Why electric motors are rated in horsepower?

Introduction: As electrical professionals, many of us would have been handling electric motors in hundreds of numbers over the years in our careers. And, whatever the SI System of units might suggest, many of us would still prefer to refer to electric motors' power rating in horse power only rather than in watts or kilowatts as is prescribed in the SI System of units. But, how many of us know why this is so? Why electric motors are rated in horsepower? Why not in Elephant Power, as somebody asked this author?

The story: One would be surprised to note that the rating of electric motors in horsepower has nothing to do with any electrical professional in the first place. It all started with one James Watt. Recall him? He is a Scottish engineer, associated with the Steam Engine. In most of the children's general knowledge books, James Watt is wrongly credited with the invention of the Steam Engine. That credit goes to one Thomas Newcomen of England, who, in fact, had invented the Steam engine in 1705. James Watt only made improvements to the Newcomen Engine, to improve its efficiency and to make it commercially viable. This was in the year 1769.

Having made this improvement, Watt started manufacturing these engines, in a partnership with a businessman called Matthew Boulton and started looking for markets for his engine. Remember! It was the 18th century. And the industrial revolution was in its primitive stages in England. The main profession of world's population was only agriculture. The only customers, to whom Watt could sell his engine, were farmers. Watt started talking to farmers, with a view to market his engine to them. They wanted to know what his engine could do for them. Watt knew that an engine is a device that could do some work. What work that a farmer is more interested in? That of drawing water from a deep well and of irrigating his fields.

Farmers in India were (and are) using bulls and oxen for the purpose. But, in Europe and in England, farmers were using horses for the job. Hence, Watt pointed out to the farmers that his engine could do the work of the horses – that of drawing water from the deep well. But the farmers wanted to know how many horses that his engine would replace. Watt did not have an answer. For, he only knew what his engine could do and not what a horse could do. But, without answering this, the farmer wouldn't buy his engine.

So, he bought a healthy horse and wanted to find out what is the power of one horse. What is power? Power is the rate of doing work. Isn't it? What is work? Work is Mass x Displacement. Watt tied a weight to the horse that he bought and chased it to move. He found out that the horse was capable of doing about 550 foot-pounds (lb-ft) of work in one second. That is, it could move 550 pounds of weight over a distance of one foot in one second or 55 pounds of weight over a distance of 10 feet in one second.

But, for his engine, one second is too early to calculate. So, he converted the power of the horse into work done per minute. That is 550 lb-ft/sec. x 60 seconds (i.e.) 33000 foot-pounds per minute. That, in fact, is the power of one horse or one HP. When he tied the same weight to his engine and measured the work done in one minute, he could compare it with the power of the horse now. If his engine moved 66000 foot-pounds in one minute it was having the power of two horses or 2HP and so on. Now, he had an answer to the farmer.

Later, when steam engines were replaced by electric motors as prime movers, the custom of rating them in horsepower continued.

Subsequently, as a mark of respect to the great inventor, the British Association gave his name to the unit of electrical power, which, later was adopted by the SI System of units.
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