RVDT

Principle It produces an electrical output which is proportional to the angular displacement of the magnetic movable core. 9AEI 306.19

Uex = excitation voltage, Uo = output voltage, 1 = excitation coil, 2 = output coil, 3 = moving core or armature, 4 = sensing shaft 9AEI 306.19

9AEI 306.19

9AEI 306.19

9AEI 306.19

Rotary Variable Differential Transformer (RVDT) Fig 1 9AEI 306.19

Operation It consists of a single primary winding P1 and two secondary windings S1 and S2. The secondary windings have an equal number of turns. They are identically placed on either side of the primary winding connected in series opposition. A shaft whose angular velocity is to be measured is connected to the core. 9AEI 306.19

The out put of RVDT is given by eo= es1-es2 When primary winding is excited by an a.c signal, voltages are induced in each secondary section. The out put of RVDT is given by eo= es1-es2 Where es1 is induced voltage in secondary s1 es2 is induced voltage in secondary s2 9AEI 306.19

when the core is at null position When the core at null position the output voltage in secondary windings S1 and S2 are equal. Therefore the differential output is e0=0 Fig 2 9AEI 306.19

When The Core Rotates In Clockwise Direction When the core rotates in clockwise direction es1>es2. The output voltage e0 is positive and in phase with input signal. Fig 3 9AEI 306.19

When the core rotates in Anticlockwise Direction When the core is in anticlockwise direction es2>es1. The output voltage e0 is negative and 1800 out of phase with the input signal. Fig 4 9AEI 306.19

The amount of the angular displacement and its direction may be ascertained from the magnitude and the phase of output voltage of the transducer. 9AEI 306.19

Advantages RVDT provide an extremely reliable solution for precision angular displacement (position) measurements. They are used wherever a physical quantity can be converted to rotary displacement. The construction of the device prevents direct contact between the moving and the stationary part Therefore insures a long operational life 9AEI 306.19

vibration 9AEI306.11

Vibration Vibration refers to mechanical oscillations about an equilibrium point . The oscillations may be periodic such as the motion of a pendulum random such as the movement of a tire on a gravel road 9AEI 306.20

Importance The need for making measurement of vibrations has arisen mainly because of The growth of environmental testing Specifications Many a times requires that the equipment should withstand stated levels of vibrations This can be done quantitatively only through vibration measurements 9AEI 306.20

9AEI 306.20

Vibration Vibration Monitoring is Important in power stations Turbines generators to give an early warning of impending conditions which may develop and lead to complete failure and destruction of equipment 9AEI 306.20

Vibration Vibration defines the motion in structure and machine components. Vibration can be due to unbalance of rotating parts, misalignment, external forces. 9AEI 306.20

Vibrations 9AEI 306.20

Vibration Measurement Are Made In General For Three Major Reasons A) Obtaining the response of a body or structure, such as the response of an aircraft wing to various load conditions. It requires the analysis of signals in addition to actual measurements. 9AEI 306.20

9AEI 306.20

(b) Defining the vibratory environment surrounding a vibratory source. Like floor vibrations surrounding a high speed compressor or generator. A judicious selection of the number of measurement stations and their location at which vibration is measured is important. The investigation includes a number of fields tests are carried out under varying environmental conditions. 9AEI 306.20

9AEI 306.20

Monitoring and control of a system (c) Such as in maintaining acceleration at a desired level in electromagnetic exciters or in an inertial navigational system. Measurements made are mainly on the acceleration levels, acceleration-time waveforms, spectral density distribution. All measurements are carried out with suitable velocity or acceleration transducer. 9AEI 306.20

9AEI 306.20

Moving Coil type Velocity Transdcuer

Principle This transducer utilizes the voltage produced in a coil on account of change in flux linkages resulting from change in reluctance. 9AEI 306.21

diaphragm coil Magnet case Output wires 9AEI 306.21

Moving Coil Type Velocity Transducer Fig 1 9AEI 306.21

Operation It consists of an arm on which coil is mounted A mass is attached to the end of the arm. The velocity to be measured is applied to the arm Therefore the coil moves in the field of a permanent magnet When the coil moves a voltage is generated which is proportional to the velocity of the coil. 9AEI 306.21

9AEI 306.21

N = number of turns of the coil R = reluctance of the coil Therefore the magnitude of the voltage is a measure of velocity and is given by Where N = number of turns of the coil R = reluctance of the coil = rate change of flux 9AEI 306.21

Advantages High stability at temperature varying conditions. Less effective to stray magnetic field 9AEI 306.21

Moving Iron Type Velocity Transducer

Principle This Transducer Utilizes The voltage produced in a coil On account of change in flux linkages Resulting from change in reluctance. 9AEI 306.22

M a g n e t coil coil O/p 9AEI 306.22

Operation Fig.1 shows a moving iron (magnet ) type linear velocity transducer. It consists of a permanent magnet which is rigidly coupled to the device whose velocity is to be measured. There is a coil surrounding the permanent magnet. 9AEI 306.22

Operation The motion of the magnet induces voltage in the coil and Amplitude of the voltage is directly proportional to the velocity. For a coil placed in a magnetic field, the voltage induced in the coil is directly proportional to the velocity. The polarity of the output voltage determines the direction of motion. 9AEI 306.22

Advantages Maintenance free due to absence of mechanical surfaces or contacts. Output voltage is linearly proportional to velocity. Used as event markers Less expensive 9AEI 306.22

Disadvantages Performance is adversely affected by stray magnetic fields. These fields cause noise. Frequency response is usually limited and is stated Susceptible to vibrations, it leads to demagnetization 9AEI 306.22

AC Tachogenerator 9AEI306.23

Principle It operates on the principle that… The relative motion between a magnetic field and a conductor results the voltage generation in that conductor. 9AEI306.23

A.C Tacho Generator The Fig. shows the A.C Tacho Generator. 9AEI306.23

9AEI306.23

Construction It consists of Permanent magnet (rotor) Coil (stator) Rectifier bridge Moving Coil (MC) voltmeter 9AEI306.23

Operation When the magnet rotates in a stationary coil , an AC voltage is generated The amplitude and the frequency of this voltage are both proportional to the speed of rotation Thus either amplitude or frequency of induced voltage may be used as a measure of rotational speed The output voltage of ac tacho generator is rectified and is measured with MC voltmeter 9AEI306.23

Advantages Maintenance free due to the absence of brushes and commutators 9AEI306.23

Disadvantages Large number of poles are required Requires high input impendence display devices 9AEI306.23

DC Tacho Generator

Principle It operates on the principle that.. The relative motion between a magnetic field and a conductor results the voltage generation in that conductor. 9AEI 306.24

For measurement of angular velocity tachogenerators are used There are two types of tacho generators 1) D.C tacho generator 2) A.C tacho generator 9AEI 306.24

DC Tacho Generator Fig 1 9AEI 306.24

Construction An armature is rotating type and this magnet is a fixed type. The armature is coupled to the machine whose velocity is to be measured. It consists of commutator and brushes is connected to the armature Output is connected to a Moving Coil (MC) type voltmeter 9AEI 306.24

9AEI 306.24

As the armature speed increases the relative motion also increases. The output voltage is induced in the armature winding. The magnitude of this voltage is proportional to the speed of the armature. A commutator and brushes are connected in the armature to give the DC output voltage. 9AEI 306.24

Operation When the armature is stationary there is no relative motion between the magnetic field and the armature winding. Hence the output voltage is ZERO. 9AEI 306.24

The polarity of output voltage indicates the direction of rotation This output voltage is measured with the help of moving coil voltmeter calibrated in terms of speed. The relationship between the DC output voltage and angular velocity is given by 9AEI 306.24

Nc = No. of conductors in armature øp = flux per pole Where ω = angular velocity e0 = DC output voltage Np = No. of poles Nc = No. of conductors in armature øp = flux per pole Npp = parallel paths between positive and negative brushes. 9AEI 306.24

Advantages The direction of rotation is indicated by the polarity of the output voltage. 9AEI 306.24

Disadvantages The commutator and brushes required periodic maintenance. The output voltage is non-linear 9AEI 306.24

Photo Electric Tachometer

Principle It converts speed of rotation into an electrical signal. This is used to determine angular speed of a rotating device. 9AEI 306.25

Photo Electric Tachometer Light source shaft Opaque Disc Light sensor 9AEI 306.25

It consists of an opaque disc mounted on a rotating shaft. Construction It consists of an opaque disc mounted on a rotating shaft. The disc has a number of equidistant holes on its circumference. At one side of the disc a light source is fixed At another side of the disc a light sensor is placed on line with the light source. 9AEI 306.25

Operation When the opaque portion of the disc is between the light source and light sensor , The light sensor is unilluminated and produces no output. But when a hole appears between the light source and the light sensor , the light falls upon the sensor and produces an output pulse. 9AEI 306.25

The frequency of output pulses depends upon the number of holes in the disc and its speed of rotation. Since the number of holes is fixed then the pulse rate is a function of speed rotation. The pulse rate can be measured by an electronic counter which can be directly calibrated in the terms of speed in rpm. 9AEI 306.25

Advantages and Disadvantages Digital output , requires no ADC Simple electronic circuitry Disadvantages :- Light source must be replaced from time to time 9AEI 306.25

Toothed Rotor Variable Reluctance Transducer 9AEI 306.26

Principle It converts speed of rotation into an electrical signal. This is used to determine angular speed of a rotating device. 9AEI 306.26

Toothed Rotor Variable Reluctance Transducer FIG 1 9AEI 306.26

Induced pulses To timer/counter /frequency meter Shaped amplifier magnet Shaped pulses Toothed rotor 9AEI 306.26

Construction It consists of a small permanent magnet with a coil wound around it. This magnet is placed near a metallic toothed rotor Rotor is made with Ferro magnetic material It is connected to shaft whose speed is to be measured. 9AEI 306.26

Operation When toothed rotor is rotating the air gap will change between the rotor and the permanent magnet. Due to change in the air gap the field expanses or collapses. The voltage is induced in the coil in the form of pulses. 9AEI 306.26

The pulses are amplified and fed to a counter or frequency meter. The frequency of pulses depend upon the number of teeth on wheel and its speed of rotation. The pulses are amplified and fed to a counter or frequency meter. 9AEI 306.26

Let T is the number of teeth on rotor. N is the number of revolutions per second. P is the number of pulses per second. Then If the rotor has 60 teeth and the counter counts the pulses in one Second. Then the counter will directly display the speed in rpm. 9AEI 306.26

Advantages Simple and rugged construction Maintenance free Easy to calibrate The information from the device can be transmitted easily 9AEI 306.26

Hall Probe

Principle It converts speed of rotation into an electrical signal. This is used to determine angular speed of a rotating device. 9AEI306.27-28

9AEI306.27-28

Hall Probe I VH Hall Voltage 9AEI306.27-28

Operation The hall probe is rigidly suspended between the poles of permanent magnet. The magnet is connected to the shaft whose angular velocity is to be measured. As the shaft rotates the hall probe remains stationary. 9AEI306.27-28

9AEI306.27-28

9AEI306.27-28

It is done by means of a constant current source A constant current is applied to the electrical contacts at the end of the probe It is done by means of a constant current source A voltage (Hall voltage ) is generated across the probe The voltage generated across the probe is directly proportional to the sine of the angular displacement of the shaft. 9AEI306.27-28

The hall voltage is given by Where KH =Hall coefficient I = electric current B= flux density t= thickness of strip A linear relationship exists between the rotation and the output voltage can be obtained up to ± 60 of the rotation. 9AEI306.27-28

Advantages 1. Small size. 2. High resolution. 3.It is a non contact type device 9AEI306.27-28

Applications Used for measurement of Velocity rpm Non contact current Magnetic field 9AEI306.27-28