What's new
What's new

RPC and 575V Transformer for a 17" Cincinnati

LukeWilson

Plastic
Joined
Nov 29, 2021
Long time lurker here,

I'm in the final building stages of my dream shop and pulled the trigger on some equipment. I grabbed a 2HP First knee-mill a few weeks back and picked up a 17 x 54 7.5HP Cincinnati lathe on the weekend. I was going to have the motor rewound on the mill and go the VFD route, but since both machines are 575V I figure its easier to just stick with a RPC and transformer and run both machines plug and play.

I'm finding very conflicting information regarding the sizing of the RPC and transformer and am hoping for some real world insight from members who have been down a similar road.

I'm hoping to use a 10hp 240V 3ph motor as my idler and a 15kVa 240V-575V 3ph transformer to step the voltage up. The lathe is clutched and I don't plan on pushing it anywhere near full load. Does this seem ok or should the RPC really be 15hp+?

Thanks in advance,
-Luke
 

Strostkovy

Stainless
Joined
Oct 29, 2017
You can run a 7.5hp motor off of a 10hp RPC no problem, but the transformer would potentially worry me with its parasitic losses.

I'd personally run a 1:2 single phase transformer and a cheap 480V VFD rated to 2x the HP of the motor. You will only get 80% of your motor speed at full torque this way (you can get to 100% with reduced torque) but it's super cheap to build.
 

LukeWilson

Plastic
Joined
Nov 29, 2021
Hmm not sure that would work for me as I need 575V (Industrial standard here in Canada). From what I've seen 15hp 600V VFD's are a little harder to come by, but good to know this is an option. Would I need a VFD for each machine or would it effectively be wired as the RPC would be for both machines?

Based on what I can find locally, I should be able to build a RPC and get a 15kVa transformer for less than $1000.

Thanks,
-Luke
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Hmm not sure that would work for me as I need 575V (Industrial standard here in Canada). From what I've seen 15hp 600V VFD's are a little harder to come by, but good to know this is an option. Would I need a VFD for each machine or would it effectively be wired as the RPC would be for both machines?

Based on what I can find locally, I should be able to build a RPC and get a 15kVa transformer for less than $1000.

Thanks,
-Luke

You can use one VFD for all machines if the VFD is suitably sized and rated for direct on line starting. The 480V idea was just because it's super cheap and probably still meets your performance needs. (A 575V 60Hz motor running on 480V 50hz has full torque but 80% of the speed).

You can also use a transformer after a VFD. Lots of options.
 

BT Fabrication

Hot Rolled
Joined
Nov 3, 2019
here in ontario also, my rpc is currently only 10 hp but the panel is rated for 20hp. 10 hp is plenty for the 3hp and the 7.5. the only thing you might have is on certain days where the input voltage is a bit lower, it might kick out a breaker on you with a 10hp when you first kick it on. biggest thing, is your supply big enough for 15 hp? that would be a dedicated 100A supply to it as 50A is too small.

and 15 kva transformer would work with the 10 hp as they are ususally 30A 3ph = 11 KVA and the 15 would be right at the max kva for the transformer before any power loss calculations.

also rewinding a motor seems ambitious, its about $800 to do so around here.
 

LukeWilson

Plastic
Joined
Nov 29, 2021
I've got a 200A panel in the shop so supply shouldn't be an issue.

I might have a line on a 20HP motor for scrap price, the price of the RPC control panel doesn't change too much whether its 10/15/20HP but at some point it's just overkill. I highly doubt I will ever have any equipment with a motor larger than the 7.5HP on the lathe.

Does the transformer need to be sized to the RPC or the machine load? I was thinking I could use the 20HP idler for the RPC and the 15kVa transformer, but if I'm understanding what you're saying, the 15kVa would be undersized?

Thanks,
-Luke
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I've got a 200A panel in the shop so supply shouldn't be an issue.

I might have a line on a 20HP motor for scrap price, the price of the RPC control panel doesn't change too much whether its 10/15/20HP but at some point it's just overkill. I highly doubt I will ever have any equipment with a motor larger than the 7.5HP on the lathe.

Does the transformer need to be sized to the RPC or the machine load? I was thinking I could use the 20HP idler for the RPC and the 15kVa transformer, but if I'm understanding what you're saying, the 15kVa would be undersized?

Thanks,
-Luke

I personally like dual idler RPCs because the big motors tend to be louder.

The transformer should be sized for the machine load if after the RPC, but would ideally have suitable overcurrent protection.

On my converter I had a contactor for each motor onto incoming line power and the generated leg for starting them, then had a separate contactor to deliver three phase power from the converter. I would go this route in your case too to avoid having transformer and idler inrush all at once.
 

jappietoutou

Aluminum
Joined
Dec 2, 2015
Location
Montreal
Luke

If it can help, have a look at my project:
search for : 240 v to 575 v 3 phase converter project, on this forum. or search with my name: jappietoutou

JP
 








 
Top