Why are the SF amps and FL amps different even though SF = 1.0


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?

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Best Answer

In NEMA MG 1 there are tables that list, by Hp rating, required service factors for motors of design A, B, and C. Motors of other designs have SF 1.0 specified. Per the nameplate, the motor in question is Design L. It appears that SF current is marked when the SF does not conform to NEMA standards. Operating at service factor > 1.0 does not have a specific time limit, but it results in reduced life expectancy for the insulation and bearings. The amount of reduced life expectancy is not specified by NEMA. SF is a multiplier applied to the Hp rating. When operating above 1.0 SF, the current and torque increase and the speed decreases by unspecified amounts. In this case, the manufacturer has chosen to mark SF as a multiplier for current with unspecified Hp and speed at current greater than rated current.

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