4 EDM&Induction Type Instrument

I. EDM INSTRUMENTII. INDUCTION-TYPE INSTRUMENT III. MAGNETIC FLUX MEASUREMENT

REFERENCE: Instrumentation Measurement and AnalysisBy BC Nakra and KK Chaudry

I. EDM INSTRUMENT - DIAGRAM

EDM and PMMC

The Electrodynamic or electro-dynamometer (EDM) instruments are very similar to PMMC instrument. But instead of using a permanent Horseshoe magnet, the EDM is using two fixed coils on either side of the moving coil

These coils generate magnetic fields proportional to either the current to be measured or to the voltage to be measured.

The field coils are generally air cored to avoid errors due to hysteresis effect and eddy currents.

The moving coil wherein the pointer is attached to is connected in series with the fixed coils.

EDM Voltmeter and Ammeter

GOVERNING PRINCIPLE AND EQUATION

The deflection torque developed in the moving coil suspended in the magnetic field of fixed coils is

TD = BINA

In EDM instrument the magnetic flux density B of the coil is directly proportional to the current I flowing through it. Thus

TD I2

EDM Scale

Because of the square law, the measuring scale of the instrument is non-linear

There is over crowding of scale markings at low current values and there is progressive spreading of scale divisions at higher current values.

ADVANTAGES

The instrument is of universal type. It can be used for ac and dc measurements

Due to the absence of an iron core, the instrument does not display hysteresis effect and eddy current losses are absent.

The instrument can be modified as a wattmeter. The instrument can measure a range of current

and voltages up to 10A and 600V respectively with an accuracy specification of 1%.

These instrument has the same calibration for dc measurements as well as ac measurements.

DISADVANTAGE

These instruments have a low torque/weight ratio as compared to D’Arsonval instrument.

Sensitivity is relatively low because of poor deflecting torque, especially at low measured ranges

The scale in non-uniform Power consumption is high The instrument can be affected by stray

magnetic field and has to covered with a laminated shield.

EDM POWER METER

The EDM movement for the measurements of voltages and current can be conveniently used to measure power for both dc inputs as well as ac inputs of any waveform of voltage and current.

For the EDM power meter, the fixed coil, known as current coil, is excited by the total line current flowing through the circuit, and the moving coil is not connected in series with this circuit.

The Moving coil, usually called the pressure coil, is exited independently by the small current proportional to the voltage by introducing a high multiplier resistance across the load circuit.

EDM POWER METER SCHEMATIC

EQUATIONS

cos2

)(

fmD

ifD

IVKT

VIKT

ADVANTAGES

The instrument indicates true power for both dc as well as ac inputs.

The scale of the instrument is linear. By careful design, an instrument with

a high degree of accuracy can be obtained and this in turn could be used as standard calibration purposes.

TYPICAL EDM POWER METERS

II. INDUCTION TYPE WATTMETER AND ENERGY METER

OPERATION

The deflecting torque is generated by the interaction of eddy currents generated in the aluminum disc supported on the jewelled bearings, and the two fluxes generated by the two magnets.

INDUCTION TYPE WATTMETER

The deflection torque produced on the aluminum disc is balanced by the controlling torque of a torsion spring. This way, the deflection of the pointer on the scale would indicate the wattage on the load.

INDUCTION TYPE ENERGY METER The deflection torque produced by the electrical load

causes the aluminum disc in this case, to rotate. The rate of rotation is proportional to the wattage of

the load. A permanent magnet can be used to control the

speed of the of the disc. The consumption of energy (Wh) is proportional to

the number of rotations of the aluminum disc, in a given time.

For counting the number of rotation, the spindle is provided with a worm-screw and a worm-wheel mechanism.

The worm wheel is connected to a train of gears which are connected to the suitable energy consumed register.

TYPICAL INDUCTION TYPE ENERGY METER

III. MAGNETIC FLUX MEASUREMENT

Tesla/Gauss Meter – is used to measure the intensity of magnetic field.

Magnetic flux measurements are commonly based on hall-effect

Hall-effect –the development of a voltage between the two edges of a current carrying conductor whose faces are perpendicular to a magnetic field.

Hall-Effect

Basic Hall

Analog output

OTHER APPLICATIONS

Linear output sensor

APPLICATION ON MECHANICAL SYSTEMS anti-skid

Transmission crankshaft/speed

Distributed mountedignition

Remote conveyor

Piston detection

Throttle angle

Automative