# Test and Measurement World

## Test and Measurement World Terminology Section

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## Mechanical Tachometer vs Electrical Tachometer | difference between Mechanical Tachometer and Electrical Tachometer

This page compares Mechanical Tachometer vs Electrical Tachometer and mentions difference between Mechanical Tachometer and Electrical Tachometer.

What is Tachometer?
• It is an instrument which indicates speed of crankshaft of reciprocating engine.
• In other words, it is an instrument which measures rotation speed of shaft or disk as in motor or machine.
• This instrument can be either direct indicating or indirect indicating.
• The dial of tachometer is calibrated to provide measurement in rpm (revolutions per minute).
• It is used in reciprocating engines to monitor engine power. It is used in gas turbine engines to monitor speed of compressor section of the engine.

### Mechanical Tachometer

• The figure-1 depicts flyweight type of mechanical tachometer.
• As shown it uses coil spring, drive shaft, rocking shaft, flyweight etc.
• It works on the principle that centrifugal force on rotating mass depends on speed of rotation and it can be used to compress/stretch mechanical spring as shown.
• The mechanical tachometer measures speed of the shaft in RPM (revolutions per minute).

### Electrical Tachometer

• The figure-2 depicts electrical tachometer with synchronous motors and drag cup indicator.
• As shown it uses alternator, synchronous motor, permanent magnet etc.
• It utilizes principle of relation motion between H (magnetic field) and shaft of coupled device.
• In electrical tachometer, relation motion induces emf in the coil (which is placed between magnetic fields of the permanent magnet ). This emf is proporional to speed of the shaft.
• Electrical tachometer are of two types viz. AC tachometer and DC tachometer. Refer DC vs AC tachometer generator>> for more information.

Following points summarize difference between mechanical tachometer and electrical tachometer.
➤Electrical tachometer provides greater accuracy compare to mechanical tachometer.
➤Electrical tachometer requires less maintenance than mechanical tachometer.
➤Mechanical tachometer uses mechanical parts where as Electrical tachometer uses electrical parts.