EE024-Power Measurement EMTS-Th-Inst.pdf

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Power Measurement EMTSTheory

No: EE 024

INDUSTRIETECHNIKINDUSTRIETECHNIK

ELECTRICAL and ELECTRONIC

ENGINEERING

Instructor Manual


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Training Unit

Power Measurement EMTS

Theoretical Part

No.: EE 024

Edition: 2008All Rights Reserved

Editor: MCE Industrietechnik Linz GmbH & CoEducation and Training Systems, DM-1Lunzerstrasse 64 P.O.Box 36, A 4031 Linz / AustriaTel. (+ 43 / 732) 6987 3475Fax (+ 43 / 732) 6980 4271Website: www.mcelinz.com


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POWER MEASUREMENT EMTS

CONTENTS Page

LEARNING OBJECTIVES...................................................................................................4

1 POWER MEASUREMENTS.........................................................................................5

1.1

Power measurements in direct current (DC) circuits............................................5

1.2

Alternating current (AC) power measurements....................................................6

1.2.1 Active (working) power.....................................................................................6

1.2.2 Reactive power ................................................................................................7

1.2.3

Apparent power................................................................................................7

1.3

Electro-dynamic meter (instrument).....................................................................7

1.3.1 Ironless (no iron-core) electro-dynamic instrument..........................................7

1.3.2

Iron-cored electro-dynamic instrument...........................................................10

1.4 Circuits for power measurements ......................................................................11

1.4.1

Power measurements in a single-alternating current circuit ..........................11

1.4.2 Power measurements in a balanced three-phase four-wire circuit ................12

1.4.3

Power measurements in a balanced three-phase three-wire circuit using an

artificial neutral point...................................................................................................12

1.4.4 Power measurements in an unbalanced three-phase three-wire circuit ........13

1.4.5

Power measurements in an unbalanced three-phase four-conductor circuit .14

2

POWER FACTOR MEASUREMENTS.......................................................................15

2.1 Determining the power factors from current, voltage and true power ................15

2.2

Measuring power factors with an electro-dynamic meter (ratio meter) in a single-

phase system.................................................................................................................15

2.3 Measuring the power factor with an electro-dynamic quotient-meter in a

multiphase (two or more phases) system ......................................................................16


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3 MEASUREMENTS OF ELECTR1CAL ENERGY.......................................................18

3.1

The motor meter ................................................................................................18

3.1.1 The magnetic motor meter.............................................................................18

3.1.2 The electro-dynamic motor meter ..................................................................19

3.1.3 The induction meter .......................................................................................20

3.2 Circuit connections for an energy meter ............................................................23

3.2.1 Circuit connection of single-phase AC energy meter .....................................23

3.2.2

Circuit connection of a three-phase meter in a four wire circuit .....................23

3.2.3

Circuit connection of a two watt-hour meter unit to measure three-phase

energy .......................................................................................................................23

3.3 Circuit connection of a reactive watt-hour meter................................................24

3.3.1

Circuit connection of a three-phase meter in a four-wire circuit.....................24


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LEARNING OBJECTIVES

The student should

state the three formulas for calculating electrical power in DC circuits

describe the basic circuits for measuring power in DC circuits and explain the errors

produced by each circuit

explain "active power", "apparent power", and "reactive power" of an AC circuit

explain the method of operation for an ironless, electro-dynamic meter movement and

state the advantages and disadvantages

explain the method of operation of an iron core electro-dynamic meter movement and

state the advantages and disadvantages

derive the formula for finding the power factor in a single-phase AC and in a three-

wire, three-phase circuit

explain the term "commercial power factor" in a three-phase system


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(Power and Work)*

1 POWER MEASUREMENTS

1.1 Power measurements in direct current (DC) circuits

Electrical power (for DC) is understood to be the product of:

voltage x current

unit = Watt (W)

The following formula, based on Ohm 's law can also be used to determine electrical

power:

Based on the above formula power measurements of DC may be accomplished by

making voltage and current measurements either simultaneously or consecutively.

For this purpose there are two basic circuit possibilities:

Here the correct voltage is measured across the load.However, the current taken by the voltmeter would cause a higher current to be

measured.

* NOTE:Work is accomplished and expressed in terms of POWER multiplied by time.

P = V x I

P = I x R P = V


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Here the correct current would be measured but the voltage drop across the ammeter

would increase the voltage measured. Therefore the appropriate type of circuit connection

must be used so that the energy consumed by the ammeter or the voltmeter is the least.

It can be accepted that with a variable current and a constant voltage an ammeter with the

scale divisions indicated in Watts could be used for making power measurements.

However, if both current and voltage are subjected to variation then only a suitable

electro-dynamic power meter may be used for making power measurements.

1.2 Alternating current (AC) power measurements

1.2.1 Active (working) power

The active power in AC (alternating current) circuits differs from that in direct current

circuits in that the active power to be measured is the product (cos is the power factor,

cosine of the phase angle between current and voltage).This indicates that, with a load which is not purely resistive and where the current and

voltage are not in phase, only the part of the current which is in phase with the voltage will

produce power.


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1.2.2 Reactive power

With reactive power (wattless

power) Q = V x I x sin .

Only that part of the current which is 90 out of phase (90 phase shift) with the voltage is

used in the measurement.

1.2.3 Apparent power

With apparent power S = V x /,the measurement can be made in the same way as for

direct current with a voltmeter and ammeter. The power factor is not indicated in the

measurements.

1.3 Electro-dynamic meter (instrument)

1.3.1 Ironless (no iron-core) electro-dynamic instrument


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1.3.1.1 Method of operation

A rotating voltage coil is mounted within the magnetic field of the fixed current coil. This

voltage coil has a large number of windings with a very small conductor cross section, as

a result of which the resistance is very large.

When connected into the circuit, current will flow through the current coil as well as

through the voltage coil. Due to the presence of the magnetic field produced by both

current coil and voltage coil, the latter will be forced to rotate (left-hand rule), and as a

result the pointer will then be deflected.

For control torque two spiral springs are attached. They serve as a current path as well as

a control string for the voltage coil.

The meter movement damping is accomplished primarily by the use of an air chamber.

The meter deflection is proportional to the product of the currents in the fixed and moving

coils.

Based an this type of design the meter movement can be used to measure current or

voltage as well as power. With direct current the product

and, therefore, the power is given by V x IWith alternating current the product is given by

V x I x cos which corresponds to the alternating current active power.

The electro-dynamic meter is suitable for direct as well as alternating current.

The reading for alternating current is the true effective value.

With an ironless, electro-dynamic meter the magnetic lines flow purely in the air. This has

the advantage that no error can result from residual induction or eddy currents.

The disadvantage of this type of construction is that external electrical and magnetic fields

can greatly affect the indication of the meter.


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1.3.2 Iron-cored electro-dynamic instrument

1.3.2.1 Method of Operation

The method of Operation is principally the same as with the ironless electro-dynamic

meter, with the difference that most of the magnetic lines (field) cut the laminated Iran

core of the fixed coil.

As a result of this the magnetic field (flux) is amplified many times and the turning torque

of the moveable coil will be much larger.

Through the use of an Iran core construction the effect of external fields will be practically

eliminated.

Instruments of this type are used mostly in plant operations (test panel mounting).

They are useable for measuring alternating as well as direct current quantities.


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With direct current it must be accepted that a small error will exist, as a residual flux will

remain in the iron core.

The disadvantage is that the frequency range of this meter is limited to 40 - 60 Hz.

By mounting several meter movements an the same spindle, power measurements of two

or three circuits can be totalled an a single scale.

1.4 Circuits for power measurements

1.4.1 Power measurements in a single-alternating current circuit

Current path: thick wires

Voltage path: thin wires

Ri= resistance of volt coil circuit


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1.4.2 Power measurements in a balanced three-phase four-wire circuit

In a balanced three-phase four-wire circuit the total power can be measured with a single-

power meter.

The power in one phase is measured. The total power is therefore three times as large as

that value of power indicated on the measuring device. Calibration of the scale for the total

power is possible.

1.4.3 Power measurements in a balanced three-phase three-wire circuit using an

artificial neutral point

If the voltage coil of the meter is not suitable for connection between two lines (380 V), but

is suitable for connection between one line and the neutral (220 V), then a measurement

may be made, by producing an artificial neutral point.

The resistance of the voltage path (plus any dropping resistance) (= Ri) and the two other

resistances must be of equal resistance.


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1.4.4 Power measurements in an unbalanced three-phase three-wire circuit

This measurement can be made with two or three watt meters.

Circuit for three watt meters

Circuit for two watt meters

In both cases the individual valves are added together to give the total power.


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By the three wattmeter method the resistance of the three voltage coils must be of equal

value, otherwise an exact measurement cannot be made.

When using the two wattmeter method the two voltage coils must be so designed that

they can withstand the line voltage applied from the mains (example 380 V).

1.4.5 Power measurements in an unbalanced three-phase four-conductor circuit

These measurements must be conducted by making three separate power

measurements. The total power is the result of adding the three readings from the three

meters. The three separate meters may be installed in the same housing, with the voltage

coil of each meter movement mounted on one spindle. The torque is added together and,

therefore, the measuring device will indicate the power of the three-phase circuit.


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2 POWER FACTOR MEASUREMENTS

2.1 Determining the power factors from current, voltage and true power

In all cases the power factor cos can be obtained mathematically from the sinusoidal

quantities, similarly in balanced three-phase three-wire circuits by measurement of

current, voltage, and active power.

For single-phase alternating current the following applies:

For three-wire three-phase circuits the following applies:

2.2 Measuring power factors with an electro-dynamic meter (ratio meter) in a

single-phase system

The method of operation for the moving coil quotient-meter is as follows. The indicator

(pointer) movement depends on the difference between the torques produced by the

voltage coils. Both coils are cut by the magnetic field produced by current flow through the

fixed coil. The first turning force is produced by the current flow through the fixed coil and

the in-phase voltage and current in one moving coil.

cos = active power = P_

apparent power (V x /) Pa

cos = active power

apparent power (V x / x 3)


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The second turning force is caused by the current flow through the stationary coil and the

current in the moving coil connected through L, which will lag the voltage by 90.

Power factor measurements in a one-phase system

2.3 Measuring the power factor with an electro-dynamic quotient-meter in a

multiphase (two or more phases) system

The total power factor of a three-phase system is also referred to as a "commercial power

factor". The direct measurement of the total power factor is not possible in a three-phase

system at any time without a balanced lad. Therefore, the power factor of each individual

circuit must be found separately.


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In a delta-connected three-phase, three-wire system with balanced loads, all voltage

paths with equal resistance are connected to an artificial neutral point (ground). The 120

phase shift of the voltage in a three-phase circuit provides the required shift of the current

in the two voltage paths.

Power factor measurements in a three-wire system


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3 MEASUREMENTS OF ELECTR1CAL ENERGY

Electrical energy is the product of power and the time during which the power is

consumed.

A measuring device which measures electrical energy is referred to as an ENERGY

METER.

Energy meters are normally motor meters but for measurement of DC power it is possible

to measure energy, in circuits where the voltage is constant, by electrolytic means.

3.1 The motor meter

Almost all of the meters used today are of the motor type. Through an electro-mechanical

arrangement a spindle is driven at a speed proportional to the power and the electrical

work is indicated by a revolution counter mechanism.

3.1.1 The magnetic motor meter

Its principle of operation is similar to that of the moving coil meter and is, therefore, used

for direct current only.

A disc armature winding turns within the field of a permanent magnet.

Electrical work W = true power P x time t

1 W h = 1W x 1h


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The current is taken to the winding through a collector, fastened to the meter spindle. The

instrument is fundamentally an ampere-hour meter, but with a supply voltage which is

sufficiently constant, this meter can also be used as a kilowatt hour meter.

3.1.2 The electro-dynamic motor meter

With this type of meter a drum armature in series with a dropping resistor acts as a

voltage path from the voltage supply. The current flows through the fixed coil of the load.

The electrical energy is proportional to the revolutions of the drum which are indicated on

a counter. The method of function for this system can best be compared with the electro-

dynamic meter. The meter can be used for direct or alternating current.


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3.1.3 The induction meter

This is used today in AC circuits. The operation of this meter depends on the induction

principle. The fixed components consist of two iron cores, one carries the voltage coil

while the other carries the current coil. In the air gap between these two iron cores is a

rotatable aluminium disc, called the rotating disc. Fixed permanently to this disc is a worm

gear, which transfers the revolutions of the disc to the recording device.

The movement of the rotating disc is damped by a permanent magnet which serves as a

stop mechanism and prevents the meter from rotating after the circuit has been broken.

This damping is caused by the generation of eddy currents.

1 current coil 5 permanent magnet

2 voltage coil 6 aluminium disc

3 worm gear 7 supply terminals

4 recording mechanism 8 load terminals


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If there is only pure resistance in the circuit being measured, current will flow through the

current coil, which has only a few windings. Due to the low inductance this current is in

phase with the supply voltage. The voltage coil because of its high number of windings

has a large inductance.

For this reason the current in the voltage coil is almost 90 out of phase with the voltage.

With the help of a balancing resistance the phase lag between current and voltage can be

adjusted to exactly 90. The magnetic field produced by the voltage and current coils will

produce a voltage across the rotating disc. Current will flow through the disc, which is

almost a short-circuited conductor. A magnetic field will be produced (a rotating field). The

turning speed of the rotating disc is dependent on the strength of this rotating field.

The proper movement of the meter, especially under a small load, will be impaired by the

initial permeability of the core material and friction of the bearings. To eliminate these

influences an adjustable booster is wound on the core of the voltage coil. The meter is

stopped from running freely by means of a small brake which is magnetized by the voltage

coil.

With a three-phase meter, three single-phase meters are mounted in the same housing.

The turning moment produced is developed on a single spindle and the reading will be

indicated on a single recorder.

With the help of an induction meter the power consumed by a load can be found.

With an induction meter only AC can be measured.

With an induction meter only true energy is metered.


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Measuring procedure for power

a) Switch on the device to be measured.

b) Count the revolutions of the rotating disc for a determined period of time.

c) Using this number, determine the revolutions of the rotating disc per hour. This value

will be divided by the constant (c)of the meter, which is indicated on the dial of each

individual meter.

The calculation gives the power consumed by the load in kW.

Example

To calculate the power consumed by an electric stove the running disc has 10 revolutions

in 20 seconds. On the dial of the meter, the constant of the motor, 1200 KWh-1, is

indicated. Calculate the power consumed by this electric stove.

The constant of the meter gives the number of revolutions per kilowatt hour (kWh).

Old indication: U/kWh new indication: 1/kWh or kWh-1

P (kW) = revolutions of the meter per hour____

the meter constant ( C )


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3.2 Circuit connections for an energy meter

3.2.1 Circuit connection of single-phase AC energy meter

3.2.2 Circuit connection of a three-phase meter in a four wire circuit

3.2.3 Circuit connection of a two watt-hour meter unit to measure three-phase energy


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3.3 Circuit connection of a reactive watt-hour meter

3.3.1 Circuit connection of a three-phase meter in a four-wire circuit


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EE024 - Power Measurement EMTS

Theoretical Test


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TEST 1

QUESTIONS:

1. Give the formulae for the active, reactive and apparent power.

2. For which type of current is the ironless electro-dynamic meter suitable.

3. State the advantages and disadvantages of an ironless electro-dynamic meter.

4. Describe the method of operation of an iron-cored electro-dynamic meter.

5. Name the basic parts of an iron-cored electro-dynamic meter.

6. For what type of current are electrolytic type energy meters suitable?

7. State a formula for the determination of the power factor for:

a) Single-phase AC circuit

b) Three-wire three-phase circuit

8. Give a formula for the determination of electrical energy.

9. Name the main parts of an induction meter.

10. State the procedure for measuring the power consumed by a load, using an induction

meter.


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TEST 2

QUESTIONS:

1. Draw the circuit diagram for power measurements in a sing le-phase alternating

current circuit.

2. Draw the circuit diagram for power measurement in an unbalanced three-phase three-

wire circuit using two-wattmeter circuit.

3. Name the instrument which has the following circuit diagram

4. Draw the circuit diagram for power factor measurements in a three-w ire system.

5. State the definition of electrical energy.

6. Name two types of direct current electrical energy meters.

7. For what type of current is the magnetic motor meter suitable?


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Test 2 - questions continued

8. Name two types of alternating current electrical energy meter.

9. What is the function of the rotating disc in an induction meter?

10. Is it possible to use the induction motor type of meter for DC current electrical work

measurement?


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TEST 1

SOLUTION:

1. Active power: P = V I cos

Reactive power: Q = V I sin

Apparent power: S = V I

2. AC and DC.

3. No error due to residual induction or eddy current.

Disadvantage: external influences greatly affect the indication of the meter.

4. In a closed circuit current will flow through both fixed and rotating coils. The magnetic

lines produced by the two coils cut in laminated iron core of the fixed coil to a high

extent and the magnetic flux is amplified many times. This flux will force the moving

coil and, therefore, the pointer to move. The iron core is used to eliminate external

influences.

5. Fixed coil, rotating coil, fixed iron core, laminated iron core, damping mechanism,

pointer, scale.

6. DC current.

7. a) cos = active power = P

apparent power S

b) cos q) = active power = Papparent power 5


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Test 1 - answers continued

8. Electrical work = true power x time

W = P x t

units Wh (or Joules).

9. Current coil, voltage coil, worm gear, recording mechanism, permanent magnet,

aluminium disc, power connections (inlet, outlet).

10. a) Switch on the device (load).

b) For a determined period of time count the revolutions of the rotating disc.

c) With this value determine the revolutions of the rotating disc for one hour. This

value is then divided by the constant (c)of the meter.


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TEST 2

SOLUTION:

1.

2. Circuit for two power measurements

(Two-wattmeter circuit)

3. Power factor meter.


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Test 2 - answers continued

4.

5. Electrical work is understood to be the product of power and time during which power

is consumed.

Electrical energy W= true power Px time t

6. a) Electrolytic type

b) The magnetic motor meter

7. DC current.

8. a) The induction meter

b) The electro-dynamic motor meter

9. After the circuit has broken it prevents the meter from running past this point, i.e. as a

stop mechanism.

10. No.


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KEY TO EVALUATION

PER CENT MARK

88 100 1

75 87 2

62 74 3

50 61 4

0 49 5

EE024-Power Measurement EMTS-Th-Inst.pdf

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