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.

A-C Indicating Meters

The moving iron-vane type is the most common a-c meter. In it induced eddy currents are used to produce magnetic force on a structure bearing a pivoted pointer and a thin iron element called a vane. The vane has no coil. The stationary magnetic field is produced by a single current-carrying coil surrounding both the fixed metal element and the pointer movement. This coil is so arranged that its own field induces a field in the moving vane and in addition generates attractive or repulsive magnetic forces with respect to its own self-produced magnetic field. Deflection is basically proportional to the current through the main coil.

Moving iron-vane meters usually have relatively low impedance and are simple and inexpensive. They measure either voltage or current, but their use must be restricted to the frequency for which they are designed.

Rectifier type meters utilize PM d-c movements actuated by current developed from rectifying the applied a-c being measured. Rectifier elements mounted within the meter case may be copper oxide, selenium, germanium, or silicon. The developed d-c is proportional to the applied a-c while the rectifiers and associated circuitry are designed for operation over as wide a band of frequency as possible.

Basic Electrical Measuring Instruments

1. In the field of electricity, as well as in all other physical sciences, accurate quantitative measurements are essential. This involves two important items — numbers and units. Simple arithmetic is used in most cases, and the units are well defined and easily understood. The standard units are those of current, voltage and resistance.

2.  The operator commonly works with ammeters, voltmeters and electron-tube analyzers; but he may also have many occasions to use watt meters, watt-hour meters, power-factor meters, synchroscopes, frequency meters and capacitance-resistance-inductance bridges.

3. Electrical equipment is designed to operate at certain efficiency levels. A good understanding of the functional design and operation of electrical instruments is important. In electrical service work one or more of the following methods are commonly used to determine if the circuits of an equipment are operating properly.

a) Use an ammeter to measure the amount of current flowing in a circuit.
b) Use a voltmeter to determine the voltage existing between two points.
c) Use an ohmmeter or megger to measure circuit continuity and total or partial circuit resistance.

4. It may also be necessary to employ a wattmeter to determine the to tal power being consumed by certain equipment. If we wish to measure the energy consumed by certain equipment or certain circuits, a watt-hour or kilowatt-hour meter is used.

5. For measuring other quantities such as power factor and frequency it is necessary to employ the appropriate instruments. In each case the instrument indicates the value of the quantity measured, and in this information helps to understand the way the circuit is operating.

6. Occasionally the operator will need to determine the value of a capacitor or an inductor. Inductance or capacitance bridges may be employed for this purpose.

7. A thorough understanding of the construction, operation and limitati ons of the basic types of electrical measuring instruments, coupled with the theory of circuit operation is most essential in servicing and maintaining electrical equipment.

DC Voltage Transducer

This delivers an output voltage proportional to the armature voltage or induced voltage. The output signal is mathematically isolated from the input voltage. The armature current signal is used to compensate the output signal for armature current variations in such a way that the output signal is approximately proportional to the induced EMF.

Where accurate EMF measurement is required, the measuring circuit also has an input which may be connected to the terminal of the machine. The voltage drop across the commutation winding of the machine is then used for IR compensation, and this arrangement therefore compensates to a considerable degree for the dependence of winding resistance on temperature.

Both static and dynamic voltage drop can be compensated by using both compensation inputs at the same time.

The unit is used for voltage control, where it replaces the tacho generator of the machine. The output signal is used as the actual value for the speed control system.

The unit is also used together with a field supply for controlled field current, in order to obtain automatic field weakening. The unit is connected at the factory for automatic field weakening if a controlled field exciter is ordered: if not, it is connected for voltage control.