Saturday, October 10, 2009

WARD LEONARD CONTROL


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. It also provided a solution to a moving sidewalk at the Paris Exposition of 1900, where many others had failed to operate properly.[citation needed] Until the 1980s, when the Ward Leonard control system started to be replaced by other systems, primarily thyristor controllers, it was widely used for elevators because it offered smooth speed control and consistent torque. Many Ward Leonard control systems and variations on them remain in use.

 Basic concept

A Ward Leonard drive is a high-power amplifier in the multi-kilowatt range, built from rotating electrical machinery. A Ward Leonard drive unit consists of a motor and generator with shafts coupled together. The motor, 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 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. Armature voltage control only controls the motor speed from zero to motor base speed. If higher 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.

A more technical description

A Ward Leonard Control system with generator and motor connected directly.
The speed of motor is controlled by varying the voltage fed from the generator, Vgf, which varies the output voltage of the generator. The varied output voltage will change the voltage of the motor, since they are connected directly through the armature. Consequently changing the Vgf will control the speed of the motor. The picture of the right shows the Ward Leonard control system, with the Vgf feeding the generator and Vmf feeding the motor.

Mathematical approach

Among many ways of defining the characteristic of a system, obtaining a transfer characteristic is one of the most commonly used methods. Below are the steps to obtain the transfer function, eq 4.
Before going into the equations, first conventions should be set up, which will follow the convention Datta used. The first subscripts 'g' and 'm' each represents generator and motor. The superscripts 'f', 'r',and 'a', correspond to field, rotor, and armature.
Wi = plant state vertor K = gain t = time constant J = polar moment of inertia D = angular viscous friction G = rotational inductance constant s = laplace operator
eq 1: The generator field equation
Vgf = RgfIgf + LgfIgf
eq 2: The equation of electrical equilibrium in the armature circuit
-GgfaIgfWgr + (Rga + Rma) Ia + (Lga + Lma) Ia + GmfaImfWmr = 0
eq 3: Motor torque equation
-TL = JmWmr+DmWmr
With total impedance, Lga + Lma, neglected, the transfer function can be obtained by solving eq 3 TL = 0.
eq 4: Transfer function
\frac{W_m^r(S)}{V_g^f(S)} = \cfrac{K_BK_v/D_m}{(t_g^fs+1)\left[t_ms+\cfrac{K_m}{D_m}\right]}
with the constants defined as below.
K_B = \tfrac{G_m^faV_m^f}{R_m^f(R_g^a+R_m^a)}
K_v = \tfrac{G_g^faW_g^r}{R_g^f}
t_m = \tfrac{J_m}{D_m}
t_g^f = \tfrac{L_g^f}{R_g^f}
K_m = D_m+K_B^2(R_g^a + R_m^a)
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