Ward Leonard control
A DC motor speed control system using a generator as an amplifier.
Invented by Harry Ward Leonard in 1891, the Ward Leonard drive system controls a DC motor's speed by adjusting the voltage from a linked DC generator. By tweaking the generator's field current, the output voltage changes, which directly alters the motor's speed—and can even reverse it. This setup acts like a high-power amplifier, using rotating machinery: a constant-speed prime mover (like an AC motor or internal combustion engine) spins a DC generator, whose armature output drives a second motor that moves the load. Small changes in the generator's field current produce large, smooth speed changes in the load motor.
The system saw early use in the U.S. Navy around 1900, powered passenger lifts in large mines, and solved a tricky moving sidewalk problem at the 1900 Paris Exposition. It was adapted for railway locomotives in World War I and later for anti-aircraft radars in World War II. For the SCR-584 radar, MIT’s Radiation Laboratory chose Ward Leonard drives in 1942 because they provided the extremely smooth, precise two-dimensional tracking needed for automatic gun directors. Elevators relied on this system for smooth speed control and consistent torque until thyristor drives took over in the 1980s, though many Ward Leonard setups and their variants still operate today.
A common variation, the Ward Leonard Ilgner control, adds a flywheel to smooth voltage dips during sudden load changes. This replaces the usual synchronous motor with a wound-rotor induction motor, and the combination of induction motor, flywheel, and generator is called an Ilgner set. It buffers the AC supply from short, heavy loads.
The basic circuit works like this: voltage applied to the generator’s field windings (Vgf) controls the generator’s output, which directly feeds the motor’s armature. Changing Vgf changes the motor’s voltage and thus its speed. The motor’s own field voltage (Vmf) can also be adjusted. The transfer function involves generator and motor field resistances and inductances, armature resistances and inductances, rotational inductance constants, and mechanical factors like inertia and friction.
- Introduced
- 1891
- Inventor
- Harry Ward Leonard
- Patent year for vehicle application
- 1903
- Adopted by us navy
- early 1900s
- Used in paris exposition
- 1900
- Selected for scr 584 radar
- 1942
- Widely used until
- 1980s
Lore & Background
In the early 1900s, the control system of Ward Leonard was adopted by the U.S. Navy and also used in passenger lifts of large mines. It provided a solution to a moving sidewalk at the Paris Exposition of 1900, where many others had failed to operate properly. It was applied to railway locomotives used in World War I, and was used in anti-aircraft radars in World War II. Connected to automatic anti-aircraft gun directors, the tracking motion in two dimensions had to be extremely smooth and precise. The MIT Radiation Laboratory selected Ward-Leonard to equip the famous radar SCR-584 in 1942. The Ward Leonard control system was widely used for elevators until thyristor drives became available in the 1980s, because it offered smooth speed control and consistent torque. Many Ward Leonard control systems and variations on them remain in use. A Ward Leonard drive can be viewed as a high-power amplifier in the multi-kilowatt range, built from rotating electrical machinery. Where the 'prime mover' is electrical, a Ward Leonard drive unit consists of a motor and generator with shafts coupled together. The prime mover, which turns at a constant speed, may be AC or DC powered. The generator is a DC generator, with field windings and armature windings. The input to the amplifier is applied to the field windings, and the higher power output comes from the armature windings. The amplifier output is usually connected to a second motor, which moves the load, such as an elevator. With this arrangement, small changes in current applied to the input, and thus the generator field, result in large changes in the output, allowing smooth speed control. A flywheel may be used to reduce voltage fluctuations during sudden load changes. The Ward Leonard system with this modification is known as Ward Leonard Ilgner Control. In that configuration, the synchronous motor, normally used for Ward Leonard control, is replaced by a wound-rotor induction motor. The combination of an induction motor, flywheel, and generator(s) is known as an Ilgner set. It effectively decouples intermittent short-term high loading of the generator from the AC supply.
Reader's Guide
The Ward Leonard control system was notable for providing smooth speed control and consistent torque, which made it widely used for elevators until thyristor drives became available in the 1980s. Its ability to smoothly vary the speed of a DC motor, including reversing it, by controlling the field and output voltage of a DC generator, gave simple speed control. The system was selected by the MIT Radiation Laboratory to equip the famous radar SCR-584 in 1942, where connected to automatic anti-aircraft gun directors, the tracking motion in two dimensions had to be extremely smooth and precise. The system was also applied to railway locomotives used in World War I and anti-aircraft radars in World War II. Many Ward Leonard control systems and variations on them remain in use. The Ward Leonard drive can be viewed as a high-power amplifier in the multi-kilowatt range, built from rotating electrical machinery. A flywheel could be added to reduce voltage fluctuations during sudden load changes, forming the Ward Leonard Ilgner Control, which used a wound-rotor induction motor and an Ilgner set to decouple intermittent short-term high loading from the AC supply.
More in Electric Motors, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced