Induction tachogenerators are employed in automatic control systems and electric drives as angular-rate transducers. One form of induction tachogenerator known as the drag-cup type is shown in Fig 42.
When the rotor is at a standstill (ɷrot = 0), the alternating magnetic flux Φt excited by the current traversing a field coil, ɷt energized from the supply line induces alternating currents itr
in the drag-cup rotor as it were the short-circuited secondary winding of a transformer. These alternating currents set up an alternating magnetic flux Φy . As this happens, no emf is induced in a measuring coil, wmeas , whose axis is at right angles with the axis of the field coil, wf .
When the rotor of the tachogenerator is spinning at an angular velocity ɷrot , a second set of alternating currents, ir will be induced in the drag-cup rotor cutting the excitation flux Φt in addition to the transformer alternating currents itr . The alternating magnetic flux Φx set up by ir will be in line with the axis of the measuring coil and. will induce in it an alternating emf whose amplitude is proportional to the angular velocity of the rotor.
Simple and reliable, induction tachogenerators are widely used in automatic control systems.
Labels: Electricity and Magnetism