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Birmingham Lathe, Milling Machine and RPC

YotaBota

YotaBota

Mike
Premium Member
Package deal or separate on Vancouver Island.
https://nanaimo.craigslist.org/tls/d/ladysmith-precision-bed-lathe-milling/7395416704.html
1634573259667.webp

1634573278735.webp
 
I thought a RPC was nothing more than a single phase motor driving a 3 phase motor with some magical electrical stuff added on. I don't see two motors in the ad image?
 
I see a RT at the base of the mill, there must be a vise around somewhere to go with it as well.
Craig - Mr. Google must have some good explanations on how RPC's work.
 
One can use a single phase “pony” motor to start a 3 phase motor (via a belt drive) on single phase and let it act as a 3 phase generator. Once the 3 phase motor runs, the single phase motor can be decoupled and switched off.

My RPC has only the 3 phase idler motor. It uses a start capacitor circuit with a potential relay and a run capacitor bank to make three phase power.
 
I'm pretty sure that this is the same people that have a bunch of round cut outs from a water jet, so I presume there was more equipment there at one time. If the mill had been listed a couple weeks earlier, I would have made an effort to get it sonce I was only a couple miles away visiting my parents.
 
It's an American Rotary 25HP - they don't make that exact model any more, but the comparable industrial model is about 3K$ US for a 20HP model

This is a good deal.

Tom Kitta said:
Wow, why so big?
Click to expand...

To start a lathe with a heavy chuck, you need a phase converter that is double the size of the main motor. It is perfectly matched to the 10HP lathe motor.
 
I have a 10hp lathe as well and have no issues with my 15hp converter even without added fly wheel & there is no clutch mechanism on the lathe. You are not going to start the lathe with a large chuck and top RPM anyways as it exceeds max speed of the chuck. My RPC already burns like 500w of electric power when idling - this is why I thought about getting a 7.5hp model as well to run this when I do not need full power. A 25hp with a fly wheel is probably around 1000w.

The only thing I can think of is a very special case of using your largest chuck and say 4" shaft over the length of the whole lathe - to maximize initial load. However, when such load is applied it just slows down the RPC (hence fly wheel) and that in turn lowers the voltage with increases the amps (I think I got that right) i.e. your system has a brown out. But in that case I just need to add either a large fly wheel or in a pinch start the milling machine or even two - with three motors running there is no way even intentionally heavily loaded lathe started at higher speed is going to brown out hard.

I.e. the rotating energy of the RPC when load is applied is converted & RPC slows down - the more rotational energy it has - the heavier the shaft - the less it slows - but if you add other motors to the system they also slow down - i.e. when I have a mill running and I start the lathe the mill will slow down a bit.
 
Everything you are saying there makes sense. It is clear that your solution works for you.

The standard for RPCs for lathes has always been 2X - with the newer digital ones many manufacturers are using 1.7X. There are good electrical reasons for these figures, but anyone can use whatever and make it work.

For instance on disk sanders the motor HP always seems too large, and on belt sanders is usually 1HP per inch width of belt -- if the belt is moving over 2000 SFPM. I have a 3/4HP and a 1HP 6X48 belt sanders, and I can use them, but under load they always slow down. The 'correct' size is typically 3HP (both my sanders operate in the 1000SFPM-1500 SFPM range).

You asked why 25HP and gave you the answer. This is an attempt to give you an nontechnical followup, It feels that the above indicates that you might not accept these guidelines, and that is fine. The technical issuses go to capacitor strain, what is an acceptable brownout voltage, and other factors, some of which I'm sure I don't know.

What I do know is that American Rotary makes very good equipment, and their stuff lasts a really long time. If I didn't have a 15HP RPC already I'd have contacted the seller.
 
One has to point out that the RPC "rule of thumb" of 2x is well over 50 years old and not based on too much though. It is propagated by American Rotary and other makers to sell you larger RPC as they make $$$ of it. In real life you do not need such large RPCs at all. Now it would not matter all that much if you go 2x or 3x is costs of going bigger were not big or none - but like I pointed out a 15hp already burns 500w when idle with very efficient motor and 25 with fly wheel and I bet not as efficient motor is probably 1000w. Also motors use reactive power - so if you are commercial guy you want to minimize size of your motors so they are as close to full load as possible - if you get charged transmissions fees for power you never use.

Here is a link to a guy that builds RPCs - https://www.hobby-machinist.com/threads/rotary-converter-size-question-and-shop-help.61980/
He run a lathe that has 7.5hp motor off 5hp idler. I.e. reverse ratio. Link under is another guy that can start his lathe in reverse ratio in low speed.

My 10hp lathe has original breaker set from 7.5hp motor. So the brown out effect is so minimal it does not even trip smaller set of breakers.

There are absolutely not good reasons for sizing RPC 2x bigger - this is why manufacturers call it rule of thumb and when pressed they say "well it depends". They can always point out to "what if you are starting this super large load" - the problem is that "super large load" does not exist in real life. People then repeat it all over the place online. There is no good electrical reasons for 2x - you can ask American rotary for such reasons and they say "in our practice" or similar.

Here is another person starting 7.5hp lathe from... 7.5hp rotary - https://bbs.homeshopmachinist.net/f...-with-added-motor-to-increase-output-question

Please also note that one 7.5hp motor is not the same as other 7.5hp motor & rotary converter capacitors may not be perfect for one motor vs. the other.

So I am not alone in noticing that you do not need such big RPCs and I am also not alone in noticing that American Rotary is promoting 20hp RPCs when much smaller are doing the job quite fine (same thread as link above).
 
All I can say is the 1.7 is for modern RPC and recommended by independent engineering. I agree with you that what you do, works. No argument there. Engineers make rules of thumb to ensure that engineers don't make silly mistakes, and is a quick check for descrete and intricate calculations.

-- do you run your 20 amp load on a 20 amp breaker? I hope not. It is the same kind of thing.

No it is not a sales ploy. I'd rather not argue about it. What you say can work fine.

Doing less is just not best practice. On this I think we will just have to disagree.
 
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