Ward Leonard Control - Basic Concept

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 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

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.

Another practical consideration is the armature usually has a much lower inductance than the field, therefore reversal of the motor can be more economically effectuated using armature reversal.

Most Ward Leonard drives are so-called "four quadrant" drives: forward motoring, forward braking, reverse motoring and reverse braking.

Braking is usually accomplished by connecting the motor's armature to a set of "dynamic braking" resistors which dissipates braking energy as heat. Full motor field is always applied during braking.

In one notable application, the motor can go from a dead stop to more than 3,000 rpm in under 2 seconds, and from more than 3,000 rpm to a dead stop, also in under 2 seconds, or from more than 3,000 rpm in one direction to more than 3,000 rpm in the opposite direction in under 4 seconds. This, from a motor with a "base speed" in the 1,100 rpm range.