sources (e.g. due to shaft-mounted fans) magnetic sources (e.g. due to magnetic forces such as Maxwell and magnetostriction forces acting on stator and rotor structures) The latter source, which can be responsible for the "whining noise" of electric motors, is called electromagnetically induced acoustic noise. Standards The following are major design, manufacturing, and testing standards covering electric motors: American Petroleum Institute: API 541 Form-Wound Squirrel Cage Induction Motors – 375 kW (500 Horsepower) and Larger American Petroleum Institute: API 546 Brushless Synchronous Machines – 500 kVA and Larger American Petroleum Institute: API 547 General-purpose Form-Wound Squirrel Cage Induction Motors – 250 Hp and Larger Institute of Electrical and Electronics Engineers: IEEE Std 112 Standard Test Procedure for Polyphase Induction Motors and Generators Institute of Electrical and Electronics Engineers: IEEE Std 115 Guide for Test Procedures for Synchronous Machines Institute of Electrical and Electronics Engineers: IEEE Std 841 Standard for Petroleum and Chemical Industry – Premium Efficiency Severe Duty Totally Enclosed Fan-Cooled (TEFC) Squirrel Cage Induction Motors – Up to and Including 370 kW (500 Hp) International Electrotechnical Commission: IEC 60034 Rotating Electrical Machines International Electrotechnical Commission: IEC 60072 Dimensions and output series for rotating electrical machines National Electrical Manufacturers Association: MG-1 Motors and Generators Underwriters Laboratories: UL 1004 – Standard for Electric Motors Indian Standard: IS:12615-2018 – Line Operated Three Phase a.c. Motors (IE CODE) “Efficiency Classes and Performance Specification” (Third Revision) Non-magnetic motors An electrostatic motor is based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high-voltage power supply, although small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. In the 1750s, the first electrostatic motors were developed by Benjamin Franklin and Andrew Gordon. Electrostatic motors find frequent use in micro-electro-mechanical systems (MEMS) where their drive voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. The molecular machinery that runs living cells is often based on linear and rotary electrostatic motors. A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a piezoelectric material when an electric field is applied. Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or ultrasonic vibrations to produce linear or rotary motion.