CNC数控机床的电能质量的思考：接地8078.pdf

1Power Quality Considerations for CNC Machines:GROUNDINGAllen G.Morinec,Senior Member,IEEEFirstEnergy Corporation and Cleveland State University 1 Presented at IEEE I&CPS ConferenceMay 9th,2000 Clearwater Beach,FloridaAbstract-Computer Numerical Control(CNC)machines areused to shape metal parts by milling,boring,cutting,drilling,and grinding.A CNC machine generally consists of acomputer-controlled servo-amplifier,servo-motors,spindlemotor,and various tooling.The machine can be programmedto shape a part by use of a front control panel.Moresophisticated models allow a CAD drawing to be uploaded tothe machine.The electronic components within a CNC machineare particularly sensitive to the grounding techniques used inthe electrical supply to the machine.Malfunction,degradation,and damage to the electronics can often be traced to problemswith the grounding system.Production downtime,product loss,and expensive repair bills result.With the wide-spread use ofCNC machines across the world,these problems have become asignificant financial concern to many CNC machine users andtheir electric utility companies.This paper begins with a brief explanation of the fundamentalsof service and equipment grounding.The basic design of CNCmachines is also explained.Based on a survey of several CNCmachine representatives,the paper will explore the commongrounding techniques recommended by many CNC machinetool builders with particular emphasis on the ground rodproblem.In addition,several actual case studies that supportthe ground rod problem will be described.Finally,arecommended powering and grounding practice is presented tohelp eliminate power quality related operating problems withCNC machines while maintaining the safety requirements ofelectrical codes.Index Terms Computer Numerical Control,CNC,powersystem grounding,ground rod problem,damage.I.BUILDING SERVICE GROUNDINGThe logical place to begin discussions on grounding isat the electrical utility service entrance.Most machineshops are supplied with a three-phase service classifiedas a“four-wire grounded”or a“three-wireungrounded”system.Common voltages of groundedsystems are 208Y/120 V and 480Y/277V,andcommon ungrounded system voltages are 240V deltaand 480V delta.Both grounded and ungroundedelectrical systems are required to be connected to earthvia the building grounding electrode system.Thispractice is referred to as grounding or earthing forsafety1.In grounded electrical systems,one of the current-carrying conductors(typically the neutral)is solidlygrounded to the building grounding electrode system,providing a stable reference to the surroundingsSource:National GeographicSource:Hitachi Seiki Source:Square D(earth).The neutral is also connected to the buildingsequipment grounding system through the main bonding jumper,which facilitates the operation ofover-current protection devices(fuses and circuitbreakers),during a ground fault.All conductive(metal)enclosures of the electrical system are alsobonded together and to the power system groundingpoint to keep metal parts at ground potentialpreventing shock hazards.Fig.1.shows a typicalsolidly-grounded building electrical system1.Fig.1.Typical grounded building service andgrounding Electrode SystemIn an ungrounded electrical system,no circuitconductors are intentionally connected to ground.Theadvantage of an ungrounded electrical system is thatthe system can continue to operate with a singleground fault,avoiding production delays.Disadvantages of an ungrounded system areundetected ground faults and uncontrolled voltagebuild-ups between the insulated electrical powersystem conductors and various exposed metal parts orequipment enclosures.For many sensitive electronicdevices these uncontrolled voltage potentials can bedamaging.Hence,unless specified by the manufacturera CNC machine may not be suited for powering froman ungrounded electrical system.For ungroundedsystems a ground fault detector should be used to alertqualified personnel that an inadvertent ground fault hasoccurred.Fig.2.shows a typical ungrounded electricalsystem.Note that exposed metal parts are still bondedto the building grounding electrode system1.Fig.2.Ungrounded building service and groundingelectrode systemBoth grounded and ungrounded building electricalservices require a grounding electrode conductor toreference the electrical power system to the buildinggrounding electrode system.The grounding electrodesystem consists of all available earth electrodes,including building steel,metal water pipes,buriedground rings,and ground rods.The intent of thegrounding electrode conductor and groundingelectrode system is to provide a low impedance path toearth for lightning surge current and to reference thebuilding electrical system to its surroundings.Thisminimizes voltage differences between exposed metalparts of the power system including connectedequipment and the surrounding parts of the buildingthus reducing the hazard of electrical shock1.II.CNC EQUIPMENT GROUNDINGBesides the grounding electrode system,anotherfundamental of safety grounding is the equipmentground.The fundamental purpose of equipmentgrounding is to provide equipment and personnelsafety,and the secondary purpose of equipmentgrounding is to enhance equipment performance byproviding a reference for the electronic components.Equipment grounding intends to effectively ground allnon-current-carrying metal parts of the electricalsystem,including equipment enclosures and raceways.The proper combination of equipment and systemgrounding should ensure that a ground fault anywherein the system would not pose a shock hazard topersonnel.In case of a ground fault with a solidlygrounded electrical system,the low-impedanceequipment-grounding path allows the fault current toflow back to the power source.With sufficient groundfault current,the over-current protection device willtrip quickly and clear the ground fault.According to the NEC,metal raceways,such asconduit,are acceptable means of equipmentgrounding.Quite often though,especially withsensitive electronic equipment,a conductor is alsoused to supplement the conduit and further ensureeffective grounding.This is usually done becauseconduit connections can become loose or corrodedover time.To maintain a low impedance ground path,the equipment-grounding conductor must always berouted in the same conduit or raceway with itsassociated power conductors1.III.CNC MACHINE COMPONENTS ANDSUSCEPTIBILITIESA CNC machine is typically composed of a controllerwith appropriate computer program describing thedesired part,servo-amplifiers and positioning motorsto control relative movement of the part and shapingtools,and spindle motors that actually work to shapethe parts.Positioning of the part or the tooling istypically accomplished by turning a screw mechanismand moving a nut in one or more axes.Some machineshave five axes,3-dimensions plus a horizontal and avertical axis of rotation.See Fig.3.illustration of atypical CNC machine and controller with 2-dimensionsand a horizontal axis of rotation2.Fig.3.Illustration of a typical CNC machine andcontrollerThe CNC machine controller contains a computer toprovide overall control of the machine.It usuallymonitors the machined parts position by feedbackfrom the resolvers or encoders to update the program.Movement is accomplished by positioning motors withvelocity-based feedback from tachometers to thecomputer.The speed of the spindle motor is alsovariable using an adjustable speed drive with eithercomputer or manual control2.The onboard computer can be programmed throughthe units front panel or through a communication portwhere the required data from a CAD drawing programcan be downloaded from a remote computer.The CNCmachine microprocessor then executes the program,setting up the machine sequence,determining thedesired speed and position for machining,etc.Whenthere is physical separation between the CNC machineand its electronic control,certain functions may besensitive to the machine powering and groundingtechniques2.Electrically,the AC power is usually converted toseveral levels of DC for servo systems and computerlogic.Fig.4.shows the AC power sources,functionalblocks and interconnections for a typical CNCmachine.Also the data link to another area of thefactory is likely to have a signal reference ground inthat area of the factorys power system.Thesedifferent reference ground points may increase theCNC machines sensitivity to power disturbances.Todays electronically controlled CNC machinesrequire a common signal reference ground for logicalcircuits to operate reliably.The term,signal referencedescribes the zero-voltage level used by digital logiccircuits.For example,five volts DC above signalreference indicates a logic level“1”and zero volts DCindicates a logic level“0.”This signal reference istypically the common of the computer logic powersupply,which is connected to the machines groundplate.The ground plate is bonded to the machinesenclosure,equipment grounding conductor or conduitand,subsequently,to the building grounding electrodesystem.So the equipment ground establishes the localsignal reference both within the CNC machine and forany remotely connected devices.Fig.4.AC power sources functional blocks andinterconnections for a typical CNC MachineCNC machine upsets that are related to grounding andground reference are usually attributed to noise orstray currents that find ways into logic circuits.Susceptibility of CNC machines to different sources ofnoise and stray currents will depend on how themachine is designed and installed.Grounding,bonding,and shielding also play a role in its immunity.For example,certain types of communication circuitsare more susceptible to high frequency noise;othersare more susceptible to stray ground currents,whereasothers(fiber optic)are not susceptible to either.IV.NOISE SOURCES AND COUPLING MODESMuch of the focus of special groundingrecommendations for electronic equipment is based onreducing electrical noise,or any unwanted signal thatmay affect logic circuits or signal.Sources of electricalnoise may be nearby transmittersboth intentionaland unintentionallike electric welding arcs orchattering relay contacts.Also,lightning or switchingsurges may capacitively-couple noise-voltages intosensitive control circuits.A common wiring-relatednoise source is simply the normal electric currents thatinductively couple noise currents into control wires ifcontrol and power wiring are run in the same racewayor run close to each other inside the CNC machine.Internal to the CNC machine,contactors,relays,solenoids,and motor drives are a large source of noiseand transients,which may also affect the logiccircuits3.CNC machines that are contained in asingle cabinet with good bonding,grounding andshielding should be relatively immune to noise.However CNC machines with remote controllers ordata links may be quite susceptible.In these machinestwo forms of this ground-related noise should beconsidered.See figure 4.One form is the noiseappearing on the input power conductors relative to theequipment ground(typically the enclosure or chassisground).The other is the noise or difference inpotential that appears between the grounds ofinterconnected equipment.When trying to avoidelectrical noise,distancing from the source of noise isusually a great help.Some grounding practices canalso help to control the noise relative to ground,butwatch out for myths about ground noise problems.Remember that electrical noise or unwanted signalswill follow the fundamental principles of electricity,i.e.currents flow only when there is a difference inpotential and a completed circuit,and they followpaths of least resistance(reactive impedance in thecase of high-frequency signals)3.V.THE GROUND ROD PROBLEMTo combat the problem of unwanted electrical noise onthe equipment grounding system,many CNC machinetool builders require or recommend that a ground rodbe driven into the earth at each CNC machine andconnected to the CNC machines frame via the groundplate.Fig.5.shows a typical installation.Some CNCmachine tool builders will void the warranty if theground rod is disconnected.Many CNC machine toolbuilders feel that the buildings equipment groundingsystem is“noisy”because of nearby equipment in thefacility such as welders,wire EDM machines,andmotor drives.The supposed purpose of the ground rodis to carry away or eliminate this unwanted electricalnoise from the CNC machines data signals.Apparently,these CNC machine tool builders believethe ground rod provides a low impedance path for thisnoise to flow.The recommended impedance of thispath ranges from no more than 5 ohms to no more than100 ohms.These CNC machine tool builders feel thatthe buildings equipment grounding system does notprovide a reliable,low impedance path to earth.CNCmachine tool builders that have recommendedinstalling a ground rod to eliminate unexplainedoperating problems report that the problems go awaymore often than not.In addition,some feel that theground rod provides lightning protection.On thecontrary,other CNC machine tool builders do notrecommend a ground rod at the CNC machine becauseof lightning problems45.Fig.5.Supplemental ground rod properly installedmay invite stray ground currentsObviously,CNC machine tool builders and servicetechnicians recommend many confusing andconflicting grounding techniques.Quite often,thesetechniques,implemented to help correct operatingproblems believed to be caused by electrical noise,canactually lead to more severe problems such as damageduring lightning storms.Of particular concern is thepractice of driving a ground rod directly at the CNCmachine and connecting that ground rod to the CNCmachines ground plate.The presence of this groundrod creates a ground loop with the earth.The groundloop is a conductive path for current to flow betweenthe grounding electrode system at the main serviceswitch through the conduit/equipment groundingconductor to the CNC machines ground plate,thenthrough the ground rod at the CNC machine and backto the grounding electrode system through the earth.Current will flow in this path when a voltagedifference develops between the two earthconnections.Current flowing through the earth,suchas that during a lightning strike or utility ground fault,can create an extremely large voltage difference.The presence of the ground rod also creates a path toearth,which allows lightning current and utility faultcurrent to travel on the conduit/equipment groundingconductor.When lightning strikes a facility,thelightning current flows into the earth at the mainservice switch(grounding electrode system)and alsotravels along the conduit/equipment groundingconductor through the CNC machine and into theground rod driven in the earth4.Fig.6.shows how utility fault current flowing throughthe earth can flow up the CNC machines ground rod,travels along the conduit/equipment groundingconductor,onto the utility neutral and back to thesource(the