Re: Re: Re: Re: Re: All HP still not created equal....???

Posted by Steve - IN on January 06, 2003 at 14:27:37 from (12.222.17.160):

In Reply to: Re: Re: Re: Re: All HP still not created equal....??? posted by Joe on January 06, 2003 at 12:27:50:

Joe,
You're right in your examples of torque and rpm varying yet giving the same horsepower. The thing Wayne is trying to get a handle on is why different types of torque generating devices produce different 1 hp results in terms of ft. lbs of work / minute. If we ignore the foreign measurement kind of stuff, there are still different kinds of torque generating machines that have different ratings for 1hp. You bring up electric motors in your last sentence, so let's take them as an example.

Say we're the IEEE making rules for rating motors. Those SAE mechanical engineer guys cheat for specsmanship and selling cars, we know, 'cause they rate their engines at their best operating rpm - not an average horsepower over all normal operating rpm - which is closer to the truth. Those boilermaker guys and their steam are dead meat anyway, so won't do the trick they use. We electrical engineers are better than that. And we know a motor starts up and has about the same horsepower all across its rpm range. No fun. But our motors have a magic property those car and tractor engines don't. They'll do more work - make more horsepower - when you put then under a bigger load. They won't do it for long, maybe an hour or two of producing more hp, then they'll burn themselves out and turn very dead -- but they will do it for short periods of time with no harm. So how do we take than magic property into account without being outright liars by describing 1 hour wonders, and still not sell short the capabilities of our motors. Answer is we get a committee together and fudge the numbers a bit so the magic of the motor shows a bit, and rate it continuous - except for that wee little bit it might sometimes be called upon to do more than expected. So our definition of a horsepower ends up being 13 more ft. lbs/min more than the SAE guys use, because a motor can rise to the occassion. Kinda like Watt's theoritical horse with an adrenalin injetion. They must make more power when they're running from a wolf than they do when they're just foolin'around or getting bored lifting coal. Make sense?

So I don't get any nasty email from guys with ieee.org addresses, I'm not saying it actually happened that way - I'm just giving a supposition to explain why any standards group might want to stretch things a bit to show off the unique properties of their type of machine.

Steve

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