Motor load calculation

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rozana81

Member
Location
keller, tx
I wonder how to calculate the actual power of a motor

For example:

1/3 hp motor rated at 230V, 1phase

Ampere listed for 1/3 hp motor in NEC table is 3.6A

So the watts is 230X3.6Ax 0.8(p.f) = 828 Watts

or is it (1/3)x746Watts = 249Watts (since 1hp is equal to 746 watts)

Which one is right?
 
FionaZuppa

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
actual power? amp clamp it under load.

to calculate "actual" you'll need some expensive software and test tools.
 
I

Ingenieur

Senior Member
Location
Earth
You have 2 forms of power
electrical input which is the v i pf
the mechanical shaft is 1/3 hp or 249 w

the efficiency is 249/828=30%
all based on nameplate, not actual
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Panel Schedule kW = HP x .746 / motor efficiency. For a small motor like that, I use .7 eff to be safe, a lot of small motors can be that bad and it's better to be worse in that value.

PF is already in the W or HP value, you don't need to do that again..
 
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Ingenieur

Senior Member
Location
Earth
FLA 3.6 A
va = 3.6 x 230 = 828 va
w = 828 va x 0.8 = 662 w

shaft power 1/3 hp
shaft power 1/3 hp x 746 w/hp = 249 w

eff = 249/662 = 38% (His math for power is incorrect)

shouldn't the 828 va be used since that is reflective of the load the xfmr or service must supply?
 
kwired

kwired

Electron manager
Location
NE Nebraska
rozanashaji said:
I wonder how to calculate the actual power of a motor

For example:

1/3 hp motor rated at 230V, 1phase

Ampere listed for 1/3 hp motor in NEC table is 3.6A

So the watts is 230X3.6Ax 0.8(p.f) = 828 Watts

or is it (1/3)x746Watts = 249Watts (since 1hp is equal to 746 watts)

Which one is right?
Click to expand...
NEC tables cover about the worst case power factor and efficiency you would ever find, so in your case your motor will not likely draw any more then 230v x 3.6A = 828 VA (I double checked and your .8 pf you tried to pile onto that must not have been entered correctly when you did that - but you still need efficiency as well to get true power). No need to know power factor or efficiency for the panel schedule. Now if you were correcting the PF (on load side of the panel) then you may consider that impact on actual VA for your panel schedule.
 
P

Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Fiona...

Not necessarily! I can be done with simple measurement of Ph-Ph voltages and Line-current ampere magnitudes (even if unbalanced) as long as you have some knowledge of the motor's pf!

Regards, Phil Corso
 
Besoeker

Besoeker

Senior Member
Location
UK
rozanashaji said:
I wonder how to calculate the actual power of a motor

For example:

1/3 hp motor rated at 230V, 1phase

Ampere listed for 1/3 hp motor in NEC table is 3.6A

So the watts is 230X3.6Ax 0.8(p.f) = 828 Watts

or is it (1/3)x746Watts = 249Watts (since 1hp is equal to 746 watts)

Which one is right?
Click to expand...
The 249W is correct for output power or shaft power as it's sometimes called.
The 828 is the input VA, not power. Important distinction. It takes account of motor power factor and efficiency.
The 3.6A tells you how to ought to rate conductors and other circuit components.
 
Besoeker

Besoeker

Senior Member
Location
UK
kwired said:
NEC tables cover about the worst case power factor and efficiency you would ever find, so in your case your motor will not likely draw any more then 230v x 3.6A = 828 VA (I double checked and your .8 pf you tried to pile onto that must not have been entered correctly when you did that - but you still need efficiency as well to get true power). No need to know power factor or efficiency for the panel schedule. Now if you were correcting the PF (on load side of the panel) then you may consider that impact on actual VA for your panel schedule.
Click to expand...
No real beef with that but would you really want to correct the PF of such a small motor?
 
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