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VFD for a Clausing Colchester 15?

If you do not want to rewire the lathe than consider and RPC or Phase Perfect digital converter, probably less expensive than doing a proper VFD install. Assuming you have a 7.5 Hp motor, if using a VFD on single phase power, you would need to derate the VFD which are mostly three phase input power. In that size than something like the Fuji Electric FRN0047C2S-2U, Hitachi WJ200-110LF, Teco E510-215-H3-U are under 1K, but adding in an enclosure, disconnect, braking resistor, rewiring the lathe will add up quickly and yo need to know what you are doing. Building a proper VFD lathe control system takes some knowledge and experience. Plug and play, build your own RPC, otherwise Phase Perfect is probably a better option.
I have a Colchester 21" x 80" running on a Fuji VFD.
The VFD is one I had, and is not really sufficient for the motor. It's probably a 15 hp motor.
So I leave the VFD display showing amps, and run the lathe such that I never draw more than 21 amps or so.
Works great, and allows me to push the lathe as hard as I want to !
My lathe has clutches, so no need for braking resistor.
The VFD is on the wall behind the lathe.
I wired the VFD directly to the motor, eliminating any contactors and other devices from the circuit, but left them in place.
I then picked up the control wires from the contactor and ran them straight back to the VFD control circuit, so when I turn the original switch on the headstock, it sends a signal to the VFD to start the motor, then I use the clutch as normal 3-phase operation.
This is not rocket science and is not expensive.
You really don't need to worry about under-rating the VFD unless you intend to push the lathe very hard with big cuts, high feedrate and high speeds.
Good luck,
A VFD is the easiest method unless you want to spend more on a Phase-Perfect or ready-built RPC, building your own RPC is quite involved - I built one for the other machnery (Mill, TIG, bandsaw, pillar drill etc and anything I should drag home from an auction or work on for someone), but have VFD on my lathe (before I built the RPC) - a VFD has a lot of other advantages, like variable speed, which can be linked to a slider potentiometer on the cross-slide for almost-constant surface speed, soft start for the motor, electronic braking if your lathe isn't equipped with a spindle brake. As Overland suggests, you can use the existing contactors and switchgear wired to the VFD control inputs, I didn't as I wanted to add a few extra features (and the existing contactors were 50 years old with 415v coils!). A VFD will give a much smaller electricity bill than either a Phase-Perfect or a RPC, too, they're very, very, efficient.

Building my RPC was an interesting challenge, not only because I wanted 415v from a 240v supply - it has a big oil-cooled welder rewound as an isolating step-up transformer, several contactors for incoming power, idler connection, start capacitor, magnetic breakers for input, motor overload, output (with an RCD too) etc. as I'm quite keen on electrical safety :)

One caveat, if you plan to use the VFD at very low frequencies (I run mine down to 5Hz for some operations, e.g. spiral milling long-lead threads) there won't be sufficient air pushed by the motor's fan - add an external fan for cooling, and use one of the VFD's relay outputs programmed to switch it on *below* a sensible frequency, e.g. 30 or 40Hz, to maintain cooling, most VFDs have relay contacts you can program for various functions, e.g. the ABB I use calls it "supervisory frequency reached" and a normally-closed contact opens *above* the setting if told to, so from motor stopped to 30Hz the external fan cools the motor - also reminds me to power the lathe off when I'm done!
Thank you for your replies. I readily admit i’m confused by all these options. I just want to plug and play a lathe with 16 speeds and no variable speed component. Does the fact that my lathe motor is English present a problem, ie wound differently?

I have a TECO VFD powering my Bridgeport currently. The variable speed works normally. Westinghouse, which owns TECO, has an interesting YouTube video out titled something like-‘is your motor suitable for a VFD?’.
Evidently new motors are being built that are inverter ready with additional insulation and bearing mechanisms to protect the motor. My motor is 40 years old, and has none(I’m assuming) of the new technology. Will running it as a straight geared lathe protect it from damage?

thank you
I am not aware that this lathe has a clutch engagement system, I have a similar lathe. You also have the issue if the lathe uses a pressurized lubrication system(s) that if the Hz is reduced the lubrication may not be adequate dependent on the system used.

Get a phase Perfect or RPC if you want a plug and play option. I will disagree with others that want to use switch gear/contactors that have been used previously for high voltage, as they build up contact resistance and when used to switch low voltage/low current VFD inputs can be quite intermittent and poor. At least this has been my experience when I tried it years ago, I no longer waste my time by not doing a proper install upfront. On VFD sizing and derating, I size them appropriately to the application, if you happen to have one laying around that is your option to use it, but on doing VFD system installs I do not want to deal with starting out with the wrong size VFD. The cost difference is nominal if buying new hardware.

Older motors will do fine when using VFD's if run on 240VAC, as they are typically dual voltage. Issues of insulation breakdown and bearing erosion is more frequent at higher voltage and long cables. Cooling can be an issue with TEFC type motors below 15-20Hz under heavy or long loading, which is typically not done. A TENV motor it is not an issue.
Just to clarify my post from earlier.
The contactors are no longer in the circuit for my installation.
There were originally control wires from a switch on the headstock to control the contactors. I extended these wires to the VFD control circuit, so now the switches on the headstock control the VFD.
The motor wires would normally go from the motor, through a terminal strip to the contactor. I ran wires from the VFD to this terminal strip.
My approach was to leave the original wiring and devices in place.
I don't use the VFD to vary the motor speed, so my motor runs at normal operating speed, so no cooling issues.
It's really not complicated. Just consider the VFD to be a very efficient phase converter.
Send me a PM if you want to discuss this over the phone.