Motor Service Factor Question

Per whatever standard you're referencing, does the service factor applying to HP (not current), only hold true when voltage is equal to nameplate value?

Never mind, I answered my own question. You must be referencing this standard?

The National Electrical Manufacturers Association (NEMA) defines service factor in section MG1 – 1.43 of their manual as: “The service factor of an alternating current (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.” The conditions under which service factor may be applied are described in NEMA MG1 – 14.36 as: “When the voltage and frequency are maintained at the value specified on the motor’s nameplate, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.”

So in order for service factor to apply, V and F have to match nameplate. They're constants.
P=I×E.
If E is a constant, then there is only one variable which can affect P, and that is I. Current.
So your argument is nothing but semantics. Service Factor is based on Current (at nameplate voltage), and therefore HP (at nameplate voltage). And the way you broadly stated it to be based on HP alone (no mention of nameplate voltage) makes the argument less than true.
 
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Never mind, I answered my own question. You must be referencing this standard?



So in order for service factor to apply, V and F have to match nameplate. They're constants.
P=I×E.
If E is a constant, then there is only one variable which can affect P, and that is I. Current.
So your argument is nothing but semantics. Service Factor is based on Current (at nameplate voltage), and therefore HP (at nameplate voltage). And the way you broadly stated it to be based on HP alone (no mention of nameplate voltage) makes the argument less than true.

I haven't made an argument. The Service Factor standard is what it is. You should not apply the SF for a motor that is designed to run on a variable frequency drive, nor should you apply the SF for a motor that is designed to operate in a very high altitude location, due to lower density ambient air providing less efficient cooling.

That said, the SF relates to HP, rather than to motor amperage. Of course, horsepower and amperage are inherently intertwined, but you will calculate an invalid maximum amperage value for a motor by applying the SF to amperage, rather than to HP.
 
I haven't made an argument. The Service Factor standard is what it is. You should not apply the SF for a motor that is designed to run on a variable frequency drive, nor should you apply the SF for a motor that is designed to operate in a very high altitude location, due to lower density ambient air providing less efficient cooling.

That said, the SF relates to HP, rather than to motor amperage. Of course, horsepower and amperage are inherently intertwined, but you will calculate an invalid maximum amperage value for a motor by applying the SF to amperage, rather than to HP.

That's because of what was left out of the copied text from NEMA MG-1:
When the motor is operated at any service factor greater than 1, it may have efficiency, power factor,
and speed different from those at rated load, but the locked-rotor torque and current and breakdown
torque will remain unchanged.
A motor operating continuously at any service factor grater than 1 will have a reduced life expectancy
compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by
the service factor load.
The only way to get more HP from the same motor is to increase torque and/or speed and in a motor running across the line, you can't increase speed. So in order to increase torque you must increase current, which means copper losses will increase and power factor will decrease. So the relationship to an increased HP at the shaft is NOT a 1:1 relationship with the increase in HP. At 1.15SF HP then, the current drawn by the motor will likely be HIGHER than 1.15x FLA.

Starting in 2002, NEMA MG-1 requires that motors designed with a service factor of greater than 1.0 must now list the SFA (Service Factor Amps) in addition to the SF itself, because of this very thing.
 
That's because of what was left out of the copied text from NEMA MG-1:

The only way to get more HP from the same motor is to increase torque and/or speed and in a motor running across the line, you can't increase speed. So in order to increase torque you must increase current, which means copper losses will increase and power factor will decrease. So the relationship to an increased HP at the shaft is NOT a 1:1 relationship with the increase in HP. At 1.15SF HP then, the current drawn by the motor will likely be HIGHER than 1.15x FLA.

Starting in 2002, NEMA MG-1 requires that motors designed with a service factor of greater than 1.0 must now list the SFA (Service Factor Amps) in addition to the SF itself, because of this very thing.

Thank you for explaining. That actually made the thread constructive.
 
At 1.15SF HP then, the current drawn by the motor will likely be HIGHER than 1.15x FLA.

Which may explain why the code states something about being allowed to set overloads "not higher than 125% of FLA" under certain conditions. I don't have the book handy to check.

Thanks to everyone for sharing your expertise- I've learned a lot!
 
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