Static converter?

[davebreeze]

I've just bought an Elliott Omnimill milling machine which I'm very pleased with (from Emtec Machine Tools in Basildon - great service, very helpful by the way). I'd planned to run it from inverter drives as I have with my Colchester lathe, but it turns out the 3-phase motors on the Elliott are not dual voltage, they're 400v only. So the options are: change all the motors to single-phase, or get a static converter.

So I'm just wondering what people's experiences are with static converters? I noted there was an earlier thread about VFDs where a couple of folks expressed satisfaction with running machines from static converters, but when you read the manufacturers websites they seem to be trying to put you off buying one rather than sell them!

I think I could change the motors for about the same cost as the cheapest converters, it's got 4 motors - one for the vertical spindle (0.75hp), one for the horizontal spindle (1.25hp) and two small ones for the suds pump and table power feed. It would be a big hassle though as I don't know how much of the switchgear would need changing too, so the converter is an attractive option.

[jackrae]

Hi Dave
Seems very much a no-brainer. With 4 motors, and a heap of control gear, it's an inverter you need.
Since the machine is rigged for 3-phase, all the control gear will be designed to run from the 400/415 mains supply, most probably through reduction transformers.
If you opt for changing the motors for single phase units, you'll also have to rework the control system which no doubt will be easier said than done.
Because of the control gear requirement you'll be best to avoid using VFD inverters, since the control circuits will require the full 400/415 volts and may not appreciate the "artificial" AC that VFD devices produce.
You then have a couple of choices, static inverter or rotary inverter. You pays your money and makes your choice.
Unfortunately all my experience is with VFDs (and they are all good) so perhaps one of our other writers can guide you towards the best choice between static or rotary.
jack

[mutley]

I've got a rotary invertor. Very happy with it, 3 wires to connect, mains excepted, and away I went. No load setting to tweak no minimum motor load to worry about. Yes you have the constant noise of a motor running light but I find that quite comforting!

Andy

[phillthy]

Gents,

I did a lot of research on inverters, and found that only rotory inverters meet the grade for running 3 phase motors under variable load. The static units are very sensitive to variations in loading, and often require a soft start to avoid being damaged or damaging the motor. There are a couple of really good rotory inverters on the market, complete with computer controled capasitor banks to ensure an even current supply and constant voltage.

At pressent, I run of a genset, but as soon as my new workshop is built, I'll be buying a rotory inverter. Fortunately for me, my new place has what is called "farm power" (here in West Aussie), which is a 500V supply as well as the standard 240V. This means that the current draw from the mains is much lower (twice the voltage = half the amps for the same power) and there are no losses through bringing the voltage up to 440V through transformers. Even with transformers, the losses are not very large except on startup.

From my research, I would not buy a static innverter.


Cheers,

Phill.

[houstonceng]

Hey guys. These things (Static and, so called, Rotary) are CONVERTERS not INVERTERS.

Bottom line is that a static converter is best suited to a single motor and needs to be matched to the load. Too little load, can lead to problems and burnt out motors because the "3 phase" would not be balanced and one winding could receive too much voltage.

A "rotary converter" is just a static converter with a pilot motor. You pick a size that will suit the maximum load of all you motors. Then add a suitable factor to ensure that the converter can handle that load and still give good balanced 3-phase. Some suggest a factor of twice the maximum load. (ie a 4kW Rotary driving a total of 2kW). The other beauty of the "rotary" over the "static" is that the pilot motor provides the miniumum load, so you can leave the suds pump running when you turn off the main 1.25hP drivev with no fear of burn-out.

You could, if you wish, wire the 3-phase from the converter around your workshop and run more than one machine from it - so long as you don't excveed the max loading

Your best bet is to talk to some knowledgable suppliers, explain your problem and get some quotes. A "good-un" won't be cheap.

[mutley]

you spotted the deliberate mistake! I shouldnt try and do two things at once ie work and play!!

Andy

[davebreeze]

A rotary converter's not really an option for me - just too expensive. It would be fine if it was powering more than one machine but my lathe is already set up with VFDs. So I was just wondering if static converters were any good - they seem to get mixed reviews but on the other hand there are several manufacturers of them , so somebody must be using them...

[AndrewP]

I use a motorun 2hp static converter for my Boxford AUD, it is switchable in 3 ranges for different size motors, not that that feature would help you a lot. Could you re-motor the suds pump and table feed for 240v without upsetting the switchgear drastically?
I have to admit the electrickery is not my field, turn it on, it buzzes, lathe works - sorted!
Took the cover off because I'm nosey, dead simple, not much to go wrong I would have thought, some big capacitors, a transformer, relay and a few switches, don't know where they get the nearly £300 price tag from.
Probably not much help but that's never stopped me before

Cheers, Andy

[davebreeze]

Andy, that's just the sort of practical experience that I'm after. I take it you're happy with the way the lathe runs - plenty of power, smooth running etc?

I'm sure I could change the table feed and suds pump motors, I was even thinking they could use 12 or 24v DC motors which are commonly available and the electrics would be nice and simple. I think I'm tending toward fitting single phase motors just on the basis of cost, but still tempted by the idea of the converter for simplicity.

Attached (hopefully) is a picture of the machine on it's way into my garage. Took all day as the 1-tonne engine crane could only just reach.

[mutley]

I picked my rotary convertor up second hand for less than the cost of a static.

Andy

[davebreeze]

They seem to hold their value, I haven't seen any second-hand bargains.

[AndrewP]

Very happy with the way the lathe runs Dave although my previous equipment has been somewhat weedier so I have little to compare with. I did notice that the build date written inside the converter is 1991 and I know the lathe has completed at least 4 5" gauge engines in the hands of the previous owner so it hasn't been idle.

I've never run a big mill but since the problem with static converters seems to be running small motors from the setting for a bigger one, what is the likelyhood of running the suds pump or table feed without one of the spindle motors? If you constrained usage on those lines then surely the static would be OK. Mine has 3 settings - 1/8 to 1/2 hp, 1/2 to 1 hp and 1 to 2 hp just by a simple 3 position switch on the side of the case.

Motorun have a website with some data and sales blurb.

[newmans]

Dave, let me say first that I'm not an electrical expertby any means but I do run a Colchester bantam from a static converter. I chose this because of the awkwardness of changing switchgear, motor etc at the time. I chose static for cost reasons. I have a Transwave 3.0kw model from Power Capacitors (usual disclaimer). It works well. Two knobs on the front, one on/off , the other to adjust to the load. Above the load adjustment knob there is a dial showing voltage. You must make sure that you use this knob to adjust voltage to be within 380/430 reading for whatever load the machinery in use is putting on the converter at the time. Settings can vary depending on what speed your lathe/mill is working at. You just adjust the knob if you change any speed /loading. It's not a big chore and your ear will very soon tell you if you forget. To date mine has worked well and the lathe runs sweetly at the right setting. The motor in the lathe is rated 1.5/0.75kW two speed and the convertor I got is Max 3kW. A bit extra capacity would be a wise precaution. Another precaution is to have a heavier than normal single phase cable feed to your converter, 4.0mm. in my case.
The documentation that came with the machine was anything but comprehensive especially to the layman. Suppliers could do a much better job here for their customers.
Considering the extra noise and cost of the rotary converter, I'm happy enough still that I went with the static. But, my experience only!!!
John

[davebreeze]

Looking at the manual for the mill it seems that the suds pump is wired so it only runs when one of the spindle motors is running anyway. Also, I'm unlikely to use the power feed other than when cutting so that's no problem.

I emailed Transwave/Power Capacitors and got a reply to say that the machine would run fine on one of their static converters but they thought I'd get fed up with changing power settings for the different size motors, and a rotary converter would avoid this.

Thanks for your help folks, very useful.

[jackrae]

Dave
Slightly off topic and probably obvious but,
when lifting high loads off a trailer or trying to get them under the garage door, letting the tyres down or even removing the wheels can often get you out of a bind.
Before you buy a rotary have you considered making your own? Fairly easy to do and requires minimal ziggy knowledge.
jack

[drjohn]

Even more off topic - any bright ideas for converting 60Hz to 50Hz to allow me to use some of the expensive appliances I brought from 50Hz Malaysia to 60Hz Philippines which fail on the 60Hz - all 220/240V

DJ

[jackrae]

DJ
How about a 24v battery bank with charger, then couple up a 50Hz sine wave inverter to supply your critical loads
jack

[drjohn]

I'd need an awfully big battery bank, Jack - about 6KVA in total load.

DJ

[mutley]

A diesel genny then!

[jackrae]

Well if you don't give the full project specification what do you expect

This gets interesting

Exactly what are you trying to run on 50Hz that won't tolerate 60Hz and eats up 6KVA

Actually the battery doesn't need to be too large, just big enough to smooth out the AC ripple off the 6KVA charger, since it's the charger that would be providing the base power. But at that sort of load both charger and inverter would probably cost more than either a genny as suggested or changing the original equipment for 60Hz tolerant stuff.

Alternatively throw a wire across the transmission line and drag the county's generation system down to 50Hz

jack