Electric Motors

There are five criteria used to distinguish electric motors:

  • Frequency characteristics
  • Alternating field generation (commutation)
  • Magnetic field excitation
  • Structural design
  • Magnetic field direction relative to the rotational axis

Depending on the frequency characteristics, a difference is made between

Asynchronous motors

The ratio of the motor speed to the frequency of supply voltage is not constant but varies depending on the load state of the machine. The higher the load, the higher the speed difference – the so-called "slip" – i.e. a specified propeller speed will not be maintained when flow resistance is greater. Thrust is therefore not available at the precise moment it is needed.

Synchronous motors

With this type of motor, the ratio of the motor speed to the frequency of supply voltage is constant. As a rule, synchronous motors are torque controlled. They therefore always draw as much current as they need in order to deliver the necessary torque at the desired motor speed. For this reason they are the preferred choice for use in fields with especially demanding torque requirements. If the motor needs more power to maintain a given propeller speed it will automatically draw more current.

>> Torqeedo motors are always synchronous motors

Depending on the type of alternating field generation (commutation), a difference is made between

Mechanically commutated motors

Brush-complemented motors generate the alternating field necessary for the motor to operate by means of sliding contacts Owing to their geometrical arrangement, these "brushes" switch the current depending on the position of the rotor. A major weakness of these motors is the wear caused to the brushes, which results in high maintenance overhead. Contact resistance also causes brush loss, which lowers the efficiency of the motor.

Electronically commutated motors

This type motor generates the alternating field required via electric switching, the "frequency converter". No brush loss is produced and the motors are maintenance-free. It is only in the last few years that major advances in the fields of electronic power components and circuit design have allowed these high-performance motors to be produced at an affordable price.

>> Torqeedo motors are always electronically commutated motors with tailor-made power electronics.

Depending on the type of magnetic field excitation, electric motors can be subdivided into

Electromagnetically excited motors

This type of motor generates the required magnetic field by means of a second coil field. This makes the motor cheap but at the price of greater volume and weight compared to a motor with excitation from a permanent magnet, and also results in significantly higher power consumption and lower efficiency.

Permanent-magnet excited motors

Permanent magnets generate the required magnetic field in this type of motor. This avoids any loss of performance due to the excitation coil.

>> Torqeedo motors are always permanent-magnet excited motors

Depending on the structural design, people speak of

  • Internal rotors:
    In this classical construction of an electric motor, the rotor is surrounded by the stator. The rotor is the part of the motor that rotates, is connected to the motor shaft and is also called the armature. Since, with an internal rotor, the coils are on the outside, the motor has advantages with regard to cooling. However, compared to other structural designs, it generates relatively low torque.
  • Disc rotor:
    It generates torque (force x lever) by arranging the axle of the magnetic field parallel to the shaft instead of radial to the shaft. This allows geometries to be realised where the electromagnetic force is generated well away from the axle, producing greater torque with the same force. Outboard motors with direct water cooling have a disadvantage with the disc rotor layout. Disc rotor motors cannot be installed direct in a pylon due to their very large diameter.
  • External rotors:
    This is the most modern structural design for a motor, with the coils being located on the inside. The magnets are situated on a bell rotating on the outside. External rotor motors thus achieve significantly greater torque than internal rotor motor with the same structural form.

>> Torqeedo motors are typically external rotor motors

Depending on the direction of the magnet field, people speak of

Radial flux motors

In this case the magnetic field is vertical to the rotational axis. Rotational movement comes from a lateral (rotational angle) offset between stator and rotor.

Transverse flux motors

The magnetic field is largely parallel to the rotational axis. Rotational movement also comes from a lateral (rotational angle) offset between stator and rotor. Transverse flux motors need 3-dimensional magnetic flow management and are therefore complex.

>> Torqeedo motors are typically radial flux motors