I've just spent a couple of hours trawling through the manual sat in front of the mill, with a soldering iron and a multimeter.

The net result is that the 2k2 pot is now in the bin, having measured various resistances between 0 and 20Meg with no consistency. I have replaced it with the only other one I have, a 4k7, and checked all the relevant parameters I can find. The result is not all that much different to be honest, still very difficult to set lower motor speeds though better than before. I have programmed a minimum frequency of 12.5Hz, which makes things easier, but I can really only control it accurately from 50 down to 40 Hz, beyond that is just pot luck.

For anyone interested, the pot is connected to +v, A1 and AC (with the centre terminal to A1). The relevant parameters are

B1-01=1, B1-02=1, B1-04=0, H3-02=0, also the min and max frequency settings I mentioned above.

There may be other parameters that are relevant but I haven't been able to decipher them.

Anyone complaining about the HuanYang inverter manual needs to read the Omron one – it is bordering on the incomprehensible.

You saw what I did there..

I've been I've been using the mill a fair bit over the last few days and the inverter works fine apart from the very non–linear speed control. To be honest I usually rely on the gearbox for nearly everything, but thought the speed control, in conjunction with the fed-rev facility, would be useful for power tapping.

Nikai- interesting to hear you recollect a similar problem. I can easily try a few different pot values (not the sort of thing I usually keep on hand so I'll have to order a few or go to Maplins). If you have a note of,your settings and parameters that might be useful (mine runs a conventional 1kW 4 pole motor and I've set it at 12.5kHz min and 50 kHz max).

Muzzer – I agree that the vfd is very versatile, but therein lies the problem with the manual – it has so much information in it, and on topics which are not relevant to my particular application, that it is difficult to sift out the bits that are relevant for me. For example, the inputs are all programmable by changing parameters, which makes it difficult to find out what the parameters should be for my use of S1 and S2. And there is one parameter that I think should be set to zero, but which steadfastly refuses to change from its current setting of F. Re the 10v – I'm using the on-board 10v reference supply from V+, with 3 wire control using A1 as the control voltage and AC as the common.

Just a thought on re-reading the manual yet again – does anyone think that parameters H3-03 and H3-04 have any bearing on the speed control issue? They are described as the gain and bias of terminal A1.