과부하율 Service Factor (S.F.)

Service Factor

 

 

 

 

Section 1.42 of NEMA Standard MG1 is titled “Service Factor – AC Motors” and states: "The service factor of an AC motor is a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor." In other  words, multiplying the AC motor's nameplate power by its service factor indicates how much output power it can produce for a short period of time without detrimental overheating. This is thus a safety factor and is not to be considered part of the motor’s normal useful horsepower, since it decreases the thermal reserve of the insulation system. A rough rule of thumb is that for every 10°C above its rating that the insulation operates at, the time until the insulation fails is halved.
 
The service factor is often listed as 1.15, which means the motor can briefly withstand a 15% overload.  A 10 HP motor with a 1.15 service factor, for example, can safely handle an occasional load of 11.5 HP. 
 
The service factor concept is unique to American motors, promulgated by NEMA. IEC rules, covering the rest of the world, make no claim that a motor produces other than its rated power. So, essentially, all IEC motors have a service factor of 1.00. 
 
The service factor is based upon the fact that general purpose AC motors are rated on various combinations of minimum pull-up torque and maximum current draw. They typically can, however, briefly operate at higher currents without damage; this is what the service factor represents.
 
A direct current (DC) motor has a speed/torque relationship that is completely different: it only produces one speed at a particular torque and the heat rise due to operating losses at that torque cannot exceed the limit of its insulation system. There is no more output available without compromising the life of the motor.  If the customer actually wants more power, he should buy a motor with that rating.
 
The “compromising the life of the motor” phrase is an important consideration. DC motors could be operated above their temperature rating, within limits, BUT this, of course, is NOT recommended.
 
NEMA makes no mention of service factor in reference to DC motors because it could not apply  given  the  speed/torque  characteristic. Thus, what you see on the nameplate is what you get.
  
Note:  This is general information and is not intended to provide operational instructions for a specific motor or application.

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Service factor is a measure of the extra horsepower a motor can deliver if it is operating under rated conditions and is located in an acceptable ambient environment. A common service factor is 1.15. This means that a motor could deliver 115% of the horsepower indicated on the nameplate.

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The term "Service Facotr" is not defined in IEC. However, an IEC motor with class F insulation meets the NEMA MG-1 definition of 1.15 service factor.

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Service Factor는 정격전압과 정격주파수 및 허용온도 (Max. ambient temperature) 아래서 허용할 수 있는 과부하 용량을 얻기 위해서 정격출력에 곱하는 계수를 말하며 S.F.가 1 이상이면 전동기 명판에 표시하도록 NEMA에 규정되어 있다. 전동기를 허용 과부하 용량으로 운전할 때의 기대운전전류 (Expected running amps)를 전동기 명판에 'SF MAX AMPS' 또는 'MAX A'라고 표시하는 경우도 있다. 허용 과부하 용량으로 전동기를 연속적으로 사용하면 전동기의 온도가 올라가서 수명이 단축되고 효율이 저하된다.

 

Rated Power x Service Factor = Allowable overload capacity

 

정격 출력이 25HP이고 Servece Factor가 1.25라면 허용 과부하 용량은 31.25HP가 되는 것이다.

 

Servece Factor를 Rated Power에 곱한 값이 필요 용량 보다 높다고 해서 해당 Motor를 이용하는 것은 바람직하지 않다. Service Factor가 높아도 Coil Size나 절연등급은 동일하며 단순히 Motor Frame만 크게 설계하여 냉각효과를 가져와 Motor 수명에 큰 영향을 주지 않도록 한 것이므로 연속적인 과부하까지 충분히 커버할 수는 없다.