Ward Leonard
Control, also known as the Ward Leonard Drive System, was a widely used
DC motor speed control system introduced by Harry Ward Leonard in 1891.
In early 1900s, the control system of Ward Leonard was adopted by the U.S. Navy and also used in passenger lift of large mines.
An outstanding contribution to the war effort was the use of Ward-Leonard Control systems in antiaircraft radars.
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.
A Ward Leonard drive unit consists of a motor and generator with shafts coupled together.
The generator is a DC generator, with field windings and 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.
Armature voltage control only controls the motor speed from zero to load motor base speed.
If higher load motor speeds are needed the motor field current can be lowered, however by doing this the available torque at the motor armature will be reduced.
This system provides constant torque (hence variable horsepower) below base speed, rated horsepower (and torque) at base speed and constant horsepower (hence variable torque) above base speed.
Another advantage for this method is that the speed of the load motor can be controlled in both directions of rotation, although for practical reasons, it is the armature which is reversed, never the field, as loss of field current, even for a moment, could lead to a motor runaway, sometimes referred to as "bird-caging" the motor, and representing a catastrophic failure of the load motor's rotor.
The speed of motor is controlled by varying the voltage fed to the generator, V, which varies the output voltage of the generator.
Consequently changing the V will control the speed of the motor.
In early 1900s, the control system of Ward Leonard was adopted by the U.S. Navy and also used in passenger lift of large mines.
An outstanding contribution to the war effort was the use of Ward-Leonard Control systems in antiaircraft radars.
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.
A Ward Leonard drive unit consists of a motor and generator with shafts coupled together.
The generator is a DC generator, with field windings and 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.
Armature voltage control only controls the motor speed from zero to load motor base speed.
If higher load motor speeds are needed the motor field current can be lowered, however by doing this the available torque at the motor armature will be reduced.
This system provides constant torque (hence variable horsepower) below base speed, rated horsepower (and torque) at base speed and constant horsepower (hence variable torque) above base speed.
Another advantage for this method is that the speed of the load motor can be controlled in both directions of rotation, although for practical reasons, it is the armature which is reversed, never the field, as loss of field current, even for a moment, could lead to a motor runaway, sometimes referred to as "bird-caging" the motor, and representing a catastrophic failure of the load motor's rotor.
The speed of motor is controlled by varying the voltage fed to the generator, V, which varies the output voltage of the generator.
Consequently changing the V will control the speed of the motor.
source wikipedia
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