What is an electric motor? Here, Leroy-Somer’s Fatma Zanzou explains the basic principles behind these devices which turn electricity into motion. For more information on Permanent Magnet (PM) motors, read our article here.
Anything that turns electricity into motion, meaning electrical energy into mechanical energy, is called an electric motor. Electric motors are everywhere! Almost every mechanical movement that you see around you can be created by an electric motor. Because of the nearly unlimited number of applications for electric motors, it is not hard to imagine that there are hundreds of millions of motors in operation across the world. Let’s understand what they are and how they work.
Electric motors operate on a very simple principle, which is that when electricity and magnetism are united into one force, it is called electromagnetic force. Electric motors work therefore on the principles of electromagnetism. When an electric current is introduced into a magnetic field, a force is generated. An electric motor utilizes looped wires – the same wires that carry the current – which are stationed at right angles to the magnetic field in the electric motor. Because the magnetic field has dual polarities, each end of the wires is moved in a different direction. This creates a turning motion.
Torque, which is the ability of a rotating element to overcome turning resistance, is controlled by adding several loops to the armatures and the magnetic field is produced by an electromagnet. This design allows the rotor to be turned by simple electro-mechanical force. There are very few parts that actually experience any wear and, with those two factors combined, it’s possible for electric motors to keep running for an incredibly long amount of time while exhibiting very little wear.
Indeed, one of the most remarkable things about electric motors is that they contain very few parts. Compared to, for example, an internal combustion engine, an electric motor is a simple device. In fact, all of the different parts of an electric motor could easily be pulled out and laid out on a very small table, depending upon the size of the motor, of course.
The stationary part of an electric motor is called the stator. The stator will be provided with permanent magnets or windings, depending on motor technology. The windings will look familiar to anybody with experience of other electrical components. They are typically simple windings of wire around a magnetic iron core. When a current is passed through these windings, they generate a magnetic field.
The rotor is the part that actually converts electrical energy into mechanical energy. These come in various designs. One of the biggest breakthroughs in electric motor design was finding a way that the rotor could operate continuously, providing uninterrupted torque to whatever was being powered by the electric motor. Today’s electric motors are capable of producing an incredible amount of torque. The commutator, meanwhile, is a device that is utilized to switch the input of the electric motor.
If we go back in history, electric motors, like many electrical devices, started out as simple experiments then used as demonstration devices until they found practical applications.
In the year 1821 British scientist Michael Faraday explained the conversion of electrical energy into mechanical energy by placing a current carrying conductor in a magnetic field which resulted in the rotation of the conductor due to torque produced by the mutual action of electrical current and field. The most primitive of machines was a D.C. (direct current) machine designed by another British scientist William Sturgeon in the year 1832. But his model was overly expensive and wasn’t used for any practical purpose. Later in the year 1886 the first electrical motor, capable of rotating at a constant speed under a varied range of load, was invented by scientist Frank Julian Sprague.
Today, there are several different types of electric motors on the market. They can first be differentiated by whether they utilize AC or DC power as their means of activating the motor. AC electrical motors are driven by alternating current, for example the synchronous motor, which always runs at synchronous speed. Here the rotor is an electromagnet which is magnetically locked with stator rotating magnetic field and rotates with it. The speed of these machines is varied by altering the frequency (f) and number of poles (P).
Induction motors are about interaction of the magnetic field and the circulating currents so that the rotor starts and keeps rotating. Induction motors, also known as asynchronous motors, run at a speed a bit less than synchronous speed. Other types of electrical motors exist, such as servo motors with special features like high torque in a compact design or high dynamic characteristics, which have been developed according to industry needs. Usually these motors integrate rare earth permanent magnet in the rotor.
Electric motors utilize various starting mechanisms. In the simplest and smallest types, the starter may connect directly to a power supply network. This is also known as Direct On Line (DOL). Larger motors require more complex devices like soft starters. A Soft Starter allows the operator to start the device with a reduced voltage. The user can define the limits for startup current and other variables. The Star Delta starter is a type of soft starter that gradually increases the voltage to its maximum load as the motor increases its speed. Soft Starting has the advantage of allowing the mechanical stress and torque output on the load to be controlled. Rather than the motor suddenly taking off at full torque and speed, as would be the case with a DOL starter, the motor gradually spins up.
Variable speed drives are more and more used with three-phase induction motors. These controllers are used in electric motors of all sizes. The most significant advantage is that they offer the highest degree of control and functionality. In industrial settings, the control they offer over torque, tension, acceleration and flow can all contribute to more efficient and controlled processes. Drives also integrate many features such as automation and PLC, communication means, fieldbuses, safety control etc.
Electric motors can be found in a huge array of applications. Everything from pumps, compressors, fans, tower cranes and material handling, textile, printing, packaging, wood machinery and test stands all make use of their capabilities. They are among the most common electrical components in use today, so it’s fair to say that electric motors have greatly influenced our everyday life.