1 - 4 of 26 Posts

#### MattsAwesomeStuff

Joined
·
1,794 Posts
Watts per Horsepower also seemed a bizarre declaration of efficiency to me.

But... if we're being picky about units and equivalencies and such...

Major said:
Efficiency is defined as the power output / power input * 100%. Of course you use the same units of measure for the numerator and denominator. So eff = watts out / watts in * 100%. Or eff = hp out / hp in * 100%.
100% is equal to 1. Yes?

In which case, why would you just multiply something by 1?

I'd amend it to: Efficiency is defined as the power output / power input * 100 ... as measured in percent... if you need that added

Myself I'm fine with saying/reading "Efficiency is 0.75" as it's an implied fraction in the concept of efficiency itself, but most people would want to read "Efficiency is 75%".

But that % should be stricken from the formula, no?

#### MattsAwesomeStuff

Joined
·
1,794 Posts
It's not about being picky about units. You miss the point. "Watts per horsepower" is not a "bizarre declaration of efficiency." It is a units conversion equal to 746w/hp, always.
3/4 of the time you "correct" me, you seem to think I misunderstand something that I don't.

I understand that watts and horsepower both are units that measure power, just different scale.

I'm saying I found it bizarre (my opinion) that Sunking tried to express efficiency (which he did, expressly) by using "watts/hp" as that doesn't make much sense.

There is zero confusion and I haven't "missed the point."

#### MattsAwesomeStuff

Joined
·
1,794 Posts
Guess who got an A+ from the teacher.
Again, I find that bizarre. The "genius" of this solution exists only when the question is perfectly framed for that exact answer, as it's 74.6% efficient and there are 746 watts in a horsepower. Any other numbers and you're doing just as much calculation, only now you have unit conversion on top of it.

A sensible real world problem would say: "We have a 7460 watt motor that is 74.6% efficient, how many watts does it take to run?" With the answer being "Gee, 10kw."

But at that point all you're calculating is efficiency, which has almost nothing to do with motors and such specifically. It could be relevant to anything.

...

I've heard other esoteric things, like, the way you know that a contactor is rated for motor loads is if it shows a Horsepower rating in addition to the wattage rating. I don't know if that's true, but it seems silly. If it's rated for motor use, it should just say that, or say Inductive, or, something. Not just change the units and imply that that means motors.

#### MattsAwesomeStuff

Joined
·
1,794 Posts
Not sure what your beef is,
Huh? I have none.

It's odd to me that you think "someone has a different opinion than me, why don't they like me?" and presume some motive of aggression, rather than simply, "I said somethign and someone said something in reply". It's a discussion forum. Just commenting. I find that way of doing things bizarre. It's not an attack, I'm not telling you that you can't or that you're wrong. It's just something that came to mind based on what you said.

I suppose it's a neat trick that with DC motors you can kind of ballpark that 1hp of output requires 1kw of input, but, beyond a chuckle, it's a bit funny to be using them in tandem, one to refer to output the other input, when they're both the same thing.

Probably a generational thing too.

1 - 4 of 26 Posts