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Difference between AC tachogenerator and DC tachogenerator

AC tachogenerator DC tachogenerator
The AC tachogenerator is use to measure speed only in one direction only. The DC tachogenerator is use to measure speed in both direction.
It consists of a stator and a rotor arrangement or a squirrel cage setup. It consists of horse shoe type permanent magnet.
Need of rectifier to convert AC output into DC. Output is in DC form therefore no need of rectifier.
No problem of brush friction and brush bounce. Problem of wear and tear brushes at high speed.
Ripples are reduced. Small ripples are appearing at output.
Maintenance is difficult. Easy to maintenance.
AC tachogenerator

AC tachogenerator

DC tachogenerator

DC tachogenerator