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Electronic Measurement Industrial Electronics - College - Dong Thap Vocational College - 7

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L1 L2 L3

V

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V

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Electronic Measurement Industrial Electronics - College - Dong Thap Vocational College - 7

* Implementation process

Step 1: Prepare measuring tools, materials and equipment. Step 2: Assemble the circuit according to the diagram.

Step 3: Check for cold. Step 4: Power on.

Step 5: Check the safety of the connection points.

Step 6: Observe on the measuring device. Read and record the results.

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2.6.4 Measurement of direct current (DC)

I

+

-

A

+

5V

-

47K

* Implementation process:

Step 1: Prepare to select measuring tools and measuring equipment). Step 2: Assemble the circuit according to the diagram.

Step 3: Check cold.

Step 4: Power on.

Step 5: Check the safety of the connection points.

Step 6: Observe on the measuring device. Read and record the results.

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2.6.5 Measurement of single-phase direct current


I

A

100W

220V


220VAC

a. Implementation process

Step 1: Prepare measuring tools, materials and equipment. Step 2: Assemble the circuit according to the diagram.

Step 3: Check for cold. Step 4: Power on.

Step 5: Check the safety of the connection points.

Step 6: Observe on the measuring device. Read and record the results.

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- Measure 3-phase direct current


CB

A


a. Implementation process

Step 1: Prepare measuring tools, materials and equipment. Step 2: Assemble the circuit according to the diagram.

Step 3: Check for cold. Step 4: Power on.

Step 5: Check the safety of the connection points.

Step 6: Observe on the measuring device. Read and record the results.

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- Measure indirect current of 3-phase load



CB

CT

A


a. Implementation process

Step 1: Prepare measuring tools, materials and equipment. Step 2: Assemble the circuit according to the diagram.

Step 3: Check for cold. Step 4: Power on.

Step 5: Check the safety of the connection points.

Step 6: Observe on the measuring device. Read and record the results.

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LESSON 4: MEASURING INDUCTION AND CAPACITY

Module code: MD 13 -04


1. Introduction

- In measuring electrical quantities we can check coils and capacitors in the ways as in the lesson.

- Present the structure, principles, and methods of measuring inductance and capacitance of components using AC bridges.

- Analyze and calculate the price of the bridge circuit to measure inductance and capacitance

2. Content

2.1. Alternating current demand theory


A, B, C, D: Are the 4 vertices of the measuring bridge.

AD, DB, BC, CA: are the 4 branches of the measuring bridge. R x : Is the resistance to be measured.

R 2 , R 3 , R 4 : are sample resistors.

G: is a highly sensitive magnetic galvanometer.

C

R X

R 3

A

GK B

I

U source

R 2

I 2

1

R 4

D

Measure with a single bridge (Wheatstone) (Figure 4.1).


Figure 4.1: Wheatstone Bridge

* Adjust the resistors R 2 , R 3 , R 4 so that the galvanometer needle points to zero. We say the bridge is balanced.

U A = U B

Or U AB = 0 (no current through branch AB) U DA = U DB I 2 . R 2 = I 1 .R 4

U AC = U BC I 2 R X = I 1 .R 3

Divide 3.9) by 3.10) we get:


R 2 R 4

RR 2 R

x

R

3

R X R 3 4

Put

R 2 k , usually adjusted according to known ratios, when measuring only need

R 4

Adjust R 3. However, after adjusting R 3 , the measuring bridge is still not balanced.

equal then we have to choose the ratio R 2 again

R 4

then adjust R 3 for balanced bridge.

This method is accurate but has a complex structure and is expensive.

2. 2 .Capacitance bridge

a. Concept of capacitance and loss angle:

An ideal capacitor is a capacitor that does not consume power (DC current does not pass through the capacitor) but in reality, due to the dielectric layer, a small current still passes from one pole to the other. Therefore, there is power loss in the capacitor.

This power loss is very small and to evaluate the loss of a capacitor, people often measure the loss angle tg ).

A capacitor is represented as an ideal capacitor in series with a resistor.

U

I

C

U C

R

U R

U C

U

I

UR

a. Capacitors have small losses

I R

R

I

I C

C

U

Low loss capacitor) or parallel connection with a resistor High loss capacitor).



I C

I

U

I R


b. Capacitors have large losses


Figure 3.27: Loss angle of capacitor

With small loss capacitors, based on the vector diagram, we determine the loss angle as follows:

U R = IR ;

U C

1

. C

time​

U R

U C

IR I

. C

tg= R..C


is the loss angle of the Capacitor.

With high loss capacitors we have:

I U

R R

; I C = U C.


time​

I R

I C

U

R U . C


time​


1

R . . C


b. Measure capacitance with Voltmeter, Ammeter:

The measuring circuit is connected as follows:

A

U

V

C X


Figure 3.28: Voltmeter and Ammeter diagram


If the power loss of the capacitor dielectric is negligible, the total impedance of the capacitor C X is determined by the Voltmeter and Ammeter as follows:

Z U

I

1

C X


C I U


x

* The signal source supplied to the measuring circuit is a sinusoidal signal source with constant amplitude and frequency.

c. Measuring capacitance with Voltmeter, Ammeter and Wattmeter:

The measuring circuit is connected as follows:


UV​

*

A * W

C X R X



Figure 3.29: Voltmeter, Ammeter and Wattmeter diagram


In case the measuring circuit uses an additional Wattmeter, the leakage resistance R X of the capacitor C X is determined by the following expression:


P

R x I 2

Capacitor impedance:

R 2 (

1

x

C

X

Z U

I

) 2. Z 2R 2

1

X

( C ) 2

C 2 2

1

Z 2 R 2 x


Z 2 R 2

x

U 2

P

2

I 2 I 2

( )

x

x

1

C x

1 1



U 2 P 2

I 2 I 4

1 ( U 2 I 2 P 2 )

I 4

1

U 2 I 2 P 2

11

I 2

I 2


U 2 I 2 P 2

Substitute R x , C x and into the formula: 3.15b)


time​


(3.17)

1

R . . C

() loss angle of capacitor


The power loss due to the dielectric of the capacitor is given by the formula: P = UIcos

The method using Wattmeter is not accurate when determining the capacitance of capacitors with small loss angle . To measure capacitors with small loss angle , people use the bridge measurement method.

d. Measure the capacitance of a capacitor using a simple measuring bridge:

C

C X

I 2

C 1

I 1

number). A

G

K

B

I 1

U

I 2

R 1

R 2

D

The measuring circuit is connected as shown in Figure 3.30:


C 1 : Sample capacitor with variable value


C X : Capacitor to be measured.

R, R: Sample resistor.

1 2


Figure 3.30: Simple measuring bridge


Principle: Adjust R 1 , R 2 and C 1 to balance the bridge. When the bridge is balanced we have:

C 1 R 1 = C X R 2 C

R 1 C

x

R

1

2

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