xai

ion of Control Equipment When selecting control equipment, the power supply sys- tem, the type and size of the connected motor, and operational and service conditions should be taken into account. Affect- ing these conditions and requiring careful appraisal are the power supply, the controller size...

ion of Control Equipment When selecting control equipment, the power supply sys- tem, the type and size of the connected motor, and operational and service conditions should be taken into account. Affect- ing these conditions and requiring careful appraisal are the power supply, the controller size and rating, and the fre- quency of starting. 6.23.1 .I Power Supply The ability of the power distribution system to satisfacto- rily handle motor starting loads is of major impor&ance and in large measure determines the selection of control. This is true, particularly where objectionable voltage disturbances are produced by the starting of a few large motors represent- ing the bulk of the system capacity. The full-voltage starting current of squirrel-cage induction motors and synchronous motors is several times full-load current (350% to 700%), so the system capacity must be able to supply the increased kilo- volt-amperes without objectionable system disturbance. If this is not practical, an alternative starting method must be employed to confine the current in-rush and voltage drop to satisfactory levels. 6.23.1.2 Controller Size and Rating Motor controllers are rated in horsepower or current-carry- ing capacity and must be capable of interrupting the motor locked-rotor current at the voltage specified. Industrial motor controllers bear the manufacturer’s nameplate specifying their size, horsepower rating, and voltage. Controllers are supplied in several duty classifications and include the fol- lowing types: a. The continuous-duty type, which is capable of indefinitely carrying full-load motor current without exceeding a speci- fied temperature rise of current-carrying parts. b. The intermittent-duty type, which is used on cranes, machine tools, or other equipment requiring less sustained duty. It is recommended that the user consult with the manufac- turer to select adequate equipment for the operating conditions. 6.23.2 Manual Operation Manual control has limited use, which is customarily for the starting of fractional horsepower, single-phase motors in the 120-V to 240-V range. Within the 240-V range, on-off control as well as motor overload protection is provided within the control enclosure. The overload protection is provided by trip- free thermal devices located in at least one side of single-phase units and in all three phases of three-phase units. Fused switches or circuit breakers providing a line-disconnect feature and short-circuit protection can be obtained in combination units (combined with the control in a common enclosure) or can be separately mounted. Low-voltage release may not be available; consequently, the control contacts remain closed dur- ing periods of power failure, thereby causing automatic restarts of the motor upon resumption of power. Manual control is not recommended for motors greater than 1 HP or for motors greater than 240 V because of increased risk to personnel. 6.23.3 Contactor Operation The application of magnetic contactor control is the stan- dard throughout the petroleum industry. Magnetic contactor control utilizes a magnetic contactor and a pushbutton, or automatic control device(s), to start and stop the motor. In general, the control voltage is at a level lower than the equip- ment utilization voltage. The equipment is applied at standard voltage levels match- ing the motor requirements. Fuses, circuit breakers, or motor circuit protectors, separately or integrally mounted within the starter enclosure, provide the required disconnect and short- circuit protection. Thermal elements or current-sensitive devices, connected in all three phases of the control equip- ment, provide the overload protection. Three-wire control provides the undervoltage protection against automatic restart of motors after the restoration of failed voltage. Starters con- trolled by automatic devices are wired for undervoltage release (two-wire control permitting automatic restart after voltage restoration). The selection of the pushbutton or con- trol location may be made to comply with desired operational requirements. Where continuity of service or operating conditions demand, time-delay relaying is available to permit motors to ride through momentary voltage dips. The delay interval is critical because full voltage, particularly in the larger sizes, should not be applied to de-energized motors having residual voltages above 25% to 35% rated voltage, unless the motors have been designed for such applications. The time-current characteristics of associated protective device and relaying equipment should be coordinated to ensure selective protection. Overload, locked-rotor, and short- circuit protection should be provided by the protection char- acteristics. Medium-voltage circuit breakers, along with protective relays, are sometimes used with large motors to serve not only as controllers but also as the means for disconnection. These breakers will be electrically operated and can provide automatic control comparable to that of magnetically oper- ated contactors; however, for frequent operation, magneti- cally operated contactors are more reliable because circuit breakers are not designed for such service. 6.23.4 Protective Relaying and Aútomatic Control As a rule, the function of protective relaying is to discon- nect the faulty equipment from the source of the electrical supply as quickly and with as little system disturbance as pos- sible. In a comprehensive installation, each circuit or piece of equipment should operate independently under distress, ~d the protective relaying should be so selective that only the affected units are de-energized. In more detail, protective relaying must distinguish between abnormal equipment oper- ation and system failures. 6.23.4.1 Overload Protection Overload protection is applied to de-energize overloaded motors automatically before winding or conductor damage has been caused by excessive operating temperature. The motor controller thermal device, actuated by a self-contained heating element responsive to motor line current and selected for coordination with the type of motor enclosure, is fre- quently used and is available with either a manual or an auto- matic reset. Overload relays used on motors controlled by automatic devices must have manual reset. Noimally, the selection of a thermal device is based on a standard air ambient temperature reference of 40°C (104°F). For engine rooms, fire rooms, and similar elevated ambient locations, 50°C (122°F) may be chosen as the reference, but consideration must be given to the choice of thermal ele- ments. This is of particular note where the ambient tempera- ture at the motor is higher than that at the remotely located controller. Thermal elements, unless temperature compen- sated, are responsive to variations in ambient temperature. The necessity for oversized thelmal elements can be pre- vented by carefully selecting control locations and by ample shielding against heat radiation. When controllers are located in areas subject to unusual ambient temperature variations or an ambient different from the motor, the temperature-com- pensated thermal relay is available. Large motors, greater than or equal to 250 H€’, frequently use embedded detection- type thermal protection as the best indication of motor tem- perature. Low-voltage motor controllers generally use one of the fol- lowing devices: a. The nonadjustable melting alloy. b. The bimetallic strip. c. Solid-state type. The sizing or setting of these devices is based on the motor nameplate full-load current, the service factor, the ambient temperature of the motor and controller (specifically, whether the motor and controller are at the same or different ambient temperatures), and NFPA 70 requirements. 6.23.4.2 Additional Protection for Large Motors Protective relays, electromechanical or solid state, are well adapted to provide all forms of protection. It is recommended that differential relay protection be provided for motors with ratings of 1,500 HP and over, and this requires that each phase of the wye connection in the motor winding be accessi- ble. Differential relays are responsive to changes in the rela- tionship between incoming and outgoing current. Undervoltage relays, responsive to voltage changes, are used to disconnect equipment from the line when the voltage fails or when dips below predetermined values are encountered. Phase-sequence voltage relays protect against reversed phase sequence; and negative-sequence voltage and current-balance relays provide protection against phase voltage unbalance and current unbalance, respectively. lnduction disk overcurrent relays are frequently used with large motors for overload and fault protection. Motors operated on grounded systems greater than 600 V should be provided with ground-fault pro- tection. Multifunction motor protection relays (solid-state type) are available that can provide some or all of the protec- tion discussed in this section. Overcurrent relays used for overload protection must be of the long-time-delay type to eliminate nuisance tripping on motor in-rush current. Protective relays with thermal detector inputs are also available. See IEEE Std 242, Chapter 9. Partial and fully automatic control are used extensively in petroleum processing facilities. Because automatic control is closely related to protective relaying in its application, both should be considered when an overall selection is made. Control power transformers are employed for magnetically operated controllers having motor voltages greater than or equal to 480 V; they are also employed where process instru- mentation is involved. Control power transformers may be applied individually to specific equipment, or may serve a bus from which several controllers operate. Figures 15, 16, and 17 illustrate the use of an individual control power trans- former in a motor controller. Both 120-V and most 240-V cir- cuits derive control power directly from the source. 6.23.5 Types of Enclosures Control enclosures are provided to protect personnel and to meet service and operating conditions. Several types are available, each designed to meet a particular application, such as being used in a corrosive, wet, dusty, or hazardous atmo- sphere, and in a general-purpose indoor location. The cost varies with the design, increasing with the severity and nature of the service conditions to be met (see Table 3). 6.23.6 Maintenance and Cost The control equipment selected should satisfactorily han- dle the assigned duty. When borderline decisions regarding equipment size are to be made, excessive long-range mainte- nance or replacement may offset an apparent first-cost sav- ings. In instances where explosionproof controllers are indicated for classified locations, a study of the comparative installed costs of remotely located general-purpose or weath- erproof controllers may show substantial savings. Special consideration should be given to severe service conditions, such as humidity and atmospheric or process cor- rosion, which may damage or render control elements inef- fective. Availability of corrosion-resistant enclosure parts should be discussed with the equipment manufacturer. Also, the installation of control centers or grouping of control equipment should be considered for the possible savings and the convenience of maintenance. Table 3-NEMA Enclosure Types for AC Motor Controllers Comparative Cost Combination Circuit NEMA Typeof Type Enclosure Characteristics 1 General A Type 1 enclosure is designed to meet purpose- ’ Underwriters Laboratories’ most recent gen- indoor eral specifications for enclosures. This enclo- sure is intended primarily to prevent accidental contact with the control apparatus. 2 Dripproof- indoor 3 Dusttight, raintight, and sleet resis- tant-outdoor 3R Rainproof, sleet resistant- outdoor 3s Dusttight, raintight, and outdoor sleet-proof- When a nonventilated enclosure is specified for equipment consisting of devices that require ventilation (electronic devices and resistors), such devices may be mounted in ventilated portions of the enclosure, provided they are capable of operating satisfactorily and without hazard when so mounted. A Type 1 enclosure is suitable for general- purpose applications indoors and under nor- mal atmospheric conditions. It protects against dust, light, and indirect splashing but is not dusttight. Flush-type enclosures (designed for mounting in a wall) have provisions for aligning with the flush plate and compensating for wall thickness. A Type 2 enclosure is similar to a Type 1 enclosure, but it also has drip shields or their equivalent. A Type 2 enclosure is suitable for applications where condensation may be severe, such as in cooling rooms and laundries. It provides pro- tection against dust, falling liquids, and light splashing but is not dusttight. A Type 3 enclosure is designed to provide protection against windblown dust and water. It is not sleet- or iceproof. A Type 3 enclosure is suitable for use out- doors if ice is not a serious problem. A Type 3R enclosure is designed to provide protection against rain. It is not dusttight or snow-, sleet-, or iceproof. A Type 3R enclosure is suitable for use out- doors and will prevent the entrance of a rod of 0.125 inch diameter, except at drain holes. Types 3 and 3R are usually combined in one enclosure type. A Type 3s enclosure is designed to provide protection against windblown dust and water and to provide for operation when covered with external ice or sleet. It may have auxiliary provisions for ice breaking. Typical Application Breaker Type Where Used in Plants Size 1 Size 4 Locations where enclo- Office buildings, 1 .o 1 .D sure prevents accidental warehouses, change contact with live parts, and houses. indoor locations where normal atmospheric con- ditions prevail. Locations where conden- sation may be severe. Locations subject to wind- blown dust and rain. Locations subject to heavy rain. Locations subject to heavy icing conditions. Refrigeration rooms and water pump- houses not classified and not corrosive. Outdoors on con- struction jobs and dusty locations. Outdoors at com- mercial installations and on construction jobs. Outdoors on ship decks or on con- struction site subject to heavy icing. Table 3-NEMA Enclosure Types for AC Motor Controllers (Continued) NEMA Type of Tp Enclosure Characteristics Typical Application Where Used in Plants Comparative Cost Combination Circuit Breaker Type Size 1 Size 4 4 Watertight and dusttight 4X Watertight, dusttight, and corrosion- resistant 6 Submersible 7 Classified location; Class I- air break (see Note 2) 8 Classified location; Class I-oil immersed (see Note 2) 9 Classified location; Class II- Groups E and G 10 MSHA, U.S. Dept. of Labor 11 Corrosion I resistant and immersed- Type 4 enclosures are usually used for this service. A Type 4 enclosure, intended for use indoors Outdoor locations or loca- ’ Outdoors at pumps or outdoors, protects the enclosed equipment tions where the starter not in classified or against splashing water, seepage of water, might be subjected to corrosive locations. falling or hose-directed water, and severe splashing or dripping external condensation. A Type 4 enclosure is water; not suitable for sub- sleet-resistant but not sleetproof. mersion in water. A Type 4X enclosure is the same as aType 4 Locations subject to Outdoor locations in enclosure but is also corrosion-resistant. splashing or dripping chemical plants. water where corrosion is also a problem. A Type 6 enclosure is suitable for applications Locations where the Normally not where the equipment may be subject to sub- equipment is subject to required. mersion in water, as in quarries, mines, and submersion in water. manholes. The design of the enclosure will depend on the specified conditions of pressure and time. It is also dusttight and sleet resis- tant. A Type 7 enclosure is designed to meet the Locations which are clas- Class I, Division 1 application requirements for Class I locations sified as Class I locations and 2 areas, Groups as defined in Article 500 of NFPA 70 and is according to Article 500 A-D. designed in accordance with the latest specifi- of NFF’A 70 due to the cations of Underwriters Laboratories. A letter presence of flammable suffix in the type designation specifies the gases and vapors. NFPA 70 group for which the enclosure is suitable. A Type 8 enclosure is designed to meet the Locations which are clas- Class I, Division 1 application requirements for Class I locations sified as Class I locations and 2 areas, Groups as defined in Article 500 of NFPA 70 and is according to Article 500 A-D. designed in accordance with the latest specifi- of NFPA 70 due to the cations of Underwriters Laboratories. The presence of flammable apparatus is immersed in oil. A letter suffix in gases and vapors. the type designation specifies the NFPA 70 group for which the enclosure is suitable. A Type 9 enclosure is designed to meet the Locations which are clas- Class II areas, application requirements for Class II locations sified as Class II locations Groups E and G. as defined in Article 500 of NFPA 70 and is according to Article 500 designed in accordance with the latest specifi- of NFPA 70 due to the cations of Underwriters Laboratories. A letter presence of combustible suffix in the type designation specifies the dusts. NFPA 70 group for which the enclosure is suitable. A Type 10 enclosure is designed to meet the Locations that must meet Normally not latest requirements of MSHA. the latest requirements of required. MSHA. A Type 11 enclosure is suitable for applica- Locations where acid or tions indoors where the equipment may be fumes are present. subject to corrosive acid or fumes, as in chem- ical plants, planting rooms, and sewage plants. The apparatus is immersed in oil. Table 3-NEMA Enclosure Types for AC Motor Controllers (Continued) Comparative Cost Combination Circuit NEMA Typeof Tm Enclosure Characteristics Typical Application Breaker Type Where Used in Plants Size 1 Size 4 12 Industrial use- dusttight and driptight- indoor 13 Oiltight and indoor dusttight- A Type 12 enclosure is provided with an oil- resistant synthetic gasket between the case and the cover. The cover is hinged to swing hori- zontally and is held in place with captive clos- ing hardware; a screwdriver or wrench must be used to release the cover from the hardware. There are no holes through the enclosure for mounting or for mounting controls within the enclosure and no conduit knockouts or conduit openings. Mounting feet or other suitable means for mounting are provided. When a Type 12 enclosure is specified for equipment consisting of devices which require ventilation (electronic devices and resistors), such devices may be mounted in a ventilated portion of the enclosure as long as they are capable of operating satisfactorily and without hazard when so mounted. A Type 13 enclosure, intended for use indoors, protects pilot devices; such as limit switches, foot switches, pushbuttons, selector switches, and pilot lights, against lint; dust; seepage; external condensation; and spraying water, oil, or coolant. A Type 13 enclosure has oil-resistant gaskets, external mounting means, no conduit knockouts or unsealed openings, and oiltight conduit entry. Locations where oil or Machine tool drive- 1.2 1.2 coolant might enter the in shops; chemical enclosure through mount- rooms in water treat- ing holes or unused con- ment plants. duit openings and where it is necessary to exclude dust, fibers, flyings, and lint. Indoor locations subject to Indoor dusty control the contaminants listed. areas or where sub- ject to liquid spray. Notes: 1. MSHA = Mine Safety and Health Administration. 2. Any individual starter, circuit breaker, fuse, switch, fused discon- necting switch, or any combination of these items may be enclosed in any of the aforementioned enclosures. When NEMA Types 7,8, and 11 enclosures are applied, a combination of enclosures may be required if the installation is outdoors or additional protection fea- tures other than the basic protection provided by the specific NEMA type are required. To standardize the practice in referring to equip- ment known as explosionproof (Types 7 and S), apparatus designed for use in Class I, Group A-D locations, should be described in one of the following ways: a. Control listed by a NRTL for use in Class I, Group (state spe- b. Control designed to conform with the manufacturer’s interpretation of the requirements of Underwriters Laboratories’ standards or from its testing facilities. c. Control of a size and nature for which there are no existing Under- writers Laboratories’ standards or tests from a NRTL testing facility. 3. When an enclosure has to meet the requirements of NEMA Type 1 1, the design will depend on the conditions of exposure. 4.’ NEMA ICs 6 can provide more detailed information on enclosures and test requirements. cific group letter) locations. 6.24 APPLICATION OF MOTOR CONTROL 6.24.1 Squirrel Cage Induction Motor Because of its simplicity and adaptability to full-voltage starting, the squirrel-cage induction motor is widely used in constant-speed applications. Magnetic full-voltage starting with remote or automatic control is customarily applied for motor starting. (The wiring for this application is dia- grammed in Figure 15.) Magnetic full-voltage starting is the most desirable and can prevent restart of motors after the return of failed voltage. There are cases, however, where automatic restarting is imperative because of critical process requirements. If automatic restarting is necessary, time- delay relaying can be provided, and if several motors are involved, the motors can be automatically restarted in sequence to prevent possible system disturbances. Reduced-voltage starting, either manual or automatic, is employed to reduce the starting current on power systems of limited capacity if the reduced-starting torque is adequate winding is brought out through slip rings and connected to a for the connected equipment. Reduced-voltage starting also variable resistor or regenerative system. This arrangement is employed when the driven load demands smooth starting. provides selective speed control under varying conditions of Several methods of applying reduced-voltage starting are load (see Figure 17). available: The primary control device for starting a wound-rotor a. The autotransformer method offers variations in starting motor is usually the same as that used for a squirrel-cage torque through selective starting taps. It provides maximum induction motor. The secondary device for regulating the starting torque with minimum line current (see Figure 16). speed of a wound-rotor motor consists of an adjustable b. The series resistor or reactor method offers simplicity of primary and secondary control devices so that the motor resistor or rheostat. It is general practice to interlock these design and uses a more economical starter. cannot b