# Motor issues



## debrular (Mar 7, 2011)

My friend is considering buying a 17X40 Harrison lathe. This piece has a 10 hp motor. How does one go about selecting a rotary unit for this. 

John DeBrular
debrular@suddenink.net


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## Zigeuner (Mar 7, 2011)

[quote author=debrular link=topic=1257.msg6258#msg6258 date=1299502983]
My friend is considering buying a 17X40 Harrison lathe. This piece has a 10 hp motor. How does one go about selecting a rotary unit for this. 

John DeBrular
debrular@suddenink.net
[/quote]

The general rule from what I've read and heard is that you need to power up at least 50%. So, if I had a 10 hp 3 ph lathe motor, I would build a 15 hp RPC. That raises the issue of startup current. If you have sufficient power available, there should be no problem. If power is at a premium, I would strongly consider either a rope start with a small pulley on the RPC motor or a pony start with something like a 1 hp single ph motor that will start the 15 hp motor with a V-belt and then shut off. 

On my 3 hp Webb Mill, I have a 5hp RPC and it's more than enough. I've tested it and it will even start the mill if I leave the mill switch on.

As to whether to buy or build an RPC....that's an interesting question. I bought the first one from a major eBay seller and I got snookered. It was cheaply built with inexpensive components that were glued into a smallish control box with silicone. The motor was overrated to 5 hp but based on its measured current draw was only a 3 hp. Even though they called it a 5 hp, it would barely keep the mill running. It didn't have a motor plate either, apparently to keep the buyer from knowing the truth - that they had used a cheap 3,600 rpm 3 hp motor and over rated it. For a bonus feature, t was almost unbelievably noisy. I got free shipping, though. LOL. 

I would definitely build my own. I sold the other one with full disclosure to a fellow who wanted it to power a 1 hp Bridgeport. I then collected quality components and made my own and haven't yet looked back. There is another thread where I described my situation in building my present RPC. I'm gonig to build another one just for fun and try a pony start. I think they are neat. Ha. 

That's my story and I'm stickin' to it. 

Here's the other thread below in this section. 

http://hobby-machinist.com/index.php?topic=731.0


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## Amigo (Mar 12, 2011)

A 10 H/P 3 Phase motor will operate just fine on a RPC utilizing a 10 HP. Delta wound motor. In fact, a 10 HP. RPC, Delta wound, will easily run 3 or 4 more 10 HP. machines if power balance capacitors are used - a rule of thumb is 70 to 75mf per hp. recommended for each cap. Thus, a 10 H.P. RPC would need 700 to 750mf (microfarad) run capacitors. The "rule" is not hard &amp; fast cause conditions can vary.
I've been running a 5 hp. lathe off of a 5 hp RPC since 87, with no ill effects...I eats my own cookin.

However, a 15 H.P. machine can't be run off of a 10 HP. RPC regardless of number &amp; size of caps. used.

Turbo recently posted a link to a Lindsay Publication titled "How to run a 3 phase motor on single phase". It is a must read.


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## Zigeuner (Mar 13, 2011)

[quote author=Amigo link=topic=1257.msg6782#msg6782 date=1299907211]
A 10 H/P 3 Phase motor will operate just fine on a RPC utilizing a 10 HP. Delta wound motor. In fact, a 10 HP. RPC, Delta wound, will easily run 3 or 4 more 10 HP. machines if power balance capacitors are used - a rule of thumb is 70 to 75mf per hp. recommended for each cap. Thus, a 10 H.P. RPC would need 700 to 750mf (microfarad) run capacitors. The "rule" is not hard &amp; fast cause conditions can vary.
I've been running a 5 hp. lathe off of a 5 hp RPC since 87, with no ill effects...I eats my own cookin.

However, a 15 H.P. machine can't be run off of a 10 HP. RPC regardless of number &amp; size of caps. used.

Turbo recently posted a link to a Lindsay Publication titled "How to run a 3 phase motor on single phase". It is a must read. 
[/quote]

When I saw your post here, it raised the question....how does one tell whether a motor is Delta wound or Wye wound, especially when looking on the internet? When I bought my nice NOS 5 hp 3 ph Boston gear motor on eBay last year, I had a picture of the motor plate but there is nothing on the plate to determine how the motor is wound. All that told me was the amperage at each of the two rated voltages and the wiring diagram for each voltage. (220/440) 

It's a nine wire motor. T1 through T-9. I have it wired for the lower voltage with T1/T7, T2/T8 and T3/T9 together to make the needed three output wires. T4, T5 and T6 are tied together. It works fine. I guess that it's a Wye wound motor but I still am not certain. 

I don't disagree with your premise that a Delta motor may have certain advantages but it's hard to tell what you are getting. Sellers never seem to have that information handy. Due to my location in a rural area, 99% of my equipment comes from internet sellers. There's lierally nothing available locally.

In any case, my 5 hp motor has more than enough power for my 3 hp mill as I mentioned elsewhere. 

Curious in Northern California.


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