Variable Voltage Control. Ward-Leonard control

Where fine control of both hoisting and lowering speed is required either booster control or a modified form of Ward-Leonard control is suitable and footbrakes are not essential. 

A magnetic brake provides against power failure or when returning the controller to off. Generally speaking for straightforward Ward-Leonard schemes the motor for the generator set can be either a.c. or d.c. and as the set runs continuously and in one direction only it is started in the conventional manner. If the supply is a.c. the exciter would be replaced by a static rectifier.

Under these Regulations vessels of 200 gross tons or less must have two lines of hawsers, one at the bow and the other at the stern quarter, each leading through a closed chock. Larger vessels must have at least four lines so arranged that they can be used on either side of the vessel. Two must lead from the bow and two from the stern quarters and not from the extreme bow or stern. For vessels between 200 and 300 gross tons the windlass forward and the capstan aft may be used for the two lines ahead but those leading aft must run from the main drum of power-driven winches and not from capstans. For all larger vessels all four lines must be power-operated and run from the main drum of power-driven winches and not from capstans.

Booster control systems are variations of the Ward-Leonard system except that the generator armature or booster as it is termed is connected in series with the winch motor. Performance can be said to equal that attained by conventional Ward- Leonard systems with the advantage of greater compactness which, for ship work, is a great asset. 

With Ward-Leonard the whole power for the winch motor is supplied by the generator of the motor-generator set so both machines of the latter are equal in output to the winch motor but with the booster system smaller motor-generator sets can be used as the booster is not called upon to handle full power. The booster voltage is reversible and can therefore add to the supply voltage or oppose it.

When adding to the line voltage the booster will be generating but when opposing it will be motoring the M.G. set and returning energy to the supply system. Since control is via the field system of the booster the change from step to step takes place smoothly and without jerk and there are no current peaks on the supply system. Main circuits are not broken while carrying current and the currents handled by the controller are small field currents with consequent elimination of excessive wear and burning of contacts. The power taken from the mains is proportional to the work done and not, as with conventional resistance control schemes, a constant demand while the winch is in operation, irrespective of the winch speed. Although this description is included under winches this system finds its most appropriate application for windlasses and capstans.

Armatures A and B constitute a tandem set, both being on the same shaft, in which B supplies the motive power for A. It differs from the booster scheme in having the armature B in parallel to the capstan armature. When A is fully excited it generates full voltage in opposition to the line voltage and there is therefore no voltage on the winch motor. By gradually reducing the voltage of A the main motor voltage is correspondingly increased and it speeds up until eventually armature A idles round at zero voltage and the motor is at full running speed. In this system the reducer set has to be big enough to carry the full current required at slow speed but at top speed it is doing no work. The overall performance is such as to give the necessary falling characteristic.

Variable Voltage Control and Ward-Leonard control are both methods used in the control of electric motors, particularly DC motors, to achieve smooth and precise regulation of speed.

Variable Voltage Control

Variable Voltage Control involves adjusting the voltage supplied to the motor to control its speed. In DC motors, the speed (N) is directly proportional to the applied voltage (V) for a given load torque. The relationship can be expressed as: N ∝ V

Key Points:

• Speed Regulation: By varying the voltage, you can adjust the speed of the motor. A higher voltage increases the speed, while a lower voltage decreases it.
• Methods: This can be achieved using various methods such as rheostats in series with the armature, solid-state devices like SCRs (Silicon Controlled Rectifiers), or chopper circuits in modern applications.
• Efficiency: Variable Voltage Control is relatively simple and efficient, but it may have limitations at low speeds due to reduced torque.

Ward-Leonard Control System

The Ward-Leonard control system is a more complex method used for the fine control of DC motor speed. It involves a motor-generator set, where a variable voltage DC generator is used to supply the armature voltage to the DC motor.

Components:

1. Prime Mover: An AC motor or a DC motor that drives the DC generator.
2. DC Generator: Generates the required variable DC voltage.
3. DC Motor: The motor whose speed is to be controlled.
4. Control Mechanism: A field regulator to adjust the field current of the DC generator, which in turn adjusts the output voltage.

Working Principle:

• The speed of the DC motor is controlled by varying the voltage of the DC generator. This is done by adjusting the field current of the DC generator.
• The prime mover drives the DC generator, and by controlling the generator's field current, the output voltage can be varied smoothly.
• This variable voltage is supplied to the armature of the DC motor, thereby controlling its speed.

Advantages:

• Smooth and Wide Range of Speed Control: The Ward-Leonard system provides very smooth and precise control over a wide range of speeds.
• High Starting Torque: The system provides high starting torque and is capable of operating at low speeds with full torque.
• Reversibility: By reversing the field current direction, the system can easily reverse the motor's direction of rotation.

Disadvantages:

• Complexity and Cost: The system is complex and expensive due to the need for additional machines (motor-generator set).
• Maintenance: Higher maintenance due to the additional rotating machinery involved.
• Efficiency: Efficiency can be lower compared to modern electronic controls because of energy losses in the motor-generator set.

Applications

• Variable Voltage Control: Common in applications where simple speed control is sufficient, such as in household appliances and some industrial machines.
• Ward-Leonard Control: Used in applications requiring precise and wide-range speed control, such as in elevators, cranes, rolling mills, and other heavy industrial equipment.

While Variable Voltage Control is simpler and less costly, the Ward-Leonard system offers superior performance in terms of smooth and precise speed control, especially for heavy-duty applications. However, it comes with higher complexity and maintenance requirements.