14.8 THE ELECTRIC MOTOR

The electric motorconverts electrical energy into kinetic energy, usually in the form of rotation. Electric motors depend on the magnetic effect of an electric current. You may study this effect later in a physics course, but for now we willfocus onthe diagram(left) which shows the principle of the electric motor.The stationary, outer part of the motor is called the stator. The most important part of this is a large magnet whose north and south poles are shown in the diagram. The rotating, inner part of the motor is called the rotor. The rotor consists of a coil of wire wrapped around an iron core. The core, and a device called a commutator,are attached to an axel that is free to rotate. The two ends of the wire in the coil are connected to the two sides of thecommutator.These two sides are conductors and they are separated by strips of insulator. The commutator is connected to twoelectric terminals through twobrushes(usually made of graphite) that brush against it as it rotates.

When the terminals are connected to a suitable battery, electricity flows through the coil on the rotor and the rotor becomes a magnet. In the diagram, the north pole of the rotor magnet is at the top and the south pole is at the bottom. It is well known that like magnetic poles repel,and unlike poles attract. The top of the rotor is repelled by the north pole of the stator (on its left) and attracted to the south pole of the stator (on its right).The bottom of the rotor is repelled by the south pole of the stator and attracted to the north pole. These magnetic forces all combine to make the rotor rotate clockwise. Without the commutator, the rotorwould stop as soon as it was horizontal. However at that point the brushes make contact with the opposite side of the commutator! Now electricity flows through the coil in the opposite direction so the poles on the rotor are reversed. They are now attracted to the opposite sides of the stator so the rotor continues to rotate clockwise. The picture (right) is the rotor froma small electric motor. This one has three coils but the principle is the same. A large electric motor may have many coils.

In the modern world, electric motors are used in many ways; infans, hair dryers, food mixers, vacuum cleanersand the compressor pumps in refrigerators and air conditioners. They are also used in DVD and CD players, the hard drives of computers, sewing machines, power tools,and the starter motors in cars and trucks. Electric motors are much more efficient than internal combustion engines with efficiencies of about 80 – 90%. A little energy is wasted by friction between the brushes and the commutator and brushes have to be replaced when they wear out.

  • 1.In an electric motor what are (i) a stator magnet, (ii) a rotor, (iii) a commutator, (iv) a brush, (v) a terminal?
  • 2. List all the things you can think of, that you have ever used, that have electric motors in them.
  • 3. For a motor, what does an efficiency of 85% mean?

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