DC Equipment and Controls

Course Number: 726.2

Covers DC power applications in industry, types of DC generators, operating characteristics of DC motors, DC armature principles, and armature maintenance and repair. Includes types of DC relays, DC controllers, over speed and overload protection, drum and reversing controllers, dynamic braking, DC power supplies, diodes, semiconductors, SCR principles, and DC maintenance practices.

Recommended Contact Hours - 15

Chapter 1: DC Power in Industry

Topics: DC generators and motors; Rectifiers; SCR speed control; Storage batteries; Electroplating; Electrolytic refining; Arc welding

Learning Objectives:

• List the advantages of dc over ac.
• List the brush problems caused by eddy currents.
• Name three types of batteries commonly used in the plant.
• Explain how the electroplating process works.
• Explain the difference between straight polarity and reversed polarity in arc welding.

Chapter 2: DC Electromagnets

Topics: Magnetic forces and fields; Magnetic shielding; Solenoids; Residual magnetism; Relays; Polarized relays; Protecting relay contacts

Learning Objectives:

• State the definition of residual magnetism.
• Explain the effects of distance on magnetic field strength.
• Discuss the characteristics and uses of solenoids.
• Discuss the characteristics of relays.

Chapter 3: DC Generators

Topics: Parts and action of DC generators; Armature reaction; Commutation; Interpoles; Types of generators; Losses; Parallel operation

Learning Objectives:

• Explain the function of each of the main parts of the dc generator.
• Explain how to increase the number of pulses during each rotation of an armature.
• Demonstrate how to combine the shunt field and series field to produce a compound generator.
• State the reasons why electrical losses, magnetic losses, and mechanical losses occur in the dc generator.

Chapter 4: DC Motors

Topics: CEMF; Armature reaction; Self-induction and commutation; Interpoles; Torque; Speed regulation; Shunt, series, and compound motors

Learning Objectives:

• Explain what happens during self-induction and commutation.
• Define CEMF.
• State the difference between speed regulation and speed control.
• Name the kinds of dc motors.
• Explain the different operating characteristics of series, shunt, and compound motors.

Chapter 5: DC Armatures

Topics: Windings; Armature, copper, eddy-current, and hysteresis loss; Commutation; Armature maintenance

Learning Objectives:

• Name the basic parts of an armature assembly.
• Describe the main differences between a lap winding and a wave winding.
• List the characteristics of a single-reentrant simplex-lap winding.
• State the definition of copper loss, eddy-current loss, and hysteresis loss.
• Demonstrate how to perform preventive maintenance on an armature.

Chapter 6: DC Relays

Topics: Operating characteristics; Shunt, series, lockout, and inductive time-delay relays; Magnetic blowout coils; Dynamic braking

Learning Objectives:

• Name three factors that determine the performance and reliability of a relay.
• Name the six types of commonly used relays.
• Explain the operation of each type of relay.
• Explain dynamic braking.
• Describe how a disc brake is attached to a motor.

Chapter 7: DC Controllers

Topics: Classification; Factors affecting motor speed; Protection; Temperature compensation; Manual starters; Overload reset; Magnetic and drum controllers

Learning Objectives:

• List the kinds of functions performed by motor-control devices.
• Name the types of motor controllers and discuss their operating characteristics.
• Explain how each of the three kinds of thermal overload relays works.
• Name the kinds of resets for overload relays.

Chapter 8: DC Power Supplies

Topics: Electron emission; Electron tubes; Vacuum-tube diode and rectifier; Mercury-vapor diodes; Semiconductors; Checking diodes; Identifying replacement semiconductors

Learning Objectives:

• Discuss the operating principles of vacuum tubes and rectifiers
• Name the four types of filters commonly used in rectifier circuits.
• Identify a mercury-vapor diode.
• List common causes of semiconductor failure.
• State the criteria for selecting replacement semiconductors.

Chapter 9: Silicon Controlled Rectifiers

Topics: Principles of SCRs; Pulse timing in DC circuits; Trigger pulses; SCR motor control; AC and DC applications; AC-DC conversion

Learning Objectives:

• State the definition of a silicon controlled rectifier.
• Explain how an SCR works.
• Explain how to increase the effective current and the power delivered to a motor by an SCR motor control.
• List four dc applications of SCRs.
• List four ac applications of SCRs.

Chapter 10: Maintenance of DC Equipment

Topics: Inspection; Replacing field coils; DC motor controllers; Relay contacts; Commutation; Brush selection and care; Maintaining relays and armatures

Learning Objectives:

• Explain how to test field coils to determine the condition of the insulation.
• List the signs of a short-circuited field coil in a machine.
• Explain how to replace a field coil in a machine.
• Discuss how to maintain relay control.
• Name the criteria for satisfactory commutation.