One of the more confusing aspects of selecting a electric motor to replace a gas engine motor is understanding HP, horsepower, and torque.
From How Stuff works:
“The term horsepower was invented by the engineer James Watt.”
“Horsepower can be converted into other units as well. For example:
- 1 horsepower is equivalent to 746 watts. So if you took a 1-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts.
- 1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).
- One BTU is equal to 1,055 joules, or 252 gram-calories or 0.252 food calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.”
“Imagine that you have a big socket wrench with a 2-foot-long handle on it, and you apply 50 pounds of force to that 2-foot handle. What you are doing is applying a torque, or turning force, of 100 pound-feet (50 pounds to a 2-foot-long handle) to the bolt. You could get the same 100 pound-feet of torque by applying 1 pound of force to the end of a 100-foot handle or 100 pounds of force to a 1-foot handle.
Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft. A dynamometer measures this torque. You can easily convert torque to horsepower by multiplying torque by rpm/5,252.”
I found a great source of information on HP and comparing electric and gas motors at Yahoo Answers.
Best Answer – Chosen by Voters from Retired EE
“One horsepower output from a shaft is the same amount of power no matter what is turning the shaft. One horsepower is 746 watts, 746 joules per second, 746 newton-meters per second or 550 foot-pounds per second. I don’t know how anyone could say that 1 hp from an electric motor is anything other that the same as one hp from a gasoline engine.
All horsepower is the same. Horsepower, torque and speed are related by:
Horsepower = Torque (lbs-ft) X RPM / 5252. If an engine or motor can produce a little torque at high speed, it can produce as much horsepower as another engine that can produce a lot of torque but can’t run as fast. A transmission doesn’t change horsepower, it just changes from high speed, low torque operation to low speed high torque.
40 years of electrical engineering experience working with motors”
Another answer from E:
“All horsepower is the same. Whether you call in 746 watts, 746 joules per second, or 550 ft lbs/second.
What IS different is the torque curve. Electric motors generate the most torque when they are standing still, and generate less torque as they speed up. In general speed x torque = power. Because E-motors generate more torque as they slow down there is no control problem.
With gasolline engines it’s different. There is a maximum horsepower speed in RPM and a maximum torque speed in RPM that is ALWAYS lower than the HP max. If you run the engine against an increasing load so it slows below the max torque RPM it will suddenly loose RPM and stall unless you shift gears, etc. because as you go slower the gap between torque needed to maintain speed and available torque widens…..
Boiler HP and shaft HP are different just like Engine HP and Brake HP… Now Boiler HP is the heat rate including waste from converting from pure thermal energy to mechanical motion >> Shaft HP in ships for example. In cars it’s called engine HP vs Brake HP. In testing cars a “brake” is used to measure the HP at the rear wheels after the water pump, radiator fan, transmission losses, drive line bearings, and differential have had their chunk of energy….