Large Industrial Electric Motor Specialists

Why Are Electric Motors Rated By HP?

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Ever wonder why electric motors are rated by horsepower (hp), and not wattage or some other form of power rating? As an informed buyer, it’s important to understand the history and rationale behind electric motor horsepower ratings. This article will examine the key reasons HP persists as the standard designation today.

Horsepower Ratings Have Been Used For Centuries

Horsepower remains the predominant system for denoting electric motor output for several reasons deeply rooted in history and marketing. Dating back to the early 20th century when motors began replacing steam engines and water wheels, electric companies found horsepower an effective sales tactic as a point of comparison.

Since mechanical power was rated in HP, it was natural to market the new electric motors in the same terms. This consistency enabled direct comparison to the old power sources that electric aimed to displace.

Even as electric motors improved and became more ubiquitous over the decades, horsepower ratings stuck around. Since horsepower measures work over time, it leaves room for approximation and vagueness in motor specifications, which manufacturers prefer. Listing peak horsepower also allows branding motors as more powerful than precise wattage ratings would suggest.

The familiarity of HP as well makes it seem less foreign to consumers than ratings in watts or kilowatts.

Electric Motor Testing Standards

Today, the persistence of horsepower for electric motors also comes from standardized testing procedures. Groups like NEMA (National Electrical Manufacturers Association) and IEC (International Electrotechnical Commission) have long-established methods for measuring, validating, and certifying the horsepower output of electric motors based on torque and rotational speeds.

Adhering to these testing standards allows manufacturers to provide consistent and verifiable motor performance ratings to consumers. While wattage ratings can also be tested and confirmed, the apparatus and methods for horsepower testing were institutionalized first. Changing to new testing procedures exclusively for watts would incur significant costs and disrupt decades of standards.

Matching Mechanical Power Ratings

Electric motors must integrate into existing mechanical systems and often directly replace previous power sources. Using the same horsepower rating conventions enables easier matching of electric motor capabilities to power transmission components, gasoline engines, and any other devices originally rated in HP.

A 5 HP electric motor can reliably swap places with a 5 HP diesel engine in a piece of industrial equipment, for example. Keeping the same rating terminology avoids needless complications when substituting electric for mechanical power. Introducing new wattage ratings would disrupt engineers’ ability to directly cross-reference and swap between power sources.

Horsepower and Watts Are Interchangeable

It’s important to note that horsepower and watts provide equivalent information. There is a direct conversion where 1 HP = 746 Watts. The units describe the same power output, just on different scales.

With this easy conversion in mind, horsepower can be viewed simply as a legacy naming convention rather than an fundamentally different rating system.

Many motor manufacturers will provide power specs in both HP and kilowatts to satisfy all customers. There are also online calculators that can convert between these units instantaneously.

So ultimately, the use of horsepower vs. watts is not an indicator of actual motor performance. Knowing the quick conversion allows you to aptly compare specs in either rating system.

Commonly Asked Questions About Electric Motor Horsepower Ratings

Below we will dive into the common questions surrounding the rationale and implications of horsepower metrics for electric motors. By answering these questions, we will gain a deeper understanding of the lasting convention of rating motors by horsepower.

Why not just use watts if it’s more precise?

While wattage is a more exact measure of power, horsepower allows for approximating a range of output. Motors can be advertised as 1 HP while varying slightly above or below the equivalent 746 watts in real-world use.

Are horsepower ratings inflated?

Manufacturers can exploit vagueness in HP ratings to suggest motors are more powerful than their true output. But certified testing procedures work to ensure accuracy and standardization. Published horsepower specs must meet validation criteria.

What unit is better for making calculations?

For engineering calculations requiring precise power values, watts are always preferable. The exactness of watts makes them better suited for mathematical computations. Horsepower should be converted to watts first before doing power, torque, efficiency or other motor calculations.

Does more horsepower equal better performance?

Not necessarily. A higher horsepower motor may have lower output at certain RPMs or be oversized for a given application. Multiple factors beyond just power affect real-world motor performance. Horsepower serves mainly as a capability benchmark.

Should I only look at horsepower when selecting a motor?

While crucial, horsepower alone does not provide enough details to ensure an optimal motor choice. Consider factors like voltage, speed ranges, start-up current, torque curve, mounting, and other specs in addition to power ratings.

Summary – Horsepower Ratings For Electric Motors

In summary, horsepower persists due to tradition, familiarity, and alignment with testing standards. But it effectively conveys motor capability all the same. Knowing the reasoning behind HP ratings helps make sense of electric motor specifications.

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