VFD or Rotary ?

[Tom Kitta]

My small lathe has a build in variable speed - the main usefulness is when facing - you can put her on power feed and change speed as diameter is changing. Other then facing there is little use for var - I guess sometimes you run her too slow or fast on first pass @ changing speed on the fly is of use.

On geared machine the work around is to face part of it on speed A and the other part on speed B (or go A B C for larger parts).

On a belt based milling machines adding var is great help as changing belt speeds is a hassle in many cases. Geared milling machine usually does not benefit much if at all from adding var (ability to run it faster and slower then designed).

Also I can see benefit of var for some machines that can be run under or over speed. On that front however, many machines are already run 20% faster due to Hz change from World -> North America. So it is not a great idea to overspeed something that is already 20% too fast - maybe on some machines that have their low speed a bit high?

Benefits of var for each machine should be evaluated to that specific machine. For example, for my large lathe I would rather have a clutch build into it then var.

 

[Dabbler]

because it is simple as you say

I agree 100% a SB is VERY easy to VFD. It gets rid of the drum switch, too (and I've fixed/replaced dozens of drum switches in my time!)

 

[Hacker]

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.

I just went through this whole issue myself. I purchased a VFD to run my lathe and two years later bought a mill. I now have a VFD and an RPC (which I just finished building) and I am going to switch my lathe over to the RPC. With an RPC it allows you to have multiple three phase voltages by adding a transformer on the load side. My mill is 400V and lathe is 240V. If I find a good deal on a machine that is wired for 600V it is a matter of adding a transformer. If you go with a single VFD you are limited to the voltage you have. Cheaper to buy a small 3 phase transformer then a transformer and VFD. Also if you are using your VFD to power multiple pieces of equipment with different size motors you need to change the settings to match the VFD to the motor.
I bought my VFD from Wolf Automation https://www.wolfautomation.com/ they also sell RPC's and have a good tech department and were very helpful sorting out between a VFD and RPC.
They other thing that you will be running into with a VFD is you will need to oversize by a factor of 2 so you are looking at a 15HP VFD. If you are into some research and want to learn the ins and outs of this subject go to the Practical Machinist website https://www.practicalmachinist.com/ and you will spend hours learning about VFDs and RPCs

 

[JohnnyTK]

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.

So if you have a RPC and put a variable VFD on the output can you then hook this directly to your 3ph lathe connection to control your speed?

 

[Tom Kitta]

So if you have a RPC and put a variable VFD on the output can you then hook this directly to your 3ph lathe connection to control your speed?

Yes, VFDs are actually mostly designed for variable frequency driving - depending on the machine some wiring changes may be needed but at least for large loads you don't need to de-rate the VFD. For complex lathe you still will have to do lot of changes and connections. Usually VFDs are wired into machine so they also control stop and start and breaking etc. You probably would want to just wire in the VFD to the lathe for the main motor - the little 3ph coolant pump would just run of RPC. If there is a control transformer it would run outside of VFD (maybe pump is run from it).

A milling machine power feeds/ coolant pump would be run off RPC 3ph not off VFD. Same as with lamp and say DRO. Only main motor would be wired into VFD.

 

[Tom Kitta]

I just went through this whole issue myself. I purchased a VFD to run my lathe and two years later bought a mill. I now have a VFD and an RPC (which I just finished building) and I am going to switch my lathe over to the RPC. With an RPC it allows you to have multiple three phase voltages by adding a transformer on the load side. My mill is 400V and lathe is 240V. If I find a good deal on a machine that is wired for 600V it is a matter of adding a transformer. If you go with a single VFD you are limited to the voltage you have. Cheaper to buy a small 3 phase transformer then a transformer and VFD. Also if you are using your VFD to power multiple pieces of equipment with different size motors you need to change the settings to match the VFD to the motor.
I bought my VFD from Wolf Automation https://www.wolfautomation.com/ they also sell RPC's and have a good tech department and were very helpful sorting out between a VFD and RPC.
They other thing that you will be running into with a VFD is you will need to oversize by a factor of 2 so you are looking at a 15HP VFD. If you are into some research and want to learn the ins and outs of this subject go to the Practical Machinist website https://www.practicalmachinist.com/ and you will spend hours learning about VFDs and RPCs

I also have 600v in the garage - you need to look long and hard to find a small-ish transformer. You can run a large motor with a small VFD - no need to over size - people test 100 hp motors with 5hp rated VFDs. You usually over size with an RPC - a heavy load on RPC will start slowing it down and cause large voltage drop. People counter that with adding a fly wheel to the RPC motor and or start with other motors running - then once you connect your huge load it will slow down all motors in the garage a bit.

 

[Hacker]

I guess I got lucky as finding a reasonably priced txer was easier then finding a reasonably priced mill.

 

[Chipper5783]

The transformer add on works great. I paid $100 for a good used (even tested) 6.5kVA 240-600 3 phase X-mer. Keep an eye out - they are around.

 

[Desab]

I did just that gutted the electrical and rewired it for the VFD ever thing works like its supposed to except no brake resistor with this unit an oversight on my part still should install a contactor as per clough42.

 

[Hacker]

You can't add a braking resistor?