Thinking about building a RPC

I have read the sticky above three times and the more I read the more cornfused I get. I just got a new toy, A Cincinnati Bickford super service/21" model. It has a 3hp, 3 phase motor on it and as you might know I don't have 3 phase service here.

What I was wondering is weather to build a 7.5 hp or 10 hp one. What can I expect as far as operational cost between the two. I doubt if I will ever get another machine with anything larger than 3 to 5hp at most. I've turned down several machines I could have gotten for little or nothing because they were 3 phase but don't plan on letting anything else slide by now that I have finished my shop.

Looking at that sticky it doesn't seem to be that big a difference between building a 7.5 or 10hp machine as far as the building cost is concerned but I was just wondering about the difference in terms of energy they consume.

Also could you power something like a large oven with one of these things? I built a big powder coat oven a few years ago that has 3/10,000w heating coils in it. When I built it I set it up to where I could use 3 phase (I had 3 phase power then) but it is now set up to run off single phase. If I could run the oven off a RPC, what would the difference in operational cost be between the two? Thanks!

lin842 said:

I have read the sticky above three times and the more I read the more cornfused I get. I just got a new toy, A Cincinnati Bickford super service/21" model. It has a 3hp, 3 phase motor on it and as you might know I don't have 3 phase service here.

What I was wondering is weather to build a 7.5 hp or 10 hp one. What can I expect as far as operational cost between the two. I doubt if I will ever get another machine with anything larger than 3 to 5hp at most. I've turned down several machines I could have gotten for little or nothing because they were 3 phase but don't plan on letting anything else slide by now that I have finished my shop.

Looking at that sticky it doesn't seem to be that big a difference between building a 7.5 or 10hp machine as far as the building cost is concerned but I was just wondering about the difference in terms of energy they consume.

Also could you power something like a large oven with one of these things? I built a big powder coat oven a few years ago that has 3/10,000w heating coils in it. When I built it I set it up to where I could use 3 phase (I had 3 phase power then) but it is now set up to run off single phase. If I could run the oven off a RPC, what would the difference in operational cost be between the two? Thanks!

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I think the difference in cost would be the losses in the converter. Cannot think of any reason i would want to run heaters with a converter.

The difference would be measured in terms of a Starbucks coffee every other month.

rons said:

The difference would be measured in terms of a Starbucks coffee every other month.

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I didn't think it would be much difference but was just wondering what it would be. I tend to over think stuff like this when I start a project like this. Right now the machines don't get run that much in my shop but the oven is another matter. That thing runs all day several days a week and I was wondering more about that than my new drill press. I didn't know if you could power something like that off one of these converters and if you could, weather it would save money on energy cost to run the oven 3 phase rather than on single phase. I need to find some way to cut the cost of operating expense in my operation and I have cut back everywhere else and this oven is the last place I can cut.

gustafson said:

I think the difference in cost would be the losses in the converter. Cannot think of any reason i would want to run heaters with a converter.

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Thanks, As I responded to rons in my other post I was just looking to save a little on utilities and was wondering if that would be an option.

Running the oven on RPC 3 phase would actually take MORE power than running it straight. You would add-in the losses of the RPC, and get no gain.

If the oven has to run all day, that is one thing, but if it can be allowed to cool slightly during periods of non-use, that would save some energy

Added insulation to reduce losses from the oven is also a consideration, although you need to watch out, since the makers may have assumed the existing outer shell would be exposed. Adding insulation around it may cause overheating of some parts.

It is also worth asking if the oven might be too big. A smaller oven might use significantly less power at times when the larger one is not going to be fully utilized, and a smaller one might be sufficient..

Hello every one. I am new to this forum. Where is the sticky that is referred to in the post?

JST said:

Running the oven on RPC 3 phase would actually take MORE power than running it straight. You would add-in the losses of the RPC, and get no gain.

If the oven has to run all day, that is one thing, but if it can be allowed to cool slightly during periods of non-use, that would save some energy

Added insulation to reduce losses from the oven is also a consideration, although you need to watch out, since the makers may have assumed the existing outer shell would be exposed. Adding insulation around it may cause overheating of some parts.

It is also worth asking if the oven might be too big. A smaller oven might use significantly less power at times when the larger one is not going to be fully utilized, and a smaller one might be sufficient..

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Thanks, I do have a smaller oven that I can process smaller parts but some of the stuff people want me to do is too big to put in the smaller one. When I have large items to do I still have to fine smaller stuff to add in just to make it worth my time and to cover the extra cost of running the big one.

I don't see how I could add any more insulation. I built this oven myself and it has 4" of mineral wool block insulation in the walls, the top and in the floor. When it's running the outside walls stay at room temp even when I'm baking something in the 500 degree range. I do lose a small amount around the door but you need to vent these things anyway so rather than having a dedicated vent in the box I just let the door act as a vent for the oven.

I don't run the big one any more than I have to and some weeks it may not get run at all then some weeks it may get run 3 or 4 times a week just depending on what comes in. Right now with utilities, along with all the other stuff that goes along with the process then add in my time it's about a break even deal and I need to figure out a way to add a little to the kitty at the end of the week.

vc3006 said:

Hello every one. I am new to this forum. Where is the sticky that is referred to in the post?

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It's the first sticky at the top of the transformer,phase converters and VFD page.

lin842 said:

..... Right now with utilities, along with all the other stuff that goes along with the process then add in my time it's about a break even deal and I need to figure out a way to add a little to the kitty at the end of the week.

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The answer to that is obvious, although not necessarily popular with customers.

If your charges do not cover costs, then you have to increase charges. That can be done by basing the new costs on a surcharge for certain types of work, such as work that requires the larger oven, etc, which may be the most acceptable way, although it can drive that type of work away.

The base rate may be unchanged if the surcharge is managed correctly. And it really is best described not as a "surcharge", but as different rates for different types of work or processes.

I built a fairly large powder coat oven about ten years ago, used it for several years and then sold it when my interests changed. It had three burners from ovens that drew a total of 12,500 watts. The unit ran from a 70 amp breaker in my shop and drew 52 amps. Since the usage was resistive in nature, in-rush current was not an issue.

I wouldn't run something like a powder coat oven from an RPC especially due to the large current drain when in use. The OP mentioned 3 X 10,000 watts (30,000 watts? I wonder if that is correct.) . At 240 Volts, that would be 125 amps. That sounds pretty big to me, even for a single phase line.

Actually, inrush can be a problem with resistive heaters because their resistance is lower when cold. I knew of one oven that popped the circuit breaker every time on every start. That heated the elements enough that the breaker would stay on after being reset. It was a commercial oven made by people who were supposed to know what they were doing.

30 kw divided by 746 is 40 horsepower, so you would have at least a 40 hp motor idling for no gain. One thing, though, you wouldn't have to worry about the phase conversion varying when you loaded your 3 hp motor when it was swamped by such a large resistive load.

Bill

9100 said:

Actually, inrush can be a problem with resistive heaters because their resistance is lower when cold. I knew of one oven that popped the circuit breaker every time on every start. That heated the elements enough that the breaker would stay on after being reset. It was a commercial oven made by people who were supposed to know what they were doing.

30 kw divided by 746 is 40 horsepower, so you would have a 40 hp motor idling for no gain.

Bill

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I guess that could happen.

I ran my power coat oven fairly regularly for several years and had no inrush problem with the 70 amp breaker and a 52 amp draw. I usually took 15 minutes to get up to 375 Deg. F. but it was rather large (24" W, 36" H and 72" D inside) It started up slowly.

JST said:

The answer to that is obvious, although not necessarily popular with customers.

If your charges do not cover costs, then you have to increase charges. That can be done by basing the new costs on a surcharge for certain types of work, such as work that requires the larger oven, etc, which may be the most acceptable way, although it can drive that type of work away.

The base rate may be unchanged if the surcharge is managed correctly. And it really is best described not as a "surcharge", but as different rates for different types of work or processes.

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Thanks, I know what you mean about the unpopular thing with customers it seems that no matter how well you price your work it always seems to high for people to justify. My problem is that I have so much in this large oven I can't just scrap it and start over. When I built this oven I was in a large building that had 3 phase service and planned on staying there until I retired or died trying. I got along well with the realtor that managed the property but because of health issues he had to retire early. The people that took over the property management were real hard to deal with and the first thing they did was raise my rent to the point I couldn't make a living there.

I was in the boat business and the powdercoat thing was something I wanted to do as a way to stay busy in the off season. My plan was to build T tops, hard tops and other accessories for the boats I sold. That's why I had to have such a large oven because some of this stuff was so big. I could have made the tops to where I could bolt them together from smaller pieces but they would be as strong as a fully welded top. So I built a 8'x 8'x 8' box where I could put the whole top in after it was welded up. Shortly after I finished the oven I was forced to look for another location.

I found another location not far away that had been a boat dealership in the past so I rented that space and started my move. This place had 3 buildings one large building that had been used for a showroom for their boats, a office/ accessory store, and a 5 bay shop to do service work. It also had 3 phase service coming into the main office building and to the shop. Everything was going great until I found out that a 8'4" high oven wouldn't fit into the 8' high doors that were on the service bays.

This sure did throw a monkey wrench into my powder coating plans. I had to put the oven in the show room building but that building was just wired for a few lights and didn't have enough juice to run the oven. My land lord told me I could do anything I wanted there and I thought about getting that building power service upgraded but to do that the inspectors wanted the whole property upgraded and that would have cost over $20,000.00 and I just couldn't make that happen.

The ovens, tube benders,welders and other equipment stayed in the corner for 5 years until the economy got so bad I had to pull the plug on the whole operation. I decided to close shop, put all my equipment in storage and build me a building at my home to work out of. That was almost 5 years ago and now that I am about finished with the building and putting the final pieces together with hopes I'll be able to at least recoup some of the money I have spent over the years with this project. I know I'll never get rich but at my age I need to get something out of what I have worked so hard to put together. At this point I'm just trying to save a little every where I can and when I do get up and running hopefully the Good Lord will bless me with 10 or 12 more years of good health and I'll be able to at least pay my taxes while I'm here on earth.

Newman109 said:

I built a fairly large powder coat oven about ten years ago, used it for several years and then sold it when my interests changed. It had three burners from ovens that drew a total of 12,500 watts. The unit ran from a 70 amp breaker in my shop and drew 52 amps. Since the usage was resistive in nature, in-rush current was not an issue.

I wouldn't run something like a powder coat oven from an RPC especially due to the large current drain when in use. The OP mentioned 3 X 10,000 watts (30,000 watts? I wonder if that is correct.) . At 240 Volts, that would be 125 amps. That sounds pretty big to me, even for a single phase line.

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The burners are 10,000W each. Each one has 3 separate tubes that are tied together as one unit and each tube has a in, lug and a out, lug that feeds that particular tube. I have a 125 A sub panel that feeds the control box and in the control box each burner is fed by a 50A contactor. I had 40A breakers in the sub panel but I will need to bump them up to 50A because each 3 tube burner draws right at 36 to 39 amps and sometimes the 40 amp breakers will trip because they are about maxed out. The 125A breaker in the main that feeds the sub panel has never tripped but if it ever becomes a problem I can go up to a 150A breaker if needed because I have 1.0 THHN copper feeding the sub panel.

lin842 said:

The burners are 10,000W each. Each one has 3 separate tubes that are tied together as one unit and each tube has a in, lug and a out, lug that feeds that particular tube. I have a 125 A sub panel that feeds the control box and in the control box each burner is fed by a 50A contactor. I had 40A breakers in the sub panel but I will need to bump them up to 50A because each 3 tube burner draws right at 36 to 39 amps and sometimes the 40 amp breakers will trip because they are about maxed out. The 125A breaker in the main that feeds the sub panel has never tripped but if it ever becomes a problem I can go up to a 150A breaker if needed because I have 1.0 THHN copper feeding the sub panel.

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Thats a big oven! In my case, I had much less draw than that but it worked quite well. Sounds lke you have it handled!