How does an electric motor work? Electric motors are everywhere! In your house, almost every mechanical movement that you see around you is caused by an AC (alternating current) or DC (direct current) electric motor. In this article we’ll look at both types.
By understanding how a motor works you can learn a lot about magnets, electromagnets and electricity in general. An electric motor uses magnets to create motion. If you have ever played with magnets, you know about the fundamental law of all magnets: Opposites attract and likes repel.
So, if you have two bar magnets with their ends marked “north” and “south,” then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion.
Inside an Electric Motor
To understand how an electric motor works, the key is to understand how the electromagnet works.
An electromagnet is the basis of an electric motor. Say that you created a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail would become a magnet and have a north and south pole while the battery is connected.
Now say that you take your nail electromagnet, run an axle through the middle of it and suspend it in the middle of a horseshoe magnet as shown in the illustration.
If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen: The north end of the electromagnet would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet.
The south end of the electromagnet would be repelled in a similar way. The nail would move half a turn and then stop in the position shown.
The key to an electric motor is to go one step further so that, at the moment that this half turn of motion completes, the field of the electromagnet flips. You flip the magnetic field by changing the direction of the electrons flowing in the wire, which means flipping the battery over.
The flip causes the electromagnet to complete another half turn of motion. If the field of the electromagnet were flipped at precisely the right moment at the end of each half turn of motion, the electric motor would spin freely.
How Does an Electric Motor Work?
There are two ways to overcome this problem. One is to use a kind of electric current that periodically reverses direction, which is known as an alternating current (AC).
In the kind of small, battery-powered motors we use around the home, a better solution is to add a component called a commutator to the ends of the coil. (Don’t worry about the meaningless technical name: this slightly old-fashioned word “commutation” is a bit like the word “commute”.
It simply means to change back and forth in the same way that commute means to travel back and forth.) In its simplest form, the commutator is a metal ring divided into two separate halves and its job is to reverse the electric current in the coil each time the coil rotates through half a turn.
One end of the coil is attached to each half of the commutator. The electric current from the battery connects to the motor’s electric terminals.
These feed electric power into the commutator through a pair of loose connectors called brushes, made either from pieces of graphite (soft carbon similar to pencil “lead”) or thin lengths of springy metal, which (as the name suggests) “brush” against the commutator.
With the commutator in place, when electricity flows through the circuit, the coil will rotate continually in the same direction.
A simple, experimental motor such as this isn’t capable of making much power. We can increase the turning force (or torque) that the motor can create in three ways: either we can have a more powerful permanent magnet, or we can increase the electric current flowing through the wire, or we can make the coil so it has many “turns” (loops) of very thin wire instead of one “turn” of thick wire.
In practice, a motor also has the permanent magnet curved in a circular shape so it almost touches the coil of wire that rotates inside it. The closer together the magnet and the coil, the greater the force the motor can produce.
How Does an Electric Car Motor Work?
Electric cars run on batteries which are a DC source. However, they can have AC or DC motors fitted. Where there is an AC motor, there are onboard inverter circuits to convert the DC from the batteries into alternating currents. The motors are usually AC induction types.
The onboard motor can also be used as a back-feeding alternator used for charging the batteries when the vehicle is descending a hill or braking. The disadvantage of using AC motors in cars is that they tend to be more expensive to implement than DC motor systems.
How Does an Electric Fan Motor Work?
A larger electric fan is often powered by an induction motor. Smaller handheld fans have DC motors. Single-phase induction motors in fans can be fitted with starter capacitors. These are wired in series with the stator and rotor coils to provide out-of-phase current.
The capacitors enable the motors to start up easily and can facilitate speed control. The capacitors are used to improve the starting and running performance of single-phase motors and they also ensure the motor runs in the correct direction.
How Does an Electric Bike Motor Work?
In electric bikes, the motors favoured are permanent magnet and brushless DC motors. They are commonly 3-phase motors (they have 3 stator coil windings). These are powered and controlled by onboard electronics which supply pulses to regulate power and speed.
Universal Motors
DC motors like this are great for battery-powered toys (things like model trains, radio-controlled cars, or electric shavers), but you don’t find them in many household appliances. Small appliances (things like coffee grinders or electric food blenders) tend to use what are called universal motors, which can be powered by either AC or DC.
Unlike a simple DC motor, a universal motor has an electromagnet, instead of a permanent magnet, and it takes its power from the DC or AC power you feed in:
- When you feed in DC, the electromagnet works like a conventional permanent magnet and produces a magnetic field that’s always pointing in the same direction. The commutator reverses the coil current every time the coil flips over, just like in a simple DC motor, so the coil always spins in the same direction.
- When you feed in AC, however, the current flowing through the electromagnet and the current flowing through the coil both reverse, exactly in step, so the force on the coil is always in the same direction and the motor always spins either clockwise or counter-clockwise. What about the commutator? The frequency of the current changes much faster than the motor rotates and, because the field and the current are always in step, it doesn’t actually matter what position the commutator is in at any given moment.
Other Kinds of Electric Motors
In simple DC and universal motors, the rotor spins inside the stator. The rotor is a coil connected to the electric power supply and the stator is a permanent magnet or electromagnet.
Large AC motors (used in things like factory machines) work in a slightly different way: they pass alternating current through opposing pairs of magnets to create a rotating magnetic field, which “induces” (creates) a magnetic field in the motor’s rotor, causing it to spin around.
You can read more about this in our article on AC induction motors. If you take one of these induction motors and “unwrap” it, so the stator is effectively laid out into a long continuous track, the rotor can roll along it in a straight line. This ingenious design is known as a linear motor, and you’ll find it in such things as factory machines and floating “maglev” (magnetic levitation) railroads.
Another interesting design is the brushless DC (BLDC) motor. The stator and rotor effectively swap over, with multiple iron coils static at the center and the permanent magnet rotating around them, and the commutator and brushes are replaced by an electronic circuit.
You can read more in our main article on hub motors. Stepper motors, which turn around through precisely controlled angles, are a variation of brushless DC motors.
FAQs
How does a toy electric motor work?
A very small electric motor has two small permanent magnets, a commutator, two brushes, three poles, and an electromagnet made by winding wire around a piece of metal. It works the same way a larger version does, but on a much smaller scale.
What is a DC electric motor?
A DC electric motor converts direct current electrical energy into mechanical energy unlike the AC version that uses alternating current.
What are the parts of a simple motor?
A simple motor has six parts: an armature or rotor, a commutator, brushes, an axle, a field magnet, and a DC power supply of some kind.
How long can an electric motor last?
Under test conditions an electric motor can last between 15 to 20 years, provided it is used under normal operating conditions.
Is a DC or AC electric motor better?
AC motors are generally more powerful and require less maintenance, however, DC motors tend to be highly efficient. The application of the electric motor tends to influence whether AC or DC is selected.
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