Motor / Electric motor | SEW-EURODRIVE

The abbreviation SEW in the name SEW-EURODRIVE stands for “Süddeutsche Elektromotorenwerke” (German for Southern German Electric Motor Plants). Electric motors in various designs still are the basis of our drive technology: From energy-efficient motors, hygienic or explosion-proof designs, linear motors or electric cylinders, we certainly have just the motor solution you need.

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.

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 .

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).

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.

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.

What types of electric motors are available?

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.

DC motors

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.

Asynchronous AC motors

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.

Linear motors

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.