Ac meters-part-1

3,361 views

Published on

ac meter

Published in: Education, Business
0 Comments
3 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
3,361
On SlideShare
0
From Embeds
0
Number of Embeds
3
Actions
Shares
0
Downloads
100
Comments
0
Likes
3
Embeds 0
No embeds

No notes for slide

Ac meters-part-1

  1. 1. AC Meters Chapter 03
  2. 2. Objectives <ul><li>At the end of this chapter, the student should be able to: </li></ul><ul><ul><li>Describe the operation of a half-wave rectifier circuit. </li></ul></ul><ul><ul><li>Trace the current path in a full-wave bridge rectifier circuit. </li></ul></ul><ul><ul><li>Calculate ac sensitivity and the value of multiplier resistors for half-wave and full-wave rectification. </li></ul></ul>
  3. 3. Outline <ul><li>Introduction: What is AC? </li></ul><ul><li>d’Arsonval with Half-wave Rectification </li></ul><ul><li>d’Arsonval with Full-wave Rectification </li></ul>
  4. 4. Outline <ul><li>Electrodynamometer movement. </li></ul><ul><li>Loading effects of AC Voltmeters </li></ul><ul><li>Summary </li></ul>
  5. 5. Introduction <ul><li>Several types of meter movements maybe used to measure AC current or voltage. </li></ul><ul><li>The five principal meter movements used in ac instruments are listed in the table below: </li></ul>
  6. 6. Introduction No Meter Movement DC Use AC Use Applications 1 Electro-dynamometer YES YES Standard meter, Wattmeter, etc… 2 Iron-Vane YES YES Indicator applications, etc… 3 Electro-static YES YES High voltage measurement. 4 Thermocouple YES YES Radio freq measurement 5 D’Arsonval YES YES-w/ rectifiers Voltage, currents, resistance, etc…
  7. 7. Introduction AC Waveforms
  8. 8. d’Arsonval MM with ½ Wave Rectification <ul><li>In the previous chapter, we have discussed in detail d’Arsonval MM (PMMC) and its applications in Ammeter, Voltmeter and Ohmmeters. </li></ul><ul><li>Now, we’ll learn about using the same MM to measure ac current or voltages. </li></ul>
  9. 9. <ul><li>In order to measure ac with d’Arsonval MM, we must first rectify the ac current by use of a diode rectifier. </li></ul><ul><li>This process will produce uni-directional current flow. </li></ul><ul><li>Several types of diode rectifiers are available: - copper oxide, vacuum diode, semiconductor diode etc. </li></ul>d’Arsonval MM with ½ Wave Rectification
  10. 10. <ul><li>Still remember our DC Voltmeter, using d’Arsonval meter movement? </li></ul>d’Arsonval MM with ½ Wave Rectification Figure 1: The d’Arsonval meter movement used in a DC voltmeter Sensitivity= 1/I fs
  11. 11. <ul><li>PMMC meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC. </li></ul><ul><li>Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage. </li></ul>d’Arsonval MM with ½ Wave Rectification
  12. 12. d’Arsonval MM with ½ Wave Rectification
  13. 13. <ul><li>If we add a diode to a DC Voltmeter, then we have a meter circuit capable of measuring ac voltage. </li></ul>d’Arsonval MM with ½ Wave Rectification
  14. 14. <ul><li>The FW biased diode will have no effects in the operations of the circuit. (ideal diode) </li></ul><ul><li>Now, suppose we replace the 10-Vdc with 10Vrms, what will happen? </li></ul>d’Arsonval MM with ½ Wave Rectification
  15. 15. <ul><li>The voltage across the MM is just the positive ½ cycle of the sine wave because of rectifying action of the diode. </li></ul><ul><li>The peak value of the ac sine wave is : </li></ul><ul><li>E p = E rms X 1.414 . </li></ul>d’Arsonval MM with ½ Wave Rectification
  16. 16. <ul><li>The MM will respond to the average value of the sine wave where the average, or DC value equals 0.318 times the peak value. </li></ul><ul><li>The average value of the AC sine wave is : </li></ul><ul><li>E ave = E p / π =0.45x E rms </li></ul>d’Arsonval MM with ½ Wave Rectification
  17. 17. <ul><li>The diode action produces an approximately half sine wave across the load resistor. </li></ul><ul><li>The average value of this voltage is referred to as the DC voltage, which a DC voltmeter connected across a load resistor will respond to. </li></ul>d’Arsonval MM with ½ Wave Rectification
  18. 18. <ul><li>Therefore, we can see that the pointer that deflected full scale when a 10-V DC signal was applied, deflects to only 4.5V when we apply a 10-V rms sine AC waveforms. </li></ul><ul><li>Thus, an AC Voltmeter using ½ wave rectification is only approximately 45% sensitive as a DC Voltmeter. </li></ul>d’Arsonval MM with ½ Wave Rectification
  19. 19. <ul><li>In order to have a full scale deflection meter when a 10-Vrms is applied, we have to design the meter with the R s having 45% of the R s of the DC voltmeter. </li></ul><ul><li>Since the equivalent DC voltage is 45% of the RMS value, we can write like this: </li></ul><ul><li>R s = (E dc /I dc )-R m = (0.45E rms /I dc ) -R m </li></ul>d’Arsonval MM with ½ Wave Rectification
  20. 20. <ul><li>Example 1 </li></ul><ul><li>Compute the value of R s for a 10- V rms AC range on the voltmeter shown in Figure 1. </li></ul><ul><li>Given that E in = 10- V rms , I fs = 1mA, R m =300Ω. </li></ul>d’Arsonval MM with ½ Wave Rectification
  21. 21. <ul><li>Example 2 </li></ul><ul><li>In the ½ wave rectifier shown below, D1 and D2 have an average forward resistance of 50Ω and are assumed to have an infinite resistance in reverse biased. Calculate the following: </li></ul><ul><li>(a) R s value </li></ul><ul><li>(b ) S ac </li></ul><ul><li>(c) S dc </li></ul><ul><li>Given that E in = 10- V rms , R sh = 200Ω, I f s = 100mA, R m = 200Ω </li></ul>d’Arsonval MM with ½ Wave Rectification Make it as your exercise at home
  22. 22. Conclusion <ul><li>d’Arsonval MM can be used to measure both DC and AC current/voltages. </li></ul><ul><li>The MM will respond to the average value of sine wave where the average, or DC value equal to 0.318 times the peak value. </li></ul><ul><li>S ac = 0.45S dc </li></ul>

×