SERVICE FACTOR

[jthjr]

I have moticed exhaust fan motors with HP ratings from 2 on down with service factors of 1.35. I then see return and supply fan motors with a HP rating from 7.5 to 20 with service factors at 1.15. Is there a general rule for the service factor to be higher in motors with a smaller HP? Would the application be the case?

 

[renasis]

jthjr most nema frame motors sold today are 1.15 sf. This is the std from fractional hp-350hp. I do know what you are talking about w/the smaller mtrs though. I have seen them rated for 1.35 sf and other ratings higher than 1.15. When you get down into the smaller mtrs you are talking about less profit. So, it may be beneficial to the company to run 1.0 hp mtr and rate it for 3/4 hp with a 1/.75 + .15=1.5sf.
Mtr companies do derate like this. They could sell the same mtr as a 1.0 hp with a 1.15 sf. Get what I am saying? We are talking about production runs. With larger mtrs(400+) they give you the option of a 1.0 sf or 1.15 sf. Copper wire could be a major part of the cost in a larger mtr, but not so much in a smaller mtr. I could be proved wrong though, it may be application.

 

[jbartos]

Suggestion: Motors manufactured to meet NEMA MG-1 will have single phase motors and three phase motors with higher service factors, e.g.
1HP down to 1/2HP SF=1.25
1/3HP down to 1/4HP SF=1.35

 

[aolalde]

jthtr:
You are right, small motors are manufactured with higher Service factors (1.25 to 1.5 or larger) as compared to motors (2 HP and up), 1.0 to 1.15 SF.
The reason has to be the volume of materials involved and the cost reduction for massive production of reduced stock of designs covering a broader range of applications.

 

[jbartos]

Suggestion: The added service factor greater than one to the motor rating can be interpreted as the higher rated motor with the lower rated or nameplate HPs shown.

 

[SteveKW]

They also purposely play with the HP and SF as a selling gimmick too. Fractional HP pump manufacturers play this game all day long with motors labeled 1 HP but have a 1.5 SF. Then they can turn around and say how much MORE water their pump can do compared the competitions 1 hp, 1.15 SF pump!

Compressors are another area they play with HP labeling. One I see a lot is the "5 HPSPCL" which is just a 3 HP motor relabeled as a 5 special.

 

[jbartos]

Comment: The cost of electricity consumption, when running motor with the higher service factor, may increase if the shaft load is designed within the service factor margin, i.e. above the motor nameplate HP.

 

[buzzp]

I think the SF also has to do with the mass of the motor. Depending on who you are, the over load may be set at the service factor amps or the full load amps. In any case, the overload is supposed to account for thermal capacity. This includes cooling and heating. When the motor shuts down it takes x amount of time to cool in order to start again (enough thermal capacity). Since smaller HP motors have less mass, they cool quicker (or heat up quicker for that matter) and they move heat away quicker than a larger motor. Thus, they can run at a relative higher current than a larger motor. This is just my thoughts and I have nothing to point you too but it makes sense to me.
I have always seen lower HP motors have a larger SF than a larger one (according to NEMA this is the case) but there is likely an exception somewhere.