Motor amperage

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fdes1879

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What is the exact amperage on a 1.5hp, 115volt motor at 90 percent efficiency?


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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.
 
fdes1879 said:
What is the exact amperage on a 1.5hp, 115volt motor at 90 percent efficiency?


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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.
 
fdes1879 said:
What is the exact amperage on a 1.5hp, 115volt motor at 90 percent efficiency?


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What answer did you come up with? Sounds like they were asking you to use the 746 watts/HP calculation.
 
infinity said:
What answer did you come up with? Sounds like they were asking you to use the 746 watts/HP calculation.
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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?
 
I agree the question is vague but since no power factor is given why not assume that it's 1.0?
 
infinity said:
I agree the question is vague but since no power factor is given why not assume that it's 1.0?
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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.
 
Jraef said:
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.
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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.
 
infinity said:
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.
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Yeah, what was I thinking. Why would someone let reality get in the way of a good test question? :roll:
 
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
:)
 
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