Editing Electric motor (section)

===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:<ref>{{Cite book|last=Miller|first=John M.|url={{google books |plainurl=y |id=BYkwEsmHqKYCPropulsion|page=68}}|title=Propulsion Systems for Hybrid Vehicles|date=2008|publisher=IET|isbn=978-0-86341-915-7|pages=68, 69|language=en}}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
{| class="wikitable"
!Electric machine type
!Specific torque density (Nm/kg)
|-
|SPM – brushless ac, 180° current conduction
|1.0
|-
|SPM – brushless ac, 120° current conduction
|0.9–1.15
|-
|IM, asynchronous machine
|0.7–1.0
|-
|IPM, interior permanent magnet machine
|0.6–0.8
|-
|VRM, doubly salient reluctance machine
|0.7–1.0
|}
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:<ref>{{Cite book|last=Rashid|first=Muhammad H.|url={{google books |plainurl=y |id=HxdHDgAAQBAJPower|page=1042}}|title=Power Electronics Handbook|publisher=Butterworth-Heinemann|year=2017|isbn=978-0-12-811408-7|page=1042|language=en}}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
{| class="wikitable"
!Characteristic
!dc
!IM
!PMSM
!SRM
|-
|Torque density
|3
|3.5
|5
|4
|-
|Power density
|3
|4
|5
|3.5
|}
Another source notes that PMSM up to 1 MW have considerably higher torque density than induction machines.<ref>{{Cite book|last=Steimel|first=Andreas|url={{google books |plainurl=y |id=Hdvr82d23|page=142}}|title=Electric Traction – Motive Power and Energy Supply: Basics and Practical Experience|date=2008|publisher=Oldenbourg Industrieverlag|isbn=978-3-8356-3132-8|page=142|language=en}}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>