Editing Electric motor (section)

===Efficiency===
To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:

: <math>\eta = \frac{P_\text{m}}{P_\text{e}}</math>,

where <math>\eta</math> is [[energy conversion efficiency]], <math>P_\text{e}</math> is electrical input power, and <math>P_\text{m}</math> is mechanical output power:

: <math>P_\text{e} = I V</math>

: <math>P_\text{m} = T \omega</math>

where <math>V</math> is input voltage, <math>I</math> is input current, <math>T</math> is output torque, and <math>\omega</math> is output angular velocity. It is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the [[stall torque]].{{citation needed|date=September 2013}}

Various national regulatory authorities have enacted legislation to encourage the manufacture and use of higher-efficiency motors. Electric motors have efficiencies ranging from around 15%-20% for shaded pole motors, up to 98% for permanent magnet motors,<ref>{{cite book | url=https://books.google.com/books?id=VCHumncaeAAC&dq=15%25+shaded+pole+motor&pg=PA141 | title=Handbook of Fractional-Horsepower Drives | isbn=978-3-540-73129-0 | last1=Stoelting | first1=Hans-Dieter | last2=Kallenbach | first2=Eberhard | last3=Amrhein | first3=Wolfgang | date=29 April 2008 | publisher=Springer }}</ref><ref>{{cite book | url=https://books.google.com/books?id=EBteVXBlF-sC&dq=shaded+pole+motor+efficiency&pg=PA72 | title=Small Electric Motors | isbn=978-0-85296-921-2 | last1=Moczala | first1=Helmut | year=1998 }}</ref><ref>{{Cite web|last=Ruffo|first=Gustavo Henrique|title=Magnax Yokeless Axial Flux Motor Promises 98 Percent Efficiency|url=https://insideevs.com/news/361185/magnax-axial-flux-electric-motor/|website=InsideEVs}}</ref> with efficiency also dependent on load. Peak efficiency is usually at 75% of the rated load. So (as an example) a 10 HP motor is most efficient when driving a load that requires 7.5 HP.<ref>{{cite web|title=Determining Electric Motor Load and Efficiency|url=https://www.energy.gov/sites/prod/files/2014/04/f15/10097517.pdf|url-status=live|access-date=22 July 2021|publisher=U.S. Department of Energy|archive-url=https://web.archive.org/web/20161130131125/http://www.energy.gov:80/sites/prod/files/2014/04/f15/10097517.pdf |archive-date=2016-11-30 }}</ref> Efficiency also depends on motor size; larger motors tend to be more efficient.<ref>{{cite web|date=September 2013|title=E3 Product Profile: Electric Motors|url=https://www.energyrating.gov.au/sites/default/files/documents/Product_profile_-_Electric_motors_September_2013.pdf|url-status=live|access-date=22 July 2021|series=E3 Equipment Energy Efficiency|publisher=Governments of Australia and New Zealand|archive-url=https://web.archive.org/web/20200328010904/https://www.energyrating.gov.au/sites/default/files/documents/Product_profile_-_Electric_motors_September_2013.pdf |archive-date=2020-03-28 }}</ref> Some motors can not operate continually for more than a specified period of time (e.g. for more than an hour per run)<ref>{{Cite web|title=Traction motors｜Transportation Systems Products｜Transportation Systems｜Products Information｜Toyo Denki Seizo K.K.|url=https://www.toyodenki.co.jp/en/products/transport/train/motor.php|website=www.toyodenki.co.jp}}</ref>