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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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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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  • \$\begingroup\$ So the "1.0" is really just a placeholder, not an actually meaningful value, in this case? \$\endgroup\$
    – StayOnTarget
    Nov 20, 2020 at 13:43
  • \$\begingroup\$ My guess is that the motor was manufactured to an OEM customer specifications. The driven machine may have a work cycle that overloads the motor for some part of the duty cycle. The OEM may tell end user customers that operation is normal if the motor does not exceed the SF current during that part of the duty cycle. Note that 6 Hp is not a NEMA standard rating. I will check to see what standard Hp ratings are for 184T frame. \$\endgroup\$
    – user80875
    Nov 20, 2020 at 14:00
  • \$\begingroup\$ Standard NEMA frames for 4 pole, 1 Ph, design L motors are 184T for 3 Hp, 213T for 5 Hp and 215T for 7.5 Hp. Even though the nameplate says continuous duty, this motor must be designed for the duty cycle of some specific load. \$\endgroup\$
    – user80875
    Nov 20, 2020 at 14:10
  • \$\begingroup\$ The 1.0 SF may be marked because it is standard for that motor. The current and power ratings may only be valid for the specific application that the motor was designed for. Operation at full load may be limited to something like being in the air flow of an HVAC blower that it is driving. \$\endgroup\$
    – user80875
    Nov 20, 2020 at 16:59
  • \$\begingroup\$ That's the correct issue. The motor SF is 1.0, because the rated current and torque is based on that 1.0SF. The OEM is planning on operating at a current above 1.0 x the continuous rating of the motor, so the motor mfr provided the motor current at the desired OEM operating point, but is acknowledging (to them and the end users) that this will have an affect on the life of the motor. This is not an uncommon thing with compressor motors in particular, because the compressor mfr knows that the motor will be cycling on and off while in use. Design wise, that's probably closer to a 5HP 230V motor. \$\endgroup\$
    – JRaef
    Nov 22, 2020 at 1:25

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