line, while voltage-source inverters require an additional converter to return energy to the supply. Regeneration is useful in VFDs only where the value of the recovered energy is large compared to the extra cost of a regenerative system, and if the system requires frequent braking and starting. Regenerative VFDs are widely used where speed control of overhauling loads is required. Some examples: Conveyor belt drives for manufacturing, which stop every few minutes. While stopped, parts are assembled correctly; once that is done, the belt moves on. A crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load during lowering. Plug-in and hybrid electric vehicles of all types (see image and Hybrid Synergy Drive). See also Adjustable-speed drive AC/AC Converter Frequency changer Pump Sub-Micro AC Drive Variable-speed air compressor Notes References Electric motor control Electric power systems components Frequency changers Mechanical power transmission Mechanical power control Electrical power control