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Posted by D Sherman [24.32.202.83] on Sunday, April 18, 2010 at 14:36:24 :
In Reply to: Gas engine replacement? posted by Bob in N. Ga. [174.131.102.134] on Sunday, April 18, 2010 at 13:21:06 :
With hydraulics it's all a speed/hp tradeoff. You need a certain amount of motor speed to make the pump work, but beyond that, more hp equals faster cycle time, but it doesn't equal more pressure, assuming you have a motor speed and drive ratio that will allow the hydraulic bypass valve to open when it's idling. Allowing for inefficiency, figure 10 amps/hp at 120V and 5 amps/hp at 240V. That's running amps. Starting amps will be more. Realistically, 1 HP pretty well maxes out a normal 120V circuit, and 3 HP is about the limit for 240V. 5 HP (5 real HP, not 5 Harbor Freight air compressor HP) wants a 30 amp circuit at an absolute minimum. A bigger issue is how far you are from the panel, since voltage drops in the wiring really degrade the performance even if the setup meets code as far as not overheating the wires. There's no law that says you can't use #10 wire on a 20 amp branch circuit or #8 on a 30 amp circuit if you want to (and can afford it) also.
So long as you're okay with slower cycle times, and you can get the drive speed right, and you're far enough from the panel that distance factors into things, I'm thinking that 2 HP would be about as powerful as you'd want to go. For that matter, honest 5 HP motors in single-phase are not all that common anyway. To save money you could probably use a motor that's not rated for continuous duty, since it's really only working hard for maybe 10% of the time it's running. On the other hand, you'd probably want to invest in a drip-proof motor or even a TEFC motor to avoid damage from rain, sawdust, bugs, etc. Regardless of what hp you end up with, definitely wire it for 240. It costs nothing extra, except maybe a slightly more expensive outlet and plug, and it doubles the amount of power you can get from the same size wire. I see a big difference just on my 1 hp table saw depending on whether I wire the motor for 120 or 240.
Now we come to the mechanical end of it. Your gas motor is probably governed at 3600 rpm (look and see). Most electric motors in the range of what you're looking for run at 1800 (minus 10-100 rpm for slip angle). You can get 3600 and 1200 rpm versions, but they're less common. If you're going to bolt the motor face right up to the pump, you might just luck out and have an 1800 rpm 2HP electric motor produce almost as much torque as the 3600 rpm gas motor, albeit at half the speed and half the gpm out of the pump, and all will be good. Otherwise, you'll need some kind of belt drive to match the motor to the pump. Unless you have the specs of the hydraulic system, it might take some experimenting to get it right.
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