# What is horsepower and how is it utilized with servo motors

June 23, 2017, by Hurley Gill

## Block and Tackle Series Volume 1 - Horsepower

Welcome to our Block and Tackle series on Blog in Motion! We went back to basics to help answer some questions covering a variety of topics. We pose the question – and then offer an answer. We welcome your feedback on other questions or topics you might have – just add a comment to this blog post and we’ll add it to our list. Here we go!

Question: What is horsepower (hp) and how is it utilized with servo motors?

Horsepower (hp) is a measure of power, which can be further described as the rate at which work is performed.  There are slightly different definitions for its conversion to the unit watts depending on the mechanism being described: mechanical, electric, boiler, metric, etc..   Our focus here will be on servo motor systems.

The SI unit Watt for the measure of power has been adopted by most countries of the world.  However, the unit hp often survives because it is typically utilized to specifically define an electric motor’s output capability, reducing potential confusion between  watts input, watts output [hp], and watts demanded by a load, while defining a system.

Just a side note – the watt (W) is named after a Scottish engineer, James Watt (1736-1819).  (Also, remember to use the capital letter for units that are named after a person; like amp (A), volts (V), etc.

For electric motors:  1 hp = 746 Watts.

The basic equation for calculating horsepower is:

The constant: 5252, is the rounded off calculation: 33,000 ft-lb/minute / 2π rad/revolution.

As the story goes, the standardized value of 33,000 ft-lb/minute was established in approximately 1783 by James Watt and Matthew Boulton based on the estimated output of a brewery horse

Since there are 1.35582 Nm/ ft-lb, the metric equivalent equation would be:

Horsepower (hp) defined by servo motor catalog data sheets is mostly used as a relative number for comparison purposes of continuous and/or peak capability when choosing a servo motor, since most applications require a knowledge of the required working torques and speeds.

Let us know if you have any questions on this or any other topic in motion!