RPC design help

Leslie said:
I have just installed a 3 HP RPC Kit that I bought on E-Bay, the components came with clear instructions (had the 3 HP Motor) assembled and connected, runs my Hardinge Lathe, Tree Mill and Boyar Schultz Surface Grinder very well.

I'm furnishing you a link to their 15 Hp and 20 HP Kits.

Hope this helps.

www.ebay.com

15HP Rotary Phase Converter Quick Build Kit | eBay

With this setup you will have a 15 hp rotary phase converter, your 15 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 35XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.
www.ebay.com

www.ebay.com

20HP Rotary Phase Converter Quick Build Kit | eBay

With this setup you will have a 20 hp rotary phase converter, your 20 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 34XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.
www.ebay.com
Click to expand...
Very cool, but they are ripping customers off.
If you were to source the parts, you could get them for 1/2 the cost of a kit
 
The previous owner probably started the 3 HP motor first then started the 5hp, Yes you can do this to start larger motors on the same buss.
Here's a link to my journey designing & building a RPC, lots of good info here including a link to Jim Dawson's epic build.

www.hobby-machinist.com

Rotary Phase Converter Rebuild / Build

Hi all, I just hooked up a used, Snyder brand, Rotary Phase Converter to my newly acquired lathe (more on that soon). It works but makes excessive noise & vibration, especially when running under no load condition. It sounds like its vibrating at 60hz so my thought is its electrical in nature...
www.hobby-machinist.com www.hobby-machinist.com
 
I built my 10hp RPC 30+ years ago to power my 1947 16" 7.5hp Redstar radial arm saw. The RPC is still working perfectly. I have run my 7.5hp RAS at the same time my father was using my 7.5hp table saw. No problems. When the RAS is running it is contributing the the generated leg so starting the 7.5hp table saw draws less current from the idler than just starting the table saw by itself without the RAS running (and vice-a-versa). I.e. You need a RPC that will start your largest machine and any bigger is just a waste load. Starting and running subsequent machines will not require a larger idler motor.

A 5.4hp lathe usually won't start off a 5hp idler (in my experience). The seller might have been using the 5hp idler to start the 3hp mill that together might supply enough current on the generated leg to start the 5.4hp lathe.

Do you really think that you will purchase a machine with a larger than 7.5hp 3ph motor? If not I would recommend staying with a 10hp idler. A larger idler will require larger contactors, larger capacitors, heavier gauge wiring, larger supply circuit and put a much higher load on your electrical panel to start the larger idler. If you think you will buy larger motors then go with the bigger idler. If you can't really see larger machines happening I would go conservative on idler. A 20hp idler can dim the lights on startup with a normal 200amp service. (The larger the starting capacitors the faster the idler will come up to speed and the more inrush current it will draw when starting the idler).

An RPC is great for when I get a new machine. I plug the new machine into the RPC and I am off and running immediately.

Most all of my 3ph machines are now running on VFD's. I can plug them into any 240v outlet in the shop making it much easier to move them around. The startup inrush current on a VFD is a fraction of the startup inrush current of the same machine on the RPC. The soft start function of a VFD brings the machine up to speed over a set programmable time rather than full inrush power as soon as you hit the on switch so the startup current is a fraction with the VFD.

A VFD will most always require some rewiring of the machine. A RPC will not require any machine rewiring. I love the VFD on my South Bend 13" lathe... it is so nice to bump up the spindle speed when making multiple cut passes to compensate for the lower SFM. On my RAS and Table saw the variable speed of the VFD is useless... BUT I LOVE the braking function of the VFD to stop the blades in a fraction of the time that they take to stop by just cutting off the power switch. Eventually EVERY 3ph machine in my shop will run on VFD's... as soon as I stop buying new machines and rewire the couple machines I havn't yet.

A 20hp idler will take a lot of start capacitance (hence current) to get the idler up to speed. This will be a HUGE draw on your circuit panel. A 3hp pony motor will start up the 20hp idler slower reducing the load on your panel greatly. This is why pony motors are often used on larger idlers... to reduce the inrush current for starting the idler. This is the only reason I can see to mess with the hassle of starting the idler with a pony. A pony motor is usually physically disconnected from the idler when the idler is up to speed. I.e. The pony motor is on a hinge. The pony motor is pivoted on the hinge to tension a belt. When the idler is up to speed the idler motor is energized and the pony motor is simultaneously slacked so the belt goes flying off into space somewhere. (at least this is how I have seen it done. It was quite a show!).
 
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MikeInOr said:
I built my 10hp RPC 30+ years ago to power my 1947 16" 7.5hp Redstar radial arm saw. The RPC is still working perfectly. I have run my 7.5hp RAS at the same time my father was using my 7.5hp table saw. No problems. When the RAS is running it is contributing the the generated leg so starting the 7.5hp table saw draws less current from the idler than just starting the table saw by itself without the RAS running (and vice-a-versa). I.e. You need a RPC that will start your largest machine and any bigger is just a waste load. Starting and running subsequent machines will not require a larger idler motor.

A 5.4hp lathe usually won't start off a 5hp idler (in my experience). The seller might have been using the 5hp idler to start the 3hp mill that together might supply enough current on the generated leg to start the 5.4hp lathe.

Do you really think that you will purchase a machine with a larger than 7.5hp 3ph motor? If not I would recommend staying with a 10hp idler. A larger idler will require larger contactors, larger capacitors, heavier gauge wiring, larger supply circuit and put a much higher load on your electrical panel to start the larger idler. If you think you will buy larger motors then go with the bigger idler. If you can't really see larger machines happening I would go conservative on idler. A 20hp idler can dim the lights on startup with a normal 200amp service. (The larger the starting capacitors the faster the idler will come up to speed and the more inrush current it will draw when starting the idler).

An RPC is great for when I get a new machine. I plug the new machine into the RPC and I am off and running immediately.

Most all of my 3ph machines are now running on VFD's. I can plug them into any 240v outlet in the shop making it much easier to move them around. The startup inrush current on a VFD is a fraction of the startup inrush current of the same machine on the RPC. The soft start function of a VFD brings the machine up to speed over a set programmable time rather than full inrush power as soon as you hit the on switch so the startup current is a fraction with the VFD.

A VFD will most always require some rewiring of the machine. A RPC will not require any machine rewiring. I love the VFD on my South Bend 13" lathe... it is so nice to bump up the spindle speed when making multiple cut passes to compensate for the lower SFM. On my RAS and Table saw the variable speed of the VFD is useless... BUT I LOVE the braking function of the VFD to stop the blades in a fraction of the time that they take to stop by just cutting off the power switch. Eventually EVERY 3ph machine in my shop will run on VFD's... as soon as I stop buying new machines and rewire the couple machines I havn't yet.

A 20hp idler will take a lot of start capacitance (hence current) to get the idler up to speed. This will be a HUGE draw on your circuit panel. A 3hp pony motor will start up the 20hp idler slower reducing the load on your panel greatly. This is why pony motors are often used on larger idlers... to reduce the inrush current for starting the idler. This is the only reason I can see to mess with the hassle of starting the idler with a pony. A pony motor is usually physically disconnected from the idler when the idler is up to speed. I.e. The pony motor is on a hinge. The pony motor is pivoted on the hinge to tension a belt. When the idler is up to speed the idler motor is energized and the pony motor is simultaneously slacked so the belt goes flying off into space somewhere. (at least this is how I have seen it done. It was quite a show!).
Click to expand...
Thanks for the reply. Very informative. The previous owner very well could have started up the mill and subsequently started up the larger motor on the lathe.

All this discussion on VFD's is piquing my interest. I have access to VFD's (Allen Bradley) but all require 480v input. What VFDs would you recommend that's reasonable in price? Obviously, I'm trying to be as economical (within reason) as possible.
 
It is completely unnecessary to oversize a RPC to the amount suggested by the first poster; in the case of a hobby shop, the RPC need not be for a HP size of double (in the extreme) of the HP rating of the largest motor in the shop, additional motors in use in the shop act as additional RPCs and add to the capacity of the main RPC; a 15 or 20HP RPC in a hobby shop would be not only unnecessary, but ill advised, for one, due to the expense and the need to oversize the main power feed. As I stated in my earlier post, my 5 HP homemade RPC works just fine, running motors even larger than itself. I do not run more than one motor at a time, so I cannot comment on that, but most folks attest that it is possible, and in fact adds capacity to the system.
 
As you can see here, you're getting a range of answers.
The reality is that RPCs are a fairly practical kludge to generate something that looks like industry standard 3-phase power. How close do you want to get to meeting those standards? At one end you are paying American Rotary prices and getting a pretty good approximation. Obviously successful RPC manufacturers are inclined to make sure you get one that works well, so they will push over size.

As you cut back you get poorer electrical results. Doesn't matter greatly for most manual machines. "It works". As you push it, you get weak/slow startup especially for motors that start under load. Eventually that weak startup and poor running electrical performance, if taken too far can heat up motors and reduce their life. Which unless taken to extreme, probably doesn't matter for as little as hobby machines are used.

But that's the "why" to the range of answers. There is a lot of variation in what a 5HP motor, or a 10HP idler do electrically. Without standardized idlers and standardized loads and all that industry standard crap, you're not going to get a single this is suitable and the best cost for all situations answer. Pick something easy to do and go with it. Avoid going crazy oversizing, you can always upgrade, and you pay for oversizing in wasted electricity although for smaller RPCs that is not a big deal if you don't leave it running all the time.
 
Greyhound said:
Thanks for the reply. Very informative. The previous owner very well could have started up the mill and subsequently started up the larger motor on the lathe.

All this discussion on VFD's is piquing my interest. I have access to VFD's (Allen Bradley) but all require 480v input. What VFDs would you recommend that's reasonable in price? Obviously, I'm trying to be as economical (within reason) as possible.
Click to expand...

The Hitachi WJ2000 is known to be a top end VFD with extensive ability for customization and a great reputation for reliability. I have a couple of these and found they are quite complex to setup with lots of interrelated parameters. One of them is on my 13" South bend lathe. I have tried multiple times to enable the external braking resistor and messed up the programming so the drive no longer functions at all leaving me with a good day of work reconfiguring the drive from scratch after doing a factory reset on the drive. I gave up on trying to get the external braking resistor to work. I know the drive can be made to use the external braking resistor but I am missing some interdependent parameter that I have not been able to figure out and I have spent WAY too long reconfiguring the drive after my failures.

I have some older model Fujitsu and Teco VFD's that have been working well and are much less customizable but much easier to setup than the WJ2000's. I would consider these top end VFD's with less customization ability than the WJ2000's. But I see no use for the extensive customization ability of the WJ2000 for a home shop.

On one of my 16" radial arm saws I have a Huanyang GT series VFD which is an inexpensive Chinese VFD. For me and my home shop use it is an excellent value basic drive which is MUCH easier to configure. It took me about an hour to configure the Huanyang GT with a external braking resistor. (Not all Huanyang VFD's are capable of using an external braking resistor so you have to be careful when ordering. It is best to contact the company through Amazon to make sure you are getting the correct drive if you plan on using a external braking resistor).


Many on here are dead set against cheap Chinese VFD's. I would be too if the reliability of the VFD meant the difference between making money / paying my bills vs not being able to work. For my hobby shop this isn't a concern so the Huanyang GT VFD's are what I will most likely use going forwards. There are enough Hunyang VFD's out there in the world that it wasn't too difficult to find people out there that are familiar with them.

I want to reemphasize part of my original post... VFD's require rewiring a machines original controls. The VFD should be wired directly to the 3ph motor with no switches or other controls between the two. You should no longer use the machines original control switches, you control the machine through the VFD. You can use the front panel buttons on the VFD to control the machine but I REALLY prefer to use simple switch circuits to control start/stop/direction/speed with external physical buttons. To me the physical buttons (as opposed to the VFD panel) feel much more natural in operating the machine.

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The black switch on the base controls power to the VFD. I turn it on in the morning and turn it off when I leave at nigh (I often forget to turn it off which hasn't caused a problem). The green and red button box are what I start/stop the saw with while actually using it. If I find (or take) a picture of the saw together I will post it later.
 
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I use both, 2 VFDs on the Mill, one on the spindle and one for the head raise/lower motor. One VFD on each, bandsaw, drill press and air compressor. The RPC runs the the 10" South Bend and 13" Clausing Colchester Lathes and the washing machine. I Like the variable speed, soft-start and other features of the VFDs but feel the RCP is more reliable. If I had to go with only one, it would be the latter.
 
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