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Electric Horsepower verses Gas Horsepower


Guest Hal Davis (MODEL A HAL)

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Guest Hal Davis (MODEL A HAL)

The air compressor folks WAY over rate their motor HP. It is really nothing more than a marketing scheme. They base it on some meaningless figure like peak HP, or something like that, that happens only for some instant in time, not a useful figure like 550 ft-lbs/sec. This is a real sore point with me. <P>I saw a rule of thumb for calculating the horsepower of an electric motor the other day. It was voltage X amperage / 800.<P>For my "6-1/2 HP" compressor, that comes to 220V x 15A divided by 800 = 4-1/8 HP. Bear in mind that this is a rule of thumb, but it should be close.

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1 hp = 746 watts<P>If you are looking at the voltage and amperage plates on your AC motor remember that the amperage listed is probably peak not average. Also, on AC equipment the peak current may not be in phase with the peak voltage. I am not an electrical engineer, so I can't tell you how to figure all that out for an AC motor. DC motors are a lot easier: volts times amps = watts. smile.gif" border="0

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Full load speed, temperature rise, efficiency, and power factor as well as breakdown torque and starting torque are parameters used in the selection of electric motors. Another qualification is service factor. The service factor of an alternating current electric motor is a multiplier applicable to the horsepower rating. Applied to your motor selection this gives a permissable horsepower loading under the conditions you specified by the service factor.<P>So the short answer is no, you cannot give a factor or multiplier to convert electric horsepower to reciprocating gas engine horsepower, there are so many variables involved that it would be impossible. Electric motor type is an important consideration that determines the motor characteristics as well as the current (AC or DC). AC or DC motor selection to replace gasoline reciprocating engines typically involves choosing a higher horsepower gas engine to replace an electric type because the electric motor has much higher torque characteristics at low speeds (depending on type). This is why clutches and torque converters are necessary for power transmission in gas engines.<P>The best way to choose a replacement is to compare engine or electric motor dyno figures and dyno the load conditions to arrive at a motor horsepower that will meet your specifications. These are usually available from your equipment suppliers for free. There is no easy calculation or rules of thumb that will assist you in selection, every load has a different profile that will change your motor requirements.

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Guest elgin16

Does anyone know the formula for comparison of Electric Horsepower to Gas horsepower?<BR>I have read it to 1 hp electric is equal to a 4 hp gas for air compressors. <BR>I have also read for boats it is 1 electric hp can do the work of 4 Gas hp<BR>Is there a real conversion formula? <BR>How about for Electric cars?

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1 HP = 550 ft-lbs/sec (Basically horsepower is torque times rpm with some scaling tossed in.)<P>1 horsepower is 1 horsepower. However in many applications you need to look at required torque in addition to or instead of horsepower.<P>A DC electric motor provides max torque when stalled. A internal combustion motor provides zero torque when stalled. An internal combustion engine generates peak power at relatively high RPMs. It even takes a fairly high RPM to generate peak torque on an internal combustion engine.<P>So look at your application and see what the torque and HP requirements are. It could be that an application that requires lots of start up torque could get by with an electric motor with a lower HP rating than you would need if you were to use an internal combustion engine.

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