Tham khảo tài liệu 'intro to marine engineering 2e episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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 . The Figure 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 . The stator has Instrumentation and control 29 Ỉ Figure Drag cup generator-type tachometer two separate windings at right angles to each other. An . supply is provided to one winding and eddy currents are set up in the rotating aluminium cup. This results in an induced . 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 . or . instruments. The . tachogenerator is a .
