Tachometer | Slipping clutch tachometer | Drag Cup or Eddy current tachometer | Variable reluctance tachometer |Types | Advantages and Disadvantages
Slipping clutch tachometer :
It is a mechanical type tachometer. It uses mechanical movements for the measurement speed. A rotary shaft whose speed is to be measured is connected to indicator shaft with the help of slipping clutch. Friction material is used to avoid metal contact. During the engagement of clutch, the shaft, speed is transferred to indicator shaft and spring attached to it. The torque on the spring is calibrated in terms of speed which is indicated by ,.pointer calibrated scale.
Advantages of slipping clutch tachometer :
1) It is simple in construction.
2) It is easy to operate.
Limitations / disadvantages of slipping clutch tachometer :
1) High speed measurements are not possible.
2) Accurate is totally depending on spring.
a) Electrical tachometer :
1) The electrical tachometers are preferred over mechanical tachometers for all applications.
2) This tachometers offers all the advantages associated with electrical transducer.
3) It converts rotational speed into an electrical signal coupled to an indicator. The electrical signal can be either in the analog form or in the form of pulses.
The electrical tachometers are classified as :
(a) Eddy current or drag type tachometer,
(b) Electric generator tachometer.
(c) Contactless type tachometer.
(d) Frequency type tachometer.
(e) Ignition type tachometer.
(f) Stroboscopic tachometer.
Drag Cup or Eddy current tachometer :
1) The arrangement of this type of tachometer is shown in Fig:_ 5.5.3. A non-magnetic cup generally made of aluminium is provided very close to magnet, which is connected to pointer through spring.
2) A magnet and a steel cup rotate at the end of machine shaft.
3) Due to inducting of eddy currents in aluminium cup, it rotates (deflects) proportional to the induced emf (proportional to speed) which is indicated by a pointer on a dial scale.
Fig. Drag Cup or Eddy current tachometer.
The various tachometers working on this principle are :
1) Mechanically coupled magnet.
2) Stationary magnet.
a) Mechanically coupled magnet :
1) The arrangement is similar as shown in Fig. 5.5.3.
2) A permanent magnet is mechanically coupled at the end of the driving shaft.
3) These are very commonly used in automobiles where linear distance is computed by assuming some average diameter of the wheel.
b) Stationary magnet :
1) In this arrangement the permanent magnet is kept stationary and soft iron rotor produces a revolving magnetic field.
2) This type is very rugged and is used to measure the locomotive speed.
3) Following are few advantages and disadvantages of drag type tachometer.
Advantages of drag cup or Eddy current tachometer :
1) Low cost.
2) Rigid in construction.
3) Very low maintenance.
4) Linear characteristics of output voltage and speed of shaft.
Disadvantages of drag cup or Eddy current tachometer :
1) At very high speed it gives non linear characteristics of output voltage and speed of shaft.
2) To calibrate input voltage, it must be maintained absolutely constant.
Magnetic and photoelectric pulse counting.
Variable reluctance tachometer :
1) It consists of a toothed rotor mounted on the shaft ,those speed is to be measured.
2) It measures the rpm of a rotating body. It is also a proximity type of instrument. Toothed wheel is mounted on the shaft, whose speed is to be measured.
3) A permanent magnet is placed near the toothed wheel which is made of Ferro magnetic material (mild steel).This material causes variation of flux in the magnetic circuit due to changes in air gap.
4) The variations of flux produce an emf in the pickup coil. Consequently a voltage is induced in the toil. This is in the form of pulses.
5) The output of the instrument is fed to a pulse counter (electronic counter). The number of counts per second is. Then displayed in the form of a digital readout.
6) A permanent magnet is placed near the toothed rotor. The toothed rotor is made ferromagnetic material which causes variation of flux in the magnetic circuit due to changes in air-gap.
7) The variations of flux, produce an induced e.m.f. in the pick-up coil. This is in the form of pulses.