A Quick and Simple Permanent Magnet Motor Demonstration
Have you ever seen a permanent magnet electric motor work from the inside? In this post, I’ll show you a quick and simple permanent magnet motor demonstration, which will help you understand how it works. Before you get into the demonstration, please take a look at my previous blog about the construction of a permanent magnet motor. It should help you follow the part names that I’ll be using.
What is the Purpose of an Electric Motor?
A motor converts electrical energy into mechanical energy. You can use any electrical supply rated for the motor and connect it; this will make the motor spin. The spinning part of the motor, in our case its the shaft, can then be attached to a mechanical load. Mechanical load is anything that you to turn, for example, one of the wheels of a bike.
Quote - The primary purpose of all motors is to provide a turning force, which is Torque.
The supply can be Alternating current (AC) or Direct Current (DC). The motor we have used in the demonstration here is a permanent magnet motor, which is a DC motor. It’s rated at 12V to 24V that means that it’ll produce a useful torque within that range of voltages. We also this type of motor in our Electromag pracbox
What Happens Inside a Permanent Magnet Motor?
The Current Path
When I connected the power supply to the terminals of this motor, The current flows inside the armature. Here is how its journey occurs.
- The current travels from the positive terminal of the power supply to the positive terminal of the motor
- One of the brushes attaches to it this terminal, so the current travels there
- The brush touches one of the commutator segments; the current then goes there.
- Each commutator segment has one end of one of the armature coils joined to it, so the current then proceeds into the coil.
- The other end of the coil is wired to another commutator segment, and that’s the path the current takes.
- The other brush touches this commutator segment and guides the current through the brush.
- This brush connects to the negative terminal of the motor, so it goes to this terminal through this path.
- Finally, the current returns to the negative terminal of the power supply through the wire that connects to the negative terminal of the motor.
What does this Current do?
An electric current has several effects like heat, light, chemical, etc; we will discuss these in another post. One of the effects of the electric current is magnetism. When current goes through a wire, it creates a circular magnetic field around it. If you loop this wire into multiple turns, the magnetic field increases that many times.
In our case, the coil is the armature coil. The magnetic field amplifies many times over when there is a ferromagnetic core inside the coil – this is the armature core.
So, when the current goes through the armature, it creates a strong magnetic around it. The strength of this magnetic field depends on the amount of current that is passing through it – More current creates a stronger magnetic field.
How does the Magnetic Field in the Armature Help?
Armature’s magnetic field is one of the crucial phenomena in our permanent magnet motor. By itself, however, it’s not very useful. To make use of it, we need another magnetic field in the mix. This is the stator’s magnetic field. A motor needs at least two magnetic fields to work. One was the armature’s and the second is the stator’s magnetic field.
You may have noticed when playing with magnets that when you bring two of them together, there is either attraction or repulsion between them. A motor is the same; it’s an interaction between two magnetic fields.
The armature’s magnetic field and the stator’s magnetic field interact with each other to produce a turning force or Torque.
How does a Motor Give Torque to a Mechanical Load - Back EMF
No-load Torque, don't need much current
Without a mechanical load, a motor spins without creating much Torque – this is called a no load torque. As the motor turns because of the interactions between the two magnetic fields, it is spinning within the stator magnetic field. Here, it also acts as a generator and creates a new current, which is in the opposite direction to the supply current – this is called Back-EMF. EMF stands for Electromotive Force.
The faster the motor spins, the higher the back-emf. A motor can only spin the fastest when there is nothing attached to it, and since it produced high back-emf, the current from the supply will be less. So high speed = high back-emf = less supply current = less torque.
What Happens to the Torque When you Load it?
As the motor is loaded, the speed reduces, which causes the back-emf to reduce as well. When back-emf decreases, the supply current automatically increases because there is less opposing current. With an increase in supply current, we get more torque, and the load turns. So added load = slower speed = lower back-emf = higher supplier current = higher torque.
Controlling the Speed of the Permanent Magnet Motor
The design of a permanent magnet motor is quite simple, which makes it easier to control. From the explanation and demonstration, you may have noticed that there is only one variable in the operation of this motor, and that is the current. If you control the current, you control the speed.
Variable Power Supply
There are many ways to control the current, and the simplest way is to change the voltage. When you change the voltage, the current will also change, and the motor speed will change. This, however, is not a practical solution because you will have to fix your voltage in a practical application.
Variable Resistor
Another way you can control the current is to connect a variable resistance like a rheostat in series with the armature. As the resistance increases, the current decreases, which also decreases the motor speed. But this solution creates power loss over the variable resistor.
Pulse Width Modulator
The most efficient solution for controlling motor speed is a Pulse Width Modulator (PWM), which is more complicated compared to the other two, but that’s the topic for another post.
Can a Motor Spin the Other Way?
Absolutely. You can make most motors spin the opposite direction if the manufacturer has allowed for it. In the permanent magnet DC motor that we have here, it’s as simple as swapping the supply terminals.
Conclusion
The permanent magnet DC motor is the simplest type of motor in the industry. It can be quite useful because of its simplicity of operation and control and of course, the lower cost.
In this post, we saw a quick and simple demonstration of a permanent magnet motor and discussed some basics of its operation and control. We will produce more demonstrations of the operations in future blog posts and videos.
Please leave me a comment to share your experiences with the electric motors or if I have missed anything.
Thanks for reading.
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About the Author
Try Mao
Try is an electrical engineer with a passion for programmable logic controllers and hardware. He loves taking photos and is a gun with a camera. His purpose is to solve problems through innovative electrical product design and development
