Service Factor 2

[charlierod]

What's exactly the meaning of a motor service factor?
Should this factor be multiplied to nominal current or power of the motor?
What are the advantages and disadvantages of operating a motor at its service factor rating?

Thanks for your comments

 

[busbar]

Discussion at

thread237-86034
thread237-45425
thread237-26230
thread237-18281

 

[aolalde]

The service factor SF for an electric machine is a multiplier which, when applied to the rated power, indicates a permissible power loading which may be carried under the conditions specified for the service factor.
An AC Motor having a SF is suitable for continuous operation at the horsepower obtained by multiplying the rated horsepower by the service factor shown , when the voltage and frequency are maintained at the value specified in the nameplate.
The Efficiency, Power Factor and Speed may be different from those at rated load, but the locked-rotor current and torque, as well as the breakdown torque will remain unchanged.
A motor operating continuously at service factor greater than 1 will have a reduced life expectancy compared to that operating at its rated nameplate horsepower.
The line current follows more or less the power increase proportion but not exactly due to the efficiency and power factor changes.

 

[jbartos]

Comment: The motor operation above SF=1.0pu follows a different motor thermal curve than the motor operation with the motor SF=1.0pu.
The motors specified with service factor greater than 1.0 are for loads that may occasionally exceed the motor rated HP for short durations of time, e.g. motor operated valves during the valve unseating.

 

[aolalde]

jbartos (Electrical) Your post (Mar 25, 2004 )states:

"Comment: The motor operation above SF=1.0pu follows a different motor thermal curve than the motor operation with the motor SF=1.0pu"

Could you explain what is the meanning?

 

[jbartos]

Suggestion to the previous posting: References (both the same):
1. Edgar F. Merrill "Temperature Rise Considerations for Large Induction Motors," IEEE Transactions on Industry and General Applications, Volt. IGA-5, No. 4, July/August 1969
2. Edgar F. Merrill "Temperature Rise Considerations for Large Induction Motors," IEEE Conference Record of IGA/1969 Fourth Annual Meeting October 12-16, 1969, Hilton Hotel, Detroit, Michigan

Essentially, the Service Factor Section outlines "thermal secrets" manufacturers imbed in motors having SF>1.0pu. Cited: "Custom, practice, and outdated standards had all finally resulted in basically increasing the thermal rating of all large motors by 15% (as related to SF=1.15pu), a 500-hp motor was now 575 hp, etc. Now, only torque and inrush values were referred to the nameplate horsepower."