掌握电机与控制系统：最新教材解析

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《电动机与控制系统》是一本由弗兰克·D·佩特鲁泽拉编著的专业教材，它涵盖了电机控制领域的核心内容。该书由麦克格劳-希尔出版公司发行，是该公司商业单元的一部分，地址位于纽约市。版权属于2010年的麦克格劳-希尔公司，所有内容受法律保护，未经该公司事先书面许可，不得以任何形式复制、分发或存储在数据库或检索系统中，包括但不限于网络传输或远程学习中的广播。本书深入探讨了电机控制系统的各个方面，从基础理论到实际应用，包括晶体管技术在电机控制中的重要作用。作者详细介绍了控制策略的设计与实施，如交流和直流调速系统的原理与优化方法。读者可以在这里了解到如何通过电子元件精确控制电机的速度和方向，以满足工业自动化和设备精密操作的需求。书中还可能涵盖了电力电子技术和电机驱动系统的结构，如变频器、伺服电机控制、矢量控制等现代控制技术。此外，对于电机的维护、故障诊断和效率优化等方面也有深入的讲解，以确保电机在各种环境和工况下都能稳定高效地运行。作者弗兰克·D·佩特鲁泽拉是一位资深的电机控制专家，他以清晰易懂的语言和丰富的实践经验，帮助读者建立起扎实的电机控制理论基础，并提供实用的工程案例分析，使读者能够将所学知识应用到实际工程项目中。值得注意的是，某些辅助材料，如电子版和印刷版，可能仅限于美国市场提供。此外，该书采用了环保的酸性-free纸张，体现了对可持续性的关注。《电动机与控制系统》是一本全面而深入的参考书籍，适合电机工程学生、工程师以及从事电力设备设计和维护的专业人员使用，是电机控制领域研究和实践的重要参考资料。

Safety in the Workplace

Chapter Objectives

This chapter will help you to:

1. Identify the electrical factors that deter-

mine the severity of an electric shock.

2. Be aware of general principles of electri-

cal safety including wearing approved

protective clothing and using protective

equipment.

3. Explain the safety aspect of grounding an

electrical motor installation.

Outline the basic steps in a lockout

procedure.

5. Be aware of the functions of the different

organizations responsible for electrical

codes and standards.

Safety is the number one priority in any job.

Every year, electrical accidents cause serious

injury or death. Many of these casualties are

young people just entering the workplace. They

are involved in accidents that result from care-

lessness, from the pressures and distractions of

a new job, or from a lack of understanding about

electricity. This chapter is designed to develop

an awareness of the dangers associated with

electrical power and the potential dangers that

can exist on the job or at a training facility.

PART 1 Protecting against

Electrical Shock

The human body conducts electricity. Even

low currents may cause sev

ere health effects.

Spasms, burns, muscle paralysis, or death can

result, depending on the amount of the current

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2 Chapter 1 Safety in the Workplace

If you were sweaty and barefoot, then your resistance to

ground might be as low as 1,000 ohms. Then the current

would be:

I =

120 V

_______

1,000 Ω

= 0.12 A = 120 mA

Voltage is not as reliable an indication of shock inten-

sity because the body’s resistance varies so widely that it

is impossible to predict how much current will result from

a given voltage. The amount of current that passes through

the body and the length of time of exposure are perhaps the

two most reliable criteria of shock intensity. Once current

enters the body it follows through the circulatory system

in preference to the external skin. Figure 1-1 illustrates the

relative magnitude and effect of electric current. It doesn’t

take much current to cause a painful or even fatal shock. A

current of 1 mA (1/1000 of an ampere) can be felt. A cur-

rent of 10 mA will produce a shock of sufficient intensity

to prevent voluntary control of muscles, which explains

why, in some cases, the victim of electric shock is unable to

release grip on the conductor while the current is flowing.

A current of 100 mA passing through the body for a second

or longer can be fatal. Generally, any current flow above

0.005 A, or 5 mA, is considered dangerous.

A 1.5-V flashlight cell can deliver more than enough

current to kill a human being, yet it is safe to handle. This

is because the resistance of human skin is high enough to

limit greatly the flow of electric current. In lower voltage

circuits, resistance restricts current flow to very low val-

ues. Therefore, there is little danger of an electric shock.

Higher voltages, on the other hand, can force enough cur-

rent though the skin to produce a shock. The danger of

harmful shock increases as the voltage increases.

The pathway through the body is another factor influ-

encing the effect of an electric shock. For example, a

current from hand to foot, which passes through the heart

and part of the central nervous system, is far more dan-

gerous than a shock between two points on the same arm

(Figure 1-2).

AC (alternating current) of the common 60-Hz fre-

quency is three to five times more dangerous than DC

(direct current) of the same voltage and current value. DC

tends to cause a convulsive contraction of the muscles,

often forcing the victim away from further current expo-

sure. The effects of AC on the body depend to a great extent

on the frequency: low-frequency currents (50–60 Hz) are

usually more dangerous than high-frequency currents. AC

causes muscle spasm, often “freezing” the hand (the most

common part of the body to make contact) to the circuit.

The fist clenches around the current source, resulting in

prolonged exposure with severe burns.

This is a lethal shock,

capable of producing ven-

tricular fibrillation (rapid

irregular contractions of

the heart) and death!

flowing through the body, the route it takes, and the dura-

tion of exposure.

The main factor for determining the severity of an electric

shock is the amount of electric current that passes through

the body. This current is dependent upon the voltage and

the resistance of the path it follows through the body.

Electrical resistance ( R ) is the opposition to the flow

of current in a circuit and is measured in ohms (Ω). The

lower the body resistance, the greater the current flow and

potential electric shock hazard. Body resistance can be

divided into external (skin resistance) and internal (body

tissues and blood stream resistance). Dry skin is a good

insulator; moisture lowers the resistance of skin, which

explains why shock intensity is greater when the hands

are wet. Internal resistance is low owing to the salt and

moisture content of the blood. There is a wide degree of

variation in body resistance. A shock that may be fatal to

one person may cause only brief discomfort to another.

Typical body resistance values are:

• Dry skin—100,000 to 600,000 Ω

• Wet skin—1,000 Ω

• Internal body (hand to foot)—400 to 600 Ω

• Ear to ear—100 Ω

Thin or wet skin is much less resistant than thick or dry

skin. When skin resistance is low, the current may cause lit-

tle or no skin damage but severely burn internal organs and

tissues. Conversely, high skin resistance can produce severe

skin burns but prevent the current from entering the body.

Voltage ( E ) is the pressure that causes the flow of elec-

tric current in a circuit and is measured in units called

volts (V). The amount of voltage that is dangerous to life

varies with each individual because of differences in body

resistance and heart conditions. Generally, any voltage

above 30 V is considered dangerous.

Electric current ( I ) is the rate of flow of electrons

in a circuit and is measured in amperes (A) or milli-

amperes (mA). One milliampere is one-thousandth of an

ampere. The amount of current flowing through a per-

son’s body depends on the voltage and resistance. Body

current can be calculated using the following Ohm’s

law formula:

Current =

Voltage

_________

Resistance

If you came into direct contact with 120 volts and your

body resistance was 100,000, then the current that would

flow would be:

I =

120 V

_________

100,000 Ω

= 0.0012 A

= 1.2 mA (0.0012 × 1,000)

This is just about at the thresh-

old of perception, so it would

produce only a tingle.

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4 Chapter 1 Safety in the Workplace

increase the severity of a burn. Instead always wear

cotton clothing.

4. Remove all metal jewelry when working on ener-

gized circuits; gold and silver are excellent conduc-

tors of electricity.

5. Confine long hair or keep hair trimmed when work-

ing around machinery.

A wide variety of electrical safety equipment is avail-

able to prevent injury from exposure to live electric cir-

cuits (Figure 1-5). Electrical workers should be familiar

with safety standards such as NFP-70E that pertain to the

type of protective equipment required, as well as how such

equipment shall be cared for. To make sure electrical pro-

tective equipment actually performs as designed, it must

be inspected for damage before each day’s use and imme-

diately following any incident that can reasonably be sus-

pected of having caused damage. All electrical protection

equipment must be listed and may include the following:

Rubber Protective Equipment —Rubber gloves are

used to prevent the skin from coming into contact

with energized circuits. A separate outer leather cover

is used to protect the rubber glove from punctures and

other damage. Rubber blankets are used to prevent

contact with energized conductors or circuit parts

when working near exposed energized circuits. All

rubber protective equipment must be marked with the

appropriate voltage rating and the last inspection date.

It is important that the insulating value of both rubber

gloves and blankets have a voltage rating that matches

that of the circuit or equipment they are to be used

with. Insulating gloves must be given an air test, along

Personal Protective Equipment

Construction and manufacturing worksites, by nature,

are potentially hazardous places. F

or this reason, safety

has become an increasingly large f

actor in the working

environment. The electrical industry, in particular, regards

safety to be unquestionably the most single important pri-

ority because of the hazardous nature of the business. A

safe operation depends largely upon all personnel being

informed and aware of potential hazards. Safety signs

and tags indicate areas or tasks that can pose a hazard

to personnel and/or equipment. Signs and tags may pro-

vide warnings specific to the hazard, or they may provide

safety instructions (Figure 1-3).

To perform a job safely, the proper protective clothing

must be used. Appropriate attire should be worn for each

particular job site and work activity (Figure 1-4). The fol-

lowing points should be observed:

1. Hard hats, safety shoes, and goggles must be worn

in areas where they are specified. In addition, hard

hats shall be approved for the purpose of the elec-

trical work being performed. Metal hats are not

acceptable!

2. Safety earmuffs or earplugs must be worn in noisy

areas.

3. Clothing should fit snugly to avoid the danger of

becoming entangled in moving machinery. Avoid

wearing synthetic-fiber clothing such as polyester

material as these types of materials may melt or

ignite when exposed to high temperatures and may

Hard hat

Goggles

Cotton only,

no polyester

Tight sleeves

and trouser

legs

No rings on

fingers

Safety shoe

POISON

HIGH

VOLTAGE

WARNING

HEARING

PROTECTION

MUST BE WORN

IN THIS AREA

CAUTION

DANGER

Figure 1-3 Typical safety signs.

Figure 1-4 Appropriate attire should be worn for each

particular job site and work activity.

Photo courtesy Capital Safety, www.capitalsafety.com.

Electric arc protection apparel

Low-voltage glove

and protector

Hot switch stick

Grounding sets

Figure 1-5 Electrical safety equipment.

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PART 1 Protecting against Electrical Shock 5

• When working on any circuit, take steps to ensure that

the controlling switch is not operated in your absence.

Switches should be padlocked open, and warning

notices should be displayed ( lockout/tagout ).

• Avoid working on “live” circuits as much as possible.

• When installing new machinery, ensure that the

framework is efficiently and permanently grounded.

• Always treat circuits as “live” until you have proven

them to be “dead.” Presumption at this point can

kill you. It is a good practice to take a meter reading

before starting work on a dead circuit.

• Avoid touching any grounded objects while working

on electrical equipment.

• Remember that even with a 120-V control system,

you may well have a higher voltage in the panel.

Always work so that you are clear of any of the

higher voltages. (Even though you are testing a

120-V system, you are most certainly in close prox-

imity to 240-V or 480-V power.)

• Don’t reach into energized equipment while it is

being operated. This is particularly important in

high-voltage circuits.

• Use good electrical practices even in temporary wir-

ing for testing. At times you may need to make alter-

nate connections, but make them secure enough so

that they are not in themselves an electrical hazard.

• When working on live equipment containing voltages

over approximately 30-V, work with only one hand.

Keeping one hand out of the way greatly reduces the

possibility of passing a current through the chest.

• Safely discharge capacitors before handling them.

Capacitors connected in live motor control circuits

can store a lethal charge for a considerable time

after the voltage to the circuits has been switched

off. Although Article 460 of the National Electric

Code (NEC) requires an automatic discharge within

1 minute, never assume that the discharge is work-

ing! Always verify that there is no voltage present.

Confined spaces can be found in almost any workplace.

Figure 1-6 illustrates examples of typical confined spaces.

In general, a “confined space” is an enclosed or partially

enclosed space that:

• Is not primarily designed or intended for human

occupancy.

• Has a restricted entrance or exit by way of location,

size, or means.

• Can represent a risk for the health and safety of any-

one who enters, because of its design, construction,

location, or atmosphere; the materials or substances

with inspection. Twirl the glove around quickly or roll

it down to trap air inside. Squeeze the palm, fingers,

and thumb to detect any escaping air. If the glove does

not pass this inspection it must be disposed of.

Protection Apparel —Special protective equipment

available for high-voltage applications include high-

voltage sleeves, high-voltage boots, nonconductive

protective helmets, nonconductive eyewear and face

protection, switchboard blankets, and flash suits.

Hot Sticks —Hot sticks are insulated tools designed

for the manual operation of high-voltage disconnect-

ing switches, high-voltage fuse removal and insertion,

as well as the connection and removal of temporary

grounds on high-voltage circuits. A hot stick is made

up of two parts, the head, or hood, and the insulating

rod. The head can be made of metal or hardened plas-

tic, while the insulating section may be wood, plastic,

or other effective insulating materials.

Shorting Probes —Shorting probes are used on deen-

ergized circuits to discharge any charged capacitors or

built-up static charges that may be present when power

to the circuit is disconnected. Also, when working on or

near any high-voltage circuits, shorting probes should

be connected and left attached as an extra safety pre-

caution in the event of any accidental application of

voltage to the circuit. When installing a shorting probe,

first connect the test clip to a good ground contact.

Next, hold the shorting probe by the handle and hook

the probe end over the part or terminal to be grounded.

Never touch any metal part of the shorting probe while

grounding circuits or components.

Face Shields —Listed face shields should be worn

during all switching operations where there is a possi-

bility of injury to the eyes or face from electrical arcs

or flashes, or from flying or falling objects that may

result from an electrical explosion.

With proper precautions, there is no reason for you to

ever receive a serious electrical shock. Receiving an elec-

trical shock is a clear warning that proper safety measures

have not been followed. To maintain a high level of elec-

trical safety while you work, there are a number of pre-

cautions you should follow. Your individual job will have

its own unique safety requirements. However, the follow-

ing are given as essential basics.

• Never take a shock on purpose.

• Keep material or equipment at least 10 feet away

from high-voltage overhead power lines.

• Do not close any switch unless you are familiar with

the circuit that it controls and know the reason for

its being open.

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