Help me with my phase converting fun

[Strouty]

I just bought a used air compressor, it is big, really big (at least for me). It runs on three phase power, 200 volts, 60 hertz, and draws 77 AMPs. I currently have a phase converter, but it will need to be upgraded to run this beast. I am trying to figure out what size breaker I need, will I have to use a dedicated 150 amp circuit? Also what size motor on my phase converter do you think will start this 25 hp motor. I have a 7.5 HP motor on the converter and it is producing about 230 volts per leg. I am going to try and set things up later this week and see if it will start the motor or even do anything. Anyone have any thoughts? I was also toying with a dedicated VFD.

 

[amolaver]

i'm guessing that 7.5hp motor on the converter will have its magic smoke escape if you try to power your 25(!?!?!)hp motor through it. you'll need a phase converter at least as big as the load motor, and usually you go bigger for a little headroom.

i'm all for going big, but what kind of usage are you planning for a 25hp air compresssor?

ahm

 

[A_Pmech]

For an air compressor it is standard to double the converter motor size. So you'll need a 50HP motor to build your phase converter. It's going to require about 130A of single phase 240.

Check your pole transformer. You're not going to run that combo on any less than 25kva. Residential electric services are always have undersized transformers.

A VFD is also an option. However, I'm not aware of any VFD's over 10HP which advertise single-phase input. It is possible to do if you can either:

A) Access the DC bus of the VFD and build an external rectifier to supply power to the VFD as the internal rectifiers are about half the size they need to be to rectify a single-phase input.

B) Buy an over-sized VFD, directly connect the single-phase to two terminals of the input and build a circuit to trick the VFD into thinking the other two phases are also present.

Some significant thought would have to be put into the VFD aspect to make sure it would work the first time out. 25 HP drives start to get expensive.

 

[Strouty]

Why is it that you would double the size of the motor? We have a 2 hp phase converter running a 7.5 hp motor currently. Is it due to the compressors load? I can unload it to start, would that help? It is a rotary screw compressor and the reason I need 100 CFM is the huge sandblast cabinet I have requires almost all of that for the larger tips.

 

[A_Pmech]

Why is it that you would double the size of the motor? We have a 2 hp phase converter running a 7.5 hp motor currently.

An air compressor runs at 100% rated load and is generally a hard load to start. Thus the need to oversize to ensure the compressor will start under all conditions.

A single rotary phase converter will not run a more powerful motor at nameplate capacity. It is magnetically limited and can't source enough power on the third leg to supply the more powerful motor. You can get away with an undersized converter motor if the motor it is powering is lightly loaded, or one of a series of motors running simultaneously, some of which are lightly loaded.

When a motor larger than the phase converter is loaded up to nameplate mechanical output, the motor slip increases due to the load applied. As slip increases the windings across the single-phase supply will pick up more and more of the total load because the third leg current is limited by the phase converter. At rated load the motor is essentially single-phasing and will soon exceed it's insulation temperature rating.

 

[msnow]

Every thing AP said is spot on. Dude I kind of hate to say this because I am big proponent of go big or go home but I think that may just be too big to run in a home. Honestly you could look at getting a generator, getting the power company to run a special service, or selling the compressor for something a little smaller. I also question your transformer as well. You may not even have the right wire size running off the pole to pull 130 Amps single phase.

 

[930dreamer]

Does the screw compressor use any computer controlled parts? If so it might not like a rpc. Pictures of the compressor and the blast cabinet please.

You might look at running multiple idlers in parallel, but a 25 hp compressor will be challenging. How big is your available panel?

 

[junk4dummies]

Don't waste your money on a converter. A 25 hp moter is 440 or more and you will not be able to convert. The largest hp you can run on 110 is a 2 hp moter. I don't care what the machine says. It will not caculate out. You loose a third of your hp with a converter. 25 hp is not worth messing with. The only time you find large compressors like that are in ship yards, air craft factories, hospitals and supper sized industry. You can look up the charts on the internet and find out what type and size converter you will need. A used one will be over $2500 I am sure. All that is cheap is not gold.

I am not at my office or I would look it up scan it and send it to you.

 

[larry_g]

Don't waste your money on a converter. A 25 hp moter is 440 or more and you will not be able to convert. The largest hp you can run on 110 is a 2 hp moter. I don't care what the machine says. It will not caculate out. You loose a third of your hp with a converter. 25 hp is not worth messing with. The only time you find large compressors like that are in ship yards, air craft factories, hospitals and supper sized industry. You can look up the charts on the internet and find out what type and size converter you will need. A used one will be over $2500 I am sure. All that is cheap is not gold.

I am not at my office or I would look it up scan it and send it to you.

1. Your wrong about the voltage http://www.baldor.com/products/prod...Efficient|vw_ACMotors_PremiumEfficiency&hp=25

2. Your wrong about loosing a 1/3 with a converter.

3. Your wrong about 2hp max on 110 maxing out at 2hp. http://www.baldor.com/products/prod...s&family=Single+Phase|vw_ACMotors_SinglePhase

lg
no neat sig line

 

[msnow]

Junk,
Typically you use a small 110V motor only to start your phase converter. Once you can get it started it will generate the 3 phase power you need to run your equipment i.e. your compressor. The problem is that the "phase converter" motor itself would have to be very large to be able to deliver the needed power to run a 25 horse motor. When you reference 440V most 3 phase motors can be hooked up to run on a variety of voltages. So while I am sure you have seen 25 horse motors run on 440 you can run a 1000 horse motor on 208V 3 phase if you have enough current or in the same respect a 1 hp motor on 440V 3 phase if your insulation is rated for it.

 

[bad_idea]

I have no valuable input. But I am really interested to see this 25 hp compressor and the sandblaster that needs it. What are you blasting?

 

[2mJps]

If you are going to sand blast it would be better to go with a gas engine. The sand blaster I ran had a 300Hp diesel. I wish I had a 25 Hp compreser but in the sandblast world its not that big.

 

[coldfusion21]

Don't waste your money on a converter. A 25 hp moter is 440 or more and you will not be able to convert. The largest hp you can run on 110 is a 2 hp moter. I don't care what the machine says. It will not caculate out. You loose a third of your hp with a converter. 25 hp is not worth messing with. The only time you find large compressors like that are in ship yards, air craft factories, hospitals and supper sized industry. You can look up the charts on the internet and find out what type and size converter you will need. A used one will be over $2500 I am sure. All that is cheap is not gold.

I am not at my office or I would look it up scan it and send it to you.

We're a medium sized manufacturing business with several hundred hp of air, 25 isn't anything to a big factory.

That said your gonna need a huge idler. And that means huge power. Might be worth looking into a gas/diesel compressor or 3 phase service (440 if your gonna be running big motors)

 

[Justin1776]

A 25 horse motor is quite large. If I were you, I would look into a 3 phase 208y/120 generator. Military generators are great for the money but they are LOUD unless enclosed and their "fuel mileage" isn't as good as current civilian generators. Someone please correct me if i'm wrong but you will need a 30 kilowatt 3 phase (208) generator. Military surplus models (30kw is a MEP 005A) can be had for 4-5K, civilian models start at 10k. On the military models, you can convert the circuitry to provide two 120v legs of split phase to power your house too. That is, if you have the space for a large genset

 

[Tscott]

Hate to say it, but it sounds like you need to consider a different compressor. 25hp is pretty tough to start across the windings on a single phase. Your are gonna be pushing your utility transformer pretty hard to crank that mother over. I'd also be willing to bet your local utility has limit on the size of motor you can run without their approval. I'm not saying it can't be done, but it is gonna be a challenge.

I don't know a lot about phase converters, but I do know the law of energy conservation. Even if your phase converter were 100% efficient, which it's not, you would need a 25hp single phase motor to power the converter. Anything that large will probably need a soft start system and is gonna cost you. Probably cheaper and easier to buy a new compressor.

Sorry,
Tom

 

[Strouty]

Great advice from all, I bought this unit for $250 so I took a gamble, it must weigh close to 3000 pounds, so I won't loose money even if I scrap the compressor. I have been considering duplexing two compressors together, but I am still limited to 7.5 HP single phase motors. I am going to post some photos of both the cabinet and the compressor, The blast cabinet manufacturer says that my current compressor with 24 CFM will run the smallest tip they make, but once it starts to enlarge from wear I will be loosing pressure like crazy. This compressor is an Ingersoll Rand SSRH 100 and it does not have anything computerized on it. It is an early 80's vintage that was in a local shop. I talked with the guys who serviced it and they said it is a very dependable and tough unit. It has 57k hours on the clock, but they tell me over 100k is common for this vintage. I was also toying with the idea of converting it to a gas or diesel drive.

 

[Slick111]

Is there even a 25 hp single phase motor made. I have never heard of or seen one over 10 hp could be wrong it has happened before.

 

[Strouty]

10 is the biggest single phase that I have seen as well, phase converters use 3 phase motors to help run other 3 phase motors by creating a third leg of power to add to the two legs that the single phase all ready has.

 

[Steve from Socal]

I have a 60 HP Gardener Denver rotary screw and would love to find a 25HP for a few hundred bucks! The 60 is such an overkill but 25 would be good for my blast room. I have though about using a diesel or gas engine on the sixty. I have 600 amps of 480 but demand charges are brutal.

Steve

 

[cnc-me]

I have a 60 HP Gardener Denver rotary screw and would love to find a 25HP for a few hundred bucks! The 60 is such an overkill but 25 would be good for my blast room. I have though about using a diesel or gas engine on the sixty. I have 600 amps of 480 but demand charges are brutal.

Steve

They hit you with demand charges on a 60 Hp?
Thought that didn't start until you got into much bigger motors than 60.

 

[mark52621]

Where I live the largest motor you're allowed to use on single phase is 15 hp.

If you can run it with a generator.

 

[Mmfh]

I am going to post some photos of both the cabinet and the compressor,

I was wondering if you were still going to show us what you are working with??