how big of a motor can I run?


Old 11-15-16, 09:29 AM
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how big of a motor can I run?

I am hoping to build a hydraulic press for my shop. I do not know how big of a motor I can run with my existing service.
I just spoke with a engineer and he says I have a 10kva transformer.

I have a 20hp three-phase motor that I would like to use to power the hydraulic unit, so I would probably be looking at a 40hp rotary phase converter.
I think I read somewhere that the in-rush current for starting the 20hp motor would be round 160-170 amps. I don't know off hand what the start up current would be for the rpc.

My existing main/meter panel is only rated for 200A. I am hoping to set things up so I have 200A available to two residential structures and 200A available to my shop.

I will be having a poco engineer out to my place, but am not 100% sure what questions to ask... I think there is a somewhat loose restriction on motors at 10hp, but I'm not real sure why that is...

Any suggestions/advice much appreciated.
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Old 11-15-16, 01:18 PM
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This is a question for the POCO engineer. The restriction on 10hp motors is that they can create such an imbalance on a single phase distribution system that it will affect power quality for your neighbors.

Running current for a 20hp motor would be close to 100A single phase. Start up can be 3-4x that, which is just too much for a residential transformer and distribution cabling.
Old 11-15-16, 01:34 PM
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How hard are you going to be working this press? If not a high duty cycle, consider getting a smaller hydraulic pump working into an accumulator. Smaller pump runs longer, less current. Like maybe a 5hp. 10kVA is not even rated for 60A continuous load, although its short term overload rating is much higher than that.
Old 11-15-16, 05:29 PM
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Seems to me you are putting the cart before the horse. You should figure out what hydraulic pump you will be using and then size the motor to run the pump. 20HP sounds WAYYY oversized.
Old 11-15-16, 06:03 PM
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I just wired a three phase, 20HP hydraulic pump in a testing lab. It produced 2,800 PSI of hydraulic power. Do you need that much?

20HP 3 240V full load amperage is around 55 amps. Besides the 1 to 3 converter, you'll also need motor starters for the converter and 20HP motor. The converter will draw around 90 amps on the single phase side.

Unless you're experienced with motor loads, over current protection, motor starters, start/stop controls and motor protection, you may want to consider hiring an electrician with industrial experience. Sizing the wire and motor current protection involves more than just full load amperage or locked rotor.
Old 11-16-16, 04:42 AM
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Physically how big is this press and what size cylinder are you planning to use?

Standard hydraulic pumps operate at 3000 psi which can produce very high pushing force depending on what diameter cylinder you use.
The HP requirement comes from how fast you want the cylinder to move.

Post some info on what you want to build and I can give you some idea as to what cylinder and HP you will need.
That is how much force and the length of the stroke.
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