Calculating SFA
Calculating SFA
(OP)
I tried to figure out the SFA of a motor using the following formula,
SFA = SF*HP/[SQRT(3)*Vll*EFF*PF]
Whereas,
SFA = Service factor Amps
SF = Service factor
SQRT(3)= 1.732
VII = line voltage
EFF = Efficiency
PF = Power Fator
Considering
SFA = 1.15*5HP / (1.732*220*0.75*0.81)
= 5.75 / 231.48
SFA = 0.02484
I would like to know how to arrive the SFA after this ?
SFA = SF*HP/[SQRT(3)*Vll*EFF*PF]
Whereas,
SFA = Service factor Amps
SF = Service factor
SQRT(3)= 1.732
VII = line voltage
EFF = Efficiency
PF = Power Fator
Considering
SFA = 1.15*5HP / (1.732*220*0.75*0.81)
= 5.75 / 231.48
SFA = 0.02484
I would like to know how to arrive the SFA after this ?





RE: Calculating SFA
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Calculating SFA
SFA = 1.15*3700 / (1.732*220V*0.75*0.81)
= 4255W / 231.48W
SFA = 18.38A
RE: Calculating SFA
RE: Calculating SFA
RE: Calculating SFA
SF = 1.15
FLA = 16.3A
SFA = 16.3*1.15
SFA = 18.745
but Nameplate SFA = 19.5A.
So i contacted the manufaturer & they replied that exact SFA cannot be arrived directly multiplying FLA & SF but the actual procedure is as follows,
SF is calculated based on the Motor power
eg: 5HP = 3700Watts
SF = 1.15 = 15%
3700 + 15% = 4255W
So the motor will be loaded to 4255W and current consumed in that load is declared as SFA.
RE: Calculating SFA
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Calculating SFA
RE: Calculating SFA
If you do not have the info giving the efficiency, power factor and current at 115% load then you really don't have the info to calculate it anyways, no matter how you try to do it. The efficiency and power factor at 115% load will be different than the 100% load values as you have already pointed out.
As Jraef has pointed out, current is what heats the motor so it is fairly safe to use the service factor times the rated current to come up with the allowable current. However, NEMA MG-1 does define service factor as the allowable over power rating of the motor, ie you can operate at 1.15 times the rated power of the motor. But, NEMA MG-1 also is vague about what the service factor means and basically says you can operate in the service factor but it will reduce the life of the machine.
If you used nameplate values and got 18.4A calculating yet the manufacturer says it is 19.5A then the power factor and the efficiency do change and it does make a difference.
Maybe if you told us what you want this number for then we could tell you if it makes a difference or not. Lots of cheap overloads, for example, have errors in the current setting dial and measurement accuracy that are greater than the difference you are concerned with.
RE: Calculating SFA
RE: Calculating SFA
Can you elaborate on how the SFA is being used to select the cable?
RE: Calculating SFA
Assuming the poster "truthfully answered what the numbers are for", I think we need to help this guy understand why he is wrong to think that way!
friend81,
I'm befriending you now! Get a copy of NEC or Google "cable sizing".
RE: Calculating SFA
The same pumpset when installed in testpit with same cable is consuming the SFA 19.5A.