Horsepower for Dummies, or The Practical Explanation of How Machines Are Like Horses

[Downunder Bob]

The original James Watt horsepower is the same one that converts to 746 watts. The horses that he used to define the measurement were genuine British pit horses, these horses were bred for doing a days work in the mine pits, hence the name use a number of horses and then calculated the average. The SAE, DIN etc are just different countries and engineering societies methods of measuring HP.

 

[hman]

To a farmer. 4 tons of horse chips is way more useful than 4 tons of machining chips.
To a smelter, 4 tons of machining chips is way more useful than 4 tons of horse chips.
It just depends on what kind of chips float your boat. {Unless your name happens to be Biff.}

... or what kind of smell-ter you prefer.

As I recall, Marty did this to Biff a whole bunch o' times.

 

[Dabbler]

If I might add to the confusion: most motors try their best to convert their 746 watts to 550 ft-lbs/sec - however that is purely theoretical! IF you are at that motors rated slip, you are SUPPOSED to get the rated efficiency in work output, but in my experience that is never the case, and providing the perfect slip angle is impossible anyway. All my machines are powered by puny underrated motors, and i like it that way - it is harder to hurt yourself, something I am regularly acquainted with! (things just take longer)

 

[Janderso]

Very entertaining Montana,
You made me smile

 

[Illinoyance]

I read recently how Sears Roebuck calculated horsepower for AC motors. The multiplied locked rotor current by line voltage and divided by 746.

 

[Downunder Bob]

I read recently how Sears Roebuck calculated horsepower for AC motors. The multiplied locked rotor current by line voltage and divided by 746.

This is a very bad way to measure HP as it will give a much overrated HP. Locked rotor current is much higher than max rated current, and will cause the motor to be rapidly burned out. If the motor has the correct fuse installed the fuse will blow, to prevent the motor burn out.

HP should be calculated using max current at rated speed, which should be about the same as rated current as shown on specs plate.

 

[higgite]

I read recently how Sears Roebuck calculated horsepower for AC motors. The multiplied locked rotor current by line voltage and divided by 746.

This is a very bad way to measure HP as it will give a much overrated HP......

I believe that was probably Sears' intent.

Tom

 

[Illinoyance]

Bob, you are absolutely correct. Obviously Sears way of calculating horsepower was cheating. If you tried to operate a motor at full nameplate rating it would not take long for the magic smoke to escape.