OK. Give your opinion.
Motor load calculation
- Thread starter rozana81
- Start date
- Status
When calculating actual power drawn by the motor, I would use nameplate data over NEC tables.
However, if I were sizing overcurrent protection devices I would use the NEC tables.
From my (limited) experience so far is that values in NEC tables are almost always higher than the nameplate data, and that is for safety purposes. Better to be oversized than undersized.
However, if I were sizing overcurrent protection devices I would use the NEC tables.
From my (limited) experience so far is that values in NEC tables are almost always higher than the nameplate data, and that is for safety purposes. Better to be oversized than undersized.
When you multiply output horsepower by 746, you are converting power from the mechanical world to the electrical world. If you are calculating efficiency by taking input watts and dividing by output horsepower then the units will not match up, so therefore you convert horsepower to watts.
This whole argument is pointless.
And as far as Watts, VARs, and VAs go: only Watts provide any real useful work, but all three are still considered units of power.
This whole argument is pointless.
And as far as Watts, VARs, and VAs go: only Watts provide any real useful work, but all three are still considered units of power.
Last edited:
And my wit was wasted.........
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
It depends on what you mean by "actual power".
To measure actual power, one would use a watt meter
To measure power for nec purposes, one would use the fLA from the nec tables.
To size a motor to say an off grid inverter or generator, or to size conductors for voltage drop, I would use an amp clamp while the motor is running at the load it will see in service (then there is the inrush which is another topic)
When saying 'actual' power, I mean the Watts that are consumed by the motor while it is running, used for efficiency purposes. You are right, to get the most accurate reading you would take a watt meter and measure the incoming power.
But if you do not have a power meter, then to calculate efficiency I would take motor hp x 746 to get output power. Input power is voltage x current (nameplate) x pf. Divide output power by input power and you have efficiency. All I was saying is that motor name plate FLA would give you a more accurate number for efficiency, because FLA values in NEC tables are often higher. Plus no two motors are the same.
Last edited:
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
What do you need to know the efficiency for? I think you would need a dynamometer to get an accurate efficiency measurement.
There were earlier mentions in this post about efficiency calculations, and what 'power' really is.
And yes, physical instruments are often the best ways of getting the most exact values. However, calculations can get you very close (at least close enough), especially if you don't have a dynomometer and would have to rent one.
tc777
Member
- Location
- Avenal,Ca. 93204
motor load calculation
motor load calculation
change 1/3 to decimal,o.333,then multiply by 746watts, 248.418 watts
motor load calculation
change 1/3 to decimal,o.333,then multiply by 746watts, 248.418 watts
GeorgeB
ElectroHydraulics engineer (retired)
- Location
- Greenville SC
- Occupation
- Retired
I believe you need to recognize facts, not what you want facts to be.
S in va total volt-amps
P in watts POWER, portion of va doing work
Q in var portion of va NOT doing work
That va is higher than watts increases losses in conductors.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Well, IMO, if they represent neither an instantaneous rate of energy transfer nor an average rate of energy transfer over time (full cycles), then they do not credibly meet the definition of "rate of transfer of energy". Might that be your view too?
kwired
Electron manager
- Location
- NE Nebraska
This is what OP's intentions were with his original question.
I believe he had been corrected that we use VA for the panel load calculations. With no power factor correction downstream of the panel, that means NEC FLA table values is what should be used. The actual power drawn is still going to depend on motor efficiency, power factor, and actual driven load conditions, but those table values give us worst case we should ever encounter as long as same motor type and horsepower are being utilized.
Power System Analysis & design
Glover & Sarma
you can have your own opinion but not your own facts
no one is debating whether S, P, Q does 'work', net average, instantaneous or otherwise
only that S, P and Q are forms of power, as are mechanical, chemical, etc.
- Status