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Which instrument is used to measure the speed of a machine?
Thermometer
Tachometer
Multi-meter
Voltmeter
Tachometer
Quick Summary: A tachometer is a mechanical or electronic instrument specifically designed to measure the instantaneous rotational speed of a shaft or disk in a machine, typically expressed in revolutions per minute (RPM). It is a vital diagnostic tool used during the installation, commissioning, and preventive maintenance of rotating electrical machines like motors and generators.
A tachometer is a mechanical or electronic instrument specifically designed to measure the instantaneous rotational speed of a shaft or disk in a machine, typically expressed in revolutions per minute (RPM). It is a vital diagnostic tool used during the installation, commissioning, and preventive maintenance of rotating electrical machines like motors and generators.
N=P120f — relationship between speed (N), frequency (f), and poles (P) in synchronous machines
RPM=t60 — basic tachometer calculation where 't' is time per revolution in seconds
Tachometers operate based on various principles including magnetic induction, optical reflection, or centrifugal force. In an optical tachometer, a laser or infrared beam is reflected off a reflective marker on the rotating object; the sensor detects these pulses, and the circuitry calculates speed based on the frequency of the returning pulses: RPM=tinterval60. Alternatively, eddy current tachometers use a rotating magnet to induce eddy currents in a metal disk, creating a torque proportional to speed.
Used for verifying rated speed during motor commissioning.
Can be contact-based (mechanical coupling) or non-contact (optical/laser).
Essential for detecting slip in induction motors.
Critical for industrial safety and mechanical health monitoring.
Provides real-time feedback on motor performance.
Non-contact types allow measurement without interfering with load.
Compact and portable for field maintenance.
Mechanical types may suffer from wear and tear.
Optical types require a reflective marker on the rotating surface.
Subject to calibration drift over time.
Maintenance of industrial electrical motors.
Speed control verification in variable frequency drives (VFDs).
Commissioning of centrifugal pumps and fans.
A thermometer measures temperature, a multi-meter measures electrical parameters (voltage, current, resistance), and a voltmeter measures potential difference.
The unit of measurement for tachometers in industrial engineering is almost exclusively Revolutions Per Minute (RPM).
B is correct — The tachometer is the standard instrument used to measure the rotational speed of machinery in RPM.
Always ensure the tachometer is calibrated to the specific frequency of the machine, as incorrect settings in digital tachometers can lead to significant measurement errors.