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External rotor motor

What is an external rotor motor?

An external rotor motor is a type of electric motor in which the moving rotor rotates externally around the internal, immovable stator. The external rotor motor transfers the torque directly from the rotating external rotor, e.g. to an impeller, which is mounted directly on the rotor.

Different technologies can be used in the design of external rotor motors.External rotor motors can be designed as EC motors or AC motors.

 ​Maximum efficiency from ZIEHL-ABEGG: Motors for ventilation systems and elevator machines designed as external rotors.

How does an external rotor motor work?

Fundamentally, all electric motors consist of a fixed, non-moving stator and the rotating rotor.
The internal stator contains windings that create an electromagnetic field of rotation through an external current supply. The external rotor generates its own magnetic field: Depending on the technological design, alternately poled permanent magnets (EC motors) or a sheet metal package with energised windings (AC motors) are installed in the rotor.
By attracting different and simultaneously rejecting identical polarities, the rotor moves with the stator’s moving field of rotation.
In the case of an external rotor motor, the rotor therefore does not rotate inside the stator, but around it.

The desired speed range can be controlled electronically by means of control electronics integrated in the external rotor motor or by connecting a frequency inverter.

How are external rotor motors constructed?

EC external rotor design:

  • Housing
  • Stator bushing with coiled sheet metal package
  • Printed circuit board with commutation electronics
  • Rotor with shaft and permanent magnets
  • Ball bearing

AC external rotor design:

  • Housing with junction box (terminal box)
  • Stator bushing with coiled sheet metal package
  • Rotor sheet package with rotor coiling or permanent magnets
  • Ball bearing

External rotor EC

Housing cover with power connection

Rotor with shaft and permanent magnets

Wound sheet package (stator)

Stator bush

Printed circuit board with commutation electronics

External rotor AC

Housing with junction box (terminal box)

Stator bushing with coiled sheet metal package

Rotor sheet package with rotor coiling or permanent magnets

 Maximum efficiency from ZIEHL-ABEGG: Motors for ventilation systems or elevator technology designed as external rotors.

What are the advantages of external rotor motors?

  • High torque: Due to the external rotor, the area of the magnetic field is significantly larger than in the internal rotor motor with the same components. This generates a greater electromechanical force that produces a high torque.
  • High levels of efficiency: The large area of interaction between the rotor and stator in EC external rotor motors generates high efficiency and comparatively low energy consumption.
  • Compact dimensions & robust design: The high performance between the rotor and stator allows the external rotor motor to save space.
  • Wear- and maintenance-free design: External rotor motors designed as EC motors have very low mechanical wear thanks to the electrical commutation.
  • Precisely adjustable speed range: Depending on the technology, it can be controlled via integrated electronics (EC) or an upstream frequency inverter (AC).
  • No need for a gearbox or mechanical transmission depending on the application.
  • Quiet operation possible.

What is the difference between external and internal rotors?

In the case of an external rotor motor, the stator is located inside the machine and surrounded on the outside by the rotating rotor, which outputs the torque. Due to the large surface of interaction between the rotor and stator, external rotor motors achieve high torque with low energy consumption. The exact opposite applies to the internal rotor motor, whose torque is transmitted outwards by the internal rotor via a stable shaft and is surrounded by the stator lamination pack. Internal rotor motors achieve a high output density with low inertia.

Where are external rotor motors used?

Due to the special performance characteristics and advantages of external rotor motors, frequent applications are, for example:

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