Motor amperage
- Thread starter fdes1879
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
NEC Table 430.248 gives a value of 20 amps. That table does not tell us anything about efficiency. The actual and exact value that you would measure can be anything. It depends on how the manufacturer built the motor, what it is connected to (i.e., its mechanical load), how old the motor is, what its environment is (temperature, humidity, etc.), and a hundred other things.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The efficiency tells us exactly how much electrical energy is needed to produce a mechanical output of 1.5hp.
But the current will depend on the power factor of the motor, which is not directly determined by the efficiency.
A motor can have a PF of .5 and still have an efficiency of 90% because the reactive current is not delivering any power.
And in fact some changes that make a motor more efficient can also lower the power factor.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Maybe it was a trick question, because the answer is, there is not enough information to give an EXACT answer, just an estimate.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What answer did you come up with? Sounds like they were asking you to use the 746 watts/HP calculation.
kwired
Electron manager
- Location
- NE Nebraska
Which is fine if they want you to exercise using that formula - they were still short on information to do it though, power factor is also important here.
If multiple choice question maybe answer selections were more obvious to what they wanted?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I agree the question is vague but since no power factor is given why not assume that it's 1.0?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Have you ever seen a single phase AC motor that runs at a PF of 1.0? I haven't. If it's a Shaded Pole motor, the PF could be as low as .6 and if it's a universal motor, it could be as high as .95, so without knowing what kind of motor it is, even guessing is fraught with inaccuracies.
A = kW/VxPFxEff. The test question only gave 2 out of 3 necessary factors. So whatever you come up with will be an estimate, not an exact value.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
No, but sometimes test or practice questions are hypothetical. It doesn't say that the PF isn't 1.0 and does not give another value. The person who wrote the question had something in mind. Assuming that the PF is 1.0 makes the problem workable even if the question is poorly written.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yeah, what was I thinking. Why would someone let reality get in the way of a good test question? :roll:
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
In my opinion, if the motor is single-phase 115 V [60 c/s] rated the rated current it could be:
I=746*HP/Vrated/efficiency/power factor
power factor: 0.66-0.75
then, I=14.4-16.38 A
I=746*HP/Vrated/efficiency/power factor
power factor: 0.66-0.75
then, I=14.4-16.38 A
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- Massachusetts
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