ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 45-2012 MOTORS MOTOR CONTROLS AND VARIABLE-SPEED DRIVES.pdf

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1、45.1CHAPTER 45MOTORS, MOTOR CONTROLS, AND VARIABLE-SPEED DRIVESMOTORS 45.1Alternating-Current Power Supply 45.1Codes and Standards . 45.2Motor Efficiency. 45.2General-Purpose Induction Motors 45.3Hermetic Motors 45.4Integral Thermal Protection. 45.5Motor Protection and Control 45.6AIR VOLUME CONTROL

2、 45.11Variable-Speed Drives (VSD). 45.12Power Distribution System Effects . 45.15MOTORSANY TYPES of alternating-current (ac) motors are available;M direct-current (dc) motors are also used, but to a more limit-ed degree. National Electrical Manufacturers Association (NEMA)Standard MG 1 provides tech

3、nical information on all types of ac anddc motors.ALTERNATING-CURRENT POWER SUPPLYImportant characteristics of an ac power supply include (1) volt-age, (2) number of phases, (3) frequency, (4) voltage regulation, and(5) continuity of power.According to AHRI Standard 110, the nominal system voltageis

4、 the value assigned to the circuit or system to designate its voltageclass. The voltage at the connection between supplier and user is theservice voltage. Utilization voltage is the voltage at the line termi-nals of the equipment. Utilization voltages are about 5% lower thantheir corresponding nomin

5、al voltages, to allow for distribution sys-tem impedance.Single- and three-phase motor and control voltage ratings shownin Table 1 are adapted to the nominal voltages indicated. Motorswith these ratings are considered suitable for ordinary use on theircorresponding systems; for example, a 230 V moto

6、r should gener-ally be used on a nominal 240 V system. A 230 V motor should notbe installed on a nominal 208 V system because the utilization volt-age is below the tolerance on the voltage rating for which the motoris designed. Such operation generally results in overheating and aserious reduction i

7、n torque. Single- and three-phase 200 V motorsare designed for nominal 208 V systems. Three-phase models up toat least 100 hp are available in NEMA Premiumefficiencies.Motors are usually guaranteed to operate satisfactorily and todeliver their full power at the rated frequency and at a voltage 10%ab

8、ove or below their rating, or at the rated voltage and plus or minus5% frequency variation. Some U.S. single-phase HVAC com-ponents that are dual-voltage rated (e.g., 208/230-1-60) may carry a5% voltage allowance (at rated frequency) from the lower voltagerating of 208 volts. Table 2 shows the effec

9、t of voltage and fre-quency variation on induction motor characteristics.Phase voltages of three-phase motors should be balanced. If not,a small voltage imbalance can cause a large current imbalance.This leads to high motor operating temperatures that can result innuisance overload trips or motor fa

10、ilures and burnouts. Motorsshould not be operated where the voltage imbalance is greater than1%. If an imbalance does exist, contact the motor manufacturer forrecommendations. Voltage imbalance is defined in NEMA Stan-dard MG 1 as% Voltage imbalance = 100 The preparation of this chapter is assigned

11、to TC 1.11, Electric Motors andMotor Control.Table 1 Motor and Motor Control Equipment Voltages (Alternating Current)System Nominal VoltageU.S. Domestic Equipment Nameplate Voltage Ratings(60 Hz)Integral Horsepower Fractional HorsepowerThree-Phase Single-Phase Three-Phase Single-Phase120 115 115208

12、208/230 or 200/230208/230 or 200/230208/230 or 200/230208/230 or 200/230240 208/230 or 200/230208/230 or 200/230208/230 or 200/230208/230 or 200/230277 265 265480 460 460 600* 575 575 2,400 2,300 4,160 4,000 4,800 4,600 6,900 6,600 13,800 13,200 *Some control and protective equipment has maximum vol

13、tage limit of 600 V. Consultmanufacturer, power supplier, or both to ensure proper application.System Nominal VoltageInternational Equipment Nameplate Voltage Ratings 50 Hz 60 HzThree-Phase Single-Phase Three-Phase Single-Phase127 127 127200 220/200 200 230/208 or 230/200220 220/240 220/240 or 230/2

14、08230/208 or 230/200230/208230 230/208 220/240 or 230/208230/208 or 230/200230/208240 230/208 220/240 230/208 230/208250 250 380 380/415 460/380 400 380/415 415 380/415 440 440 460 480 500 Note: Primary operating voltage for a dual-voltage rating is usually listed first (e.g.,220 is primary for a 22

15、0/240 volt rating).Maximum voltage deviationfrom average voltageAverage voltage-45.2 2012 ASHRAE HandbookHVAC Systems and Equipment In addition to voltage imbalance, current imbalance can be pres-ent in a system where Y-Y transformers without tertiary windingsare used, even if the voltage is in bala

16、nce. Again, this current imbal-ance is not desirable. If current imbalance exceeds either 10% or themaximum imbalance recommended by the manufacturer, correctiveaction should be taken (see NFPA Standard 70).% Current imbalance = 100 Another cause of current imbalance is normal winding imped-ance imb

17、alance, which adds or subtracts from the current imbalancecaused by voltage imbalance.CODES AND STANDARDSThe National Electrical Code(NEC) (NFPA Standard 70) andCanadian Electrical Code, Part I (CSA Standard C22.1) are impor-tant in the United States and Canada. The NEC contains minimumrecommendatio

18、ns considered necessary to ensure safety of electri-cal installations and equipment. It is referred to in the OccupationalSafety and Health Administration (OSHA 2007) electrical stan-dards and, therefore, is part of OSHA requirements. In addition,practically all communities in the United States have

19、 adopted theNEC as a minimum electrical code.Underwriters Laboratories (UL) promulgates standards for vari-ous types of equipment. UL standards for electrical equipmentcover construction and performance for the safety of such equip-ment and interpret requirements to ensure compliance with theintent

20、of the NEC. A complete list of available standards may beobtained from UL, which also publishes lists of equipment thatcomply with their standards. Listed products bear the UL label andare recognized by local authorities.The Canadian Electrical Code, Part I, is a standard of the Cana-dian Standards

21、Association (CSA). It is a voluntary code with min-imum requirements for electrical installations in buildings of everykind. The Canadian Electrical Code, Part II, contains specificationsfor construction and performance of electrical equipment, in com-pliance with Part I. UL and CSA standards for el

22、ectrical equipmentare similar, so equipment designed to meet the requirements of onecode may also meet the requirements of the other. However, agree-ment between the codes is not complete, so individual standardsmust be checked when designing equipment for use in both coun-tries. The CSA examines an

23、d tests material and equipment for com-pliance with the Canadian Electrical Code. MOTOR EFFICIENCYSome of the many factors that affect motor efficiency include(1) sizing the motor to the load, (2) type of motor specified,(3) motor design speed, (4) number of rewinds, (5) voltage imbal-ance, (6) curr

24、ent imbalance, and (7) type of bearing specified. Over-sizing a motor may reduce efficiency. As shown in the performancecharacteristic curves for single-phase motors in Figures 1, 2, and 3,efficiency usually falls off rapidly at loads lower than the rated fullload. Three-phase motors usually reach p

25、eak efficiency around 75%load, and the efficiency curve is usually fairly flat from 50 to 100%(Figure 4). Motor performance curves (available from the motormanufacturer) can help in specifying the optimum motor for an ap-plication. The U.S. Department of Energys (DOE) MotorMaster+software gives part

26、-load efficiency as well as efficiency at ratedload. Larger-output motors tend to be more efficient than smallerTable 2 Effect of Voltage and Frequency Variation on Induction Motor CharacteristicsVoltage and Frequency VariationStarting and Maximum Running TorqueSynchronous Speed % SlipFull-Load Spee

27、dEfficiencyFull Load 0.75 Load 0.5 LoadVoltage variation120% Voltage Increase 44% No changeDecrease30%Increase1.5%SmallincreaseDecrease0.5 to 2%Decrease7 to 20%110% Voltage Increase 21% No changeDecrease17%Increase1%Increase0.5 to 1%Practicallyno changeDecrease1 to 2%Function of voltage Voltage2Cons

28、tant 1/Voltage2Synchronous speed slip90% Voltage Decrease 19% No changeIncrease23%Decrease1.5%Decrease2%Practicallyno changeIncrease1 to 2%Frequency variation105% Frequency Decrease 10%Increase5%Practicallyno changeIncrease5%SlightincreaseSlightincreaseSlightincreaseFunction of frequency 1/Frequency

29、2Frequency Synchronous speed slip95% Frequency Increase 11%Decrease5%Practicallyno changeDecrease5%SlightdecreaseSlightdecreaseSlightdecreaseVoltage and Frequency VariationPower FactorFull-Load CurrentStarting CurrentTemperature Rise,Full LoadMaximum Overload CapacityMagnetic Noises,No Load in Parti

30、cularFull Load 0.75 Load 0.5 LoadVoltage variation120% VoltageDecrease5 to 15%Decrease10 to 30%Decrease15 to 40%Decrease11%Increase25%Decrease9 to 11FIncrease44%Noticeable increase110% VoltageDecrease3%Decrease4%Decrease5 to 6%Decrease7%Increase10 to 12%Decrease5 to 7FIncrease21%IncreaseslightlyFunc

31、tion of voltage Voltage Voltage290% VoltageIncrease3%Increase2 to 3%Increase4 to 5%Increase11%Decrease10 to 12%Increase11 to 13FDecrease19%DecreaseslightlyFrequency variation105% FrequencySlightincreaseSlightincreaseSlightincreaseDecreaseslightlyDecrease5 to 6%DecreaseslightlyDecreaseslightlyDecreas

32、eslightlyFunction of frequency 1/Frequency 95% FrequencySlightdecreaseSlightdecreaseSlightdecreaseIncreaseslightlyIncrease5 to 6%IncreaseslightlyIncreaseslightlyIncreaseslightlyNote: Variations are general and differ for specific ratings.Maximum current deviationfrom average currentAverage current-M

33、otors, Motor Controls, and Variable-Speed Drives 45.3motors at the same percentage load. Four-pole induction motorstend to have the highest range of efficiencies It is important to understand motor types before specifying one.For example, a permanent split-capacitor motor is more efficientthan a sha

34、ded-pole fan motor. A capacitor-start/capacitor-runmotor is more efficient than either a capacitor-start or a split-phasemotor. Three-phase motors are much more likely to have publishedefficiency: NEMA and the DOE promulgate efficiency standardsfor three-phase motors between 1 and 500 hp.Motor manuf

35、acturers offer motors over a range of efficiencies.NEMA Standard MG 1 describes two efficiency categories: energy-efficient and premium. These standards pertain to most three-phaseinduction motors between 1 and 500 hp. Note that “energy-efficient”no longer represents a remarkable level of efficiency

36、; it was madea mandatory minimum for general-purpose induction motors from1 to 200 hp in the United States by the Energy Policy Act of 1992.Today, it has been significantly exceeded by the NEMA premiumstandard.Higher-efficiency motors are available in standard frame sizesand performance ratings. Pre

37、mium-rated motors are more costlythan less efficient counterparts, but the additional costs are usuallyrecovered by energy savings very early in the motors service life;most manufacturers also cite extra reliability features added intopremium-rated motors. NEMA Standards MG 10 and MG 11 havemore inf

38、ormation on motor efficiency for single-phase and three-phase motors, respectively.GENERAL-PURPOSE INDUCTION MOTORSThe electrical industry classifies motors as small kilowatt (frac-tional horsepower) or integral kilowatt (integral horsepower).Fig. 1 Typical Performance Characteristics of Capacitor-S

39、tart/Induction-Run Two-Pole General-Purpose Motor, 1 hpFig. 2 Typical Performance Characteristics of Resistance-Start Split-Phase Two-Pole Hermetic Motor, 0.25 hpFig. 3 Typical Performance Characteristics of Permanent Split-Capacitor Two-Pole Motor, 1 hp45.4 2012 ASHRAE HandbookHVAC Systems and Equi

40、pment In this context, kilowatt refers to power output of the motor. Small-kilowatt motors have ratings of less than 1 hp at 1700 to 1800 rpmfor four-pole and 3500 to 3600 rpm for two-pole machines. Single-phase motors are readily available through 5 hp and are most com-mon through 0.75 hp, because

41、motors larger than 0.75 hp are usuallythree phase.Table 3 lists motors by types indicating the normal power rangeand type of power supply. All motors listed are suitable for eitherdirect or belt drive, except shaded-pole motors (limited by low start-ing torque).ApplicationWhen applying an electric m

42、otor, the following characteristics areimportant: (1) mechanical arrangement, including position of themotor and shaft, type of bearing, portability desired, drive connec-tion, mounting, and space limitations; (2) speed range desired; (3)power requirement; (4) torque; (5) inertia; (6) frequency of s

43、tarting;and (7) ventilation requirements. Motor characteristics that are fre-quently applied are generally presented in curves (see Figures 1 to 4).Torque. The torque required to operate the driven machine at alltimes between initial breakaway and final shutdown is important indetermining the type o

44、f motor. The torque available at zero speed orstandstill (starting torque) may be less than 100% or as high as400% of full-load torque, depending on motor design. The startingcurrent, or locked-rotor current, is usually 400 to 600% of thecurrent at rated full load.Full-load torque is the torque deve

45、loped to produce the ratedpower at the rated speed. Full-load speed also depends on themotors design. For induction motors, a speed of 1750 rpm is typicalfor four-pole motors, and a speed of 3450 rpm is typical for two-pole motors at 60 Hz.Motors have a maximum or breakdown torque, which cannotbe ex

46、ceeded. The relation between breakdown torque and full-loadtorque varies widely, depending on motor design.Power. The power delivered by a motor is a product of its torqueand speed. Because a given motor delivers increasing power up tomaximum torque, a basis for power rating is needed. NEMA basespow

47、er rating on breakdown torque limits for single-phase motors,10 hp and less. All others are rated at their power capacity withinvoltage and temperature limits as listed by NEMA.Full-load rating is based on the maximum winding temperature.If the nameplate marking includes the maximum ambient tempera-

48、ture for which the motor is designed and the insulation designation,the maximum temperature rise of the winding may be determinedfrom the appropriate section of NEMA Standard MG 1.Service Factor. This factor is the maximum overload that can beapplied to general-purpose motors and certain definite-pu

49、rpose mo-tors without exceeding the temperature limitation of the insulation.When the voltage and frequency are maintained at the values speci-fied on the nameplate and the ambient temperature does not exceed104F, the motor may be loaded to the power obtained by multiply-ing the rated power by the service factor shown on the nameplate.Operating a motor c