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

Posted by Joe on January 07, 2003 at 12:14:48 from (216.148.246.98):

In Reply to: Re: Re: Re: Re: Re: All HP still not created equal....??? posted by Steve - IN on January 06, 2003 at 14:27:37:

I see what you are asking. The reason is that HP is again -a DEFINITION based on measured units. Example: I hearby create the Joe HP which I define to be 33,333. You measure other units and derive HP you dont directly measure HP. It is defined as 550lb-ft/s for almost any use today, this is the value. Alot of your units, like boiler HP are units derived from long ago. How do you take a boiler full of steam and compare it to an electric motor? Someone determined way back when that the number listed in the beginning post was a close approximation. Not because some engineer just wanted to be different. Its because somebody who is not an expert with boilers, steam tables, and pressure curves needs to understand how to compare a boilers output with a simple number common with that of various other powerplants.

Now lets move to our freind the AC motor. Motors being electric and the common units for electric power being watts are usually calculated in metric units. The actual conversion of 550lb-ft/s converted to watts is 745.699. How ever someone decided to simplify things and set the standard electric horsepower to be 746 so that electricians dont need to mess with the decimal places. There is your variation from mechanical to electrical HP it has nothing to do with an electric motor being able to make more power.

Electric motors get their power from external sources. While your gas or diesel engine gets it from within. A piston or turbine engine has only a built-in size of displacement. Given fuel and air it can only produce a maximum amount of power per its size. You need a bigger engine to make more power. The electric motor gets all the power it needs from your electric service provider. Its a supply so big that as far as the motor is concerned it could be considered infinite. The only thing limiting the max power consumption of the motor is the circuit breaker. You can get way more horsepower out of an electric motor only because its that power comes from the outside. We push an old 5HP motor upwards of 12HP but only do it for short periods of time. WHat happens as stated in the previous post, they heat up and cook. Being less than perfectly efficient, they generate heat and therefore the the need for a larger motor to dissipate the heat. If someone were to develop a motor that was 100% efficient, the only size contstrain would be that the shaft and body and bearings be able to physically handle the load. Hope this makes more sense. Unless otherwise specified 1HP = 550lb-ft/sec = 745.69 watts or round up if your want and make it 746!

Now for the SAE rating on cars and the change in the early 70's. The rating before was gross HP. The engine was measured on the dyno with a tuned carb and headers with ideal air temp and humidty in test lab conditions. The standard was changed to make a more realistic number. Engine in car with full exhaust and mufflers. Standard airfilter installed and the whole drive train in place. Basically power as delivered to the rear wheels in a production car with accessories. Big difference there topped with the government starting to require the pollution controls is why you see a big drop in HP numbers when you look in those old reference manuals.

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