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Patent & Technology
External rotor topology
  • TM4's permanent magnet electric motors are based on an external rotor topology, resulting in higher power and torque densities as well as increased long-term reliability.
  • The main goal of a motor designer is to maximise motor ouput ratings while minimizing its mass. Since its inception, TM4 has chosen the external rotor design for its permanent magnet surface-mounted electric motors. TM4’s SUMO and MOTIVE electric powertrain systems are based on this topology.
♦ Increased performance           
Compared to rotor-centric permanent magnet motors, the external rotor motor topology (where the rotor rotates on the outside of the stator) has a greater magnetic flux.

 TM4 PM external rotor technology results in:
  • Higher power density.
  • Higher torque density.
  • Increased long-term reliability.
 Increased effective air gap radius:
  • The torque of an electric motor is the product of the magnetic force and the length of the radius of the magnetic flux. Thus, the farther the magnetic flux is from the axle, the more torque is generated.
  • In a set diameter, the external rotor topology has a longer radius than any other topology, thus generating more torque with less material.
 Surface permanent magnet technology
A motor with a high number of poles and a rounded magnet shape allows for:
  • Lighter rotor and stator (reduced back iron).
  • Near sinusoidal current and voltage generates fewer harmonic frequencies resulting in higher efficiency.
  • Thinner magnets that use a smaller amount of magnetic material.
  • Low cogging torque.

Patented stator technologies

TM4 stator motor technologies are the result of more than 30 years of R&D that have led to the design of some of the world’s most advanced drivetrains.


Patented winding stator technology with rectangular copper sections for:
Better fill factor than round copper sections:

  • Better fill factor than round copper sections: Reduced winding head length.
  • Improved compactness and thermal management: Improved efficiency.
Controlled winding insertion for optimized insulation:
  • Limits the risk of voltage breakdown.
  • Provides lower thermal resistance: Heat transfer from conductors to laminations is better.
The water-glycol mixture circulates right underneath the laminations and windings where the losses are predominant. This allows:
  • Low thermal resistance between the copper of the winding, the laminations and the cooling liquid.
  • Cooler magnets due to larger rotor surface and location of the rotor.
  • Steel core stator circuit (laminations) improves effectiveness of the magnetic path and reduces the amount of the permanent magnet material used.
  • Patented lamination profile that facilitates assembly (winding insertion, mechanical assembly).