Electricity Principles & Applications Sixth Edition

1. ©2003 Glencoe/McGraw-Hill Electricity Principles & Applications Sixth Edition Richard J. Fowler Chapter 15 Instruments and Measurements

2. INTRODUCTION • Digital Ohmmeter • Basic Meter Movement • Analog Meters • Current Transformer • Meter Loading • Bridges

3. Facts About Instruments and Measurements • Many analog instruments use a d’Arsonval meter movement. • Analog wattmeters use electrodynamometer meter movements. • Rectifiers convert ac to pulsating dc. • Meter movements have voltage, current, and resistance ratings. • A DVM measures the time to charge a capacitor, from a constant-current source, to the value of the voltage being measured.

4. More Facts About Instruments and Measurements • Shunts and multipliers are used to extend the range of meter movements. • High-frequency currents can be measured with thermocouple meters. • Digital capacitance meters measure the time required to charge the capacitor. • Digital inductance meters measure the inductor’s cemf. • Wattmeters measure true power. • Impedance can be measured by the equivalent-resistance method.

5. 1 mA constant current source Voltmeter circuitry Ohmmeter Function of a DMM The ohmmeter function uses the voltmeter section of the DMM. A constant current source develops a voltage across the resistor. and the current source is 1mA, If the voltmeter measures 0 to 999.9 mV, then the displayed output will be in ohms. This resistance of this resistor is very close to its coded value.

6. Basic Meter Movement 50 µA 1 kW 50 mV The basic meter movement has a full-scale current (Ifs) rating and an internal resistance (Ri) rating. Using these two ratings, the full-scale voltage rating (Vfs) is: 50 µA x 1 kW = Vfs = Ifsx Ri = 50 mV

7. 50 µA 1 kW 50 mV Analog Ammeters This meter movement is a 50 µA ammeter. Its range can be extended by adding a shunt resistor. The shunt resistance for a 1-mA range is calculated thus: 52.63 W Rshunt = Vfs/ (Irange - Ifs) = 50 mV / (1 mA - 50 µA) =

8. 50 µA 1 kW 50 mV Analog Voltmeters This meter movement is a 50 mV voltmeter. Its range can be extended by adding a multiplier resistor. The multiplier resistance for a 20-V range is calculated thus: 399 kW Rmult = (Vrange - Vfs)/Ifs= (20 V - 50 mV)/ 50 µA =

9. 50 µA 1 kW 50 mV R1 R2 Analog Ohmmeters This meter movement can be converted to anohmmeter by adding a fixed and a variable resistor and a cell. RT=R1 +R2(mid value) +R i= 1.5 V / 50 µA = 30 kW Make R2(mid value) = 0.05RT=1.5 kW. Thus, R1= 30 kW - 1.5 kW - 1 kW = 27.5 kW

10. Output pdc 50 µA - + 1 kW 50 mV Input ac Rectifier-Type Ac Ammeter A polarized (dc) meter can respond to ac when a rectifieris used. Therectifierallows current to flow in only one direction. The converted meter can be used in extended-range ammeters and voltmeters.

11. Analog-Meters Quiz 80 mV The Vfs rating a meter movement is ____ when Ifs = 100 mA and Ri = 800 W. shunt 1.5 To extend the range of a 500-mA, 298-W meter movement to 100 mA requires a____ resistor of ____ W. multiplier 499.5 k To extend the range of a 200-mA, 500-W meter movement to 100 V requires a____ resistor of ____ W. cell An analog ohmmeter requires a(n) ____ as well as resistors and a meter movement. rectifier A ____ allows a polarized meter movement to measure ac.

12. AMPERES Current Transformer Principle The current transformer, with a split core, is used in the clamp-on meter.

13. AMPERES Current Transformer Principle The current transformer, with a split core, is used in the clamp-on meter. To measure current in a conductor, open the core and insert the conductor.

14. AMPERES Current Transformer Principle The current transformer, with a split core, is used in the clamp-on meter. To measure current in a conductor, open the core and insert the conductor. The conductor is now a single-turn transformer primary. The current induced in the secondary is measured by the digital meter.

15. R1 V V 45 V 45 V 2 kW B1 90 V R2 2 kW Voltmeter Loading (Case 1) Circuit analysis tells us that each resister should drop 45 V. Using a 20-kW/V meter on the 50-V range, the voltages are as expected. The 1 MW of Rmeter in parallel with 2 kW does not significantly change the resistance in the circuit. Thus, there is no meter loading!

16. R1 V V 20 V 20 V 1 MW B1 60 V R2 1 MW Voltmeter Loading (Case 2) Circuit analysis tells us that each resister should drop 30 V. Using a 20-kW/V meter on the 50-V range, the voltages are much lower than expected. The 1 MW of Rmeter in parallel with 1 MW gives an equivalent resistance of 0.5 MW in series with 1 MW. The meter-resister combination drops only 20 V. There is meter loading!

17. R1 R2 1.6 kW V C1 Cu 0.002 µF Capacitance Bridge 2.2 kW .00275 µF Bridge null occurs whenR1is adjusted to2.2 kW. Therefore, the measured capacitance is: Cu=(C1/R2)R1= (0.002/1.6)x2.2= .00275 µF

18. Meter-Use Quiz A ____ ammeter can measure current without physically interrupting the circuit. clamp-on A clamp-on ammeter uses a ____ ____ connected to another ammeter. current transformer Voltmeter ____ occurs when the meter’s Ri is not many times larger than the resistance to which it is connected. loading A DMM with 10 MW of Ri should indicate ____ V when connected to a 10-MW resistor that is series connected with a 5-MW resistor to a 40-V source. 20 A capacitor bridge uses ____ capacitor(s) and ____ resistor(s) in the bridge. two two

19. REVIEW • Digital Ohmmeter • Basic Meter Movement • Analog Meters • Current Transformer • Meter Loading • Bridges