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Motor and power electronics
Background information on electric motors
There are four criteria for differentiating among electric motors: the frequency response, the generation of the alternating field
(commutation), the excitation of the magnetic field, and the structural shape.
Depending on the frequency response, we speak of:
Induction motors: the ratio between the engine speed and the frequency of the supply voltage is not constant: it depends on
the loading condition of the machine. The higher the load, the higher the speed difference – the so-called “slip”, i.e. a specified
propeller speed is not maintained at higher flow resistances. Hence, thrust is not available at the very time it is required.
Synchronous motors: with this type of motor, the ratio between the supply voltage frequency and the engine speed is constant.
As a rule, synchronous engines are torque controlled. This means that they always draw as much power as they need in order
to provide the necessary torque at the desired speed. For this reason, they are the preferred motor in areas with particularly
demanding torque requirements. Should the motor require more power in order to maintain a specified propeller speed, the
motor automatically draws more power.
Depending on the type of the generation of the alternating field (commutation), we divide electric motors into:
Mechanically-commutated motors: The brush-complemented motors generate the alternating field necessary for the motor to
operate by means of sliding contacts. Based on their geometric organization, these “brushes” convert the power depending on
the rotor position. A shortcoming in these motors is the wear-and-tear of the brushes, hence making the motors maintenanceintensive.
The contact resistance also causes so-called brush losses, impairing the degree of effectiveness of the motor.
Electronically-commutated motors: they generate the alternating field necessary for the motor to operate by means of an electronic
circuit – the “frequency converter”. This prevents the occurrence of brush losses, and the motors are maintenance-free.
The enormous progress that has been made in the area of electronic power components and circuit design has only made it
possible in recent times for high-power motors to be manufactured at a marketable price.
Depending on the type of generation of the magnetic field, electric motors are divided into
Electromagnetic-excited motors: this type provides the necessary magnetic field by means of a second loading section. This
makes this option more economical: however, it is considerably bulkier and heavier than the permanent magnet-excited motor.
Further, it is also considerably less advantageous with regard to power consumption and degree of effectiveness.
Permanent magnet-excited motors: in this case, the permanent magnets generate the necessary magnetic field. Hence, there are
no performance losses in the field coils.
Depending on the structural shape, we speak of:
Internal rotor motor: in this classical model of electrical motor, the rotor is surrounded by the stator. The rotor is a revolving motor
component attached to the motor shaft: it is also known as a “rotor motor” or “armature”. Since the coils of the internal rotor
motor are located on the outside, the motor has advantages when it comes to cooling. Compared to other structural shapes,
however, it is relatively low-torque.
Disc armature motor: it generates the torque (= force times lever) by arranging the axle of the magnetic field parallel to the shaft
instead of radial to the shaft. This enables the realization of geometries in which the location of the electromagnetic generation
of power is a good distance from the axle. Hence, a higher torque is achieved at the same power. The disc armature geometry is
disadvantageous to outboard motors with direct water cooling. Due to its extremely large diameter, it isn’t possible to build disc
armature motors directly into a pylon.
External rotor motor: this is the most modern type of motor: the coils are arranged inside. The rotating magnets are located on
an externally-running bell. With the same structural shape, external rotor motors hence have a significantly higher torque than
internal rotor motors.
Catalogue 2007
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