Otis Singleton
In the British Imperial System, one horsepower equals 33,000 foot-pounds of work per minute, or 746 watts in the International System of Units (SI). The term was coined by Scottish engineer James Watt in the late 18th century, based on his experiments with horses. Today, horsepower is used to describe the power output of electric motors and engines, with one hp being equivalent to approximately 746 watts or 0.746 kilowatts. In this context, horsepower refers to the maximum speed an engine can produce and its ability to maintain that speed. This concept is also applied to electric vehicles, where it is used to measure power output.
| Characteristics | Values |
|---|---|
| Definition | Horsepower is a unit of power that measures the work done or energy used over time. |
| Origin | The term was coined by Scottish engineer James Watt in the late 18th century based on experiments with horses. |
| British Imperial System | 1 horsepower = 33,000 foot-pounds of work per minute. |
| International System of Units (SI) | 1 horsepower = 746 watts. |
| Heat Equivalent | 1 horsepower = 2,545 BTU (British Thermal Units) per hour. |
| Metric Horsepower | 1 horsepower = 4,500 kilogram-meters per minute = 32,549 foot-pounds per minute = 0.9863 horsepower. |
| Boiler Horsepower | 1 boiler horsepower = thermal energy to evaporate 34.5 pounds (15.6 kg) of water at 212 °F (100 °C) in one hour. |
| Electrical Motors | Horsepower = volts x amps x rated efficiency / 746 watts. |
| Internal Combustion Engines | Horsepower is based on the amount of work done over a period of time. |
| Torque | Torque is the true metric of an engine's power. Mechanical horsepower = torque x engine rpm / 5,252. |
| Ambient Air Temperature | Horsepower is altered by about 1% for every 10°F change in air temperature. |
| Informal Usage | The term is sometimes used to describe a person's physical strength. |
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What You'll Learn
Horsepower is a unit of power
The term was coined by Scottish engineer James Watt in the late 18th century. Watt was studying the work of horses, which at the time were used to provide mechanical power for industry. He calculated that a horse could exert 33,000 foot-pounds of work per minute, which is equivalent to 746 watts or 0.746 kilowatts. This value, adopted into the British Imperial System, became the definition of one horsepower.
Horsepower can be used to measure the power of engines, motors, and other mechanical devices. It is also used to describe the amount of work done by machines such as generators and pumps. In the context of engines, horsepower is calculated from torque production working against a load, known as a brake. The faster an engine can perform the same amount of work, the higher its horsepower.
In electrical terms, horsepower refers to the power generated by electric motors. To determine the horsepower of an electric motor, one must know the voltage input, current draw in amps, and the rated efficiency of the motor. The formula for calculating the horsepower of an electric motor is: volts x amps x rated efficiency / 746 watts.
It is worth noting that horsepower is rarely used to express power in any form other than mechanical. While it is a useful metric for comparing the performance of engines and motors, other units such as watts and kilowatts are more commonly used to describe electrical power.
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How to calculate the horsepower of an electric motor
Horsepower is a unit of measurement for power, often used to describe the power of engines and other mechanical devices. One unit of horsepower is equal to 33,000 foot-pounds of work per minute, or 746 watts in the International System of Units (SI). The term is sometimes used informally to describe a person's physical strength.
When it comes to electric motors, horsepower can be used to describe the amount of power they can generate. A small electric motor might have one unit of horsepower, while a large one could have 10. To calculate the horsepower of an electric motor, you need to first determine its wattage. This is done by multiplying the amperage rating by the voltage rating. For example, let's consider a motor with a voltage rating of 460 volts and an amperage rating of 30 amps.
Now, we need to consider the efficiency of the motor. No motor is 100% efficient, and there are inherent losses in work potential that must be factored in. Motor efficiency ratings are usually represented as a percentage on the motor data plate. For our example, let's assume the motor has an efficiency rating of 85%. To use this value in our calculations, we need to convert it from a percentage to a decimal, which would be 0.85.
Finally, we can calculate the wattage of the motor by multiplying the amperage and voltage ratings, and then multiplying that result by the efficiency factor. In our example, the wattage would be calculated as follows:
460 volts x 30 amps x 0.85 efficiency = 11,730 watts
To get the horsepower, we divide the wattage by 746 (or 756, according to another source), as one unit of horsepower is equal to 746 watts.
11,730 watts / 746 = 15.71 horsepower, or approximately 15 horsepower when rounded down.
So, for this example motor with a voltage of 460 volts, an amperage of 30 amps, and an efficiency rating of 85%, we would expect a horsepower of around 15. It's important to note that when sizing a motor, other factors like speed, torque, frame size, ramp-up, and load should also be carefully considered.
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The history of horsepower
The history of the term "horsepower" dates back to the late 18th century when Scottish engineer James Watt conducted experiments with strong dray horses to develop a unit of power. Watt's calculations determined that one horsepower equalled 33,000 foot-pounds of work in one minute, which is about 50% more than the average horse's capacity in a working day. This unit of power was adopted as a standard in the British Imperial System.
The term "horsepower" was coined by Watt to make engine power more relatable and understandable to people. In the context of engines, one horsepower is the power necessary to lift a total mass of 33,000 pounds by one foot in one minute. This unit became particularly relevant with the development of the steam engine, which provided a comparison between the output of horses and the power of engines.
In the early 19th century, nominal horsepower (nhp) was used as a rule of thumb to estimate the power of steam engines. It assumed a steam pressure of 7 psi (48 kPa) and was calculated using the area of the piston and its speed. This was followed by the introduction of brake horsepower (bhp) or SAE gross horsepower, which was measured according to the standards set by the Society of Automotive Engineers (SAE).
However, in the 1970s, the term bhp fell out of use in the United States as automakers transitioned to quoting power in terms of SAE net horsepower, which accounts for engine accessories but not transmission losses. Around the same time, the electrical equivalent of one horsepower was standardised in the International System of Units (SI) as 746 watts, which is approximately equal to 0.746 kilowatts.
In the 19th century, revolutionary-era France used its own unit called the poncelet, based on a 100 kgf⋅m/s standard, to replace cheval vapeur (horsepower). This unit was abbreviated as "p". Similarly, tax or fiscal horsepower was introduced as a non-linear rating of a motor vehicle for tax purposes, directly related to the size of the engine.
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How horsepower is used to measure engine power
The term "horsepower" was coined by Scottish engineer James Watt in the late 18th century. Watt, who lived from 1736 to 1819, is renowned for his work on enhancing steam engine power output. The concept of horsepower was conceived while Watt was working with ponies used for lifting coal in a coal mine. He discovered that a draft horse could, on average, do 22,000 foot-pounds of labour in a minute. He then increased that amount by 50% and set the measurement of one horsepower at 33,000 foot-pounds of work per minute. In the British Imperial System, this is equivalent to the power required to lift a total mass of 33,000 pounds by one foot in one minute.
Horsepower (hp) is a unit of measurement in the foot-pound-second (fps or ft-lb/s) or English system, used to express the rate at which mechanical energy is expended. It is defined as the work done by a force of 550 pounds acting through one foot in one second, or the unit of power needed to raise 550 pounds one foot in one second. This is equivalent to 33,000 foot-pounds of work per minute.
Horsepower is commonly used to gauge the power of cars, trucks, and other vehicles, as well as to express the power generated by a machine, motor, or engine. It is also employed to assess the performance of engines in cars and even lawnmowers. The term "brake horsepower" (bhp) is used to refer to the horsepower at the output shaft of an engine, turbine, or motor. For instance, a small car may have a brake horsepower of 100, whereas a large truck could have a brake horsepower of 500.
Horsepower can also be used to measure the power output of electric motors and electric cars. A small electric motor may have a horsepower of 1, while a large one could have a horsepower of 10. One horsepower is approximately equal to 746 watts (W) or 0.746 kilowatts (kW).
While most countries now use the SI unit watt for power measurement, the use of horsepower is still permitted in the EU as a supplementary unit.
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The different types of horsepower
The term "horsepower" was coined by Scottish engineer James Watt in the late 18th century, following experiments with strong dray horses. Watt defined one horsepower as the amount of energy expended by a single horse lifting 33,000 pounds of water one foot in the air from the bottom of a 1,000-foot-deep well in 60 seconds. This value is about 50% higher than the rate at which an average horse can work in a day.
Over time, the term "horsepower" has evolved to encompass various types, depending on the specific application and region. Here are some of the different types of horsepower:
- Brake Horsepower (BHP): This is a measure of the power of an engine, taking into account internal frictional losses such as piston movement within the cylinder and bearing friction. It is calculated using a brake-type (load) dynamometer at specified locations like the crankshaft. In Europe, the DIN 70020 standard tests the engine with all ancillaries and the exhaust system, while older American standards used an engine without these components, resulting in higher figures.
- Indicated Horsepower: This was an earlier measure of engine power that considered steam pressure. However, it did not account for power losses due to internal friction.
- Nominal Horsepower (NHP): An early 19th-century rule of thumb for estimating steam engine power, assuming a steam pressure of 7 psi (48 kPa). It is calculated using piston area, piston speed, and a constant (33,000) in a specific formula.
- Boiler Horsepower: This measures a boiler's capacity to deliver steam to a steam engine. It is defined as the thermal energy rate required to evaporate 34.5 pounds (15.6 kg) of water at 212 °F (100 °C) in one hour.
- Equivalent Shaft Horsepower (ESHP): Used to rate turboprop engines, including the power derived from residual jet thrust from the turbine exhaust.
- Drawbar Horsepower (DBP): This is the power available to a railway locomotive or agricultural tractor to pull a load. It is calculated using a dynamometer car that measures drawbar pull and speed, from which the power generated can be determined.
- Tax or Fiscal Horsepower: A non-linear rating of a motor vehicle used for tax purposes, originally based on engine size but now often based on CO2 emissions.
In addition, horsepower is also used to describe the power output of electric motors and vehicles, measured in watts or kilowatts. One horsepower is equivalent to approximately 746 watts or 0.746 kilowatts.
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Frequently asked questions
The electrical equivalent of one horsepower is 746 watts in the International System of Units (SI).
To calculate the horsepower of an electric motor, you need to know the voltage input, current draw in amps, and the rated efficiency of the motor. The formula is: volts x amps x rated efficiency / 746 watts.
Horsepower is a measure of how much power an engine can produce and how well it can maintain that power output. Generally, the higher the horsepower, the faster the car.
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