Service Factor of Induction Motor

we begin with the assumption that the rated voltage and frequency are maintained during operation.

you have not mentioned if the higher service factor consideration is because the motor will see only periodical overloads? or be continuously over loaded by 50%. your choice should be influenced by that information.

a 75kW motor running at full load has a defined life expectancy at service factor 1.0.

a 50kW motor running at full load has a defined life expectancy at service factor 1.0.

when a 50kW motor is used at service factor 1.5, it results in higher heat dissipation than a 50kW motor with service factor of 1.0.

therefore for this service factor the motor would have to be built in a larger size.

even after building the motor in a larger size it would still mean higher insulation stress and faster bearing wear.

more on subject . . . .

we generally select motors with higher service factors to run them "cooler than normal" and thereby improve insulation life expectancy.

we also apply higher service factors in cases where the motor runs in hostile environments.

if you wish to apply the stated conditions then you might do well to select a 75kW motor with a higher service factor than unity.

"when a 50kW motor is used at service factor 1.5, it results in higher heat dissipation than a 50kW motor with service factor of 1.0."

In other words, it will work at 1.5 SF, but not far as long.

I'd order a 75 kW motor with a SF of 1.2.

Answer given by Mr. soebfatehi very much explains your query. adding a further information, here is an extract from baldor.

Service Factor

Some motors carry a service factor other than 1.0. This means the motor can handle loads above the rated HP. A motor with a 1.15 service factor can handle a 15% overload, so a 10 HP motor with a 1.15 service factor can handle 11.5 HP of load. Standard open drip-proof motors have a 1.15 service factor. Standard TEFC motors have a 1.0 service factor, but most major motor manufacturers now provide TEFC motors with a 1.15 service factor.

The question often arises whether to use service factor in motor load calculations. In general, the best answer is that for good motor longevity, service factor should not be used for basic load calculations. By not loading the motor into the service factor, the motor can better withstand adverse conditions that occur. Adverse conditions include higher than normal ambient temperatures, low or high voltage, voltage imbalances, and occasional overload. These conditions are less likely to damage the motor or shorten its life if the motor is not loaded into its service factor in normal operation.

If you know the meaning of service factor, you can decide which motor to select based on your need. See this link for motor service factor: http://cr4.globalspec.com/thread/54464/Service-Factor-of-Electrical-Motor

- MS

A study of loading conditions is required.If motor is likely to run at 50 kw most of the time but likely to get overloaded for 75 kw under some conditions then a 50KW motor with SF 1.5 is to be specified.Here the motor performance like EFF PF and temp rise over ambient will be designed for 50KW.If amotor is likely constantly operate at 75 kw then better to go in for a t 75 kw motor where all performance figs. will be designed considering 75 kw.When a motor is operating Service factor range the performance figures quoted for the rated conditions will not apply .In particular a higher temp. rise is permitted.SF 0f1.5 is normally not used.SF of 1.1,1.15or1.2 are normally used.You will find this mostly in varying load applications like reciprocating compressors where loads keep fluctuating.