Question on VFD vs RPC

Hi All,

I currently have a 5hp RPC (an older FW design) in my shop which has been, up to now, just fine to start a couple of 5hp and smaller motors - Cabinet saw, misc other tools. I have a a new-to-me 5hp Lathe that can't start with this thing. I tried hooking up a 1hp motor in addition to the existing 5hp motor, that did not work a spare 5hp motor, that didn't work. Then found that when I started my 5hp saw, that seemed to allow me to start the lathe.

The lathe seems to have a (measured) 50A / 220v inrush current - at least that is what my meter indicated...

Bottom line is that I need to build either a 7.5 or 10HP RPC or buy a 'good' 5hp VFD.

If I buy a VFD, I can just leave the existing 5hp RPC intact and not loose that, and I get an advantage of soft start and braking...

The motors for the RPC will cost ~$130-$150, then I need switches, caps, etc. Not sure what a respectable VFD costs - I would assume the $110 Flea-Bay ones would not represent the best use of my money...

Thoughts or suggestions?

Thanks

Ray

I'd just buy a 10hp RPC panel (mine is from WNY Supply: they are decently priced and very well built) and purchase a used/rebuilt low rpm motor from a local motor repair shop or any other source. Low RPM ones are WAY less noisy. Mine is a 15HP 1200 rpm (the nameplate states 1165rpm) monster. You have to come very close to hear it running.

Start with the motor and then tell WNY what motor you have. Sometimes they need to adjust capacitors value to fit you motor better.

I doubt you'll save any money if you decide to build the panel yourself.

I wouldn't go below 10HP.

rbahr said:

Hi All,

I currently have a 5hp RPC (an older FW design) in my shop which has been, up to now, just fine to start a couple of 5hp and smaller motors - Cabinet saw, misc other tools. I have a a new-to-me 5hp Lathe that can't start with this thing. I tried hooking up a 1hp motor in addition to the existing 5hp motor, that did not work a spare 5hp motor, that didn't work. Then found that when I started my 5hp saw, that seemed to allow me to start the lathe.

The lathe seems to have a (measured) 50A / 220v inrush current - at least that is what my meter indicated...

Bottom line is that I need to build either a 7.5 or 10HP RPC or buy a 'good' 5hp VFD.

If I buy a VFD, I can just leave the existing 5hp RPC intact and not loose that, and I get an advantage of soft start and braking.

Click to expand...

"Respectable" VFD, rated for single-phase input by the maker? Two to three times the cost of less-respectable "white bread" ones. See prices, Yaskawa's industrial line, then "see below".

The motors for the RPC will cost ~$130-$150, then I need switches, caps, etc. Not sure what a respectable VFD costs - I would assume the $110 Flea-Bay ones would not represent the best use of my money...

Thoughts or suggestions?

Thanks

Ray

Click to expand...

I'd suggest finding a used 7.5 HP for new pilot idler. Might not even have to upgrade the Fitz Williams components, but if so, not hard.

Add one contactor, no extra caps unless fussy, your old 5 HP can be dropped online as a supplemental idler, dropped back off line after any heavy start. Modest extra effort, you can drop the 7.5 HP off, run on the 5 HP only.

As you have been. If that part ain't broke...

Not a lot of spend there, regardless, is there?

rbahr said:

Bottom line is that I need to build either a 7.5 or 10HP RPC or buy a 'good' 5hp VFD.

If I buy a VFD, I can just leave the existing 5hp RPC intact and not loose that, and I get an advantage of soft start and braking...

Click to expand...

The real bottom line is if you are willing to bypass any controls on the lathe and wire the lathe motor directly to the VFD.

You can connect a second 5hp idler to the RPC when starting the lathe. If you existing RPC can handle the additional load.

If your current RPC will bring it up with the table-saw running, that means you just need more gusto on the generated leg. I'd take Bill's suggestion one step closer to Neandrethal... get an old knife switch and an extra 5hp motor, wire it in with your RPC's output, mebbie a couple more capacitors on hand for trimming it... and wire it up... start your current system, throw the switch to bring on the second idler (+capacitors), and then start your lathe.

Once it's an operable system, take time to acquire a VFD, plan the controls, etc, and wire it all up to a test-motor to prove it out... then a quick shop-down of the machine to convert over to the variable. Lathe spindles are the perfect application for VFD.

I run my Monarch 10EE with an old Allis-Chalmers 1800rpm motor (OTH, I thing it is a 7.5hp), and an Allen-Bradley 1336... the EARLIEST type of that line... red LED display and crude (no menu) interface... best I recall, the drive is a 10hp unit... 480v 3ph, and I only feed it single. It was used, but extremely clean, I gave about $40 for it. I step the 240 mains up to 480 using a 10kva dry transformer that I acquired for less-than-scrap price from a junkyard.

I removed the motor's shaft-mount cooling fan (it's a TEFC motor), and bolted a constant-speed 220v 6" computer fan (the ball-bearing, cast aluminium type that run forever at a nice, slow, quiet pace) to the shroud, and wired it to the incoming power, so that it's running anytime the main power contactor is closed.

The early 1336 has NO phase-loss protection. Matter of fact, it has terminals to directly access the DC bus, in the event that an application has 500-800vdc on hand... but it's built so big, that I never worried about overloads... I probably never use more than 2.5hp of spindle power for any length of time... the rest of this beast's heft just protects the machine against lateral migration circumstances.

I wired the VFD's speed control to a 10k potentiometer, and made a mechanical adapter so that the Monarch's stock speed control knob would modulate the pot accordingly. I also pulled the machine's original spindle control switch apart and re-arranged the contacts so the VFD's control inputs would match. I mounted an E-stop button and a kick-switch on the front so that if something got really bad, I could shut it down hands-free.

My Monarch uses a D1-3 spindle nose, and I like it to stop with authority, so I programmed the drive for braking and quick reversal. Motor drives the spindle with a 3:1 reduction through a Gates Polychain toothed belt. Not as 'silk smooth' as a flat belt, but it doesn't slip. I COULD have gone through considerable work to adapt my original drive system's 2 speed gearbox to the motor, but with that reduction, and speed programming set from basically 3hz to 210ish, I have no problem running the full range of original spindle speed, with an obscene amount of spindle torque available.

And I didn't spend much money. I'ts on a 50A/240v range plug. I've got 120v receptacle on back to some LED lights, and there's been swarf and chips everywhere, and a big smile on my face, ever since.

jim rozen said:

Who's this Fitz guy?

Click to expand...

He was a member here many years ago and perhaps pioneered a RPC design that tried to optimize the 3 phase output

AT least one of his designs: https://www.practicalmachinist.com/FitchWConverter.pdf

WRT adding another idler motor - I found that not all 5hp motors worked, not certain why.

WHat are the better names for VFD's these day - I see a lot of stuff that seems pretty cheap.

Thanks

Ray

jim rozen said:

Who's this Fitz guy?

Click to expand...

SP error. We all know of him. The late Fitch Roberts Williams. Contributor of a better-than-average RPC design as a hobby:

https://www.practicalmachinist.com/FitchWConverter.pdf

Despite the old saying that an RPC "ain't exactly rocket science"? Fitch actually WAS akin to a "rocket" scientist - propulsion and power innards, not the fires and noise end - in his career "Day Job".

His power conversions & such were a great deal more exotic and "bleeding edge" than a simple RPC!

We were well blessed by his contribution..

Late? What, when??

HI All,

I picked up an AB 1336 7.5hp vfd. Now to figure out how to use this thing!

Thanks

Ray

rbahr said:

I picked up an AB 1336 7.5hp vfd.

Click to expand...

Good man.....