Ward-Leonard system of speed control

The Ward-Leonard system controls the speed of a separately excited dc motor by varying its armature voltage, which is obtained from a separately excited dc generator driven at constant speed by an induction (or synchronous) motor. The motor field is kept constant.

Diagram

 3-ph   +------+  shaft  +-----+      +-----+
 ac ----|  IM  |=========|  G  |------|  M  |===> load
        +------+         +-----+      +-----+
                           |            |
                        gen. field   motor field
                        (variable,   (constant,
                         reversible)  from exciter)
           exciter --[rheostat + reversing switch]

• M–G set: induction motor (IM) drives the dc generator G at constant speed.
• The generator armature is connected directly to the motor M armature.
• Generator field is fed from an exciter through a potential divider and reversing switch.

Working

1. Generator voltage $V_g \propto \phi_g$ (speed constant). Changing the generator field current changes $V_g$ smoothly from zero to rated value.
2. Motor speed $N \propto \dfrac{V_g - I_a R}{\phi_m}$, so speed rises from zero to base speed as $V_g$ is raised (constant torque region).
3. Reversing the generator field reverses $V_g$, so the motor reverses without switching the armature circuit.
4. Above base speed, the motor field can be weakened (constant power region).
5. Regenerative braking: when $V_g$ is reduced below the motor emf, current reverses; the motor acts as a generator, drives G as a motor, and G drives the IM above synchronous speed, returning energy to the supply. Hence four-quadrant operation is possible.

Advantages

• Very wide and smooth speed control (about 1 : 10 or more) in both directions.
• Speed from zero upward; good starting, no starting resistance.
• Inherent regenerative braking and four-quadrant operation.
• Speed regulation good; small control currents (field circuits only).
• With a synchronous motor or flywheel (Ilgner set), power factor and load peaks on supply are improved.

Disadvantages

• Three machines of nearly the same rating: high cost, large floor space and heavy foundation.
• Low overall efficiency (about 60–70 %), especially at light load.
• Noise, maintenance of commutators and brushes.
• Slower response than static drives.

It is used for rolling mills, elevators, mine winders, paper machines and cranes; today it is largely replaced by the static Ward-Leonard (thyristor converter) drive.