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VFD or Rotary ?

cuslog

cuslog

Super User
Premium Member
In the process of buying a Lathe. Nardini 1640 - 208V 3Ph, 6.5 hp. I have a VFD on a 2hp (on another machine) where speed control and fwd / rev on the motor are big advantages. On a lathe though, I'm thinking I don't really need speed changes or fwd/rev at the motor, they're already built into the lathe. Never had a RPC, wouldn't mind trying one. Factors though will be price as well as space (I'm in just one side of a 2 car garage and have a couple other machines in there as well.
What do you like ? Sources ?
Thanks in advance.
 
It’s the one for sale in Calgary?

Those lathes do not have clutches. So when you have a big hunk of steel in the chuck, the inrush current of the 220V, 3ph motor is huge. So a 7.5hp VFD should be able to help you there. OTH, the lathe also has a coolant pump - it will be 3ph as well. So you’ll need a second, dedicated VFD for it (in case you want to run it as well).
Nardini’s also have brakes. So you will need to rewire the lathe: VFD directly to motor (no other circuitry in between); spindle brake will have to use another signal to activate as the VFD will take care of the motor deceleration. If not rewired correctly, you can have the spindle brake fight the VFD - the VFD internals will probably loose that battle.

A RPC of appropriate size (15 hp, say) will run the whole machine and any other 3 ph equipment you have. No rewiring, Just plug it in and go. It will handle the inrush current of a heavily loaded motor. If you go smaller, it will probably trip the breaker.

https://www.phase-quest.com/product...MIsfu_3LGK7gIVscmUCR0bfwa-EAAYASAAEgKmQfD_BwE

https://toewspower.com/collections/...MIsfu_3LGK7gIVscmUCR0bfwa-EAAYAiAAEgIRKfD_BwE

https://www.americanrotary.com

These are some RPC suppliers. (Not affiliated with any of them).
 
Yeah, that's the one.
Thanks for the info - I'm no expert on the electrics but somehow I thought a RPC might be the better choice for this machine.
Having never owned (or seen one in person), I'm thinking they're bigger than a typical VFD ?
 
+1 on @RobinHood comments

My 7.5 HP lathe has a 15HP RPC, and it works fine. 10HP is recommended by Rotophase for a lathe with a large chuck on, for a 7.5 HP start. It's all about the inrush current, as @RobinHood says... The older RPCs were make more 'stoutly' and you could get away with a 30% overage, but the newer ones seem to be made more 'delicate', making 15HP more appropriate.

Inrush current can damage the balancing capacitors, if they are not specified appropriately.

The trouble to rewire a lathe for VFD makes it a serious and time consuming activity. Just ask @John Conroy

"Are they bigger than a typical VFD"

Yes, mine is 24" X 24" X 20" high.
 
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Almost all people I know that are serious have RPC. It is cheap to make and it is hard to kill. Some work for decades without any issues.

Think size of a motor of your choice plus a box the size of a small electrical distribution panel.

Total cost of RPC of 15hp power is at most $500 (motor and panel). Plus cables etc.

My 15hp has no issues starting 10hp lathe. It is capable of running multiple machines at once (I think I have 3 on video).

If you want I can sell you a motor.
 
With an RPC, are you not running two electric motors to run one machine? I'm looking at it from the cost of operation perspective.
 
A 15hp idler will consume about 0.5kw of energy turning it into heat. This will vary with load - more load less waste. VFD waste is smaller - maybe for a 5hp VFD it is at most 0.2kw also exposed as heat except here more waste is produced with higher load.

VFD also cannot run multiple motors / has issues with resistive loads / cannot start larger loads "fast" etc. VFD also has issues with the wave form while RPC usually has one "wild" leg.

Let just sum this up as follows - for electric current change VFD is like a dial up modem is for internet while RPC is like a broadband. Once you go RPC you wonder why you ever bothered with converting voltage from 1ph to 3ph with a VFD. Main use of VFD will then be to convert 3ph to 3ph variable Hz for speed control.

There are commercial motor generators available that are build as one unit having two "motors" inside. These are very efficient (more then VFD for close to full load) but are expensive. They are offered by https://www.americanrotary.com/ and others.
 
Just thought of this:

a) the factory brake is most likely external to the motor, running on DC. If the bridge rectifier is 3 phase, you will burn up the VFD (because, as Tom mentioned above, VFDs hate resistive loads).

b) If the brake is integral to the motor, VFD won’t work either.

c) if the bridge rectifier is only using one leg (well, two technically) of the 3 ph input, that is a better situation, but you still need to feed in single phase to power all other safety/control circuitry on top of the power to the VFD.

d) depending on the factory brake set up (see points above), it might be easier to disable it all together and use braking resistors as part of the VFD. That can get speedy in a hurry though for a 6.5 hp motor as there is a lot of back EMF that needs to be dealt with especially if it is loaded (lots of inertia).
 
Every VFD over 5HP that I have seen (3800W and above) seem all to have external braking resistors.

The cost to operate a VFD and/or RPC is very small. I wouldn't run one all day long if I'm not using it, but for the hour or 2 I use it, It is a trivial cost.

I prefer to VFD my mills, and use the RPC for stationary woodworking stuff and the metal lathe. None of the woodworking stuff needs to run at variable speed, and the lathe is a lot of trouble to rewire, so RPC.
 
Brake on my lathe is scaled down version of what is on the car - friction based. VFDs usually can handle tiny resistive loads OK in my experience - like a lamp or similar. The way one has to think of a VFD is a tiny power plant with max output of the rated plate (or de-rated output for larger VFDs that convert 1ph to 3ph) vs. an RPC which can handle heavy over loads for a tiny fraction of a second - as it is connected to a grid which has a lot of "spare" capacity. For example, a 5hp VFD running a milling machine that runs at say just 1hp load should still handle power feed motor starting that has just 1/6hp - even "hard start" should not exceed 1hp draw over say 0.01 sec. Your VFD should simply show and error code and turn off it hates you or with cheap Chinese unit it will produce some smoke.

VFDs use some "hacks" to get around some limits - for example they try to minimize harmonics and undesirable wave forms by expecting a single motor on the connected end. You can run a regular "heater" but VFD will have little control over heat output and I am sure you get rather strange wave forms (which for primitive heater should not be an issue as long as you don't mind a lot of interference being created all over the place). Induction heaters will work great - you can use VFD with these.

Also you can get cheap small 5hp VFD to run your lathe - it will simply not run at full 6.5hp.
 
Which of the two, VFD or RPC, is the one you hear can cause issues with the household lines like driving the TVs & monitors crazy. Is that really a thing? If so why?
 
Any VFD, correctly installed is fine, but incorrectly installed can mess up electronics. For a great installation video, follow Clough42's installation at:

 
Dabbler said:
Any VFD, correctly installed is fine, but incorrectly installed can mess up electronics. For a great installation video, follow Clough42's installation at:

Click to expand...
That is an excellent series, it was instrumental in my choosing a Teco VFD and getting me through my mill conversion.
 
Vid all the way ....you can use the speed control for facing or finding that sweet spot if chatter is an issue when machining light parts
 
Ken Mach said:
Vid all the way ....you can use the speed control for facing or finding that sweet spot if chatter is an issue when machining light parts
Click to expand...

You can have that with a RPC. My small lathe has infinitely variable speed and can change it as needed. Or I can connect a VFD to one that does not have it to 3ph input.
 
I agree completely that a VFD on a lathe is insanely useful! It is just very hard to retrofit on many modern lathes, such as the Modern 14X40 - the contactor box has 9 contactors in it! To rewire it properly, would take the better part of a week, full time, and perhaps as much in planning and redesign of the electrical system.

On some smaller lathes without electric brake, apron mounted controls, and other luxuries, it can be much simpler.
 
Dabbler said:
I agree completely that a VFD on a lathe is insanely useful! It is just very hard to retrofit on many modern lathes, such as the Modern 14X40 - the contactor box has 9 contactors in it! To rewire it properly, would take the better part of a week, full time, and perhaps as much in planning and redesign of the electrical system.

On some smaller lathes without electric brake, apron mounted controls, and other luxuries, it can be much simpler.
Click to expand...
A VFD on my SB 10K would be something I would tackle because it is simple as you say. Fwd/off/rev switch and frequency pot would be all it would require.
 
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