Hardinge KL-1

[Swarfing]

Paul the money you are going to pump into stepping up for the 415v, you might as well just swap it out for something else. a good quality 3 phase motor that will work at 240v delta with a vfd would be far more logical. I had to do the same for mine and even put a lower hp in, a 2hp instead of the 3hp it replaced. Never regretted it at all.

[JAZZCNC]

Would imagine a rotary converter would be the cheapest and easiest way.? Keeps every thing original plus neat and tidy.

[Swarfing]

Trouble is they are not very efficient and take a bit of room. The OEM motor was meant for a purpose. A replacement motor would be for the new purpose. No problem changing a motor to do job it is meant for. A darn site lot cheaper as well unless you build? Most lathes are sold with different motors for the environment they are to be used in.

[m_c]

Rotary converter effiency depends on how well they're built.

I built an ammeter into mine, and when sitting with no load the needle is barely above zero. The noise of the idler motor does annoy me, although that's probably more to do with it sitting on top of a bit plywood with minimal vibration damping!

I am however considering upgrading to a digital convertor, as the latest purchase is going to be far too much hassle to convert to single phase, and may be pushing the power limits of the current rotary.

[pauly45]

I hear what you are saying about changing the motor, but the motor in the Hardinge lathe is a highly balanced motor, with two sets of windings for a low and high speed so there is just no sense in removing it and changing it for an inferior motor. Plus, the cost to convert using a transformer and VFD's isn't really that large.

Yes, I may be going over the top with 3 VFD's, but this is a project and a challenge.
I've also seen another 'mycnc' contributors hardinge powered this way, and it's impressive. It also makes sense to me as it retains all the original lathe controls.


Some calculations for the transformer :

The motor is 1.5hp which is 1.1kW (1hp = 0.746kW)
Assuming a 90% efficiency of the VFD then 1.1kW/0.9 = 1.22kW required.
Approximately, Transformer kVA = 1.22kW/0.7 = 1.74kVA.

However, this rating would be for heavy, continuous operation, which I'm just not going to do, so I have gone for a 1.5kVA Autowound Transformer. This will keep costs and physical size of the transformer down to an acceptable level.
An isolation transformer will be bigger and more expensive, and I've been told it's just not necessary.

This will step me up from 240V to 415V, then this will connect into two of the connections of a 3 phase input VFD.
I'll common up two of the inputs to share the loading on one AC cycle a bit more across the VFD rectifier diodes.

[EddyCurrent]

then this will connect into two of the connections of a 3 phase input VFD.
I'll common up two of the inputs to share the loading on one AC cycle a bit more across the VFD rectifier diodes.

I've never heard of this done before. Devices inside the drive use a particular phase for synchronisation, that's why they specify which terminal to use if you are on single phase input, also a 3phase rectifier relies on phases being 'out of phase' for the return current but 2 of yours would be 'in phase'.
I hear what you are saying about the original motor but to be honest a modern 240v 3 phase motor is going to be just as good, the 240v drive will be cheaper and you won't need a transformer.

[bikepete]

"I've also seen another 'mycnc' contributors hardinge powered this way, and it's impressive. It also makes sense to me as it retains all the original lathe controls."

Not sure if it's mine you saw but if so thanks! Mine's written up at

HLV conversion to VFD - circuit and pics

Six years on it's still working fine. If I were to do it again I'd probably somehow add an interlock to the speed change as various people suggested just to be on the safe side and also an external speed pot. But neither is likely to rise to the top of the projects list for a while.