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How much power do I need to run 2hp 3 phase motor thru converter from genset?

stoneaxe

Stainless
Joined
Mar 2, 2010
Location
pacific northwest
I was thinking of a propane generator as backup for the well pump and refrigerators.

Then I started thinking about a propane welder/genset like a Miller bobcat, around 9 kw on propane,and having a portable welder as well.

Now I am wondering if a genset like that has the power to run a RPC big enough to turn a 2 hp lathe. Taiwan 1340.

Right now the lathe is powered up by a 10 hp RPC, which is too big for my current generator (5000w)

So if I downsize the RPC to 5hp, could I run it with a 9KW (11KW peak) genset?


It would really be nice to have an non-utility option to run the tiny "machine shop" we have.
We are at the very end of the power line in a rural area-lines get minimal maintenance and always the last ones to get power back.

The mill is 1 1/2hp, lathe 2, and bandsaw 1hp. Not much draw but I do not know how to figure all the startup draw, single to three phase changes etc.

The goal is to have enough but not too much, to economize fuel cost.

Is this a reasonable thing to do, electrically?
 

SomeoneSomewhere

Hot Rolled
Joined
Dec 24, 2019
Starting induction motors across the line pulls a lot of current. A 2-3HP single phase motor would be pushing it. Running a motor of that power (let alone 5HP) as an idler and starting another would be unlikely to work.

1 mechanical HP is very roughly 1 electrical kVA when losses are taken into account. A bit more on less efficient small motors. Starting current is roughly 5-8x running current.

On the other hand, a VFD will pull basically no starting current, and would run easily on a generator in that class, and is much more efficient.

Upgrade the lathe to a VFD and get a smaller converter (2, maybe 3HP) (or more VFDs) for the others?

I'm sure there will be folks along shortly to tell you to get a 3~ surplus generator, but that doesn't really work if you want a welder and the generator is a side benefit.

Some kind of reduced voltage starting might be feasible for the RPC idler to reduce the startup current at the expense of a slower spinup. It's done regularly in larger motors.
 

stoneaxe

Stainless
Joined
Mar 2, 2010
Location
pacific northwest
Starting induction motors across the line pulls a lot of current. A 2-3HP single phase motor would be pushing it. Running a motor of that power (let alone 5HP) as an idler and starting another would be unlikely to work.

1 mechanical HP is very roughly 1 electrical kVA when losses are taken into account. A bit more on less efficient small motors. Starting current is roughly 5-8x running current.

On the other hand, a VFD will pull basically no starting current, and would run easily on a generator in that class, and is much more efficient.

Upgrade the lathe to a VFD and get a smaller converter (2, maybe 3HP) (or more VFDs) for the others?

I'm sure there will be folks along shortly to tell you to get a 3~ surplus generator, but that doesn't really work if you want a welder and the generator is a side benefit.

Some kind of reduced voltage starting might be feasible for the RPC idler to reduce the startup current at the expense of a slower spinup. It's done regularly in larger motors.

I will look into a VFD, did not think about that as all our stuff runs on RPC's.
A three phase generator , at a guess, is going to be a lot bigger than necessary and consume a lot of fuel.
 

scsmith42

Aluminum
Joined
Jul 28, 2020
Location
New Hill, NC
From the circumstances that you describe, I'd definitely look into a VFD. It's the most simple, and least expensive solution, IMO.
 








 
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