Mechanical vs. Electrical Tachometers: A Comparison

This article explores the differences between mechanical and electrical tachometers, instruments used to measure rotational speed.

What is a Tachometer?

A tachometer is an instrument designed to measure the rotational speed of a shaft or disk, commonly found in engines and machinery. This measurement is typically displayed in revolutions per minute (RPM).

• Purpose: Indicates the speed of a crankshaft in reciprocating engines or the rotation speed of a shaft in motors and machines.
• Types: Can be direct or indirect indicating.
• Calibration: Dials are calibrated to display measurements in RPM.
• Applications:
  • Monitors engine power in reciprocating engines.
  • Monitors compressor speed in gas turbine engines.

Mechanical Tachometer

Figure 1: Mechanical Tachometer

Mechanical tachometers, like the flyweight type depicted above, operate on the principle of centrifugal force.

• Components: Includes elements such as a coil spring, drive shaft, rocking shaft, and flyweights.
• Principle of Operation: The centrifugal force generated by the rotating flyweights, which is directly proportional to the speed of rotation, compresses or stretches a mechanical spring. This movement is then translated into a speed reading.
• Measurement: Displays the shaft’s speed in RPM.

Electrical Tachometer

Figure 2: Electrical Tachometer

Electrical tachometers, such as the type shown with synchronous motors and a drag cup indicator, utilize electromagnetic principles.

• Components: Consists of an alternator, synchronous motor, and a permanent magnet.
• Principle of Operation: The relative motion between a magnetic field (H) and the shaft of a coupled device induces an electromotive force (EMF) in a coil positioned within the magnetic field of a permanent magnet. This induced EMF is proportional to the speed of the shaft.
• Types: Includes AC tachometers and DC tachometers. (For more details, see “DC vs AC tachometer generator”).

Key Differences

Here’s a summary highlighting the differences between mechanical and electrical tachometers:

• Accuracy: Electrical tachometers generally offer greater accuracy compared to mechanical tachometers.
• Maintenance: Electrical tachometers typically require less maintenance than their mechanical counterparts.
• Components: Mechanical tachometers rely on mechanical parts, while electrical tachometers use electrical components.