DC Equipment and Controls

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.

Lesson 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.