Electrical Systems > Three-Phase Systems

# Three-Phase Systems

Course Number: 728.1

The Three-Phase Systems textbook covers three-phase motor principles for induction, synchronous, and multi-speed dual-voltage motors. Gives recommended maintenance practices for large AC motors. Covers principles of three-phase motor starters, part winding, reversing, jogging, alternator principles and operation. Describes three-phase power distribution.

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Recommended Contact Hours – 15

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### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

### Chapter 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.

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