Electric machinery (English: "electric motor") is an electromagnetic device that converts or transmits electric energy according to the law of electromagnetic induction. It is represented by the letter M (old standard D) in the circuit. Its main function is to generate drive torque as a power source for electrical appliances or various machines. The generator is indicated by the letter G in the circuit. Its main function is to use mechanical energy to convert into electrical energy. At present, the most common use is to use thermal energy, water energy, etc. to drive the generator rotor to generate electricity.