Service Factor

Economy of time and resources dictate using the smallest sized faciHty possible to assure that projected larger scale performance is within tolerable levels of risk and uncertainty. Minimum sizes of such laboratory and pilot units often are set by operabiHty factors not directly involving internal reactor features. These include feed and product transfer line diameters, inventory control in feed and product separation systems, and preheat and temperature maintenance requirements. Most of these extraneous factors favor large units. Large industrial plants can be operated with high service factors for years, whereas it is not unusual for pilot units to operate at sustained conditions for only days or even hours. 

Operating Costs Annual operating costs for the example in Table 22-18 are shown in Table 22-19. For this 0.25 mVs plant, a service factor of 90 percent is assumed. 

FIG. 29-1 Motor prices in dollars per horsepower for 1800 rev/min sqnirrel-cage induction motors from 3 to 10,000 hp. Dripproof and TEFC motors shown from 3 to 400 horsepower have 1.15 service factor for other motors above 250 horsepower, the service factor is 1.0. The basis of these data is July, 1994. To convert dollars per horsepower to dollars per kilowatt, multiply by 1.340 to convert horsepower to kilowatts, multiply by 0.746. 

Indusirial Power Engineering and Applications Handbook Table 8.5 Service Factors for flat and V-belts... 

For speed-increasing drives of speed ratio 1.00 to 1.24 multiply service factor by 1.00... 

The service factors do not apply to light duly drives. 

W = maximum load iransmiiling capacity of ihe belt SF = service factor... 

Service conditions on all the ratings so determined, one may apply the applicable service factors as noted... 

Textile motors Crane motors Determining the size of motor Sugar centrifuge motors Motors for deep-well pumps Motors for agricultural application Surface-cooled motors Torque motors or actuator motors Vibration and noise level Service factors Motors for hazardous locations Specification of motors for Zone 0 locations Specification of motors for Zone I locations Motors for Zone 2 locations Motors for mines, collieries and quarries Intrinsically safe circuits, type Ex. f Testing and certifying authorities Additional requirements for ciritical installations Motors for thermal power station auxiliaries Selection of a special-purpose motor... 

Over the years, oldtimers came to expect a 10-15% service factor for motors. Things are changing, as shown in the following section from Evans. ... 

For many years it was common practice to give standard open motors a 115% service factor rating that is, the motor would operate at a safe temperature at 15% overload. This has changed for large motors, which are closely tailored to specific applications. Large motors, as used here, include synchronous motors and all induction motors with 16 poles or more (450 rpm at 60Hz). 

New catalogs for large induction motors ai e based on standard motors with Class B insulation of 80°C rise by resistance, 1.0 service factor. Previously, they were 60°C rise by thermometer, 1.15 service factor. 

Service factor is mentioned nowhere in the NEMA standards for lai ge machines there is no definition of it. There is no standard for temperature rise or other characteristics at the service factor overload. In fact, the standards are being changed to state that the temperature rise tables are for motors with 1.0 service factors. Neither standard synchronous nor enclosed induction motors have included service factor for several years. 

Users should specify standard hp ratings, without service factor for these reasons ... 

The practice of using 1.0 service factor induction motors would be consistent with that generally followed in selecting hp requirements of synchronous motors. 

Induction motors with a 15% service factor are still available. Large open motors (except splash-proof) are available for an addition of 5% to the base price, with a specified temperature rise of 90°C for Class B insulation by resistance at the overload horsepower. This means the net price will be approximately the same. At nameplate... 

The newest catalogs show standard induction motors designed with Class B insulation for operation in a 40 C ambient with 80°C rise by resistance at 100% load for motors with 100% service factor. Class F insulation, with the capability of operating up to a 105°C rise by resis taiice, is today frequently offered as standard for machines with a Class B rise, particularly the larger sizes. Many users specify this as a standard Previously, induction motor ratings were based on temperature rise thermometer. 

NEMA standards do not give any fixed maximum operating temperature by any class of insulation. Briefly, NEMA states that insulation of a given class is a system that can be shown to have suitable thermal endurance when operated at the temperature rise shown in the standard.s for that type of machine. Standards for synchronous motors and indue tioii motors with a 100% service factor specify 80°C rise by resistance... 

Today, almost all large motors are designed specifically for a particular application and for a specific driven machine. In sizing the motor for the load, the horsepower is usually selected so that additional overload capacity is not required. Therefore, customers should not be required to pay for capability they do not require. With the elimination of the service factor, standard motor base prices have been reduced 4—5% to reflect the savings. Users should specify standard horsepower ratings, without service factor for these reasons ... 

The conunon practice of using Class F insulated motors with a Class B rise at 1.0 SF in effect provides some obtainable service factor above 1.0 ii the user is willing to operate the motor up to the Class F limits in response to some contingency. In many cases this provides at least 15% margin. 

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