Question on a practice test I can’t figure out how they got the answer.

[Rjsutt]

What is the efficiency and power of a 1 hp 208 single phase motor?
The answer is 40.7% but I can’t figure out how they solved it. Code refs or equations would be awesome.

 

[retirede]

That doesn’t make sense. There has to be more information given.

 

[Rjsutt]

That doesn’t make sense. There has to be more information given.

That’s what I was thinking. It must be a typo, because I feel like I need the output power to solve it.

 

[SceneryDriver]

What is the efficiency and power of a 1 hp 208 single phase motor?
The answer is 40.7% but I can’t figure out how they solved it. Code refs or equations would be awesome.

And how long is a piece of string?

There's definitely some info missing here. There's no way to figure out the answer with the info provided.



SceneryDriver

 

[don_resqcapt19]

My guess would be that they took (746÷208)÷8.8=0.40756. The 8.8 comes from the full load current found in Table 430.248 for 1 hp motor at 208 volts.

 

[wwhitney]

My guess would be that they took (746÷208)÷8.8=0.40756. The 8.8 comes from the full load current found in Table 430.248 for 1 hp motor at 208 volts.

In which case the question should be something like "What is the NEC-assumed minimum efficiency of a 1 hp 208 single phase motor?" Still not the best question, but at least fairly well defined.

Cheers, Wayne

 

[zbang]

What is the efficiency and power of a 1 hp 208 single phase motor?

The "power' part is easy- 1HP or 746 watts, but that's the rating, not the maximum power it will deliver (or consume). A lightly loaded 1 HP motor will consume and deliver less power. To know the actual effeciency, you need to know how much power is being delivered to the load itself.

It sometimes helps to think of a motor as a transformer- it has it's own losses just for existing, but the amount of power going in depends on the amount being pulled out. A 1000 watt rated transformer doesn't consume 1000 watts unless the secondary is delivering that almost much (accounting for losses).

 

[rambojoe]

And how long is a piece of string?

There's definitely some info missing here. There's no way to figure out the answer with the info provided.



SceneryDriver

What did i have for dinner last night....
Like i said in a previous post, its weird that everyone is posting test answer requests. Some moderator didnt approve of that statement- however- this one is a head scratcher..

 

[Jpflex]

I’m a

That’s what I was thinking. It must be a typo, because I feel like I need the output power to solve it.

lm also confused when it comes to the use of output wats 746 per 1 horse power and when this can be used divided by volts to get approximate amperes (excluding power factor not mentioned)

Or use table for amperes based on hp?

 

[zbang]

1 horsepower =746 Watts. Since they're both units of power, you can use Ohm's Law to convert between voltage/current/power. (The USA is about the only country that rates motors in HP, everyone else uses W or KW, or MW sometimes.)

 

[wwhitney]

lm also confused when it comes to the use of output wats 746 per 1 horse power and when this can be used divided by volts to get approximate amperes (excluding power factor not mentioned)

A motor converts electrical energy into mechanical energy. So you can use the above conversion where the HP is the output mechanical power, and it will tell you how much current the motor would need if it were 100% efficient at converting electrical energy into mechanical energy. But it's never 100% efficient.

Or use table for amperes based on hp?

The table will give you a conservative (generally higher than actual) estimate of how many amps the motor will actually need. The ratio of the computed current above to the table or measured current would be a measure of motor efficiency. [Not exactly the efficiency of energy conversion, unless the power factor is 1.]

Cheers, Wayne