Remote and Automatic Control

7/30/2019 Remote and Automatic Control

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Remote and Automatic ControlThe motor may be controlled by remote control usingpush buttons (Figure 17). When push-button remotecontrol is used or when automatic devices do not havethe electrical capacity to carry the motor starting and

running currents, magnetic switches must be included.Magnetic switch control is accomplished by electromagneticmeans. The effort required to actuate the electromagnetis supplied by electrical energy rather thanby the human operator. If the motor is to be automaticallycontrolled, the following two-wire pilot devicesmay be used.

Float SwitchThe raising or lowering of a float that is mechanicallyattached to electrical contacts may start motordrivenpumps to empty or fill tanks. Float switches arealso used to open or close piping solenoid valves to

control fluids (Figure 18).10 Section 1 Solid-State DevicesFigure 111 Line voltage thermostat. (Courtesy White Rogers.)Figure 19 Pressure switch with cover removed.NormallyopenNormallyclosedFigure 110 NEMA symbols for pressure switch contacts.

Pressure SwitchPressure switches are used to control the pressure ofliquids and gases (including air) within a desired range(Figure 19). Air compressors, for example, are started

directly or indirectly on a call for more air by a pressureswitch. Electrical wiring symbols are shown as normallyclosed and normally open in Figure 110.

Time ClockTime clocks can be used when a definite on andoff period is required and adjustments are not necessary for long periods of time. A typical requirementis a motor that must start every morning at the sametime and shut off every night at the same time, or thatswitches the floodlights on and off.

ThermostatIn addition to pilot devices sensitive to liquid levels,

gas pressures, and time of day, thermostats sensitive totemperature changes are widely used (Figure 111).Thermostats indirectly control large motors in air conditioningsystems and in many industrial applicationsto maintain the desired temperature range of air, gases,liquids, or solids. There are many types of thermostatsand temperature-actuated switches.

Limit Switch


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Limit switches (Figure 112) are designed to passan electrical signal only when a predetermined limit isreached. The limit may be a specific position for amachine part or a piece of work, or a certain rotatingspeed. These devices take the place of a human operatorand are often used under conditions where it would

be impossible or impractical for the operator to bepresent or to efficiently direct the machine.Unit 1 General Principles of Electric Motor Control 11Figure 112 Limit switch with cover removed to show internalconnections.

Limit switches are used most frequently as overtravelstops for machines, equipment, and products inprocess. These devices are used in the control circuitsof magnetic starters to govern the starting, stopping, orreversal of electric motors.

Electrical or Mechanical Interlockand Sequence ControlMany of the electrical control devices described inthis unit can be connected in an interlocking system sothat the final operation of one or more motors dependsupon the electrical position of each individual controldevice. For example, a float switch may call for moreliquid but will not be satisfied until the prior approvalof a pressure switch or time clock is obtained. To design,install, and maintain electrical controls in anyelectrical or mechanical interlocking system, the electricaltechnician must understand the total operationalsystem and the function of the individual components.With practice, it is possible to transfer knowledge of

circuits and descriptions for an understanding of additionalsimilar controls. It is impossiblein instructionalmaterialsto show all possible combinations ofan interlocking control system. However, by understandingthe basic functions of control components andtheir basic circuitry, and by taking the time to trace anddraw circuit diagrams, difficult interlocking controlsystems can become easier to understand.

Starting and StoppingIn starting and stopping a motor and its associated machinery,there are a number of conditions that may affect

the motor. A few of them are discussed here.Frequency of Starting and StoppingThe starting duty cycle of a controller is an importantfactor in determining how satisfactorily the controllerwill perform in a particular application. Magneticswitches, such as motor starters, relays, and contactors,actually beat themselves apart from repeated openingand closing thousands of times. An experienced electriciansoon learns to look for this type of component failure


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when troubleshooting any inoperative control panels.NEMA standards require that the starter size bederated if the frequency of start-stop, jogging, or pluggingis more than 5 times per minute. Therefore, whenthe frequency of starting the controller is great, the useof heavy duty controllers and accessories should be considered.

For standard duty controllers, more frequent inspectionand maintenance schedules should be followed.

Light or Heavy Duty StartingSome motors may be started with no loads and othersmust be started with heavy loads. When motors arestarted, large feeder line disturbances may be createdthat can affect the electrical distribution system of theentire industrial plant. The disturbances may even affectthe power companys system. As a result, the powercompanies and electrical inspection agencies place certainlimitations on across-the-line motor starting.

Fast or Slow Start (Hard or Soft)To obtain the maximum twisting effort (torque) ofthe rotor of an ac motor, the best starting condition is toapply full voltage to the motor terminals. The drivenmachinery, however, may be damaged by the suddensurge of motion. To prevent this type of damage to machines,equipment, and processed materials, some controllersare designed to start slowly and then increasethe motor speed gradually in definite steps. This type isoften used by power companies and inspection agenciesto avoid electrical line surges.N.O.N.C.

12 Section 1 Solid-State DevicesFigure 113 Typical electric brake.DPDT SWITCHVOLTS DC+ A1 A2LOADRESISTERS1 S2F1 F2

Figure 114 Dynamic braking for a dc compound motor.

Smooth StartingAlthough reduced electrical and mechanical surgescan be obtained with a step-by-step motor startingmethod, very smooth and gradual starting will require

different controlling methods. These are discussed indetail later in the text.

Manual or Automatic Starting and StoppingWhile the manual starting and stopping of machinesby an operator is still a common practice, manymachines and industrial processes are started and restartedautomatically. These automatic devices result intremendous savings of time and materials. Automatic


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stopping devices are used in motor control systems forthe same reasons. Automatic stopping devices greatlyreduce the safety hazards of operating some types ofmachinery, both for the operator and the materials beingprocessed. An electrically operated, mechanicalbrake is shown in Figure 113. Such a brake may be required

to stop a machines motion in a hurry to protectmaterials being processed or people in the area.

Quick Stop or Slow StopMany motors are allowed to coast to a standstill.However, manufacturing requirements and safety considerationsoften make it necessary to bring machinesto as rapid a stop as possible. Automatic controls canretard and brake the speed of a motor and also apply aUnit 1 General Principles of Electric Motor Control 13

torque in the opposite direction of rotation to bringabout a rapid stop. This is calledplugging. Pluggingcan only be used if the driven machine and its load will

not be damaged by the reversal of the motor torque.The control of deceleration is one of the importantfunctions of a motor control.

Another method of braking electric motors isknown as dynamic braking. When this method is usedto reduce the speed of dc motors, the armature isconnected across a load resistor when power isdisconnected from the motor. If the field winding of themotor remains energized, the motor becomes a generatorand current is supplied to the load resistor by thearmature (Figure 114). The current flowing throughthe armature winding creates a magnetic field around

the armature. This magnetic field causes the armatureto be attracted to the magnetic field of the pole pieces.This action in a dc generator is known as countertorque. Using counter torque to brake a dc motor isknown as dynamic braking.

Ac induction motors can be braked by momentarilyconnecting dc voltage to the stator winding(Figure 115). When direct current is applied to thestator winding of an ac motor, the stator poles becomeelectromagnets. Current is induced into the windings ofthe rotor as the rotor continues to spin through the magneticfield. This induced current produces a magnetic

field around the rotor. The magnetic field of the rotor isL1 L2 L3MOTORMMMMTRDBRDBR DBRMM TR


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OLOLHTRsMDBRCONTROL TRANSFORMERSTEP-DOWNTRANSFORMER480 VAC 3

BRIDGE RECTIFIER CONVERTSAC VOLTAGE INTO DC VOLTAGE.STOPSTART480/120FUSE

Figure 115 Dynamic braking for an ac motor.

14 Section 1 Solid-State DevicesFigure 116 (Courtesy Tennessee Valley Authority.)

attracted to the magnetic field produced in the stator.The attraction of these two magnetic fields produces abraking action in the motor.

An advantage of using dynamic braking is that motorscan be stopped rapidly without wearing brake linings

or drums. It cannot be used to hold a suspendedload, however. Mechanical brakes must be employedwhen a load must be held, such as with a crane or hoist.

Accurate StopsAn elevator must stop at precisely the right locationso that it is aligned with the floor level. Such accuratestops are possible with the use of automatic devicesinterlocked with control systems.

Frequency of Reversals RequiredFrequent reversals of the direction of rotation of themotor impose large demands on the controller and theelectrical distribution system. Special motors and

special starting and running protective devices may berequired to meet the conditions of frequent reversals.

A heavy duty drum switch-controller is often used forthis purpose.

Speed Control of MotorsThe speed control is concerned not only with startingthe motor but also with maintaining or controlling themotor speed while it is running. There are a number ofconditions to be considered for speed control.

Constant SpeedConstant speed motors are used on water pumps

(Figures 110 and 116). Maintenance of constantspeed is essential for motor generator sets under all loadUnit 1 General Principles of Electric Motor Control 15Figure 117 Cutaway view of a speed reducing cycloidal gearmotor. Cycloidal gear boxes use a concentric cam with rollersinstead of conventional gears. (Courtesy Sumitomo MachineryCorporation of America.)

conditions.Constantspeedmotorswithratingsaslowas80rpm and horsepower ratings up to 5000 hp are used in direct


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drive units. The simplest method of changing speedsis by gearing. Using gears, almost any predeterminedspeed may be developed by coupling the input gear to theshaft of a squirrel-cage induction motor. A speedreducingcycloidal gear motor is shown in Figure 117.

Varying SpeedA varying speed is usually preferred for cranes andhoists (Figure 118). In this type of application, themotor speed slows as the load increases and speeds upas the load decreases.

Adjustable SpeedWith adjustable speed controls, an operator cangradually adjust the speed of a motor over a wide rangewhile the motor is running. The speed may be preset,but once it is adjusted it remains essentially constant atany load within the rating of the motor.Figure 118 Large traveling overhead crane. (Courtesy Harrington Hoists and Cranes.)

16 Section 1 Solid-State Devices

MultispeedFor multispeed motors, such as the type used onturret lathes in a machine shop, the speed can be set attwo or more definite rates. Once the motor is set at adefinite speed, the speed will remain practicallyconstant regardless of load changes.

Protective FeaturesThe particular application of each motor and controlinstallation must be considered to determine whatprotective features are required to be installed andmaintained.

Overload ProtectionRunning protection and overload protection refer tothe same thing. This protection may be an integral partof the motor or be separate. A controller with electricaloverload protection will protect a motor from burning upwhile allowing the motor to achieve its maximum availablepower under a range of overload and temperatureconditions. An electrical overload on the motor may becaused by mechanical overload on driven machinery, alow line voltage, an open electrical line in a polyphasesystem resulting in single-phase operation, motor problemssuch as too badly worn bearings, loose terminalconnections, or poor ventilation within the motor.

Open Field ProtectionDc shunt and compound-wound motors can be protectedagainst the loss of field excitation by field lossrelays. Other protective arrangements are used withstarting equipment for dc and ac synchronous motors.Some sizes of dc motors may race dangerously with theloss of field excitation while other motors may not race


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due to friction and the fact that they are small.

Open-Phase ProtectionPhase failure in a three-phase circuitmay be causedby a blown fuse, an open connection, a broken line orother reasons. If phase failure occurs when the motor isat a standstill during attempts to start, the stator currentswill rise to a very high value and will remainthere, but the motor will remain stationary (not turn).Since the windings are not properly ventilated whilethe motor is stationary, the heating produced by thehigh currents may damage them. Dangerous conditionsalso are possible while the motor is running. When themotor is running and an open-phase condition occurs,the motor may continue to run. The torque willdecrease, possibly to the point of motor stall; this condition is called breakdown torque.

Reversed Phase Protection

If two phases of the supply of a three-phase inductionmotor are interchanged (phase reversal), the motorwill reverse its direction of rotation. In elevator operationand industrial applications, this reversal can resultin serious damage. Phase failure and phase reversalrelays are safety devices used to protect motors,machines, and personnel from the hazards of openphaseor reversed-phase conditions.

Overtravel ProtectionControl devices are used in magnetic starter circuitsto govern the starting, stopping, and reversal ofelectric motors. These devices can be used to control

regular machine operation or they can be used as safetyemergency switches to prevent the improper functioningof machinery.

Overspeed ProtectionExcessive motor speeds can damage a driven machine,materials in the industrial process, or the motor.Overspeed safety protection is provided in controlequipment for paper and printing plants, steel mills,processing plants, and the textile industry.

Reversed Current ProtectionAccidental reversal of currents in dc controllers canhave serious effects. Direct-current controllers used

with three-phase alternating-current systems that experiencephase failures and phase reversals are alsosubject to damage. Reverse current protection is animportant provision for battery charging and electroplatingequipment.

Mechanical ProtectionAn enclosure may increase the life span and contributeto the trouble-free operation of a motor and


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Unit 1 General Principles of Electric Motor Control 17Figure 119 Explosion proof enclosure for a magnetic motorstarter.

controller. Enclosures with particular ratings such asgeneral purpose, watertight, dustproof, explosionproof,and corrosion resistant are used for specific applications

(Figure 119). All enclosures must meet the requirementsof national and local electrical codes andbuilding codes.

Short Circuit ProtectionFor large motors with greater than fractional horsepowerratings, short circuit and ground fault protectiongenerally is installed in the same enclosure as the motordisconnectingmeans. Overcurrent devices (such asfuses and circuit breakers) are used to protect the motorbranch circuit conductors, the motor control apparatus,and the motor itself against sustained overcurrentdue to short circuits and grounds, and prolonged and

excessive starting currents.Classification of Automatic MotorStarting Control SystemsThe numerous types of automatic starting and controlsystems are grouped into the following classifications:current limiting acceleration and time delayacceleration.

Current Limiting AccelerationCurrent limiting acceleration is also called compensatingtime. It refers to the amount of current orvoltage drop required to open and close magneticswitches when used in a motor accelerating controller.The rise and fall of the cur rent or voltage determines atiming period that is used mainly for dc motor control.Examples of types of current limiting acceleration are: Counter emf or voltage drop acceleration Lockout contactor or series relay acceleration

Time Delay AccelerationFor time delay acceleration, definite time relays areused to obtain a preset timing period. Once the periodis preset, it does not vary regardless of current or voltagechanges occurring during motor acceleration. The

following timers and timing systems are used for motoracceleration; some are also used in interlocking circuitsfor automatic control systems. Pneumatic timing Motor-driven timers Capacitor timing Electronic timers

Troubleshooting


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One of the primary jobs of an industrial electrician istroubleshooting control circuits. An electrician that isproficient in troubleshooting is sought after by most ofindustry. The greatest troubleshooting tool an electriciancan possess is the ability to read and understandcontrol schematic diagrams. Many of the circuits shown

in this text are accompanied by detailed explanations ofthe operation of the circuit. If the circuit and explanationare studied step by step, the student will have anexcellent understanding of control schematics when thistext is completed.Most electricians follow a set procedure whentroubleshooting a circuit. If the problem has occurredseveral times in the past and was caused by the samecomponent each time, most electricians check thatcomponent first. If that component proves to be theproblem, much time has been saved by not having totrace the entire circuit.

18 Section 1 Solid-State DevicesAnothermethod of troubleshooting a circuit is shotguntroubleshooting. This method derives its name fromthe manner in which components are tested. Instead offollowing the circuit in a logical step-by-step procedure,the electrician quickly checks the major componentsof the circuit. This approach is used to save timebecause in many industrial situations an inoperativepiece of equipment can cost a company thousands ofdollars for each hour it is not working.When neither of these methods reveals the problem,the electrician must use the control schematic to

trace the circuit in a logical step-by-step procedure. Theprimary tool used to trace a circuit is the volt-ohmmilliammeter(VOM), which measures voltage, current,and resistance. It is often necessary to use jumper leadsto bridge open contacts when using the VOM. When a

jumper lead is used for this application, it should beprovided with short circuit protection. This can be doneby connecting a small fuse holder or circuit breaker inseries with the jumper lead. In this way if the jumper isaccidently shorted, the fuse or circuit breaker will openand protect the rest of the circuit.When troubleshooting a circuit, most electricians

work backward through the circuit. For example, oneline of a control schematic is shown in Figure 120.M relay coil is connected in series with a normallyclosed overload contact, a normally open limit switchcontact, a normally closed pressure switch contact, anormally closed CR relay contact, and a normallyopen float switch contact. The problem with the circuitis that M relay coil will not energize. The first testshould be to measure the voltage at each end of the


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circuit to confirm the presence of control voltage. Thenext procedure is to connect the voltmeter across eachof the circuit components to determine which one isopen and stopping the current flow to the coil. Whenthe voltmeter is connected across a closed contact,there is no voltage drop, and the meter indicates

0 volts. If the voltmeter is connected across an opencontact, the meter indicates the full voltage of thecircuit.

Assume in this circuit that the full circuit voltage isindicated when the meter is connected across floatswitch FS. This reading signals that float switch FS isopen. The next step is to determine if the switch is bador if the liquid lever it is sensing has not risen highenough to close the switch. Once that has been determined,the electrician can correct the problem.FS PS120 VOLTS ACLS

MCR CR

Figure 120 Troubleshooting a circuit.

Review Questions1. What is a controller and what is its function? (Usethe Glossary and the information from this unit toanswer this question.)2. What is meant by remote control?3. To what does current limiting, or compensatingtime, acceleration refer?4. List some devices that are used to control a motorautomatically. Briefly describe the purpose of

each device.Unit 1 General Principles of Electric Motor Control 19

Select the bestanswer for each of the following.5. The general purpose of motor control isa. to start the motorb. to stop the motorc. to reverse the motord. all of the above6. A motor may be controlled manually by using aa. float switchb. pressure switchc. toggle switch

d. time clock7. A motor may be controlled remotely orautomatically by using aa. drum controllerb. thermostatc. safety switchd. faceplate control8. Conditions that may affect starting and stoppingof motor driven machinery are


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a. fast or slow startsb. light or heavy duty startingc. frequency of starting and stoppingd. all of the above9. Which factor is notto be considered for motorspeed control when the motor is running?

a. Constant speedb. Varying speedc. Multispeedd. Starting protection10. Which is not considered a motor controllerprotective feature?a. Overloadb. Short circuitc. Adjustable speedd. Mechanical11. Which function is not a fundamental job of amotor controller?

a. Start and stop the motorb. Protect the motor, machine, and operatorc. Reverse, inch, jog, speed controld. Motor disconnect switch and startingprotection12. What factors are to be considered when selectingand installing a controller?a. Electrical serviceb. Motorc. Electrical codes and standardsd. All of the above13. Dynamic braking for a dc motor is

accomplished bya. connecting ac voltage to the armatureb. maintaining dc current flow through the fieldand connecting the armature to a load resistorc. maintaining dc current flow through thearmature and connecting a load resistor to thefieldd. disconnecting dc power from the motor andreconnecting the armature to a load resistor14. Dynamic braking for an ac motor isaccomplished bya. disconnecting ac power from the motor leads

and reconnecting the motor to a load resistorb. reversing the direction of rotation of the motorc. connecting dc voltage to the stator leadsd. connecting a load resistor in series with themotor leads

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Remote and Automatic Control