ASHRAE JOURN SUPP SEPT-1997 ASHRAE Journal (Supplement September 1997).pdf

1、 Center. And while we have no control over human communications, we have total control over the thousands of field devices and subsystems. The key is a Metasys“ Facility Manage- ment System, which has provided total building system integration. Metasys fearures true open-system architecture, and the

2、 result is complete, centralized control over chillers, eie- vators, CW, lighting, fire, security, carbon monoxide monitoring, emergency power and other subsystems. One integrated solution, covering the entire six million sq. ft. facility. With Metasys, theyre getting a more comfortable, pro- ductiv

3、e and safer facility. Plus one that attracts and retains qdty tenants. Proof positive. While others talk open systems, Metasys deiiv- ers. For a case study, call 1-800-972-8040, ext. 370. Old or new, large or small, now or in the future, building control is truly moving beyond connectivity. Honeywel

4、l is leading the industry toward combining the two best standards available to create an open environment; BACnet, for communicating between different vendorsL build- ing control systems, and LONMARK“, for open communica- tion at the distributed control level. A truly open system environment gives y

5、ou the freedom to choose the best products, applications, services - and best value. Developing our solutions on BACnet and LONMARK standards gives you the flexibility to easily change or expand as your needs evolve. With a history of leadership and proven performance in pneumatic, electronic and di

6、gital building control, Honeywell is now guiding the industry beyond con- nectivity toward open solutions. headed. Talk to the company thats leading the way. Comfort from Experience“. offer you. Cali 1-800-345-4770 ext. 938. Take a look at where the future of building control is Find out more about

7、the unique solutions HoneyweIl can Honeywell O 1996 Honeywell Helping You Control Your World (Circle No. 6 on Reader Service Card) . STD-ASHRAE JOURN SUPP SEPT 1777-ENGL 1777 = 0757b50 053238b 771 = TA BLE OF CONTENTS 14 18 22 26 35 40 44 47 50 54 56 Types of Control By Gene R. Strehlow, P.E., Membe

8、r ASHRAE, and Christopher R. Amundson, P.E. Member ASHRAE Power for Electric/ Electronic Control Systems By Jim Vick Understanding Controllers and Control Termina By Ryan Rosandich, Ph.D. Actuator Sizing By Christopher R. Amundson, P.E., Member ASHRAE Understanding the Control Loop By Gideon Shavit,

9、 Ph.D., Fellow ASHRAE, and Richard A. Wruck, Member ASHRAE Dynamic Response and Tuning By Douglas C. Hittle, Ph.D., Fellow ASHRAE Air Supply Systems for HVACion Engineer for Electro-Mechanical Products at the Home md Building Control division of Honeywell in Arlington Heights, IL. Power for Electrid

10、Electronic Control Systems lim Vick is a regional manager for Kele and Associates in Memphis, Tenn. He has a degree in electrical engineering from the University of Memphis and experience in the indus- trial and utility industry. Vick has been involved in product development and sales of building au

11、tomation products at Kele. Refrigeration Control Devices Leroy Manor, Member ASHRAE, is district sales manager for Sporlan Valve Company. A graduate of University of Mis- souri-Fblla (BSME), Manor has been a member of ASHAE since 1967 and served as president of the Boston Chapter in 1978- 1979. He h

12、as been a guest speaker at every ASHRAE chapter in New England. He also is a member of RSES and received the RSES international “Speaker of the Year” award in 1994. Controlling Variable-Volume Systems Doug Hittle, Ph.D., Fellow ASHRAE, is professor and director of the Solar Energy Applications Labor

13、atory at Colo- rado State University. Hittle has consulted on many control applications projects and published many technical papers. He and Roger Haines Co-authored the book, Control Systems for Heating, Ventilating and Air Conditioning (Chapman and Hall 1993), which was the source for much of this

14、 article. BACnet” Q- Both DataPad“ This year, well spend more on research and development than the total sales of most competitors. For you that means innovative products precisely matched to your new applications. Not to mention improved reliability, better performance and greater value. Trained to

15、 support you before and after the sale, well conduct site and energy cost analyses. Assist in system planning and design. Supervise installations and start-ups. Even provide on-site education. Its no wonder so many Fortune 1000 companies choose Liebert. For a free “Heat Energy Calculator,“ call *J-3

16、!J94YY-XLZ, Youll also receive brochures cm ow complete range of products and services. ebsb at .Jiri And see why so and Little Glass and modular protection that “computer room“ %o th+ new Liebert. not an overly large response. In electronic controls the most common amplifier device is call an opera

17、tional amplifier or Op-Amp for short (Figure 6). It is a prepackaged, transistorized elec- tronic circuit that boosts any voltage con- nected to it by a fixed multiplier. It has both a positive and a negative input as shown in the figure. If the Op-Amp had a 100: 1 amplification ratio, a O. 1 volt v

18、oltage fed into the positive (+) terminal would raise the output by 1 O volts. Because this may be too great a response to a small input change, a sensitivity adjustment must be added. This is also done with a variable resistor, R7, connected between the output and the negative (-) input. This becom

19、es an adjustable amount of negative feedback that will cancel out a preset amount of the Op-Amps amplification. In this way the controller is made adjustable in the size of its proportional response to a change in its input value. If this were a proportional electronic con- troller for a lm centrifu

20、gal chiller, its gain (amplification) might be set up to signal the inlet vane actuator for 50% capacity at a leaving chilled water setpoint of 45“E If chilled water dropped to 42“E the control- ler would request 10% capacity, while at 48F it would request 100% capacity. This 6F throttling range typ

21、ically produces sta- ble operation, whereas trying to tighten it to 2F (via higher gain) could produce rapid cycling of the chiller and possible shutdown by one of its safety functions. Some controllers have used separate Op-Amps for the positive comparator error outputs above setpoint and negative

22、error signals below setpoint. The Op-Amp on the negative signal is connected in the opposite orientation. Separate gain adjust- ments are then possible for controller out- puts that may control heating when below setpoint and cooling when above setpoint. Other electronic controllers bias their out-

23、put upward to be at a certain voltage when they are at setpoint (balanced) and then vary above and below this biased value as they perform their control functions. All of this bridge comparator and Op- Amp methodology deals with direct current (DC) voltage. While the basic voltage divider works the

24、same whether alternating current (AC) or direct current is applied to it, the rest of the circuit requires the relative values of positive and negative voltages. Because AC continually varies from posi- tive to negative 60 times per second, there are no positive or negative relative values. Some old

25、er electronic controls used AC bridges, but these have several unique issues to account for (capacitance, phase shifts, transformers, etc.), which have caused them to fall out of common use. Electromechanical (135 ohm slidewire) This type of control uses a variable-cur- rent flow to achieve a balanc

26、e between a 16 ASHRAE Journal September 1997 STD*ASHRAE JOURN SUPP SEPT L777-ENGL 1997 0757b50 0532378 973 Gain Adjustment R7 1 Output Input Op - Amp Figure 6: Operational amplifier (op-amp). controllers request and an actuators posi- tion, Low resistance is used, because these controls use a larger

27、 current flow than todays electronic controls. The comparator is usually a mechanical setpoint spring opposing a sensor-generated force. (temper- ature, humidity, flow, or pressure), which proportions the amount of its 135 ohms between RI and 2 by moving a slider along a 135 ohm coil of resistance w

28、ire. A similar variable-sliding resistor in the actua- tor represents its position via the ratio of its 135 ohms divided between R3 and R4. A magnetically activated balance relay between the comparator and the actuator then carries out the controllers function of deciding which direction and for how

29、 long to move the actuator (Figure 7). If a hot water temperature sensor causes the com- parator to lower the RI resistance, the total of RI + FU will be less than R2 + R4, and the resulting current flow through Coil I will be greater than that through Coil 2. Coil 1s greater current will create mor

30、e magnetism than that of the opposing Coil 2, and the movable armature will close the counterclockwise contact until the actuator feedback of a hot water mixing valve causes the new RI + R3 to equal the new R2 + R4. When the currents in the two circuits are in balance, the actuator is not directed S

31、etpoint I R1 135 OHM R2 L I Figure 8: Bleed-type thermostat. September 1997 -+ It1 I Power :ONTROLLED DEVICE Figure 7: Balancing relay controller. to move until such time as the comparator again reproportions its 135 ohm resistance. If this actuator/valve position is not quite right, the sensor and

32、comparator will detect the temperature error, move the 135 ohm potentiometer, and reposition the actuator to a new position. The magnetic relay is typically located within the actuator, so with this type of control the “stat” and actu- ator must both be of this type. Pneumatic Controls Pneumatic con

33、trols, although not as widely used as in the past, are still a popu- lar method of control in certain applica- tions. When large valves require high close-off pressures, pneumatic actuators are often selected because they can pro- vide significantly higher close offs at a much lower cost than electr

34、ic actuators. piped to the controller at a constant pres- sure, usually between 15 to 25 psig. This In all pneumatic controllers, supply air is FEEDBACK supply flow provides both volume, to fill large areas within the controlled devices and connecting pipes, and pressure, which provides the force to

35、 do the required work There are two basic types of pneumatic controllers: low-volume bleed and high- volume relay type. The bleed-type control- ler consists of the sensing element, setpoint dial, sensitivity slider, control port, and lid. The low-volume controller requires a restricted air supply to

36、 limit the supply air capacity to the controller and controlled device. This bleed-type controller will bleed the restricted supply pressure to the pressure required by the control device to satisfy the controller setpoint. This is achieved through movement of the sensing element that is transmitted

37、 to the dia- phragm though the post and levers, and finally to the lid and control port. See Strehlow, Page 21 SUPPLY AIR PILOT CIRCUIT - LOW VOLUME I OUTPUT I VOLUME AMPLIFIER - HIQH VOLUME Figure 9: Pneumatic relay. ASHRAE Journal 17 STD-ASHRAE JOURN SUPP SEPT 1777-ENGL 1997 W 0759650 0532377 32T

38、W 105C (221 OF) 95C (203F) 40C 11111.1 (1 04F) POWER FOR ELECTRIC/ ELECTRONIC CONTROL SYSTEMS 10C (1 8F) COIL HOT SPOT t 4 55C (99F) AVERAGE COIL RISE 111.11.11111.1 40C (1 04F) Maximum Ambient by Jim Vick dequately powering a control circuit is an important consideration for building systems. Trans

39、formers, power supplies, or wiring sized incorrectly can lead to system problems and premature failure of components. The power requirements for a control system can either be AC or DC. Typical building voltages may be too high (e.g. 480 VAC) to safely use for control, so transformers are used to lo

40、wer the voltage to a safer level (24 VAC or 120 VAC). Power sup- plies are used to convert AC voltage to DC for use typically in analog signal applications such as 4-20 mA or 0- 1 O VDC. Sizing Transformers A transformer changes AC voltage level by magnetic induc- tion. A transformer has two coils w

41、rapped around a laminated steel core. When voltage is applied to the primary coil, the core is magnetized, inducing a voltage into the secondary coil. The ratio of the number of turns of each coil determines the voltage change. Control transformers are specially designed for control circuits and the

42、 momentary current inrush associated with relay and contactor coils that are the normal loads. Five values are needed to properly select a transformer. They 1. Primary voltage available. 2. Secondary voltage required. 3. Sealed loads in VA (Volt-Amperes to hold-in the devices). 4. Inrush loads (VA r

43、equired to start the devices). 5. Ambient temperature (important if mounted outdoors). NEMA standards require relays and contactors to operate at 85% of their rated coil voltage. That means a 120 VAC contactor must pull-in with as littie as 102 VAC applied to it. A transformer sized too small will n

44、ot deliver the 85% voltage level when every- thing turns on. There is also a possibility that the transformer will run hot, which will shorten its life. A low secondary voltage can also cause relay or contactor contacts to chatter and can bum out coils. are: Here are the steps that are required to s

45、elect the proper trans- former: Step 1 : Total the sealed (steady-state) VAS of the maximum number of devices that will be turned on by the secondary of the transformer at a given time. The product data sheets should show this value. If only the current is known, then multiply this by the voltage to

46、 get the VA. Step 2: Total the inrush VA of the maximum number of devices that will be simultaneously turned on. Most coils, sole- noids etc. will have a published inrush value, but on some devices such as small relays and pilot lights, the inrush will be the same as the sealed VA. Step 3: This step

47、 will vary with transformer manufacturer. Some manufacturers select the transformer size based on a combi- nation of sealed and inrush VA. Others simply select based on maximum inrush VA. Either method should result in the same size transformer. Voltage regulation is the ability of a transformer to

48、maintain secondary voltage under different load conditions. As load increases, current flow through the transformer windings creates a voltage drop within the transformer, reducing the voltage at the output terminals. The larger the size (amperage) of the windings used, the less voltage dip under hi

49、gh load conditions. Figure I: Transformer temperature rise. 18 ASHRAE Journal September 1997 STD-ASHRAE JOURN SUPP SEPT 1777-ENGL 2977 m 075b50 0532400 771 m CONTINUOUS VA 50 75 1 O0 150 250 Manufacturers offer different transformer size recommenda- tions based on maintaining 95%, 90%, or 85% secondary volt- age. If your system voltage is erratic, you may want to choose a size to maintain 95% secondary voltage. Otherwise, maintaining 90% secondary voltage would be a good choice. Below is a typical selection