- Ward Leonard Method Definition: The Ward Leonard method is defined as a speed control system for DC motors using a variable voltage supplied by a motor-generator set.
- Basic Connection Diagram: The system involves a DC motor (M1) powered by a generator (G) driven by another motor (M2), with speed controlled by adjusting the generator’s output voltage.
- Advantages: This method offers smooth and wide-range speed control, bidirectional control, uniform acceleration, good speed regulation, and regenerative braking.
- Disadvantages: The system is costly, inefficient when lightly loaded, large, heavy, noisy, and requires frequent maintenance.
- Applications: The Ward Leonard method is used in applications needing precise and sensitive speed control, such as cranes, elevators, steel mills, and locomotives.
Harry Ward Leonard introduced the Ward Leonard control system in 1891 for variable speed of a DC motor. This armature-voltage control system supplies a DC motor M1 from a separately excited DC generator G. Changing the generator field controls motor M1 by varying its armature voltage. The variable supply comes from a motor-generator set driven by a constant-speed prime mover M2, which may be an AC or DC motor. Electronic converters have replaced this historical method in most new installations, but the same armature-voltage principle remains central to speed control of DC motor.
Principle of Ward Leonard Method
The figure shows the basic Ward Leonard speed control system.
Generator G supplies the armature of motor M1. A separate DC source excites the motor M1 field. Prime mover M2 runs the generator. The speed of motor M2 is held approximately constant.
Generator field control varies the voltage applied to motor M1. With nearly constant motor flux, speed is approximately proportional to the motor back EMF, (Va – IaRa), divided by flux.
A generator field regulator sets the output voltage from zero to either polarity. Reversing the generator field reverses its output polarity and therefore the direction of motor M1, while the excitation current and armature current must remain within their ratings. The motor-generator set continues to rotate in the same direction. Reducing motor field flux can extend operation above base speed, but it reduces available torque and requires overspeed protection.
Advantages of Ward Leonard System
- Generator-field control provides smooth speed adjustment from zero to base speed.
- Reversing generator voltage provides controlled operation in either direction.
- A field-current ramp can provide controlled acceleration and deceleration.
- Closed-loop Speed regulation of DC motor can compensate for load-dependent voltage drops.
- The reversible power path supports regenerative braking when the machines and supply can return or absorb the generated energy.
Disadvantages of Ward Leonard System
- The prime mover and generator add equipment cost compared with a static converter.
- Losses in all three rotating machines reduce efficiency, especially at light load.
- The rotating set is large, heavy and requires substantial floor area.
- Commutators, brushes, bearings and cooling systems require maintenance.
- Multiple rotating machines produce more noise and vibration than a modern electronic drive.
Application of Ward Leonard System
The Ward Leonard method of speed control system was used where smooth low-speed operation, reversal, braking and a wide control range were required. Historical applications included colliery winders, cranes, excavators, mine hoists, elevators, rolling mills, paper machines and diesel-electric locomotives. Existing installations may retain the motor-generator set while replacing its field control, but new drives normally use power-electronic converters.
