Energy-Efficient Electric Motors, Revised and Expanded

Ali Emadi

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780824757359
• eBook ISBN: 9781351836678
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria elettronica e telecomunicazioni

This detailed reference provides guidelines for the selection and utilization of electric motors for improved reliability, performance, energy-efficiency, and life-cycle cost. Completely revised and expanded, the book reflects the recent state of the field, as well as recent developments in control electronics, the economics of energy-efficient motors and systems, and advanced power electronic drivers. It includes five new chapters covering key topics such as the fundamentals of power electronics applicable to electric motor drives, adjustable speed drives and their applications, advanced switched reluctance motor drives, and permanent magnet and brushless DC motor drives.

1

Induction Motor Characteristics

1.1 THREE-PHASE INDUCTION MOTORS

In the integral horsepower sizes, i.e., above 1 hp, three-phase induction motors of various types drive more industrial equipment than any other means. The most common three-phase (polyphase) induction motors fall within the following major types:

NEMA (National Electrical Manufacturers Association) design B: Normal torques, normal slip, normal locked amperes

NEMA design A: High torques, low slip, high locked amperes

NEMA design C: High torques, normal slip, normal locked amperes

NEMA design D: High locked-rotor torque, high slip

Wound-rotor: Characteristics depend on external resistance

Multispeed: Characteristics depend on design—variable torque, constant torque, constant horsepower

There are many specially designed electric motors with unique characteristics to meet specific needs. However, the majority of needs can be met with the preceding motors.

1.1.1 NEMA Design B Motors

The NEMA design B motor is the basic integral horsepower motor. It is a three-phase motor designed with normal torque and normal starting current and generally has a slip at the rated load of less than 4%. Thus, the motor speed in revolutions per minute is 96% or more of the synchronous speed for the motor. For example, a four-pole motor operating on a 60-Hz line frequency has a synchronous speed of 1800 rpm or a full-load speed of

$\begin{array}{ll}1800-(1800\times \text{slip)}& \text{=}\text{1800}–(1800\times 0.04)\\ & =1800-72\\ & =1728\text{rpm}\end{array}$

or

$1800\times 0.96=1728\text{rpm}$

In general, most three-phase motors in the 1- to 200-hp range have a slip at the rated load of approximately 3% or, in the case of four-pole motors, a full-load speed of 1745 rpm. Figure 1.1 shows the typical construction for a totally enclosed, fan-cooled NEMA design B motor with a die-cast aluminum single-cage rotor.

Figure 1.2 shows the typical speed-torque curve for the NEMA design B motor. This type of motor has moderate starting torque, a pull-up torque exceeding the full-load torque, and a breakdown torque (or maximum torque) several times the full-load torque. Thus, it can provide starting and smooth acceleration for most loads and, in addition, can sustain temporary peak loads without stalling. The NEMA performance standards for design B motors are shown in Tables 1.1, 1.2, 1.3.

FIGURE 1.1 NEMA design B totally enclosed, fan-cooled polyphase induction motor. (Courtesy Magnetek, St. Louis, MO.)

In the past, there were no established standards for efficiency or power factor for NEMA design B induction motors. However, NEMA had established standards for testing and labeling induction motors. Recently, NEMA has established efficiency standards for energy-efficient polyphase induction motors. These standards are discussed in detail in Chapter 2.

1.1.2 NEMA Design A Motors

The NEMA design A motor is a polyphase, squirrel-cage induction motor designed with torques and locked-rotor current that exceed the corresponding values for NEMA design B motors. The criterion for classification as a design A motor is that the value of the locked-rotor current be in excess of the value for NEMA design B moto...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• 1 Induction Motor Characteristics
• 2 Energy-Efficient Motors
• 3 Fundamentals of Electric Motor Drives
• 4 The Power Factor
• 5 Applications of Induction Motors
• 6 Adjustable-Speed Drives and Their Applications
• 7 Induction Motors and Adjustable-Speed Drive Systems
• 8 Brushless DC Motor Drives
• 9 Switched Reluctance Motor Drives
• 10 Utility Interface Issues
• Index