# MODULE 3 MEASUREMENT OF RESISTANCE,...

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MODULE 2 MEASUREMENT OF RESISTANCE, POWER,

ENERGY

1

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Measurement of resistance

Measurement of low resistance

(upto 1 ohm)

Measurement of medium resistance

(1 to 0.1M )

Measurement of high resistance

(greater than 0.1M )

Measurement of earth resistance

2

Measurement of low resistance

Measurement of low resistance

Ammeter Voltmeter method

Potentiometer method

Kelvin Double Bridge method

Ohm meter method

Series type

Shunt type

Crossed coil Ohm meters

Fixed magnet moving coil Ohmmeter

Crossed coil moving magnet Ohmmeter

3

Measurement of medium resistance

Measurement of medium resistance

Ammeter Voltmeter method

Substitution method

Wheatstone bridge method

Carey Foster Bridge method

Ohmmeter method

66

Measurement of high resistance

Measurement of high resistance

Direct deflection method

Megger method

Loss of charge method

Mega Ohm bridge method

99

Direct deflection method

108

Measurement of insulation resistance of a cable

Direct deflection method

Case (a) cables with metal sheath

Galvanometer G measures the current between core and metal sheath

Leakage currents over the surface of insulating material are carried by the guard wire wound on the insulation and does not flow through the insulation

The ration of voltage applied between the core and metal sheath and current flowing between them(galvanometer deflection) gives insulation resistance of the cable

109

Direct deflection method

Case (b)

Cable is immersed in water for at least 24hrs, so that it enters pores of the cable

Initially galvanometer should be shunted, if possible it should be connected in series with a high resistance(MegaOhms)

Leakage current flows through guard wire

Ration of voltmeter reading to galvanometer deflection gives the value of insulation resistance

110

Direct deflection method

Limitations

Galvanometer should be highly sensitive

Galvanometer should be prevented from initial inrush of currents

Battery should be at least 500V and its emf should remain constant

111

Loss of charge method

124

Loss of charge method

Step 1: Capacitor C is charged by battery- by keeping switch in position 1

Step 2: Capacitor C is discharged via Rx and Rleak

125

Loss of charge method

Step 3: Time (t) taken for the potential difference to fall from V1 to V2 is noted during discharge.

=

+

126

Loss of charge method

At the time of discharge:

=

=

=

127

Loss of charge method

=

=

128

Loss of charge method

=

0

21

2

1=

2

1= ( )

From the above

expression Reff can be determined

129

Loss of charge method

The test is then repeated with Rleak only.

So value of Rleak is found and from the expression of resistance RX that is unknown value of resistance is found.

130

EARTHING AND MEASUREMENT OF EARTH RESISTANCE

What is meant by earthing?

How will you measure earth resistance?

136

EARTHING

The connection of electrical machinery/equipment to a general mass of earth, with a conducting material of low resistance is called earthing or grounding

The conducting material used is known as earth electrode

137

ADVANTAGES OF USING EARTH ELECTRODE

All parts of the electrical equipment will be at zero potential.

Leakage current flows through low resistance path provided by the earth electrode so human protection

Voltage spikes/ current spikes due to lightning or short circuits or other faults will easily get dissipated to earth.

138

ADVANTAGES OF USING EARTH ELECTRODE

In the case of three phase system, neutral is earthed which helps to maintain line voltage constant

For telephone and traction work, earthing acts as return path. So cost of cable and cast of such cable is avoided.

Earth electrode ensures low resistance path and hence able to carry leakage currents without deterioration.

139

MEASUREMENT OF EARTH RESISTANCE

Measurement of earth resistance

Fall of potential method

Megger earth tester

140

MEASUREMENT OF EARTH RESISTANCE

Measurement of earth

resistance

Fall of potential

method

Megger earth tester

141

FALL OF POTENTIAL METHOD

142

FALL OF POTENTIAL METHOD

Potential E is applied

Current I circulates through the E and Q

Voltage between E and P is noted

143

FALL OF POTENTIAL METHOD

Pattern of current flow through earth:

144

FALL OF POTENTIAL METHOD

Current diverge from E

Current converge at Q

Current density is high near E and Q

Near electrodes, voltmeter reads high, where as between the electrodes

145

FALL OF POTENTIAL METHOD

The potential V rises near E and Q due to current density

In the middle section V remains constant

146

FALL OF POTENTIAL METHOD

Value of earth resistance is given by:

=

147

FALL OF POTENTIAL METHOD

The measurement of VEP is done at various points between E and Q

The potential variation curve is shown in fig

Resistance RE is determined when the potential curve is absolutely flat To get accurate reading,

distance between P and Q should be large

148

FALL OF POTENTIAL METHOD

Variation of resistance with distance is shown

149

MEASUREMENT OF EARTH RESISTANCE

Measurement of earth

resistance

Fall of potential method

Megger earth tester

150

MEGGER EARTH TESTER

151

MEGGER EARTH TESTER

It consists of

DC generator

Current reverser

Rectifier

Current coil

Potential coil

Electrodes E,P and Q

152

MEGGER EARTH TESTER

Current reverser and rectifier have L type commutators

These are mounted on the shaft and rotated with handle

Two brushes of commutator are arranged so that it they make contact alternately with each segment of the commutator

153

MEGGER EARTH TESTER

Other two brushes of commutator are placed in such away that they always make contact with the commutator

154

MEGGER EARTH TESTER

Earth tester consists of the terminals P1, C1, P2,C2

P1 andd C1 is shorted and connected to common point E

P2 and C2 are connected to auxiliary electrodes P and Q respectively

155

MEGGER EARTH TESTER

The ratio of voltage sensed by voltage coil and current passing through current coil, directly gives the value of earth resistance RE

Deflection of the pointer gives RE

It can be used for dc purposes only, but to measure ac reverser and rectifier is used

156

MEGGER EARTH TESTER

AC current through soil prevents back emf in the soil due to electrolytic action

157

MEASUREMENT OF POWER

Measurement of power is done by wattmeters

Wattmeter is a combination of Ammeter and Voltmeter.

So it contains current coil and voltage coil(pressure coil)

158

MEASUREMENT OF POWER

159

Wattmeters

Dynamometer type

Induction type

Electrostatic type

Wattmeter

160

Current coil carries load current

Pressure coil carries current proportional to voltage

Inductance of pressure coil should be minimum to avoid phase lag between current and voltage

MEASUREMENT OF POWER

161

Wattmeters

Dynamometer type

Induction type

Electrostatic type

Dynamometer type wattmeter

162

Dynamometer type wattmeter

Fixed coil current flowing is proportional to load current

Moving coil- current flowing is proportional to load voltage

163

Dynamometer type wattmeter

Strength of magnetic field- proportional to currents through two coils

164

Dynamometer type wattmeter

165

V- supply voltage

I load current

R- resistance of moving

coil circuit

Dynamometer type wattmeter

Fixed coil current: =

Moving coil current:

=

Deflecting torque:

=

166

Dynamometer type wattmeter

For dc circuit deflecting torque is proportional to power

For ac circuit deflecting torque is proportional to voltage, current and power factor

=

167

Dynamometer type wattmeter

1

2

M- mutual inductance between

two coils

Instantaneous torque is given by:

= 12

168

Dynamometer type wattmeter

Instantaneous voltage in pressure coil is:

= 2

Instantaneous current through pressure coil is:

=2

= 2

- resistance of

pressure coil

169

Dynamometer type wattmeter

Current through current coil lags voltage by an angle

= 2 I sin

170

Dynamometer type wattmeter

Instantaneous torque is given by:

171

Dynamometer type wattmeter

Average deflecting torque is given by:

172

Dynamometer type wattmeter

Control torque is =

K- spring constant

- final steady state deflection

At balance position

= ()

173

Dynamometer type wattmeter

At balance condition:

174

Shape of scale of dynamometer wattmeter

175

Shape of scale of dynamometer wattmeter

Deflection is proportional to power measured

and scale is uniform since

is constant.

Wattmeters are designed such that

remains over 40 to 50 degree on each side

of zero mutual inductance position.

M varies linearly in this zone with respect to

176

Shape of scale of dynamometer wattmeter

If zero mutual inducatnce position is kept in the middle then M varies linearly for deflection upto 80 to 100 degrees

177

Shape of scale of dynamometer wattmeter

178

Dynamometer type wattmeter

Ranges:

Current coil: 0.25A to 100A

Pressure coil : 5V to 750V

179

Dynamometer type wattmeter

Dynamometer type wattmeter

Suspended coil torsion type

Pivoted coil indicating type

180

Suspended coil torsion type dynamometer wattmeter

181

Suspended coil torsion type dynamometer wattmeter

The moving, or voltage, coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil.

This coil is situated entirely inside the current or fixed coils and the winding in such that the system is a static.

Errors due to external magnetic fields are thus avoided. The torsion heads carries a scale, and when in use, the

moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power.

Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument.

182

Suspended coil torsion type dynamometer wattmeter

The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero.

The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern.

183

Pivoted coil indicating type dynamometer wattmeter

184

Pivoted coil indicating type dynamometer wattmeter

In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained.

The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position.

The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil.

185

Pivoted coil indicating type dynamometer wattmeter

The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box.

The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale.

The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material.

186

Electrodynamometer type wattmeter

Advantages:

1) In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.)

2) High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes.

187

Electrodynamometer type wattmeter

Disadvantages:

1) The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect)

2) In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.

188

Errors in dynamometer type wattmeter

Errors

Due to pressure coil inductance

Due to pressure coil capacitance

Due power loss in pressure coil and current coil

Due to Eddy current

Due to friction

Due to temperature

Due to stray fields

189

Errors in dynamometer type wattmeter

Errors

Due to pressure coil inductance

Due to pressure coil capacitance

Due power loss in pressure coil and current coil

Due to Eddy current

Due to friction

Due to temperature

Due to stray fields

190

ENERGY

261

ENERGY

262

ENERGY METER TYPES

263

Energy meter

Single phase

Three phase

SINGLE PHASE ENERGYMETER CONSTRUCTION

264

SINGLE PHASE ENERGYMETER CONSTRUCTION- PARTS

265

Driving system

Moving system

Braking system and

Registering system.

Driving system

consists of two electromagnets, called shunt magnet and series magnet, of laminated construction.

266

Driving system

A coil having large number of turns of fine wire is wound on the middle limb of the shunt magnet.

This coil is known as pressure or voltage coil and is connected across the supply mains.

This voltage coil has many turns and is arranged to be as highly inductive as possible.

In other words, the voltage coil produces a high ratio of inductance to resistance.

This causes the current, and therefore the flux, to lag the supply voltage by nearly 900.

267

Driving system

An adjustable copper shading rings are provided on the central limb of the shunt magnet to make the phase angle displacement between magnetic field set up by shunt magnet and supply voltage is approximately 90degrees.

The copper shading bands are also called the power factor compensator or compensating loop

268

Driving system

The series electromagnet is energized by a coil, known as current coil which is connected in series with the load so that it carry the load current.

The flux produced by this magnet is proportional to, and in phase with the load current.

269

Moving system

The moving system essentially

consists of a light rotating aluminium disk mounted on a vertical spindle or shaft.

The shaft that supports the aluminium disk is connected by a gear arrangement to the clock mechanism on the front of the meter to provide information that consumed energy by the load

270

Moving system

The time varying (sinusoidal) fluxes produced by shunt and series magnet induce eddy currents in the aluminium disc.

The interaction between these two magnetic fields and eddy currents set up a driving torque in the disc.

The number of rotations of the disk is therefore proportional to the energy consumed by the load in a certain time interval and is commonly measured in killowatt-hours (Kwh).

271

Braking system

Damping of the disk is provided by a small permanent magnet, located diametrically opposite to the a.c magnets.

The disk passes between the magnet gaps.

272

Braking system

The movement of rotating

disc through the magnetic field crossing the air gap sets up eddy currents in the disc that reacts with the magnetic field and exerts a braking torque.

By changing the position of the brake magnet or diverting some of the flux there form, the speed of the rotating disc can be controlled.

273

Registering or Counting system

The registering or counting system essentially consists of gear train, driven either by worm or pinion gear on the disc shaft, which turns pointers that indicate on dials the number of times the disc has turned.

274

Registering or Counting system

The energy meter thus determines and adds together or integrates all the instantaneous power values so that total energy used over a period is thus known.

Therefore, this type of meter is also called an integrating meter

275

Working/operation of single phase energy meter

276

Working/operation of single phase energy meter

Induction instruments operate in alternating-current circuits and they are useful only when the frequency and the supply voltage are approximately constant

The rotating element is an aluminium disc, and the torque is produced by the interaction of eddy currents generated in the disc with the imposed magnetic fields that are produced by the voltage and current coils of the energy meter.

277

Working/operation of single phase energy meter

Let us consider a sinusoidal flux (t) is acting perpendicularly to the plane of the aluminium disc, the direction of eddy current (Ie) by Lenzs law is indicated in figure

278

Working/operation of single phase energy meter

279

=0 since reactance of Al dsc is zero

Working/operation of single phase energy meter

So in all induction type meters, two eddy currents are there so that resultant torque will be there

280

Working/operation of single phase energy meter

281

Working/operation of single phase energy meter

Current coil produces two fluxes in opposite directions

So torques produced by the interaction of eddy current due to voltage and current coil is opposite at two points.

Hence the disc will start to rotate

282

Derivation of Torque equation

Phasor Diagram

283

Derivation of Torque equation

284

Derivation of Torque equation

285

Derivation of Torque equation

286

Derivation of Torque equation

287

ENERGYMETER CONSTANT

288

SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER

Incorrect magnitude of fluxes due to abnormal voltages and load currents

Incorrect phase relation of fluxes due to defective lagging, abnormal frequencies, changes in iron loss

Unsymmetrical magnetic structure disc rotates when pressure coils alone is excited

289

SOURCES OF ERRORS IN SINGLE PHASE ENERGYMETER

Changes in resistance of disc due to change in temperature

Changes in strength of drag magnets due to temperature and ageing

Phase angle errors due to lowering of power factor

Abnormal deflection of moving parts

Badly distorted waveform

Changes in retarding torque of the disc

290

ERRORS IN SINGLE PHASE ENERGY METER

Erro

rs

Phase error

Frictional Error

Creeping

Speed Error

Temperature error

Overload compensation

Voltage compensation

291

Phase error

An error due to incorrect adjustment of the position of shading band results an incorrect phase displacement between the magnetic flux and the supply voltage (not in quadrature)

This is tested with 0.5 p.f. load at the rated load condition

292

Compensation for phase error

By adjusting the position of the copper shading band in the central limb of the shunt magnet this error can be eliminated.

293

Frictional Error

Frictional forces at bearings and registering mechanism give rise to unwanted braking torque on the disc

So additional driving torque is required

295

Creeping Error

In some meters a slow but continuous rotation is seen when pressure coil is excited but with no load current flowing.

This slow revolution records some energy.

This is called the creep error.

This slow motion may be due to

(a) incorrect friction compensation,

(b) stray magnetic field

(c) for over voltage across the voltage coil.

297

Compensation for creeping error

This can be eliminated by drilling two holes or slots in the disc on opposite side of the spindle.

When one of the holes comes under the poles of shunt magnet, the rotation being thus limited to a maximum of 180 degrees

298

Compensation for creeping error

In some cases, a small piece of iron tongue or vane is fitted to the edge of the disc.

When the position of the vane is adjacent to the brake magnet, the attractive force between the iron tongue or vane and brake magnet is just sufficient to stop slow motion of the disc with full shunt excitation and under no load condition.

299

Speed Error

Due to the incorrect position of the brake magnet, the braking torque is not correctly developed.

This can be tested when meter runs at its full load current alternatively on loads of unity power factor and a low lagging power factor

300

Temperature Error

Energy meters are almost inherently free from errors due to temperature variations.

Temperature affects both driving and braking torques equally (with the increase in temperature the resistance of the induced-current path in the disc is also increases) and so produces negligible error.

A flux level in the brake magnet decreases with increase in temperature and introduces a small error in the meter readings

302

Voltage compensation

When supply voltage varies, energymeter causes errors

This is due to:

Non linear magnetic characteristics of shunt magnet core

Braking torque is proportional to square of supply voltage

307

ADVANTAGES OF INDUCTION TYPE ENERGYMETER

309

DISADVANTAGES OF INDUCTION TYPE ENERGYMETER

310

THREE PHASE ENERGYMETER

Three phase system

Four wire Three element energy meter

Three wire Two element energy meter

311

THREE PHASE THREE ELEMENT ENERGYMETER

312

THREE PHASE THREE ELEMENT ENERGYMETER

It consists of three elements

Each element is similar to that of single phase energy meter

Pressure coils are P1,P2 and P3

Current coils are C1, C2 and C3

All elements are mounted in a vertical line in common case and have a common spindle, gearing and recording mechanism

313

THREE PHASE THREE ELEMENT ENERGYMETER

The coils are connected in such a manner that the net torque produced is equal to sum of torques produced by each element

These are employed for three phase four wire system, where fourth wire is the neutral wire

Current coils are connected in series with the lines where as pressure coils are connected in parallel across line and neutral

314

THREE PHASE THREE ELEMENT ENERGYMETER

One unit of three phase energy meter is cheaper than three individual units

Due to interaction of eddy currents between all elements, errors are produced which are reduced by suitable adjustments

315

THREE PHASE TWO ELEMENT ENERGYMETER

316

THREE PHASE TWO ELEMENT ENERGYMETER

It is provided with two discs for an element

Shunt magnet is carrying pressure coil

Series magnet is carrying current coil

Pressure coils are connected in parallel

Current coils are connected in series

Torque is produced in same manner as that of single phase energy meter

The total torque on registering mechanism is sum of torques on two discs

317

ELECTRONIC ENERGY METER

318

ELECTRONIC ENERGY METER

Average power = mean product of instantaneous voltage across the load and instantaneous current through load

Potential divider- for making voltage to required level

Voltage is scaled in the required range using voltage scaling device

Current scaling device scales load voltage which is proportional to load current

319

ELECTRONIC ENERGY METER

Both scaled voltages are connected to voltage and current multiplier unit

Voltage and current multiplier unit outputs current as a result of product of ac voltage and current

The current is proportional to instantaneous power applied to voltage controlled oscillator

VCO works on the principle of constant current charging capacitor

320

ELECTRONIC ENERGY METER

VCO basically voltage to frequency converter Output of VCO is square wave The frequency of square wave is proportional to

output current of VCO So power dependent current and frequency

dependent current decides the value of consumed energy

ADC (analog to digital converter) converts analog signal to digital signal

Display unit displays energy in watt-hour

321

ADVANTAGES OF ELECTRONIC ENERGYMETER

High sensitivity

No frictional losses

Less loading effect

Low load, full load, creeping adjustments are not required

High frequency range

High accuracy of 1%

322

TOD METER

Electric company supplies electricity to various loads such as domestic, industrial and commercial purposes

These loads varies over various time periods

For some time load is maximum and for sometime load is minimum

The hours in which load is maximum is called peak load hours

330

TOD METER

The hours in which load is minimum is called off-peak hours

During on-peak hours company has to generate more power to supply for the demand.

This causes difficulties

331

TOD METER

Time of delay rate is special service offered by electric company that allows consumer to take advantage of lower electricity price during a certain time period

Consumer can save money being on TOD rate

To lessen the load during a particular time of a day, company offers special rate to the consumers who are willing shift the load or portion of the load to off-peak hours

332

TOD METER

A special metering arrangement is done to measure energy consumption during different time zones of the day including on-paek and off-peak hours

Trivector meter itself is provided with capability required

It is provided with a time of delay registser (TOD register) which is capable of being progarammed during off-peak and on-peak hours

333

TOD METER

TOD meters are time- of delay meter which is suitable of recording and indicating consumption during specific time periods of the day

Trivector meter with such an arrangement is called TOD meter

334

TOD METER

335