EET 251

Electricity and Electronics

Lesson Eight

ELECTRICAL INSTRUMENTS

Instruments of many types are in use today. They measure many different quantities. Instruments are used to measure both electrical quantities and other physical quantities.

All instruments have some common characteristics. Some quantity is monitored either periodically or continuously. Some type of visual display of the quantity must be presented. Several types of instruments for measuring electrical quantities are used. The basic types of instruments may be classified as: (1) analog instruments, (2) comparison instruments, (3) cathode-ray tube instruments, (4) numerical readout instruments, and (5) chart recording instruments.

8.1 Analog Instruments

Basic part of the analog meter: meter movement

Other components

8.2 Measuring Direct Current

Shunt resistors: formula to find the value of shunt resistance

8.3 Measuring DC Voltage

Multiplier resistor

Formula to find full-scale deflection

Sensitivity of meters

8.4 Measuring Resistance

Procedure using meter movement

Non-linear scale

8.5 Measuring Electrical Power

Wattmeter

Dynamometer

Current and potential coils

8.6 Measuring Electrical Energy

Watt-hour meter

8.7 Measuring Three-Phase Electrical PowerThree-phase wattmeter and three-phase watt-hour meters

Power analyzer

8.8 Measuring Power Factor

Power factor meter

8.9 Measuring Power Demand

Power demand meter

8.10 Measuring Frequency

Electronic counter

Vibrating-reed frequency meters

8.11 Ground-Fault Indicators

Faulty grounding conditions:

(1) hot and neutral wires reversed

(2) open equipment ground wires

(3) open neutral wires

(4) open hot wires

(5) hot and equipment grounds reversed

(6) hot wires on neutral terminals

8.12 Measuring High Resistance

Megohmmeter

Insulation tests

8.13 Clamp-On Meters

Components and procedure

Use: ac circuits

8.14 Wheatstone Bridge

Use, components and procedure

8.15 Cathode-Ray Tube Instruments

General purpose oscilloscopes: to measure ac and dc voltages, frequency, phase relationships, distortion in amplifiers, etc.

Digital storage oscilloscopes

8.16 Numerical Readout Instruments

8.17 Chart Recording Instruments

Pen and ink recorders; inkless recorders

REVIEW

1. What are the five basic types of instruments?

2. Discuss the construction of a meter movement.

3. What is a single-function meter?

4. What is a multifunction meter?

5. How is the range of a meter movement extended to measure high current values?

6. What is meant by the following abbreviations? (a) I_FS (b)R_m (c)R_sh, and (d) I_sh

7. How is a meter movement used to measure voltages?

8. What is meant by the following abbreviations? (a) V_FS and (b)R_mult

9. What is meter sensitivity?

10. How is a meter movement used to measure resistances?

11. What is the difference between a linear and nonlinear scale?

12. What is the purpose of the following in an ohmmeter circuit? (a) R_lim, (b) voltage source, (c) ohms adjust resistance

13. Discuss the method used to calibrate an Ohm's scale

14. Describe the construction of an electrical power meter or wattmeter

15. What is the difference between dc power and ac power measurement?

16. Describe a kilowatt-hour (kWh) meter.

17. How is three-phase ac power measured?

18. How could power factor of an ac circuit be determined if a power factor meter is not available?

19. Why is power demand important to industries?

20. What is the purpose of a ground-fault indicator?

21. What is the purpose of a Wheatstone bridge?

22. What is the purpose of a megger?

23. Discuss the operation of an oscilloscope?

24. What is an advantage of using a numerical readout instrument?

25. Why are chart recording instrument used?