Page 316 - Introduction to Marine Engineering

290 Instrumentation and control

conduct the current to the coil. The moving coil assembly is surrounded
by a permanent magnet which produces a radial magnetic field. Current
passed through the coil will result in a force which moves the coil against
the spring force to a position which, by a pointer on a scale, will read
current or voltage.
The instrument is directional and must therefore be correctly
connected in the circuit. As a result of the directional nature of
alternating current it cannot be measured directly with this instrument,
but the use of a rectifying circuit will overcome this problem.

Tachometers
A number of speed measuring devices are in use utilising either
mechanical or electrical principles in their operation.

Mechanical
A simple portable device uses the governor principle to obtain a
measurement of speed.
Two masses are fixed on leaf springs which are fastened to the driven
shaft at one end and a sliding collar at the other (Figure 15.15). The

Spring Leaf
spring

Mass Scale

Sliding
collar

Figure 15.15 Mechanical tachometer

sliding collar, through a link mechanism, moves a pointer over a scale.
As the driven shaft increases in speed the weights move out under
centrifugal force, causing an axial movement of the sliding collar. This
in turn moves the pointer to give a reading of speed.

Electrical
The drag cup generator device uses an aluminium cup which is rotated
in a laminated iron electromagnet stator (Figure 15.16). The stator has