section_path: Operating principles/Efficiency
section_name: Efficiency
paragraph_text: Efficiency
  
  To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power:
  
  where η {\displaystyle \eta } is energy conversion efficiency, P e {\displaystyle P_{\text{e}}} is electrical input power, and P m {\displaystyle P_{\text{m}}} is mechanical output power:
  
  where V {\displaystyle V} is input voltage, I {\displaystyle I} is input current, T {\displaystyle T} is output torque, and ω {\displaystyle \omega } 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.
  
  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, 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. Efficiency also depends on motor size; larger motors tend to be more efficient. Some motors can not operate continually for more than a specified period of time (e.g. for more than an hour per run)