replaceing and engine with a motor

[guilio2010]

This has been beat down probably alot, and I have found multiple sites that have 20 different explanations, but none of which allows me to understand how to size and replace them. Is there a simple walk through that show how one has a XHP engine @ XXXXrpm and torque and this is the electric motor to use to replace the engine?

I know electric motors have a higher torque at startup, but I just can't see where some are saying you can use a smaller HP electric motor. I can see it, but I can't.

Thanks,
guilio

 

[waross]

What is your application??
Forget the gas engine. Evaluate the load and spec the electric motor to meet the load.
Some applications require a larger electric motor than gas engine.
Some applications require a smaller electric motor than gas engine.
Some applications require a flywheel.
Some applications require an electric motor with special characteristics. (Shears, punch presses, hay balers.)
In some instances a smaller or standard motor may be used in conjunction with a VFD.
For some cyclic loads the motor may be sized according to the RMS loading.
Then again it has been suggested that half of us may just make a WAG based on twice the size of the existing gas engine.
The gas engine may be grossly oversized.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

Maybe what you need is to understand how the device is rated. Most internal conbustion engines are rated at peak HP, which maybe a short duration of it's life, but the engine must be able to deliver enough HP to do the job at any time during it's life. So the HP rating needs to be higher for an internal combustion engine.

Electric motors, are sometimes rated with a service factor greater than one, which translates to being able to produce more than it's rated HP. Also the HP of an electric motor is an averige HP for which it will not over heat. This means it can produce more than it's rated HP times it's service factor for a short amount of time, if it has a time to cool down.

Then there are other factors, which i don't wish to take the time right now to explain.

Hope that helps a little.

 

[jraef]

To add to the points already made, the difference, in my experience, is in the response to a step-change in the load. An electric motor responds almost immediately with added torque to re-accelerate, an engine cannot so it must be over sized to be capable of the needed accelerating torque all of the time. But if, as Waross mentioned, there is a big flywheel effect in the machine, that may not be the case.

I made the mistake once of reading the HP rating of a diesel engine at the RPM it was used at to determine torque, then sized an AC motor and VFD to provide that same torque. I smoked the drive belts... The machine was incapable of accelerating as fast as the AC motor could, even with a slow ramp of the VFD, but the inherent delay in providing torque from the engine allowed for the machine and drive train to match their capabilities. The issue I discovered, after that, was what cranky108 said about the HP rating of the engine being the PEAK rating, not the continuous rating. I fixed it by programming very low torque limits into the VFD so that it automatically extended the ramp time, but I could have used a much smaller AC motor and drive.

Oh well, it wasn't my money...

"Will work for (the memory of) salami"

 

[guilio2010]

I think that is where I need to do is scrap the gas engine and size the electric motor to the load. The load is a compressor so I'm looking at the PSI and flow as to find what HP i need.

I do understand the gas engine is peak HP at a certain RPM. From what I am getting though is the compressor may need lets say 5HP to run, however, the gas engine needs to deliver at least 5HP throughout it's RPM range. You may have 5 HP at 1000 rpm, 6 at 1100, 7 at 1200, etc....but your sizing the gas engine to guarantee your getting the 5 HP needed regardless where at the electric motor is 5HP all through the RPM range correct?

What HP I need on the compressor would go with the psi and flow rate I want. From there, I could get the torque? How do I determine what RPM to run the compressor? Would I find the torqe then use a HP to torque conversion chart to the find the RPM and find a motor that can run that?

Thank again...

 

[cranky108]

The compressor should have a rating curves of pressure, volume, and speed. The manufacturer should have those.