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How do you bring things in motion and keep them moving without moving a muscle? While steam engines create mechanical energy using hot steam or, more precisely, steam pressure, electric motors use electric energy as their source. For this reason, electric motors are also called electromechanical transducers.
The counter piece to the electric motor is the generator, which has a similar structure. Generators transform mechanic motion into electric power. The physical basis of both processes is the electromagnetic induction. In a generator, current is induced and electrical energy is created when a conductor is within a moving magnetic field. Meanwhile, in an electric motor a current-carrying conductor induces magnetic fields. Their alternating forces of attraction and repulsion create the basis for generating motion.
In general, the heart of an electric motor consists of a stator and a rotor. The term "stator" is derived from the Latin verb "stare" = "to stand still". The stator is the immobile part of an electric motor. It is firmly attached to the equally immobile housing. The rotor on the contrary is mounted to the motor shaft and can move (rotate).
In case of AC motors, the stator includes the so-called laminated core, which is wrapped in copper wires. The winding acts as a coil and generates a rotating magnetic field when current is flowing through the wires. This magnetic field created by the stator induces a current in the rotor. This current then generates an electromagnetic field around the rotor. As a result, the rotor (and the attached motor shaft) rotate to follow the rotating magnetic field of the stator.
The electric motor serves to apply the created rotary motion in order to drive a gear unit (as torque converter and speed variator) or to directly drive an application as line motor.
All inventions began with the DC motor. Nowadays however, AC motors of various designs are the most commonly used electric motors in the industry. They all have a common result: The rotary motion of the motor axis. The function of AC motors is based on the electromagnetic operating principle of the DC motor.
As with most electric motors, DC motors consist of an immobile part, the stator, and a moving component, the rotor. The stator consists either of an electric magnet used to induce the magnetic field, or of permanent magnets that continuously generate a magnetic field. Inside of the stator is where the rotor is located, also called armature, that is wrapped by a coil. If the coil is connected to a source of direct current (a battery, accumulator, or DC voltage supply unit), it generates a magnetic field and the ferromagnetic core of the rotor turns into an electromagnet. The rotor is movable mounted via bearings and can rotate so that it aligns with the attracting, i.e. opposing poles of the magnetic field – with the north pole of the armature opposite of the south pole of the stator, and the other way round.
In order to set the rotor in a continuous rotary motion, the magnetic alignment must be reversed again and again. This is achieved by changing the current direction in the coil. The motor has a so-called commutator for this purpose. The two supply contacts are connected to the commutator and it assumes the task of polarity reversal. The changing attraction and repulsion forces ensure that the armature/rotor continues to rotate.
DC motors are mainly used in applications with low power ratings. These include smaller tools, hoists, elevators or electric vehicles.
Instead of direct current, an AC motor requires three-phase alternating current. In asynchronous motors, the rotor is a so-called squirrel cage rotor. Turning results from electromagnetic induction of this rotor. The stator contains windings (coils) offset by 120° (triangular) for each phase of the three-phase current. When connected to the three-phase current, these coils each build up a magnetic field which rotates in the rhythm of the temporally offset line frequency. The electromagnetically induced rotor is carried along by these magnetic fields and rotates. A commutator as with the DC motor is not required in this way.
Asynchronous motors are also known as induction motors, as they function only via the electromagnetically induced voltage. They run asynchronously because the circumferential speed of the electromagnetically induced rotor never reaches the rotational speed of the magnetic field (rotating field). Due to this slip, the efficiency of asynchronous AC motors is lower than that of DC motors.
AC synchronous motors
In synchronous motors, the rotor is equipped with permanent magnets instead of windings or conductor rods. In this way the electromagnetic induction of the rotor can be omitted and the rotor rotates synchronously without slip at the same circumferential speed as that of the stator magnetic field. Efficiency, power density and the possible speeds are thus significantly higher with synchronous motors than with asynchronous motors. However, the design of synchronous motors is also much more complex and time-consuming.
In addition to the rotating machines that are mainly used in the industry, drives for movements on straight or curved tracks are also required. Such motion profiles occur primarily in machine tools as well as positioning and handling systems.
Rotating electric motors can also convert their rotary motion into a linear motion with the aid of a gear unit, i.e. they can cause it indirectly. Often, however, they do not have the necessary dynamics to realize particularly demanding and fast "translational" movements or positioning.
This is where linear motors come into play that generate the translational motion directly (direct drives). Their function can be derived from the rotating electric motors. To do this, imagine a rotating motor "opened up": The previously round stator becomes a flat travel distance (track or rail) which is covered. The magnetic field then forms along this path. In the linear motor, the rotor, which corresponds to the rotor in the three-phase motor and rotates in a circle there, is pulled over the travel distance in a straight line or in curves by the longitudinally moving magnetic field of the stator as a so-called carriage or translator.
The invention of the electric motor cannot be traced back to a single person. Its discovery was the result of the research of several inventors. In the 19th century the interest in electrical engineering grew more and more and inspired researchers worldwide. One after the other, new inventions came along.
Since the first electric motors were dependent on the current supply of zinc batteries, there was still a long way to go before they could seriously compete with the predominant steam engines. This changed with the development of the first power generators.
But here, too, there were restrictions. The direct current generated by the generators could not be transported over long distances. The breakthrough came only with the introduction of alternating and three-phase current, which could be provided over long distances without great losses, and with the invention of the AC motor.
Here a small, not complete insight into the historical data and facts:
Everything started with electric motors. Electric motors are still part of our core business – mainly in form of gearmotors and in conjunction with frequency inverters that match the desired application. As one of the world's leading manufacturers of drive and automation solutions, we offer you a wide range of asynchronous and synchronous motors. Whether energy-efficient motors, linear motors, electric cylinders, motors in hygienic or explosion protection design, extra-low voltage drives, etc. – the optimum electric motor solution for you is guaranteed. A comprehensive range of accessories, such as brakes, built-in encoders, and further options complete our motor range.