Simplicity of A Permanent Magnet Electric Motor Technology
The permanent magnet DC motors are the simplest electric motors that were mainly useful in low power applications but not anymore. In this blog post, I will crack open a permanent magnet electric motor to show you how simple its design is and discuss each component of it. By the end of this article, you will know the names and functions of each part of a permanent magnet electric motor.
How does a motor work - A simple science
When you bring two magnets with opposite poles close to each other, they will pull together. We call this attraction. And when you bring two magnets with same poles close to each other, they will try to move away. We call this repulsion.
If we fix one magnet on an axis so that it’s free to rotate and then use another magnet to repel it, the fixed magnet will turn, and that’s the basis of a simple motor. You need at least two magnetic fields to make a motor work.
Problems with this setup
The purpose of an electric motor is to provide a turning force, technically called ‘Torque’. The problem with the setup with two magnets is that it cannot provide torque for varying mechanical loads. The only way to get that is by changing the magnetic field strength of the magnet as the torque requirement changes, and this is only possible with electromagnets.
Another problem is that once the magnet on the axis turns, the other side will now attract and won’t want to turn any more. The only solution is to swap the magnet poles, which is not possible if its a permanent magnet but it is with electromagnets. The poles of an electromagnet change when the current direction changes, we will see what makes this possible later.
So what's inside a permanent magnet electric motor
The motor that I will pull apart is a 775 DC motor that we use in our Electromag pracbox with their rotor shaft coupling to each other to as a motor/generator configuration.
The permanent magnet electric motor has three main sub-assemblies
- The stator
- The rotor
- The brush system
The stator - Not everything in a motor rotates
A stator is a part of an electric motor that is stationary. For the permanent magnet electric motor, the stator has
- Two permanent magnets
- Case of the motor
The two permanent magnets give us one of the two magnetic fields that we discussed in the previous section. They are “Ferrite or Ceramic” magnets placed opposite to each and opposite polarity with the north pole facing the south pole. Without these, the motor won’t work.
The case provides a path to complete the magnetic field. Completing the magnetic field makes it stronger inside the stator. The motor may work without this case, but it won’t provide enough torque because the magnetic field will be weak.
The rotor rotates
The rotating part of the motor is called the rotor. In DC electric motors, this part is also known as the armature. The armature is as an electromagnet, and its primary function is to produce the second magnetic field to make the motor work. When the current-carrying armature is inside a magnetic field from the stator, it will experience a force and will start to rotate.
For the permanent magnet electric motor, the rotor assembly has
- Armature winding
- Armature core
- Cooling fan
The armature winding is made from enamelled copper wire wound around the armature core and can be of many turns depending on the application. The primary function of the armature winding for a DC electric motor is to produce the magnetic field whenever the current is going through it.
The DC motors in our Electromag pracbox have five coils that make the armature winding. More coils create a smoother rotation, and fewer coils will cause a jerky rotation.
The magnetic field that armature winding generates because of the current flowing in it is not very strong. The main role of the armature core is to strengthen the magnetic field that the armature windings produce.
All electromagnets need a core for a strong and reliable magnetic field. The only problem is that when the armature rotates in the stator magnetic field, the core ends up generating some internal current since it’s also a conductor; this is called Eddy current. Eddy currents create heat in the armature core and reduce its capacity to increase the magnetic field.
So instead of using a block of material as the armature core, it is made with a stack of several thin sheets of the same material. These sheets are called laminations, and they have a non-conductive coating so that each of them is electrically disconnected from the other.
Since these laminations are thin, their resistance is higher, and the Eddy current generated in them is a lot smaller than a block of material.
But how does the current reach the windings? Via the commutator
As mentioned earlier, the permanent magnet electric motor can only work if the magnetic poles change, and this is only possible in electromagnets. A commutator is a component that swaps the magnetic poles of the armature, which is an electromagnet.
The motor that I pulled apart here is using “Split ring” commutator, which has multiple segments or bars depending on the number of the armature coils. Each commutator segment connects to each end of an armature coil, and each of them is electrically separate from the next segment. Usually, there is an odd number of commutator segments in a DC electric motor.
The brushes rest on the commutator. When the current is entering the brushes, they go to the armature windings via the commutator.
Any high performing electric motor has some cooling equipment to reduce the heating in the armature windings during operation. As the motor operates, it draws current, and when the current increases, the heat in the armature windings increase, which then results in power loss called the I^2 R loss. If windings aren’t cooled down, the coating on the copper winding can deteriorate and eventually create a short circuit within the coils.
The cooling fan either extracts this heat or pulls in air to expel the heat on the other side and attempts to keep the motor cool.
All other parts of the rotor assembly, including one bearing, rest on the armature shaft. This shaft transmits the torque to the mechanical load coupled to the motor.
The purpose of the bearings is to allow the rotor assembly to rotate freely with as less friction as possible. There are two sets of bearings in this motor; one is on the shaft, and the other is in the motor casing. The armature shaft sits on these and is precisely aligned to maintain a smooth rotation.
The brush system
The brush system of a permanent magnet electric motor has two main components
- Motor terminals
The power from the battery or a DC power supply connects to the terminals of the motor. The current from the supply flows from the terminals to the brushes.
Motors of this type have carbon brushes and are a critical part of a permanent magnet electric motor. Their function is to provide an electrical path between the electromagnetic windings (Rotor) and the motor terminals. Brushes are a link between the moving part and the stationary terminals of the motor.
The permanent magnet DC motors are the simplest electric motors that were mainly useful in low power applications but nowadays, their output has increased significantly because of the improvements in the construction techniques and magnets.
In this post, we discussed the parts of a permanent magnet electric motor and their purpose. I hope this has given you a good basic understanding of the topic. Please leave me a comment to share your experiences with the electric motors or if I have missed anything.
Thanks for reading.