Motors service factor

The feed water pumps shown in Figure 4.3 are 316L stainless steel (SS) horizontal centrifugal pumps rated for a flow rate of 70.5 m /h at 2.8 bar g each. The totally enclosed fan cooled (TEFC) motors are rated for 18.7 kW, 3000 rpm, 460 V/3 Ph/50 Hz. The motor service factor is 1.15. 

Application Motor Service Factor Prime Mover Internal-Combustion Turbine Engine (Multicylinder) ... 

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. 

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. 

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. 

In NEMA size ranges, motors with service factors are still available however, for compressor drives, it would be better if they were iiul Experience with operation into the service factor rating has not been sat isfactoiy. 

Molecular weight, effect on centrifugal sizing, 159 Mollier charts, 27 Monitoring system, 356 Motor, 146 enclosure, 260 equations, 267 insulation, 257 locked rotor torque, 270 selection, 270 service factor, 262 starting characteristics, 270 starting time, 273, 274 synchronous vs induction, 265 variable frequency drives, 27/, 280 voltage, 258 Motors... 

Most standard motors are manufactured using non-hygroscopic NEMA Class B insulation. For added protection in an offshore environment, open drip-proof or weather protected motors should be specified with a sealed insulation system. NEMA Class F insulation is also available in most motor sizes and is advisable to provide an improved service factor. 

Motors with 1.0 service factor, other than those given in items a.3 and a.4... 

Totally-enclosed nonventilated motors with 1.0 service factor... 

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