Theselection of a suitable starting method for a motor/load combination isbecoming increasingly important as additional loads are added to existingutility and distribution systems. In the past, ample capacities were availablein the utility networks to support the needs of industry. As well, thedistribution systems of most North American industries were established duringthe industrial and technological boom years preceding the Second World War. Inthe 1990’s, the viability for the construction of new electrical generationplants became prohibitive. With lower North American utility generatingcapacity and increased risks involved in the de-regulation of this utility,stability and voltage drop concerns have become very real issues that face allelectrical power consumers.
Problem
Fullvoltage starting of motors can produce objectionable voltage flicker,mechanical stress to gear boxes or belt drive systems and create pressure surgesor water hammer in pumping applications. Starting a motor at reduced voltagecan help reduce or overcome these problems. If the load cannot be acceleratedto full speed using full voltage and current, it cannot be accelerated to fullspeed using reduced voltage and current.
ApplicationSolution
Thereare several factors to be considered when selecting the starting equipment forany electric motor driven load. These include, but are not limited to:
1.The source of power and the effects the motor starting currents will have onthe source and the stability of the system voltage
2.The starting and breakdown torque characteristics of the motor (motor speedtorque characteristics)
3.The motor starting characteristics (torque) that correspond to the motor bestsuited to the load
characteristicsat full load and speed
4.The starting characteristics of: motor torque load torque, accelerating loadtorque (load inertia) the motor starting, accelerating and running torque onthe driven load
5.The available short circuit capacity of the distribution system
6.The operating speed range of the connected load.
7.Process considerations: shock, vibration, mechanical hammer, the control andmaintenance of different starting methods.
Theinitial inrush currents, locked rotor currents and the resulting torque valuesproduced, are the factors that determine whether the motor can be applieddirectly across the line, or whether the current has to be reduced to get therequired performance to match the load requirements and/or utility line voltageflicker or voltage dip specifications.
Fullvoltage starting can be used whenever the driven load can withstand the shockof instantaneously applying full voltage to the motor and where linedisturbances can be tolerated. Full voltage starting uses a main contactor toapply the motor stator windings directly across the main system voltage. Thistype of starting method provides the lowest cost, a basic and simple design ofcontroller, resulting in low maintenance and the highest starting torque.
Reducedvoltage starting may be required if full voltage starting creates objectionableline disturbances on the distribution system or where reduction of mechanicalstress to gear boxes or belt drive systems is required. It must be noted thatwhen the starting torque will decrease proportional to the percent squared ofvoltage applied (i.e. 50% voltage produces 25% torque =0.50 squared). Thisphenomenon also occurs in the opposite manner when the voltage is increased.
Thereare three main reasons to apply reduced voltage to medium voltage motors:
1.To reduce the mechanical effect of across the starting and stopping
2.To limit the inrush current inherent with full voltage starting
3.To reduce the effects of pressure surges and water hammer in pumping systems.
MechanicalShock
Thisreason for applying reduced voltage has various different names; it can becalled mechanical shock, mechanical stress, or various other names. The effectis the same. When a medium voltage motor is started at full voltage the torquebeing applied from the motor to the driven load rises to a very high valuealmost immediately. This can cause damage to the bearings in the motor or theload, the rotor of the motor or to the mechanical coupling method whichconnects the motor to the load. The load itself can sustain damage depending onwhat the application may be. In the case of conveyor application if the load isstarted to quickly the belts of the conveyor can be stretched or broken. If themotor is connected to the load via chains or belts these coupling means can bedamaged as well by sudden starting techniques.
UtilityRestrictions
Asutility power systems continue to be run at maximum capacity the effect ofstarting medium voltage motors across the line can put stress on the factory’spower distribution system. The lights go dim, process control systems can failor trip out or you may be restricted as to when and how often you are allowedto start the motor.
LoadRelated Reasons
Bysoft starting the load you may see improvements in the way the equipmentperforms. For example, when a soft starter is applied to an agitator by slowingramping up the speed of the motor the material being agitated tends to splashless and causes fewer problems than when started across the line. In the caseof mill applications the material will start to move slower than when startedacross the line and cause less wear on the driven load. The ways in which asoft start can improve system performance are only limited to the number ofapplications that the end user can think of. New uses are being thought of andapplied all the time.
TorqueRequirements
Itis important to reiterate that when the voltage is reduced when starting amotor, so are the current and torque values. It should be apparent that a motorthat will not start a load at full voltage, it will not start that same loadunder reduced voltage conditions. This conflict between torque and currentrequirements of induction motors is one typical dilemma facing the user ofreduced voltage starting equipment. It may be only one of several problems butis the most common and most important.
Reducedvoltage starting can be accomplished in several different ways.
ReactorStarting
Thismethod also reduces the voltage, current and torque to the motor according tothe reactor tap setting. It is possible to reduce the motor terminal voltage asrequired by placing a primary reactor in series with the motor windings, for aperiod during starting. The use of a reactor during starting results in anexceptionally low starting power factor. Reactors must be carefully designedand applied since any saturation in the reactor will produce in-rush currentsclose to those seen during full voltage starting. Reactor starting has onemajor advantage; the voltage to the motor is a function of the current takenfrom the line. It can therefore be assumed that during acceleration the motorvoltage will rise as the line current drops. This relationship results ingreater accelerating energy at higher speeds and less severe disturbancesduring the transition to full voltage.
AutotransformerStarting
Autotransformerstarting automatically switching between taps of an autotransformer reduces thevoltage, current and torque to the motor according to the tap setting used onthe autotransformer. There are two very distinctive characteristics of anautotransformer starter.
1.The motor terminal voltage is not a function of load current and remainsconstant during the
accelerationtime.
2.Due to the turn ratio advantages, the primary line current is less than thesecondary motor currents. A three-coil autotransformer is connected in a wyeconfiguration and connected to the motor in such a way as to supply reduced voltageto the motor when the line voltage is applied to the Autotransformer. Severalsets of taps are usually available to the user to provide different values ofreduced voltage (NEMA standards are 80%, 65% and 50% of the full line voltage).
SolidState Reduced Voltage Starting
Theuse of solid state Reduced Voltage Starting can provide a smooth steplessmethod of accelerating and smoothly decelerating a squirrel cage inductionmotor. This type of starting method, when properly applied can provide an efficientand reliable means of smoothly starting and stopping a motor and load. The useof solid-state reduced voltage starting will perform, in most cases, moreefficiently than field coupling, eddy current drives and clutches. The steplessramped acceleration and deceleration capabilities of these types of starterreduces the inrush currents to the motor, eliminating transitional shocks tothe load and reducing voltage flicker on the distribution system.
Selectionof Appropriate Starting System
Theselection of an appropriate starting system requires the reviewer to compare orweigh the importance of several factors.
Costand Economics :
Whendetermining the starting method, the economics of the decision can also provideimportant tips to the selection of an appropriate controlling means.
Maintainability :
Thecapabilities of the mechanical and electrical support facilities and personnelcan have an important impact on the starting method determination.
RemoteControl Requirements :
Asbusinesses become increasingly more competitive, the reductions in the area ofpersonnel related to the control and operation of industrial processes arebecoming dramatic. This required reduction of personnel has subsequentlyhastened the development of the remote control capabilities of modern motorcontrol equipment. In many cases, production flow and efficiency rateadjustments can be critical to the quality of the end product. For example inthe case of remote pumping stations, the ability to control the speed of abooster pump, based on the product in the line at the time, can allow a remotelocation to monitor and adjust flow rates to maximize the capacity of thepipeline.
ProcessControl Requirements :
Intoday’s competitive environment, industry is endeavoring to continually improveprocesses to produce high quality products, at accelerated periods, at theleast possible cost. If the process requires variation of speeds, a controllerthat will vary the motor speed would be appropriate. If depressed distributionvoltage, during motor start cycles, is an issue, a solid state or other reducedvoltage starting method may suffice.
Physicalsize restraints :
Thelimitations of available physical floor space could be a major concern whenretrofitting new equipment into an existing control area. The physicalconstruction of equipment housing the newer technology may not be suitableincorporated within existing facilities. Careful consideration may be requiredfor the removal of heat generated by semi-conducting devices, printed circuitcards, transformers and other electronic control devices. Cooling systems maybe needed to aid in the maintenance of a suitable environment for these typesof equipment.
Easeof use :
Oneconsideration that is sometimes overlooked is the capabilities of the personnelrequired to service and maintain the equipment. Simpler more traditionalstarting means may be suitable where basic personnel training levels aremaintained. The simpler relay control logic systems may be adequate for thesystem configuration required for the specific process.
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