Page 319 - Introduction to Marine Engineering

Instrumentation and control 293

signal is then transmitted without contact to a digital frequency receiver.
When a torque is applied to the shaft, readings of strain and hence
torque can be made.

Differential transformer torsionmeter

Two castings are used to provide a magnetic circuit with a variable air
gap. The two are clamped to the shaft, as shown in Figure 15.19, and
joined to each other by thin steel strips. The joining strips will transmit
tension but offer no resistance to rotational movement of the two

Transformer
soft iron core

differential
transformer
Figure 15.19 Differential transformer torsionmeter

castings with respect to each other, A differential transformer is fitted
between the two castings, the two coils being wound on one casting and
the iron core being part of the other. Another differential transformer is
fitted in the indicating circuit, its air gap being adjusted by a micrometer
screw. The primary coils of the two transformers are joined in series and
energised by an a.c. supply. The secondary coils are connected so that
the induced e.m.f.s are opposed and when one transformer has an air
gap different to the other a current will flow.
When a torque is applied to the shaft the air gap of the shaft
transformer will change, resulting in a current flow. The indicator unit
transformer air gap is then adjusted until no current flows. The air gaps
in both transformers must now be exactly equal. The applied torque is
directly proportional to the width of the air gap or the micrometer screw
movement. Shaft power is found by multiplying the micrometer screw
reading by the shaft speed and a constant for the meter.

Viscosity measurement

Viscosity control of fuels is essential if correct atomisation and
combustion is to take place. Increasing the temperature of a fuel will