The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without damage. The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0.
A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.
NEMA Service Factor at Synchronous Speed (rpm) for drip proof motors:
|Service Factor - SF|
|Synchronous Speed (rpm)|
|1/6, 1/4, 1/3||1.35||1.35||1.35||1.35|
|1 1/2 and up||1.115||1.15||1.15||1.15|
Example - Service Factor
A 1 HP motor with a Service Factor - SF= 1.15 can operate at
(1 hp) x 1.15
= 1.15 hp
without overheating or otherwise damaging the motor if rated voltage and frequency are supplied to the motor.
Insulation life and bearings life are reduced by the service factor load.
Related DocumentsSours: https://www.engineeringtoolbox.com/service-factor-d_735.html
Understanding motor "Service Factor"
In my opinion, understanding SF requires understanding how it is typically used.
Scenarios 1: Let’s say you want to build a machine powered by an AC induction motor. You know the torque you need and the speed you want, but it calculated out to be 8.47HP continuously. If you are the end user and you want the maximum life out of this machine, you select a 10HP motor, because the next size down is 7-1/2HP and that is too small. But if you are an ORM selling this machine and you need to be competitive against other lower cost suppliers, you select a 7-1/2HP that has a 1.15 SF, because it can be used at 8.625HP, albeit with an increased current draw, lower efficiency and a reduced lifespan. Years ago the NEMA design specs used to quantify some of these issues in terms of time frames, but that was removed and now they just vaguely refer to the use of the SF as “reducing life”. Here is the wording:
So for the OEM, so long as it outlasts their warranty, they use it. The end user is who bears the cost of the reduced life by having to replace it sooner.
A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy as compared to operating at its rated nameplate horsepower. Insulation life and bearing life can be reduced by the service factor load.
Click to expand...
Scenario 2: the machine needs 6HP most of the time, but occasionally there is a change in the load and the motor is loaded down to 8.47HP for a few minutes, then returns to normal for a while longer, enough time for it to safely dissipate that extra heat. A common example is an air compressor that runs continuously but an unloader valve cycles on and off as the demand for air changes with shop use. When the unloader closes, the compressor loads the motor more heavily until the pressure is satisfied and the unloader opens again, venting the compression chamber to atmosphere. In that case, the 7-1/2HP motor makes sense.
What is a Motor Service Factor?
Motor service factor (SF) is the percentage multiplier that a motor can handle for short periods of time when operating within its normal voltage and frequency tolerance. In other words, it is a fudge factor that give extra horsepower when it's occasionally needed.
For instance, this 1/2 horsepower shown in the photo has a service factor of 1.25 so it can actually output 25% more power required for short periods of time. This comes in handy if the density of the liquid increases or a higher than normal flow rate is required.
Fractional horsepower motors usually have a higher service factor up to 1.5 since their power consumption does not lead to significantly higher winding temperatures. Motors of 10 hp and up usually have a service factor of 1.15.
The Canadian Electrical Code defines service factor as a multiplier that, when applied to:
- The rated horsepower of an AC motor,
- To the rated armature current of a DC motor, or
- To the rated output of a generator,
... indicates a permissible loading that may be carried continuously at rated voltage and frequency. Note how the CEC allows for continuous operation while we reserve it for only short periods of time to ensure reliability.
As electrical consultants, using the service factor gives us a margin of safety that allows our design to:
- Extend the life of the motor by lowering the temperature of the insulation winding.
- Compensate for low or unbalanced supply voltages.
- Accommodate the variability in horsepower. A 15% buffer is a nice margin to have especially for those occasional overload conditions.
Why Have a Service Factor
Operating a motor at its limit makes it more prone to overheating. A service factor allows the motor to operate below its theoretical maximum so it can run continuously with a cooler winding temperature at rated load. This leads to a longer life and better reliability.
Designing with the Service Factor
We never design systems to operate continuously at the maximum level (redundancy is a different topic). It is good practice to size a motor for continuously operation that is below the service factor percentage. An Electrical load monitoring test help to see how efficient a motor is running.
Operating in the service factor area may result in:
- Decreased efficiency (more energy usage).
- Decreased power factor (more reactive power usage).
- Overheating and damage to the wire insulation.
- Incorrect starter sizing which may lead to inadequate starting & pull-out torques.
A motor services factor is a margin of safety which increases the reliability of building systems.
This motor has a service factor (SF) of 1.0 and yet the SF amps > full load amps (FLA).
As I understand it, service factor is the allowed total load in an overload situation (not sure for how long, but I assume something like high starting torque situations which are brief).
So this motor seems to be saying, in effect, that no overload is allowed. Therefore I would have expected that the FLA = SFA, but it does not.
Comparing the actual SFA and FLA values, SFA is about ~1.14 of FLA. So you'd think that would be roughly the SF as well.
This was used on a machine (air compressor) labelled as "6 HP" and the motor nameplate also reads 6 HP in the crease of the label (to the right of the model #).
Why are the SFA and FLA different for this motor?
Amps service factor
Proper selection of ag motors key
SPONSORED BY GrowerSELECT
Today’s livestock and poultry operations rely on electric motors for a variety of feeding and ventilation functions. Used in buildings that alternate between dusty and humid, along with fluctuations in voltage and varying workloads, we subject our motors to a very hostile work environment.
When selecting replacement motors, it is important to select motors that are efficient and designed with a “safety factor” that will allow them to last under harsh conditions.
While you may be familiar with the term Service Factor or S.F., there are a couple of important designations found on a motor nameplate that may need more clarification.
Service Factor is defined as a motor's ability to operate under a short-term load. The higher a motor’s S.F. rating is, the more durable the motor. Motors with a high S.F. are expected to last longer. To illustrate, a 1-1/2 hp motor with a 1.5 S.F. can provide 2.25 hp for short-term use. However, it is not a good practice to continuously operate a motor above the rated workload. In other words, the same 1-1/2 hp motor with a 1.5 SF would not be selected to power a 60″ fan that was originally shipped with a 2 hp motor.
Full Load Amps, or F.L.A., represents the amount of current the motor is designed to draw at the rated horsepower. In the example nameplate, this means that when the motor is running under a full load at 230 volts, we can expect it to draw 5.4 amps. Motors with a lower F.L.A. with the same amount of horsepower are considered more efficient to operate.
Service Factor Amps, or S.F.A., represents the amount of current the motor will draw when running at the full Service Factor. In the example nameplate, the S.F.A. is eight amps at 230 volts.
Continually exceeding the S.F.A. shown on the nameplate can shorten motor life. Motors with a higher S.F.A. with the same horsepower have an increased “safety factor” and are expected to last longer under harsh conditions.
The most efficient, rugged motors are designed with a higher S.F., lower F.L.A., and higher S.F.A ratings. By comparing the information on a motor nameplate, we can select the best replacement motors for feeding and ventilation equipment.
How to Calculate Motor Service Factors
A service factor, or "SF," is a factor representing the capacity at which a motor can operate without overloading or damaging its core or components. The reference point is a service factor of 1, which would mean a motor cannot operate over its rated horsepower, or "hp," without overheating. Similarly, for example, a service factor of 1.25 says the motor can operate 25% over its rated hp without damage or overheating.
Determine the horsepower of your motor. If you do not know it, reference the manufacturer's specifications.
Refer to the National Electrical Manufacturers Association table, or "NEMA table", for service factors. The table relates service factors to hp and revolutions per minute, or "RPMs" . Calculate or determine your service factor by choosing the service factor that corresponds to your hp and various RPM levels. For example, according to the table, if you have a 1HP motor and you operate it at 3600 RPM continuously, your service factor is 1.25.
Calculate your operating or "effective" HP level. Multiply your motor horsepower by the service factor. For example, if you have a 1HP motor and your service factor is 1.25, then you can operate safely at an HP = 1.25HP without overheating or damaging the motor.
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Dwight Chestnut has been a freelance business researcher and article writer for over 18 years. He has published several business articles online and written several business ebooks. Chestnut holds a bachelor's degree in electrical engineering from the University of Mississippi (1980) and a Master of Business Administration from University of Phoenix (2004).
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