Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 7th Industrial Motor Control Edition Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part This is an electronic version of the print textbook Due to electronic rights restrictions, some third party content may be suppressed Editorial review has deemed that any suppressed content does not materially affect the overall learning experience The publisher reserves the right to remove content from this title at any time if subsequent rights restrictions require it For valuable information on pricing, previous editions, changes to current editions, and alternate formats, please visit www.cengage.com/highered to search by ISBN#, author, title, or keyword for materials in your areas of interest Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 7th Edition Industrial Motor Control Stephen L Herman Australia • Brazil • Japan • Korea • Mexico • Singapore • Spain • United Kingdom • United States Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Industrial Motor Control, 7th Edition Stephen L Herman Vice President, Editorial: Dave Garza Director of Learning Solutions: Sandy Clark Acquisitions Editor: Jim DeVoe Managing Editor: Larry Main Senior Product Manager: John Fisher Editorial Assistant: Aviva Ariel Vice President, Marketing: Jennifer Baker Director, Market Development Management: Debbie Yarnell Marketing Development Manager: Erin Brennan Director, Brand Management: Jason Sakos Marketing Brand Manager: Erin McNary Senior Production Director: Wendy Troeger Production Manager: Mark Bernard Content Project Manager: Barbara LeFleur Production Technology Assistant: Emily Gross Senior Art Director: David Arsenault Technology Project Manager: Joe Pliss © 2014, 2010 Delmar, 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a leading provider of customized learning solutions with office locations around the globe, including Singapore, the United Kingdom, Australia, Mexico, Brazil and Japan Locate your local office at: international.cengage.com/region Cengage Learning products are represented in Canada by Nelson Education, Ltd To learn more about Delmar, visit www.cengage.com/delmar Purchase any of our products at your local college store or at our preferred online store www.cengagebrain.com Notice to the Reader Publisher does not warrant or guarantee any of the products described herein or perform any independent analysis in connection with any of the product information contained herein Publisher does not assume, and expressly disclaims, any obligation to obtain and include information other than that provided to it by the manufacturer The reader is expressly warned to consider and adopt all safety precautions that might be indicated by the activities described herein and to avoid all potential hazards By following the instructions contained herein, the reader willingly assumes all risks in connection with such instructions The publisher makes no representations or warranties of any kind, including but not limited to, the warranties of fitness for particular purpose or merchantability, nor are any such representations implied with respect to the material set forth herein, and the publisher takes no responsibility with respect to such material The publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or part, from the readers’ use of, or reliance upon, this material Printed in the United States of America 14 13 12 11 10 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Contents Preface • xi New for the Seventh Edition • xii Accessing the Instructor Companion Web Site • xii Content Highlights • xii Acknowledgments • xiii General Principles of Motor Control Installation of Motors and Control Equipment Types of Control Systems Functions of Motor Control Review Questions 27 31 33 34 76 83 84 85 87 87 88 88 94 Pressure Switches and Sensors Pressure Switches Pressure Sensors Review Questions 35 52 53 61 65 66 67 75 Timing Relays Pneumatic Timers Clock Timers Motor-Driven Timers Capacitor Time Limit Relay Electronic Timers Review Questions 27 35 36 37 43 47 50 51 52 The Control Transformer Review Questions 10 16 19 20 22 26 Overload Relays Overloads Dual-Element Fuses Thermal Overload Relays Magnetic Overload Relays Overload Contacts Protecting Large Horsepower Motors Review Questions 10 Manual Starters Fractional Horsepower Single-Phase Starters Manual Push Button Starters Troubleshooting Review Questions 4 Relays, Contactors, and Motor Starters Relays Electromagnet Construction Contactors Mechanically Held Contactors and Relays Mercury Relays Motor Starters Review Questions Symbols and schematic diagrams Push Buttons Switch Symbols Basic Schematics Sensing Devices Selector Switches Review Questions 95 95 97 100 Float Switches Mercury Bulb Float Switch The Bubbler System Review Questions 101 102 103 106 v Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part vi Contents 10 Flow Switches and Sensors Flow Switches Flow Sensors Review Questions 107 107 110 117 11 Limit Switches Micro Limit Switches Subminiature Micro Switches Limit Switch Application Review Questions Effects of Voltage Variation on Motors Review Questions 119 120 121 123 Expansion of Metal Resistance Temperature Detectors Expansion Due to Pressure Smart Temperature Transmitters Review Questions Principles of Operation Hall Generator Applications Review Questions Applications Types of Detectors Mounting Review Questions 143 143 144 147 148 148 150 151 151 153 Review Questions 196 197 197 199 201 202 205 28 Multiple Push Button Stations Developing a Wiring Diagram Review Questions 206 208 212 Interlocking Developing a Wiring Diagram Reversing Single-Phase, Split-Phase Motors Review Questions 213 213 215 215 224 30 Jogging and Inching 154 154 159 161 Jogging Circuits Inching Controls Review Questions 162 165 166 167 170 195 29 Forward–Reverse Control 154 19 Schematics and Wiring Diagrams (circuit 1) 189 27 Hand-Off-Automatic Controls Review Questions 148 18 Basic Control Circuits Three-Wire Control Circuits Review Questions Component Location Point-to-Point Connection Using Terminal Strips Review Questions 225 225 227 232 31 Sequence Control Sequence Control Circuit Sequence Control Circuit Sequence Control Circuit Automatic Sequence Control Stopping the Motors in Sequence Review Questions 185 186 26 Installing Control Systems 132 137 141 141 142 181 182 25 Reading Large Schematic Diagrams Review Questions 178 180 24 Developing a Wiring Diagram (circuit 3) Review Question 132 17 Photodetectors 177 23 Developing a Wiring diagram (circuit 2) Review Question 126 127 131 16 Proximity Detectors Applications Metal Detectors Mounting Capacitive Proximity Detectors Ultrasonic Proximity Detectors Review Questions Review Questions 126 15 Hall Effect Sensors 174 22 Developing a Wiring Diagram (circuit 1) 124 125 14 Temperature-Sensing Devices 171 21 Float Switch Control of a Pump and Pilot 175 Lights (circuit 3) Review Questions 124 13 Solenoid and Motor-Operated Valves Solenoid Valves Motor-Operated Valves Review Questions Review Questions 118 12 Phase Failure Relays 20 Timed Starting for Three Motors (circuit 2) 233 233 233 234 236 237 247 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part vii Contents 32 DC Motors Application Speed Control Motor Construction Identifying Windings Types of DC Motors Direction of Rotation Standard Connections Review Questions 248 248 248 249 249 250 251 252 254 33 Starting Methods for DC Motors Review Questions The Shunt Field Power Supply The Armature Power Supply Voltage Control Field Failure Control Current Limit Control Speed Control Review Questions 261 262 262 263 264 264 265 268 Starting Methods for Single-Phase Motors Centrifugal Switch Hot-Wire Starting Relay Current Relay Solid-State Starting Relay Potential Starting Relay Review Questions Resistor Starting Reactor Starting Step-Starting Review Questions Open and Closed Transition Starting Review Questions 279 282 282 283 284 284 286 313 313 314 314 315 315 317 319 326 331 42 Variable Voltage and Magnetic Clutches Voltage Control Methods Magnetic Clutches Eddy Current Clutches Review Questions Mechanical Brakes Dynamic Braking Plugging Review Questions Manual Control of a Wound Rotor Motor Timed Controlled Starting Wound Rotor Speed Control Frequency Control Review Questions 287 290 292 292 338 338 339 342 349 294 350 352 353 353 354 357 45 Synchronous Motors Starting a Synchronous Motor Excitation Current The Brushless Exciter Direct-Current Generator Automatic Starting for Synchronous Motors The Field Contactor Out-of-Step Relay The Polarized Field Frequency Relay Power Factor Correction Applications Review Questions Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 332 333 334 336 337 44 Wound Rotor Induction Motors 287 295 299 311 43 Braking 277 38 Autotransformer Starting Overload Protection Dual-Voltage Motors Motor Applications Three-Step Starting Automatic Shut-Down Review Questions Three-Speed Consequent Pole Motors Four-Speed Consequent Pole Motors Review Questions 269 271 271 272 272 273 276 37 Resistor and Reactor Starting for AC Motors 301 302 303 304 307 310 41 Consequent Pole Motors 269 36 The Motor and Starting Methods 300 40 Part Winding Starters 260 35 Stepping Motors Theory of Operation Windings Four-Step Switching (Full Stepping) Eight-Step Switching (Half Stepping) AC Operation Motor Characteristics Review Questions Wye–Delta Starting Requirements Dual-Voltage Connections Connecting the Stator Leads Closed Transition Starting Overload Setting Review Questions 255 34 Solid-State DC Drives 39 Wye–Delta Starting 358 358 359 359 360 360 361 361 361 362 364 365 viii Contents 46 Variable Frequency Control Alternator Control Solid-State Control IGBTs Inverter Rated Motors Variable Frequency Drives Using SCRs and GTOs Review Questions 366 367 367 368 371 371 375 47 Motor Installation Motor Nameplate Data Horsepower Determining Motor Current Overload Size Example Problems Review Questions 376 376 377 387 394 398 405 48 Developing Control Circuits Developing Control Circuits Review Questions Safety Precautions Voltmeter Basics Test Procedure Example Test Procedure Example Test Procedure Example Review Questions The AND Gate The OR Gate The INVERTER The NOR Gate The NAND Gate Integrated Circuits Testing Integrated Circuits Review Questions 420 420 421 423 426 427 433 Review Questions Differences Between PLCs and PCs Basic Components Review Questions 478 479 479 Conductors Insulators Semiconductors Review Questions 480 480 481 481 485 486 486 489 58 The Zener Diode The Zener Diode Review Questions 490 490 492 59 Light-Emitting Diodes and Photodiodes 442 445 446 453 53 Programmable Logic Controllers 476 56 Semiconductors The PN Junction Review Questions 435 436 437 438 438 439 439 441 52 Start–Stop Push Button Control 464 466 468 470 470 470 471 474 475 57 The PN Junction 434 51 The Bounceless Switch Review Questions Installation The Differential Amplifier Review Questions 407 419 50 Digital Logic Circuit Operation Developing a Program Converting the Program Programming in Boolean Developing the Program Parameters of the Programmable Controller Operation of the Circuit Entering the Program Review Questions 464 55 Analog Sensing for Programmable Controllers 407 49 Troubleshooting 54 Programming a PLC 454 LED Characteristics Testing LEDs LED Lead Identification Seven-Segment Displays Connecting the LED in a Circuit Photodiodes Photovoltaic Photoconductive LED Devices Review Questions 494 494 496 496 497 497 497 498 498 499 60 The Transistor The Transistor Review Questions 493 500 500 503 454 455 463 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 160 Chapter 17 Photodetectors transmitter and receiver units, the light beam is broken and the detector activates Notice that no physical contact is necessary for the photodetector to sense the presence of the object Figure 17–13 illustrates another method of mounting the transmitter and receiver In this example, an object is sensed by reflecting light off of a shiny surface Notice that the transmitter and receiver must be mounted at the same angle with respect to the object to be sensed This type of mounting only works with objects that have the same height, such as cans on a conveyor line Photodetectors that have both the transmitter and the receiver units mounted in the same housing depend on a reflector for operation Figure 17–14 shows this type of unit mounted on a conveyor line The transmitter is aimed at the reflector The light beam is reflected back to the receiver When an object passes between the photodetector unit and the reflector, the light to the receiver is interrupted This type of unit has the advantage of needing electrical connection at only one piece of equipment This permits easy mounting of the photodetector unit and mounting of the reflector in hard to reach positions that would make running control wiring difficult Many of these units have a range of 20 feet and more Another type of unit that operates on the principle of reflected light uses an optical fiber cable The fibers in the cable are divided in half One-half of the fibers are connected to the transmitter, and the other half are connected to the receiver (Figure 17–15) This unit has the advantage of permitting the transmitter and the receiver to be mounted in a very small area Figure 17–16 illustrates a common use for this type of unit The unit is used to control a label-cutting machine The labels are printed on a large roll and must be cut for individual packages The label roll contains a narrow strip on one side that is dark colored except for shiny sections PHOTODETECTOR © Cengage Learning 2014 REFLECTOR Figure 17–14 The object is sensed when it passes between the photodetector and the reflector TRANSMITTER PHOTODETECTOR RECEIVER 45° RECEIVER 45° Figure 17–13 Object is sensed by reflecting light off a shiny surface FIBER-OPTIC CABLE Figure 17–15 Optical cable is used to transmit and receive light Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 © Cengage Learning 2014 TRANSMITTER 161 Figure 17–16 © Cengage Learning 2014 © Cengage Learning 2014 Chapter 17 Photodetectors Figure 17–17 Optical cable detects shiny area on one side of label Photodetector unit with both transmitter and receiver units spaced at regular intervals The optical fiber cable is located above this narrow strip When the dark surface of the strip is passing beneath the optical cable, no reflected light returns to the receiver unit When the shiny section passes beneath the cable, light is reflected back to the receiver unit The photodetector sends a signal to the control circuit and tells it to cut the label Photodetectors are very dependable and have an excellent maintenance and service record They are used to sense almost any object without making physical contact with it and can operate millions of times without damage or wear A photodetector is shown in Figure 17–17 ●● Review Questions List the three major categories of photodetectors What is the advantage of a photodetector that uses a reflector to operate? In which category does the solar cell belong? An object is to be detected by reflecting light off a shiny surface If the transmitter is mounted at a 60° angle, at what angle must the receiver be mounted? In which category phototransistors and photodiodes belong? In which category does the cad cell belong? The term cad cell is a common name for what device? What is the function of the transmitter in a photodetector unit? How much voltage is produced by a silicon solar cell? 10 What determines the amount of current a solar cell produces? Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Chapter 18 Basic Control Circuits Objectives After studying this chapter, the student will be able to ●● Describe the operation of a two-wire control circuit ●● Describe the operation of a three-wire control circuit motor (Figure 18–1) A good example of this type of control is the single-phase manual starter shown in Figure 3–1 Two-wire control circuits also control the operation of three-phase motors by controlling Control circuits can be divided into two major types: two-wire control circuits and three-wire control circuits A two-wire control circuit can be a simple switch that makes or breaks connection to a NEUTRAL CONDUCTOR HOT CONDUCTOR SW + ITC H Figure 18–1 A two-wire control can be a simple switch that controls a motor 162 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 + 163 Chapter 18 Basic Control Circuits FUSED DISCONNECT CONTROL TRANSFORMER MOTOR STARTER FLEXIBLE CONDUIT TWO WIRE CONTROL AIR OUTLET © Cengage Learning 2014 PRESSURE SWITCH Figure 18–2 The motor starter is controlled by running two wires to a pressure switch L1 L3 M OLHTR M OLHTR M OLHTR T1 T2 MOTOR T3 CONTROL TRANSFORMER OL M PRESSURE SWITCH Figure 18–3 Schematic diagram of the circuit shown in Figure 18–2 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 the power applied to the motor starter coil A good example of this type of control is an air compressor (Figure 18–2) The pressure switch is used to control the motor starter A schematic diagram of the circuit in Figure 18–2 is shown in Figure 18–3 Two-wire control circuits are so named because only two wires are required to control the operation of the circuit Two-wire circuits may incorporate several different external sensing devices, as shown in Figure 18–4 This circuit is a basic control for a hot water boiler The thermostat controls the action of the burner Two float switches are used to sense low and high water conditions in the boiler A high-limit temperature switch stops the burner if the water temperature becomes excessive It is not unusual for two-wire control circuits to use line voltage controls Line voltage controls L2 164 Chapter 18 Basic Control Circuits are simply controls that not employ the use of a control transformer to change the voltage to a lower value The coils of motor starters and contactors are available that operate at different voltages L1 L2 Common voltage values for motor starter coils (in volts AC) are: 24, 120, 208, 240, 277, 480, and 560 A two-wire line voltage control circuit is shown in Figure 18–5 L3 M OLHTR M OLHTR M OLHTR T1 T2 MOTOR T3 © Cengage Learning 2014 CONTROL TRANSFORMER OL M THERMOSTAT LOW WATER HIGH WATER HIGH TEMPERATURE Figure 18–4 Two-wire control circuits may contain any number of external sensing devices L1 480 VAC ϴ L2 L3 480 VAC COIL OL M M OLHTR M M OLHTR OLHTR T2 MOTOR T3 Figure 18–5 A two-wire line voltage control circuit Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 T1 165 Chapter 18 Basic Control Circuits Three-Wire Control Circuits Three-wire control circuits are characterized by the use of momentary contact devices such as push buttons When push buttons control the operation of a motor, three wires are run from the pushbutton control station to the starter (Figure 18–6) A simple three-wire push button control circuit is shown in Figure 18–7 Three-wire control is used to a much greater extent throughout industry than two-wire control because of its flexibility Pilot control devices such as push buttons, float switches, and limit switches can be mounted in remote locations, whereas the motor starter can be located close to the motor it controls or in a control cabinet with other control components Another advantage © Cengage Learning 2014 THREE WIRES ARE REQUIRED TO CONTROL THE STARTER Figure 18–6 Three wires are required to control a starter with momentary contact devices, such as push buttons Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 166 Chapter 18 Basic Control Circuits of three-wire control circuits is that in the event of a power failure they not restart automatically when power is restored This can be a major safety issue in many instances Three-wire controls depend on a set of normally open contacts, generally L1 L2 called holding, maintaining, or sealing contacts, connected in parallel with the start push button to maintain the circuit once the normally open start button is released These contacts are labeled M in Figure 18–7 L3 M OL M OL MOTOR M OL CONTROL TRANSFORMER STOP START OL © Cengage Learning 2014 M M Figure 18–7 A basic three-wire start–stop control circuit ●● Review Questions What are the two major types of control circuits? How is it possible for a two-wire control circuit to control the operation of a three-phase motor? Refer to the schematic shown in Figure 18–4 What type of switch is the thermostat? a Normally open temperature switch b Normally closed temperature switch c Normally open, held closed temperature switch d Normally closed, held open temperature switch Refer to the schematic shown in Figure 18–4 What type of switch is the low water switch? a Normally open float switch b Normally closed float switch c Normally open held closed float switch d Normally closed held open float switch What generally characterizes a three-wire control circuit? Explain the function of a holding contact How are holding contacts connected? Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part Chapter 19 Schematics and Wiring Diagrams (circuit 1) OBJECTIVES After studying this chapter, the student will be able to ●● Interpret schematic diagrams ●● Interpret wiring diagrams ●● Connect control circuits using schematic and wiring diagrams ●● Discuss the operation of circuit Schematic and wiring diagrams are the written language of control circuits Maintenance electricians must be able to interpret schematic and wiring diagrams to install control equipment or troubleshoot existing control circuits Schematic diagrams are also known as line diagrams and ladder diagrams Schematic diagrams show components in their electrical sequence without regard to physical location Schematics are used more than any other type of diagram to connect or troubleshoot a control circuit Wiring diagrams show a picture of the control components with connecting wires Wiring diagrams are sometimes used to install new control circuits, but they are seldom used for troubleshooting existing circuits Figure 19–1A shows a schematic diagram of a start–stop, push button circuit Figure 19–1B shows a wiring diagram of the same circuit When reading schematic diagrams, the following rules should be remembered Read a schematic as you would a book—from top to bottom and from left to right Contact symbols are shown in their de-energized or off position When a relay is energized, all the contacts controlled by that relay change position If a contact is shown normally open on the schematic, it closes when the coil controlling it is energized The three circuits shown in this and following chapters are used to illustrate how to interpret the logic of a control circuit using a schematic diagram Circuit 1, shown in Figure 19–2A, is an alarm- silencing circuit The purpose of the circuit is to sound a horn and turn on a red warning light when the pressure of a particular system becomes too 167 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 168 Chapter 19 Schematics and Wiring Diagrams (circuit 1) CONTROL TRANSFORMER FUSE 120 VOLTS AC START STOP OL © Cengage Learning 2014 M M Figure 19–1A Schematic diagram of a start–stop push button station L1 L2 L3 CONTROL TRANS M MOTOR © Cengage Learning 2014 THIS CONNECTION IS GENERALLY FACTORY MADE Figure 19–1B Wiring diagram of a start–stop push button station great After the alarm has sounded, the RESET button can be used to turn the horn off, but the red warning light must remain on until the pressure in the system drops to a safe level Notice that no current can flow in the system because of the open pressure switch, PS If the pressure rises high enough to cause pressure switch PS to close, current can flow through Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 169 Chapter 19 Schematics and Wiring Diagrams (circuit 1) the normally closed S contact to the horn Current can also flow through the red warning light Current cannot, however, flow through the normally open RESET button or the normally open S contact (Figure 19–2B) If the reset button is pushed, a circuit is completed through the S relay coil When relay coil S energizes, the normally closed S contact opens and the normally open S contact closes When the normally closed S contact opens, the circuit to the horn is broken This causes the horn to turn off The normally open S contact is used as a holding contact to maintain current to the coil of the relay when the RESET button is released (Figure 19–2C) The red warning light remains turned on until the pressure switch opens again When the pressure switch opens, the circuit is broken and current flow through the system stops This causes the red warning light to turn off and de-energizes the coil of relay S When relay S de-energizes, both of the S contacts return to their original position The circuit is now back to the same condition it was in in Figure 19–2A L1 L2 PS S HORN R RESET © Cengage Learning 2014 S S Figure 19–2A Circuit Alarm silencing circuit L1 L2 PS S HORN R S S Figure 19–2B Pressure switch closes Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 RESET 170 Chapter 19 Schematics and Wiring Diagrams (circuit 1) L1 L2 PS S HORN R S S Figure 19–2C The alarm has been silenced, but the warning light remains on ●● Review Questions Define a schematic diagram Define a wiring diagram Referring to circuit (Figure 19–2A), explain the operation of the circuit if pressure switch PS is connected normally closed instead of normally open Refer to the circuit shown in Figure 19-2A When the pressure switch closes, both the Horn and warning light turn on When the reset button is pressed, both the horn and warning light continue to operate Which of the following could NOT cause this problem? A The normally open S contact connected in parallel with the reset button did not close B The reset button is defective C S relay coil is defective D The pressure switch contacts are shorted Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 RESET Chapter 20 Timed Starting for Three Motors (circuit 2) OBJECTIVES After studying this chapter, the student will be able to ●● Discuss the operation of circuit ●● Troubleshoot circuit using the schematic A machine contains three large motors The current surge to start all three motors at the same time is too great for the system Therefore, when the machine is to be started, there must be a delay of 10 seconds between the starting of each motor Circuit 2, shown in Figure 20–1, is a start–stop, push button control that controls three motor starters and two time-delay relays The circuit is designed so that an overload on any motor stops all motors When the START button is pressed, a circuit is completed through the START button, M1 motor starter coil, and TR1 relay coil When coil M1 energizes, motor starts and auxiliary contact M1, which is parallel to the START button, closes This contact maintains the current flow through the circuit when the START button is released (Figure 20–2) After a 10-second interval, contact TR1 closes When this contact closes, a circuit is completed through motor starter coil M2 and timer relay coil TR2 When coil M2 energizes, motor starts (Figure 20–3) Ten seconds after coil TR2 energizes, contact TR2 closes When this contact closes, a circuit is completed to motor starter coil M3, which causes motor to start (Figure 20–4) If the STOP button is pressed, the circuit to coils M1 and TR1 is broken When motor starter Ml de-energizes, motor stops and auxiliary contact Ml opens TR1 is an on-delay relay; therefore, when coil TR1 is de-energized, contact TR1 opens immediately When contact TR1 opens, motor starter M2 de-energizes, which stops motor 2, and coil TR2 d e-energizes Because TR2 is an on-delay relay, contact TR2 opens immediately This breaks the circuit to motor starter M3 When motor starter M3 de-energizes, motor stops Although it takes several seconds to explain what happens when the STOP button is pressed, the action of the relays is almost instantaneous If one of the overload contacts opens while the circuit is energized, the effect is the same as pressing the STOP button After the circuit stops, all contacts return to their normal positions, and the circuit is the same as the original circuit shown in Figure 20–1 171 Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part 172 Chapter 20 Timed Starting for Three Motors (circuit 2) L1 L2 STOP START OL1 OL2 OL3 M1 M1 TR1 TR1 M2 TR2 © Cengage Learning 2014 TR2 M3 Figure 20–1 Circuit Time delay starting for three motors L1 L2 STOP START OL1 OL2 OL3 M1 M1 TR1 TR1 M2 TR2 M3 Figure 20–2 M1 motor starter and TR1 timer relay turn on Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 TR2 173 Chapter 20 Timed Starting for Three Motors (circuit 2) L1 L2 STOP START OL1 OL2 OL3 M1 M1 TR1 TR1 M2 TR2 © Cengage Learning 2014 TR2 M3 Figure 20–3 Motor and TR2 have energized L1 L2 STOP START OL1 OL2 OL3 M1 M1 TR1 TR1 M2 TR2 M3 Figure 20–4 Motor has energized Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part © Cengage Learning 2014 TR2 174 Chapter 20 Timed Starting for Three Motors (circuit 2) ●● Review Questions (Refer to circuit 20–1.) Explain the operation of circuit (Figure 20–1) if contact M1 did not close Explain the operation of circuit (Figure 20–1) if relay coil TR2 is burned out Refer to circuit 2, shown in Figure 20–1 Assume that both times are set for a delay of seconds When the START button is pressed, motor starts running immediately After a delay of 10 seconds, motor starts running, but motor never starts Which of the following could cause this problem? a The TR1 coil is open b The M2 starter coil is open c The TR2 coil is open d The OL2 contact is open Refer to circuit 2, shown in Figure 20–1 Assume that the timers are set for a delay of seconds When the START button is pressed, nothing happens No motors start running for a period of minute Which of the following could not cause this problem? a The M1 holding contacts did not close b The STOP push button is open c The OL1 contact is open d The M2 coil is open Copyright 2012 Cengage Learning All Rights Reserved May not be copied, scanned, or duplicated, in whole or in part ... in connection with any of the product information contained herein Publisher does not assume, and expressly disclaims, any obligation to obtain and include information other than that provided... continues to run until the stop button is pressed Push–pull buttons that contain two normally closed contacts can be obtained also (Figure 2–15) These buttons are generally employed to provide... Lead Identification Seven-Segment Displays Connecting the LED in a Circuit Photodiodes Photovoltaic Photoconductive LED Devices Review Questions 494 494 496 496 497 497 497 498 498 499 60 The