Motors starter sizes

Document the existing exhaust air systems to determine what exhaust fan modifications wiU be needed with the addition of exhaust air filters and energy recovery water coEs. These changes will have been identified in the original ECM assessment and would take into account the changing out of fan motor, motor starter size, etc., based on exhaust fan curves, performance data, and increase in fan system static pressure... 

Centralized Control As mentioned previously, motor starters may be located either at the motor or at some remote point. Frequently they are grouped at a location convenient to the source of power. The feeders radiate from this point to the individual motor loads. A convenient method is the control-center modular structure for low-voltage control, into which are assembled motor starters and other control devices. The individual starters can be drawn out of the structure for rapid, easy maintenance and adjustment. With this construction it is easy to change starter size or add additional starters. All the starters are in one location, so that interwiring is simple and easy to check. Auxiliary relays, control transformers, and other special control devices can also be included. See Fig. 29-7. 

NEMA Starter Ratings. NEMA-type starters are rated for motor loads as shown on the NEMA ratings chart, Table 5.37. Some manufacturers also supply custom or motor-matched sizes which do not have standardized ratings, but may offer an economical alternative to the user. 

Since it is not practical to manufacture a llameproof enclosure due to its size and bulk and the number of knockouts and openings on the doors for switches, metering, indicators, and pushbuttons (PBs) etc., it is common practice to locate the.se assemblies some distance from the affected area in a separate well-ventilated room. Depending upon the location and intensity ol contamination, it may be permissible to meet the requirement by using a pressurized enclosure by maintaining a positive pressure inside the enclosure similar to that for motors (Section 7.1.3..3). When there arc many switchgear assemblies, the room itself can be pressurized, which is safer and easier. Small enclosures, however, such as a PB station, switch or a switch fuse unit or an individual starter unit etc., which can be easily made of MS plates or cast iron, as discussed in Section 7.13, can be mounted in the hazardous area while the main MCC can be installed in the control room, away from the contaminated area and from where the process can be monitored. 

The detailed-t eoff method can rarely if ever be used. When detailed drawings are available, costs may be estimated by pricing materials and components from suppliers catalogs or, for special items, from quotations. Handbooks are available which give typical values of the labor-hours required to perform units of installation work, such as installation of switches, starters, motors, conduit wiring, and push buttons of various sizes, for both hazardous and nonhaz-ardous areas. Labor rates can be obtained from various government statistical sources or elsewhere. For the United States the National Electrical Contractors Association publishes an excellent manual of electrical costs. From the complete plans and specifications, the estimator can take off materials, estimate the labor cost, apply appropriate factors for labor efficiency, productivity, and local conditions, and achieve good results. 

Constant speed direct-current motors are usually started by decreasing the amount of an external resistance in the armature circuit. The starting current may exceed full-load current by about 50 per cent. From 15 to 30 sec. should be allowed to bring a motor up to speed. This may be insured by automatic operation. The rheostat may incorporate low-voltage and overload releases, and should contain the motor-field circuit closing mechanism. Ordinary rheostats (starters) cost from 3 to 6 per cent as much as motors, the smaller sizes costing relatively more. 

A 6600 volt induction motor is fed from a fuse-contactor starter. Find the most appropriate fuse rating and the appropriate size of a PVC cable for the motor. The following data are known -... 

The precise motor specification will depend upon a number of other factors such as the type of drive and starter, and how much contingency, or expansion, for which the user wants to cater. The losses between the motor and the centrifuge, such as belt friction, and fluid coupling losses if applicable, all need to be taken into account. However, the motor will be no smaller than 125 kW, the next standard size above the power so far calculated. If a much larger size of motor is chosen, the power factor for the motor efficiency will be reduced, imposing a greater penalty on the cost of electricity. 

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