the direction of the current too.
What happens then is that the magnetic poles are reversed in the electromagnet. By changing the polarity, the south pole becomes the north pole and vice versa, so that the coil continues to rotate to reorient with the poles of the stator magnets, making another half turn. At the end of the second round, the polarity changes again and rotational motion continues indefinitely.
1.- The coil goes on rotating.
2.- It reverses the polarity (sign of the electric current going through the coil) every half a cycle.
Note: If we reverse the polarity of the battery or power supply of a DC motor, we reverse the direction of rotation of the engine, since in this case, the coil current flows backwards and repulsion forces change direction.
In the pictures bellow, we can see an electric motor with several coils like those we are going to use in our technology projects.
ACTIVITIES ON MECHANISMS AND THE ELECTRIC MOTOR
1. Write the definition of “Machine element”.
2. Fill out this table:
CLASSIFICATION OF MACHINE ELEMENTS
TYPE DESCRIPTION EXAMPLES
Structural
Devices that transmit and convert forces and motions.
Electrical Devices that use electric current.
Devices that use air and other gases.
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 5 of 8 3. Explain briefly what a speed reducer is and how it works. Draw it writing the main parts.
4. Give some examples of machines that use a speed reducer.
5. What is a magnet? How does it work?
6. Mark with a cross (X) which of the following metals are attracted to magnets (To resolve this problem, you must remember the experiment made in class).
BRONZE LEAD IRON ALUMINIUM
7. What kind of machine element is a magnet? Why?
8. Write in one line what an electromagnet is.
9. Why do we say that an electromagnet is a temporary magnet?
10. Explain what elements you need to build a simple electromagnet.
11. What happens to the magnetic poles of an electromagnet when we change the polarity of the battery that gives you electric power?
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 6 of 8 13. Write the name and describe the elements of an electric motor:
NAME DESCRIPTION
1.
2.
3.
4.
5.
14. Write the parts of the electric motor:
15. What type of energy transformation occurs in an electric motor?
16. Explain briefly how an electric motor works.
17. Explain the two main functions of the commutator.
18. What would happen to the rotation of an electric motor if there were no commutators?
19. What if we reversed the polarity of the battery or the power supply of a DC electric motor?
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 7 of 8
V
ISUAL DICTIONARY OF
W
ORKSHOP
T
OOLS AND
M
ATERIALS
PICTURE NAME USE
Carpenter’s hammer (Eng) Martillo de peña (Spa) Martelo de ebanista (Gal)
A tool with a heavy metal head at the end of a wood handle, used for hitting the head of a nail so as to join pieces.
Nail (Eng)
Clavo o punta (Spa) Cravo (Gal)
Used to join pieces. It’s a thin straight piece of metal, usually with a head on it, used to join pieces of wood.
File (Eng) Lima (Spa) Lima (Gal)
A tool with a roughened metal surface used for smoothing or shaping metals and other hard materials.
Rasp (Eng) Escofina (Spa) Lima grosa (Gal)
A coarse file metal tool with a roughened surface for smoothing surfaces and edges of wood.
Wood chisel (Eng) Formón (Spa) Trencha (Gal)
Used to carve out wood. The bevelled steel blade has a cutting edge.
Gimlet (Eng) Barrena (Spa) Barrena (Gal)
Used to drill through thin pieces of wood and to make small holes.
Hacksaw (Eng) Sierra de arco (Spa) Serra de arco (Gal)
A saw with a narrow fine-toothed blade set in a frame, used for cutting metal.
Tenon saw (Eng)
Serrucho de costilla (Spa) Serrón de cota (Gal)
Saw used to make precise cuts on wood pieces (not for metals).
Coping saw (Eng)
Segueta o sierra de pelo (Spa) Serra de contornar (Gal)
Used to cut thin wood such as plywood.
Screwdriver (Eng) Destornillador (Spa) Desaparafusador (Gal)
Used to insert and remove screws.
Screw flat and cross-headed (Eng)
Tornillo plano y estrella (Spa) Parafuso plano e estrela (Gal)
Technology 2nd ESO – 2nd term – Mechanisms. Electromagnetism. The electric motor. 8 of 8
PICTURE NAME USE
Pliers (Eng) Alicates (Spa) Alicates (Gal)
Used for extracting nails, tightening screws, holding and cutting.
Spanner (Eng) Llave fija (Spa) Chave fixa (Gal)
Used to tighten or loosen nuts.
Nut (Eng) Tuerca (Spa)
Rosca ou Porca (Gal)
A small flat piece of metal with
a hole through it for screwing
on to a bolt as a fastener.
Clamp (Eng)
Sargento o gato (Spa) Sarxento ou gato (Gal)
Used for strengthening or holding things together.
Bench vise (Eng)
Tornillo de banco (Spa) Torno de banco (Gal)
Used to hold an object firmly in place while work is being done on it, typically attached to a workbench.
Electric drill (Eng) Taladro eléctrico (Spa) Trade eléctrico (Gal)
Electric tool used for making holes in wood and metal.
.
Drill bit (Eng) Broca (Spa) Broca (Gal)
Thin solid metal cylinder used in the drill to make different size holes.
Soldering iron (Eng) Soldador (Spa) Soldador (Gal)
Electrical tool used for melting
tin and applying it to metals
that are to be joined.
Tin (Eng) Estaño (Spa) Estaño (Gal)
A silvery-white metal used in
the soldering iron.
Sandpaper (Eng) Papel de lija (Spa) Papel de lixa (Gal)
