the generator using a rheostat to adjust the excitation current in the field winding. The motor field current is usually not adjusted, but the motor field is sometimes reduced to increase the speed above the base speed. The prime mover is usually an alternating current (AC) motor, but a DC motor or an engine might be used instead. To provide the DC field excitation power supply, Ward Leonard systems usually include an exciter generator that is driven by the prime mover. Although only technical specialists are likely to be familiar with Ward Leonard systems, many millions of people have ridden in elevators powered by Ward Leonard drives. From the 1920s through the 1980s most electrically driven elevators used Ward Leonard control and many systems remained in use at the beginning of the 21st century. Many variations of the Ward Leonard system have been implemented, but they have generally continued to be called Ward Leonard systems. H. Ward Leonard and many others have patented auxiliary control systems used to regulate the motor speed as required to automatically perform specific tasks such as controlling pump speed to regulate flow. Mechanical types of adjustable-speed drives and other electrical types continued to be used and new types developed after the Ward Leonard system was introduced. Electron tube types of DC motor controls began to be developed in the 1920s but electronic controls didn’t seriously begin to displace the Ward Leonard system until thyristor controlled drives were developed in the late 1960s. By the mid-1970s, Ward Leonard drives were rapidly becoming obsolete, but the replacement of existing Ward Leonard drives has continued past the end of the 20th century. Sudden death H. Ward Leonard was an active member of the American Institute of Electrical Engineers, publishing technical papers, attending meetings and making presentations. He died suddenly on February 18, 1915, in New York while attending the annual dinner of the American Institute of Electrical Engineers. See also Booster (electric power) Motor-generator Rotary transformer Ward leonard control References Citations General references 1861 births 1915 deaths American electrical engineers American inventors Massachusetts Institute of Technology alumni People from Cincinnati Engineers from Ohio