Three-Phase Systems

Lesson 1 - Principles of Three-Phase Motors

Topics:

Induction motors; Squirrel-cage rotors; Pole-phase relationships; Torque, rotor speed, slip, frequency, resistance, reactance, and power factor

Learning Objectives:

– Describe a squirrel-cage rotor.
– List the factors that determine the strength of the magnetic field in an induction motor.
– Discuss pole-phase relationships.
– Demonstrate how to reverse the rotation direction of the magnetic field.
– Discuss the relationship between rotor speed and frequency.

Lesson 2 - Induction Motors

Topics:

Squirrel-cage motors; Stator and rotor construction; Air gap; Operating features; Wound-rotor motors; Brushes and slip rings; Maintenance

Learning Objectives:

– List the main parts of the stator.
– List operating characteristics of a wound-rotor motor.
– Demonstrate how to check rotor windings for short circuits.
– State the definition of a standard motor.

Lesson 3 - Synchronous Motors

Topics:

Motor fields, characteristics, and applications; Pull-in torque; Slipping pole; Power factor; Brushless motors; Motor efficiency and care

Learning Objectives:

– List factors that contribute to the torque of an industrial synchronous motor during starting.
– Explain the effects of an amortisseur winding in a synchronous motor.
– State the definition of pull-in torque.
– State the reason why using synchronous motors can increase a low power factor in a plant.
– List the characteristics of brushless synchronous motors.

Lesson 4 - Multispeed Motors

Topics:

Consequent-pole, constant-horsepower, constant-torque, variable-torque, and dual-voltage motors and connections

Learning Objectives:

– Discuss the operating characteristics of multispeed induction motors.
– Select the best motor for driving equipment that requires the same torque at both high and low speeds.
– State the definition of a variable-torque motor.
– Explain the difference between a constant-horsepower motor and a constant-torque motor.

Lesson 5 - Maintaining Three-Phase Motors

Topics:

Cleaning; Care of stator and rotor windings; Air gap; Overload and single-phase operation; Motor shaft currents; Bearings; Maintenance schedule

Learning Objectives:

– List the steps in measuring the resistance of the insulation on motor windings.
– Explain how to raise the temperature of a motor winding.
– List the steps in lubricating motor bearings.
– List the conditions that must exist before you can lubricate bearings.

Lesson 6 - Motor Starters

Topics:

Limitations; Full-voltage and across-the-line starting; Reducing starting current; Primary- and secondary-resistance starters; Maintenance

Learning Objectives:

– Explain how a motor starter works.
– Explain the difference between open transition and closed transition.
– Name the common kinds of reduced-voltage starters.
– List the steps in inspecting motor starters.

Lesson 7 - Three-Phase Motor Controllers

Topics:

Multiple start-stop controls; Across-the-line reversing starters; Plugging control; Jogging; Controlling surge and backspin

Learning Objectives:

– Explain how to select the best motor starter for a particular application.
– Explain the difference between low-voltage release and low-voltage protection.
– Describe the plugging process.
– Explain how to prevent backspin.

Lesson 8 - Alternators

Topics:

Characteristics, ratings, and windings; Three-phase alternators; Air gap; Slip rings; Exciters; Voltage regulation; Load characteristics and effects

Learning Objectives:

– Discuss the operating characteristics of alternators.
– List the characteristics that must be considered when you work on alternator windings.
– Name the causes of change in potential difference between terminals as the load changes.
– Demonstrate how to calculate three-phase power in an alternator.

Lesson 9 - Auxiliary Generator Systems

Topics:

Requirements; Control equipment; Transfer systems; Safety switches; Engine protection; Prime movers and output control; Maintenance

Learning Objectives:

– Explain how an automatic auxiliary generator works.
– List the methods of overcoming voltage-drop problems when starting loads.
– List the parts of a hydraulic starting system.
– State the definition of a prime mover.
– List the four guidelines to follow when troubleshooting or performing routine maintenance on generators.

Lesson 10 - Power Distribution Systems

Topics:

Distribution voltages; Heat loss; System grounding; Overcurrent protection; Ground relays; Tripping; Network protection

Learning Objectives:

– State the reasons why 240-volt systems are not as widely used as are 480-volt systems.
– Explain the difference between system grounding and equipment grounding.
– List the benefits of system grounding.
– Explain how an overcurrent relay works.
– Name common circuit-opening devices.