Engine HP vs Electric motor HP

[Farkel]

Can't remember the ratio, since one is peak HP and other is constant. Jog my memory.

 

[electricpete]

Electric motor "horsepower" rating is a steady state limit, usually related to thermal performance.

I'm not sure about engine horsepower.

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[aolalde]

I do not understand why it is a difference.

HP is a unit to measure power. Power is the ratio of work made in certain amount of time.

1 HP electric = 33,013.3 LB*FT/minute =746 watts
1 HP mechanic= 33,000 LB*FT/minute= 745.69 watts
1 HP boiler =434,107 LB*FT/min = 9809.50 watts
1 HP metric= 32,549 LB*FT/min = 735.499 watts
1 HP water = 33,015.6 LB*FT/min = 746.043 watts
1 HP U.K. = 33,000.44 LB*FT/min = 745.70 watts

I hope that; 1 watt = 1joule/sec is a constant measure of power.

 

[Skogsgurra]

I would like to add 1 HP "SI" = 75 kg lifted 1 m in 1 second.

In other words: 1 HP(SI) = 75x1xg = 75x9.80665 = 735,49875 W. We use 735 W for 1 HP.

 

[jraef]

There is a difference though, not necessarilly in the actual HP, but in the step-change torque response capability. I don't know of a formula myself, but I recently did a conversion of a deisel engine to an electric motor. The electric motor was able to respond to a load change so fast that the mechanical components, which worked fine on the engine, slipped and spun when connected to an electric motor of equivalent HP. We had a VFD on the electric motor and could monitor output kW. We never exceeded 25% of the electric motor's kW output capacity before we started smoking belts. They had NEVER been able to smoke belts with the deisel.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Farkel]

Found what I was looking for; here it is as follows:
Combution engine HP rating=approx. True HPx2.5; Electric motors are rated by true HP.

 

[Skogsgurra]

Farkel,

Is it the old French Cheval Vapeur ("steam horses") you have found. The ones that explain why the Citroën 2 CV (Cheval Vapeur) had about eleven - or was it seventeen? - normal HP?

 

[HCBFlash]

O.K. since I'm an electric motor guy, it should be OK for me to ask what on earth does 434,104 lb/ft / minute have to do with 1 horsepower. French horses may have more steam than pull, but I still don't get it.

Could it be that about 476W of well-placed steam can make somebody's horse do his 660 pounds 1 foot in only about 1/13th of a second? ....and then there's the question of how much latent heat does a horse-boiler have,,, at, say, 6' above sea-level.

remember: An opinion is only as good as the one who gives it!

 

[Skogsgurra]

Sens moral: Stick with SI. And confusion is gone!

 

[capnrazz]

The 2.5 peak HP to 1 true Hp for comparing IC engines to electric motors for practical application makes sense. If either is delivering 1HP constant at the shaft at a given RPM, the work done is the same. But the IC engine will have very little torque at idle, while the electric motor will produce full torque if the current is available. I have been researching electric motors for sailboat auxilliaries, and the typical comparator is 2:1 IC HP to electric. The electric motor can turn a larger prop that would stall the IC engine at idle, so provides more thrust per HP than the IC engine as well as not having parasitic losses from pumps, gearing, alternators, etc.

Razz

 

[mc5w]

The issue is that that horsepower rating of an engine is an absolute upper limit - if you exceed it the engine stalls under constant torque overload.

The horsepower rating of a motor is based on how much constant power you can put through it and have it live for say 10 years. The breakdown torque electric motors other cheap fan motors is some where around 150% to 300% of "full load" which means that you can deliberately overload the motor for short periods provided that there are periods of less than full load for the motor to cool.

Cheap fan motors have full load torque just below breakdown torque so as to get as much out of a given iron structure which keeps down weight and cost. A portable fan needs to weigh no more than a brick and not like how concrete blocks are more like birth control blocks. You only have to work as a laborer for a stonemason for a few hours to know what I mean.

Mike Cole

 

[jraef]

Previous similar thread just for reference.
thread237-95838

If anyone is interested, here is an update on the project I mentioned above where we replaced a diesel engine with an electric motor and VFD. We are tracking costs and productivity. It has been running 3 months now, and the raw electricity cost is $1750 less than the diesel fuel consumed in a similar operating/production period last year. The machine is a rock crusher, and the ouptput is so much more efficient that the machine is producing approx. 35% more tons-per-hour as well. There have been 2 hours of down time, only the slipping drive belts we discovered early on because the torque response of the electric motor was so much quicker. That is in comparison to the engine downtime last year of 22 hours over 3 months. The numbers are not final yet, and production quantities need to be avaeraged over a longer time frame to be meaningful, but it is looking like it was a very good idea.

"Venditori de oleum-vipera non vigere excordis populi"