Electrical question how to solve?

[junkhound]

Question:
What is the Amp load of a single phase motor at 240 volts with an efficiency rating of 83%?

How do I solve this and with what formulas do I use? Thanks.


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Forty-two

 

[ptonsparky]

Forty-two

no, four hundred twenty. you needed to multiply by the sq root of 100. You know that.

 

[JoeStillman]

I think they are looking for an answer like I = HP x K and solve for K. Assuming unity power factor,


Which is totally bogus if you look at table 430.248 where K ranges from 5 to 13.2 for 230V motors. You would need a power factor between 28% and 75% to come close to the table.

 

[Jraef]

I think they are looking for an answer like I = HP x K and solve for K. Assuming unity power factor,

View attachment 2563867
Which is totally bogus if you look at table 430.248 where K ranges from 5 to 13.2 for 230V motors. You would need a power factor between 28% and 75% to come close to the table.

But…
That “P” is the mechanical power that the motor is CAPABLE of, not the power it is ACTUALLY CONSUMING to do the intended work on the “load”. The original question asked for “amp load”, which to my mind implies the amps pulled under load. We do not know what the actual load on that motor is.

 

[Besoeker3]

SI is much simpler. I know, I know......

 

[ptonsparky]

But…
That “P” is the mechanical power that the motor is CAPABLE of, not the power it is ACTUALLY CONSUMING to do the intended work on the “load”. The original question asked for “amp load”, which to my mind implies the amps pulled under load. We do not know what the actual load on that motor is.

We also don't know if it's a 1 HP or 100 trying to do the work of a 5.

 

[Jraef]

We also don't know if it's a 1 HP or 100 trying to do the work of a 5.

True, but if the questioner was just looking for a formula rather than a value, then that is irrelevant. We are left to interpret what they wanted however…

My point is that EVEN IF what they want is the formula, then the best anyone can provide is the MAXIMUM amps that this motor could draw at its rated mechanical power, whatever that rating was. Hv&Lv posted the correct formula, other than the fact that “P(out)” is not a value that is going to be known unless something is actually measured.

So if you want to know A(load), why bother trying to measure P(out), why not just measure A(load)? Using a clamp on ammeter is not rocket surgery…

The real point is that this was, from the very start, a poorly worded question.

 

[ggunn]

The real point is that this was, from the very start, a poorly worded question.

That's the difference between exam questions and those in the real world, innit? An exam question might be to calculate the minimum wire size that could be used in a certain application under specific conditions, while the real world equivalent might be "Can I use some of the buttload of #10 that I have on the truck?"

 

[kwired]

That’s not correct
I got a 10kW motor. Divide by 1000??
Nope

Never mind.

bottom line you must convert kW to W then plug that value into the formula. I for some reason was going the wrong way with the conversion.

 

[kwired]

True, but if the questioner was just looking for a formula rather than a value, then that is irrelevant. We are left to interpret what they wanted however…

My point is that EVEN IF what they want is the formula, then the best anyone can provide is the MAXIMUM amps that this motor could draw at its rated mechanical power, whatever that rating was. Hv&Lv posted the correct formula, other than the fact that “P(out)” is not a value that is going to be known unless something is actually measured.

So if you want to know A(load), why bother trying to measure P(out), why not just measure A(load)? Using a clamp on ammeter is not rocket surgery…

The real point is that this was, from the very start, a poorly worded question.

As a stand alone question as we seen it yes. If on a test and is one question of several based on a previously mentioned situation it may be a valid question. At very least for such stand alone question we would also need to know the voltage rating to be able to come up with any valid amps answer though power factor still is the final detail that gets us more precision in the answer.

 

[JoeStillman]

But…
That “P” is the mechanical power that the motor is CAPABLE of, not the power it is ACTUALLY CONSUMING to do the intended work on the “load”. The original question asked for “amp load”, which to my mind implies the amps pulled under load. We do not know what the actual load on that motor is.

You're right. This is not an electrical engineering problem, it's an ESP problem.