section_path: Performance parameters/Continuous torque density
section_name: Continuous torque density
paragraph_text: Continuous torque density
  
  The continuous torque density of conventional electric machines is determined by the size of the air-gap area and the back-iron depth, which are determined by the power rating of the armature winding set, the speed of the machine, and the achievable air-gap flux density before core saturation. Despite the high coercivity of neodymium or samarium -cobalt permanent magnets, continuous torque density is virtually the same amongst electric machines with optimally designed armature winding sets. Continuous torque density relates to method of cooling and permissible operation period before destruction by overheating of windings or permanent magnet damage.
  
  Other sources state that various e-machine topologies have differing torque density. One source shows the following:
  
  where—specific torque density is normalized to 1.0 for the surface permanent magnet (SPM)—brushless ac, 180° current conduction.
  
  Torque density is approximately four times greater for liquid cooled motors, compared to those which are air cooled.
  
  A source comparing direct current, induction motors (IM), PMSM and SRM showed:
  
  Another source notes that PMSM up to 1 MW have considerably higher torque density than induction machines.