I've recently received a really good question from a PM forum member regarding the HLV-H VFD conversion that I completed ten years ago. I t asks what i would do different now .

I thought it might be worth posting my answer here for other's to see and comment on.

Switch or not to switch?

In 2009, new VFDs were very expensive so , I bought my VFD from eBay (3ph ones then were cheap!) .It was a conventional switched output VFD and so it tends to lack power at lower output frequencies, leaving me little option but to switch motor windings.

There have been suggestions that there is no need to switch motor speeds, if one used a modern sensor-less vector type VFD. These VFDs can compensate at low frequencies and provide much more torque. I have not tried one , so I can't be certain but: The Hardinge motor has a 3:1 speed change and that is asking a lot of the VFD (in 'low' speed it would be only putting out 17Hz) and motor.

It is notable that when Hardinge produced an all electric vary speed HLV-H they up graded the motor to 5hp .

I remain a switcher :-)

When to switch motor speeds?

There is at least one forum member that will tell you that VFDs are happy to be switched under load . I strongly disagree, and so ,as it happens, do Siemens and every VFD maker whose manual I have read!

I played around trying to make the VFD 'catch' the spinning motor, to allow seamless speed changes, after powering down swapping contactors. In the end , that proved unreliable, so the stop /switch/ start logic was used.

When I started the conversion , I had been building electronics devices for years and wanted to get away from microprocessors and complex logic, which is why I chose to use simple relay logic.

It turned out to be quite complex relay logic and made the machine somewhat 'clunky', so IF I were to do the SAME conversion, I think I'd now wrap the logic up in a PIC micro-controller and use solid-state output to drive the contactors.

However , I have been thinking about different ways to do the conversion: Someone on this forum suggested two VFDs ,one for each speed.

The two windings are separate and, provided the other VFD is disabled, each VFD could be optimised to drive its 'motor'. I have a couple of spare Siemens VFDs (one for the speed jack) and intend to try this at some point. I may be able to get the 'flying motor catch' to work with this setup.

Bigger VFD and more power ?

Something that has come up recently, in relation to milling machines , is that the VFD tends to limit the maximum motor power : Motors are very robust and can easily produce far more than their plate rating for short periods if the current is available. A bigger than 1.5hp VFD (and transformer) could be easily used and would provide more power , but I can't say I've ever needed extra power on the lathe.


Transformer to voltage doubler?

In most of the conversions (240v 1ph to 415v 3ph) I have used a step-up transformer to get the higher voltage single phase for the VFD. In a couple I have used a capacitor voltage doubler. In one factional Hp device all it required was a simple wire link inside the VFD . For my brother's mill conversion I built a large 4kW unit with a couple of large high voltage caps (from a dead VFD) that is nice and compact and has been working fine for a couple of years.

There are VFDs available now with a capacitor doubler inside , so perhaps that's the way to go rather than a difficult to find transformer?

What else?

One thing I would suggest is a VFD with braking resister option (do not confuse this with dc injection braking); The spinning motor and gubbins generate a lot of volts when the VFD tries to slow them down , this power has to be dumped somewhere and without a braking resister , the bus voltage will rise causing the VFD to give up with an over-voltage fault (requiring a reset) . I managed, to get the braking time down to something usable but would be better with a BR.