- An elevator: an electric motor moves the elevator up and down. Another engine operates the doors.
- A car: Cars have several electric motors. The starter motor starts the gasoline engine to start it. Other engines run windshield wipers. Some cars have electric motors to operate the windows and even the rear-view mirrors.
- An electric train: An electric train has a powerful motor to drive it.
What is inside an electric motor?
- Coil: the coil is made of copper wire - because it is an excellent conductor. It is rolled on an armor. The coil becomes an electromagnet when the current flows through it.
- Armor: the armor supports the coil and can help reinforce the electromagnet. This makes the engine more efficient.
- Permanent magnets: there are two permanent magnets. They produce a stable magnetic field in such a way that the coil will rotate when a current flows in it. Some motors, instead of permanent magnets, have electromagnets, made with more coils of copper wire.
- Switch: each end of the coil is connected to one of the two halves of the commutator. The switch reverses the contacts every half turn.
- Brushes: the brushes press the switch. They maintain contact with the switch even if it is spinning. The current flows in and out of the motor through the brushes.
- Piece made of steel: the shaped piece is made of magnetic material that joins the two permanent magnets and, consequently, makes them a unique magnet shaped horseshoe. Commercial engines often use a horseshoe magnet.
How does it work?The motor is connected to a battery. When the switch is closed, the current starts to flow and the coil becomes an electromagnet. In this case, the current flows in the opposite direction to the time at the head of the coil. This turns the head into a north pole. This north pole is attracted to the south pole on the left, so the head turns to the left. Notice that the lowest part of the coil is a south pole and that it is attracted by the magnet on the right.
When the coil is placed in vertical position, there is no rotating force in it because the coil electromagnet is aligned with the permanent magnets. If the current of the coil were constant, the coil would stop at that position. However, to keep it turning, the switch interrupts the contact in that position. Thus the current stops flowing for a moment. The impulse of the coil keeps it spinning and the contacts are reconnected. However, they are now in the other direction. Therefore, the side of the coil that used to be a south pole is now a north pole.
The switch will continue to reverse contacts every half turn (when the coil is in the vertical position). In this way, the motor keeps turning.
The motor effectThe motor (electric) effect is what turns an engine. We can see its operation on a single piece of copper wire.
Electric catapultImagine a loose piece of copper wire on copper rails. The loose piece of cable is between the poles of a magnet. The rails are connected to a power supply. What will happen when we connect the voltage? The magnetic field causes the cable to move but only when there is electrical current in the cable.
The magnetic field points from the north pole of the magnet to its south pole and is at right angles to the current. This arrangement produces the greatest force and causes the cable to move. The current in a cable at right angles to the magnetic field produces a force in the cable.
Spinning itWe can understand an engine in the same way. When the current flows through the coil:
- One side of the coil feels a push up.
- The other side feels an impulse down.
When to invert the currentWhen the coil is upright, there is no rotary force trying to rotate it. The two forces try to pull the ends of the coil out. It is at this point when the switch reverses the contacts.
If the coil was already rotating, its momentum will cause it to exceed the vertical position. When the contacts are reconnected, the switch will have reversed the current. So the side of the coil that was previously pulled up, is now down, and vice versa.
Therefore, the coil continues to rotate in the same direction.
Energy transferElectric motors get hot when used. The heating comes from energy dissipated in the form of heat and is a loss. We want the engines to move things, not use them as heating!
The energy is lost when the electric current flows through the motor coils. The coil cables have electrical resistance, the higher the resistance the harder it is for the current to flow and therefore more energy is lost.
Copper is a good metal for use in motor coils because:
- It has less resistance than almost any other metal,
- It can be easily transformed into copper cables,
- It is not too expensive,
- can withstand high temperatures,
- It can be easily recycled when the motor is replaced.
See also "Concepts of Physics Used in Industrial Machines: Electric Motors"