Page 317 - Introduction to Marine Engineering

Instrumentation and control 291

Insulation
Winding Stater

Bearing

A.C.supply
Figure 15.16 Drag cup generator-type tachometer

two separate windings at right angles to eaeh other. An a.c. supply is
provided to one winding and eddy currents are set up in the rotating
aluminium cup. This results in an induced e.m.f. in the other stator
winding which is proportional to the speed of rotation. The output
voltage is measured on a voltmeter calibrated to read in units of speed.
Tachogenerators provide a voltage value which is proportional to the
speed and may be a.c. or d.c. instruments. The d.c. tachogenerator is a
small d.c, generator with a permanent field. The output voltage is
proportional to speed and may be measured on a voltmeter calibrated in
units of speed. The a.c. tachogenerator is a small brushless alternator
with a rotating multi-pole permanent magnet. The output voltage is
again measured by a voltmeter although the varying frequency will
affect the accuracy of this instrument.
Various pick-up devices can be used in conjunction with a digital
counter to give a direct reading of speed. An inductive pick-up
tachometer is shown in Figure 15.17(a). As the individual teeth pass the
coil they induce an e.m.f. pulse which is appropriately modified and
then fed to a digital counter. A capacitive pick-up tachometer is shown in
Figure 15.17{b). As the rotating vane passes between the plates a
capacitance change occurs in the form of a pulse. This is modified and
then fed to the digital counter.

Torsionmeters
The measurement of torsion is usually made by electrical means. The
twisting or torsion of a rotating shaft can be measured in a number of
different ways to give a value of applied torque. Shaft power can then be
calculated by multiplying the torque by the rotational speed of the shaft.