How does an electric motor work?
By understanding how an electric motor works you can learn a lot about magnetism and its relationship with electricity, and what better way to learn how a motor works than by building your own basic motor. But first, a little about the fundamentals of electricity and magnetism.
The basic operation of a motor, the rotational movement, relies on the most basic laws of magnetism; that like poles repel and opposite poles attract. This creates the force required to run the motor. Of course, there is more to it than that as any working electrical motor requires an arrangement of permanent magnets and electromagnets.
All electric motors are made of several consistent parts, including a stator, a rotor, an output shaft, casing and for DC motors (which we will look at in more depth) commutators and brushes. These parts, with a careful arrangement of permanent magnets and electromagnetic coils create a working motor.
In its most basic form, a motor converts an electrical current (either AC or DC) into rotational movement. The current is applied to an electromagnetic coil that creates a temporary magnetic field with a particular direction of magnetism creating a north and south pole at each end of the electromagnet, which can be reversed by reversing the direction of the current in the coil. This magnetic field interacts with the magnetic field produced by a series of permanent magnets aligned with alternating polarities. The interacting fields then either attract or repel each other to create rotational movement on the output shaft. As mentioned above, there are two types of motor that use alternating current and direct current to provide movement, to understand how each works, we’ll look at both in turn.
Direct current (DC) motor
The most basic form of an electric motor is a brushed direct current (DC) motor that uses a free to rotate rotor made of a current carrying conductor (simply a piece of wire bent into a rectangular loop) and static permanent magnets as the stator. The permanent magnets are aligned with adjacent opposite poles so the magnetic field flows from one to the other, through the air gap between the two. The direct current is then applied to the coil attached to the rotor; this applied current induces a magnetic field around the rotor which interacts with the magnetic field produced by the permanent magnets. This induced magnetic field is then repelled by one magnet and attracted by the other creating a rotational force. However, this rotating force would only be enough to rotate the shaft through one half turn as ultimately this would cause the poles of the coil to align with attracting poles of the permanent magnets and the motor would halt. By using what is known as commutator on the end of the axle, which resembles a ring cut in half, the direction of current delivered through the coil by conducting brushes is flipped at just the right time. This change in direction of current changes the polarity in the coil and therefore completes a full rotation of the axle. This is repeated indefinitely while ever a current is applied.
AC current (induction) motor
Most items we use every day aren’t powered by batteries (direct current) and therefore they rely on alternating current which reverses its direction about 50 times per second, as opposed to direct current which is consistent.
In a basic AC motor the electromagnetic coils form part of the stator, rather than the rotor. It is these coils that produce the magnetic field that creates the rotational movement. In the centre of the stator ring there is a further cylindrical arrangement that is free to rotate and made of metal bars. In an AC motor, the current is applied to the electromagnetic coils on the stator in sequence producing a magnetic field that alternates and rotates around the stator. Because the direction of the magnetic field in the coils is constantly changing it produces an electric current inside the rotor, which in turn produces its own magnetic field, which opposes that of the field that created it. The interaction between the two fields causes the rotor to spin.
How to build a basic DC motor
Below, we will describe how to create a basic DC motor. You can also follow a similar tutorial on instructables. You will need:
- 10mm diameter x 5mm thick neodymium magnet
- AA battery
- Half a meter of copper wire
- Two pieces of insulated wire, approximately 15cm long
- Two paperclips
- Two drawing pins
- A small piece of softwood or board
Step one – create a coil
To create a coil of copper wire you will need a length of approximately half a meter of wire. Take your AA battery and begin winding the coil around the battery leaving a couple of inches spare at each end. Make sure you wind a nice tight coil and wrap the trailing ends around the coil a couple of times to keep it together, still leaving plenty of uncoiled wire. Now, you need to sand the top surface of each of the trailing ends to remove the insulating enamel while taking great care not to sand the sides or the bottom, just the top.
Step two – make a rig
To create a simple base for your motor you need to take the small piece of softwood or board and two paperclips. Bend half of each paperclip into an ‘L’ shape with a supporting shelf so that you can create two vertical stanchions and position them approximately three inches apart. Fix the two stanchions in place with the two drawing pins.
Step two – connect the battery to the paperclips
Take two pieces of electrical wire and strip the insulation from both ends of each piece of wire. Wrap the exposed wire around each paperclip and connect one piece of wire to each end of the battery.
Step three – position the strong magnet
Take a strong neodymium magnet, a 10mm diameter x 5mm thick neodymium magnet is ideal, and glue it in place in a central position between the two paperclips.
Step four – test your motor
Now, all there is left to do is to position the coil resting on the two paperclips above the neodymium magnet and give it a twist to get it going. The current flowing from the battery through the paperclips and intro the coil will induce a magnetic field around the coil, causing it to spin.