SPEED MEASUREMENT

Made by: الله حبيب نور الرحمن عبد رضوي محمد هيثم العصيمي الرحمن عبد

Introduction

■Angular Velocity. ■Mechanical Tachometer.

■DC or AC Tachogenerator. ■Stroboscope.

■Electronic Tachometer. A) Inductive Tachometer.

B) Photoelectric Tachometer.

Angular Velocity

ω=2**N/60The angular velocity of rotating machine is usually expressed in radian per seconds, ω (rad/sec) or revolutions per minutes, N, (RPM).

Mechanical Tachometer

A tachometer is an instrument, which measure directly the angular velocity of a rotating shaft.Two masses are mount on leaf springs which are attached at one end to a driven shaft. the other ends of the springs are attached to a grooved collar which can slide on the shaft and Move a pointer.As speed increases the centrifugal force on the on the masses increases.

Centrifugal force= mrw2

M: mass R: radius from the center of motion.

angular velocity. : w

Thus causing the masses to move outwards and the sliding collar to move up the shaft. The movement of the collar is transmitted through a quadrant and pinion to a pointer .

This instrument my be used to measure a rotational speeds up to 40,000 rpm with an accuracy of ±1%

Dc or Ac Tachogenerator

D.C. or A.C. generators are used to measure the angular velocity .

The generator shaft is fixed with the driven shaft .

The output voltage is proportional to the rotational speed. The output voltage may be measured using a voltmeter calibrated in revolution per minutes .

These instruments my be used up to 5000 rpm with an accuracy of ± 2%

D.C tachogenerator

A.C. tachogenerator

Stroboscope

The instrument operates on the principle that if a repeating event is only viewed when at one particular point in it’s cycle it appears to be stationary. A mark is made on rotating shaft, and a flashing light is subjected on the shaft. The frequency of the flashing is one very short flash per revolution.To determine the shaft speed we increases the frequency of flashing gradually from small value until the rotating shaft appears to be stationary, then note the frequency. The frequency then doubled, if there is still one apparent stationary image, the frequency is again doubled. This continued until two images appear 180 degrees apart. When first appear for these two images

the flash frequency is twice the speed of rotation .

Stroboscopes are used to measure angular speed between 600 to 20000 rpm.

It’s advantage is that it doesn’t need to make contact with the rotating shaft .

Electronic Tachometer

A ) Inductive tachometer: Is a typical system using an inductive pick up. the teeth on the wheel changes the inductance of the magnetic coil as it passes in front of it. This will produces an e.m.f. (electro magnetic force) change in the form of pulse. The pulses are fed into a digital counter ,which counts the number of pulses

for some preset time interval .

B) Photoelectric tachometer: An incident light ray is directed to a small reflecting surface on the rotating shaft. The reflecting light rays fall on the photodiode produces a voltage pulses, which is then amplified and counted; the shaft speed determined by measuring the time between two successive pulses. The speed will be the reciprocal of this time.

Note : photo voltaic cell-: Is a sandwich construction consist of a metal base plate, a semiconductor material, and thin transparent metallic layer, this layer may be in a sprayed form. When light strikes the barrier between the transparent layer and the semiconductor material, a voltage is

generated .