Unit V: Electrical Power Control Systems

In this unit, electrical power control systems are studied. The fundamentals of electrical power control are discussed in Section 15. This Section examines several of the fundamental types of equipment that are used for electrical power control. This equipment includes electromechanical contactors, relays and switches.

Section 16 examines the mechanical and electronic power control systems that are used with electrical machinery, and the motor control systems that are used extensively in day-to-day industrial, commercial, and residential applications. In addition, programmable logic controllers (PLCs) are introduced, since they now lay a significant role in machine control.

Section 17 discusses electronic control devices such as silicon con trolled rectifiers (SCRs), triacs and diacs.

Figure V shows the electrical power systems model used in this guide, and the major topics of Unit V--Electrical Power Control Systems.

Figure V. Electrical power systems model

• Power Control (Section 15)
• Operational Power Control Systems (Section 16)
• Control Devices (Section 17)

UNIT OBJECTIVES

Upon completion of this unit, you should be able to:

1. Describe the function of standards organizations, such as NEMA, ANSI, and IEEE, in electrical power control.

2. Define important terms used with electrical power control systems.

3. Recognize and sketch symbols used for electrical power control systems.

4. Describe the following types of switches used for electrical power control:

• Toggle Switch
• Rocker Switch
• Pushbutton Switch
• Rotary Switch
• Limit Switch
• Temperature Switch
• Float Switch
• Pressure Switch
• Foot Switch
• Drum Controller

5. Describe indicating lights used with electrical power control.

6. Recognize and describe various motor starting systems.

7. List the functions of motor starters.

8. Describe the method of assigning size ratings for motor contactors.

9. Describe bimetallic and melting alloy thermal overload devices and their current ratings.

10. Describe the classification system used for motor starters.

11. Describe the method of assigning sizes to manual starters.

12. Describe a combination starter.

13. List criteria used to select motor controllers.

14. List types of motor controller enclosures used by NEMA.

15. Explain the operation of a relay.

16. Explain the operation of a solenoid.

17. Describe an SCR, triac, and diac control devices that may be used for electrical power control.

18. Explain the use of programmable logic controllers and other computerized controllers used for electrical power control applications.

19. Explain the operation of the following power control circuits:

• Start-Stop Control
• Start-Stop Control from Multiple Locations
• Start-Stop Control with Safe-Run Feature
• Start-Stop Control with Run-Jog Feature
• Start-Stop-Jog Control
• Forward -Reverse-Stop Control
• Full-Voltage Starting
• Primary Resistance Starting
• Primary Reactor Starting
• Autotransformer Starting
• Wye-Delta Starting
• Part-Winding Starting
• Forward-Reverse Starting

20. Describe dynamic braking for direct current (DC) and alternating current (AC) motors.

21. Describe universal motor speed control circuit operation.

22. Describe frequency conversion systems.

23. Describe silicon controlled rectifiers (SCRs), triacs and diacs.