Everything in motion
October 2010
Electric, hydraulic, pneumatic: How do the different types of drives work, and which drives are the best choice for a specific purpose.
Electric drives
A fixed magnet (stator) (1) generates a magnetic field (2). A coil carrying current (rotor) (3) is set in motion by the force of this field.
How they work
Electrical energy is converted into mechanical kinetic energy. Two magnetic fields, one each at a stator and a rotor, interact. As the magnetic fields constantly try to align themselves with each other, a torque is generated. The rotor turns inside the stator, creating movement.
Use
“Electrics have precise hands.” They offer high speed and great accuracy and are used, for instance, in machine tools, packaging machines and printing presses.
DC motor
A DC winding or a permanent magnet generates a fixed magnetic field in the stator. Coils are wound onto the rotor and connected so that the rotor field is perpendicular to the stator field; this configuration delivers the maximum torque. The motor requires a commutator and uses carbon brushes to switch the current.
Advantage: Speed and power can be controlled very effectively.
Disadvantage: The carbon brushes of the commutators are subject to wear and this affects the service life.
Synchronous motor
The stator has a three-phase winding configured in a circle. Three phase-shifted currents generate a rotating magnetic field. As the rotor has a fixed magnetic field, it can only develop effective torque at a synchronous speed. Thanks to modern current converters, synchronous motors can be controlled as precisely as DC motors, but without any wearing parts.
Advantage: Excellent energy efficiency and high operating efficiency, well above 90 percent.
Disadvantage: Requires a complex electronic regulation system. Strong permanent magnets are made of rare materials and are thus expensive.
Asynchronous motor
Here too, the stator generates a rotating field. The rotor contains a squirrel-cage winding. When the rotor is not following synchronously, a current is induced that counteracts the change in the magnetic field. Together with the magnetic field of the stator, this generates a torque that pulls the rotor along.
Advantage: Can be easily and inexpensively manufactured in large quantities.
Disadvantage: Because the current flowing through the rotor generates heat, this motor has a low efficiency level.