Variable speed motor ?

I'm finally planning to start on getting my Atlas 12" 3000 series lathe up and running for next year, can anyone tell me what the current options are for a variable speed motor system ?

1. It won't be used a lot, not even once a week, so doesnt need to be high spec.

2. My workshop is offgrid so it must run from a generator (with AVR.)

3. I like salvaging stuff rather than buying new when possible.

VFD and 3 Phase motor (uprate the power of you need high torque at low motor speeds) is the standard solution, it should be fine on a generator supply (in fact it's easier on the genny than a direct starting single phase motor)

Or fixed speed motor (of any type) and a CVT.

Given your interest in salvaging things it would be interesting to see the CVT from a Quad or Snow-Mobile used to give a lathe stepless speed control with no loss of torque. It's definitely unduly hard compared to just using a VFD, but would be an interesting approach with its own advantages.

Not knowing the spec of No.2 and its expectations, I would be particularly careful to avoid recommending a ‘low spec’ device for No.1.

I might suspect that a ‘low end’ inverter may not tolerate a ‘low end’ generator supply, especially if of barely sufficient capacity or with variable/inconsistent output wave-form.

Surely, of equal importance to Automatic Voltage Regulation is Frequency control, which means that the engine driving the alternator needs to held to within 5% or less of rated speed; ideally isochronous governed.

Thus, for 50 Hz, with a 2 pole alternator, the engine would need to run between 1500 rpm on full load, and 1570 rpm, no load.

For 60 Hz the speeds would be 1800 to 1890 rpm.

For a 4 pole machine, the speeds would be halved, and the governing need to be tighter.

If the supply frequency varies, the speed selected on the VFD is likely to vary as well.

(It will certainly affect anything dependent up supply frequency such as clocks or motors supplied direct off the supply. )

Howard

I only have basic electrical knowledge so getting into anything experimental or complex is out of my range I'm afraid.

What's a well proven value for money VFD ?

Howard, you are assuming direct drive ! The one I built was belted to bring the engine up to 2000RPM and run on no load at 52Hz using a frequency meter on the output. This was set manual and once under load just kept an eye on. It was 3Ph feeding a 4.5Kw and 11Kw motors, the small one was DOL starting and would make the engine struggle, the BIG one on star delta hardly made a sound. If both were running the 4.5 had to be started first ! The control of both frequency and voltage very much depend on what your powering. Noel

You might consider a DC motor and battery bank, perhaps solar charged since the duty cycle gives time for recharging. This also gives you power for lighting and security.

Unless the intended turning jobs really are large and needed in a desperate hurry you won't require anything like the 1HP normal power for a 12" lathe so consider 1/3HP rather than 1.5HP. A DC motor also has good starting torque to overcome the cold gummy grease.

Hardly! The VFD will generate its output frequency electronically, most likely digitally in the control microprocessor with a crystal.

For a 50Hz supply frequency the speed is 3000 rpm for a 2 pole alternator, 1500rpm in the case of a 4 pole alternator to operate electronics you need a stable alternator. I have never had issue with a 5.5Kva alternator that I have. I know people who have tried chinese 8kva alternator and they had a lot of issues with computer failures.

A vfd will give use a lower start current than a conventional starter fi the source is stable and correctly sized.

Peter

Using an IC engine to drive an alternator and then feeding its output into a VFD to drive an electric motor involves lots of losses due to all the conversions involve. Why on use an engine to drive the lathe direct?

If the generated electricity is being stored and you already have a 240V supply then yes, the VFD route make some sense but if you have to run the generator whilst you are machining why not take trip to the scrapyard?

A small petrol engine can have quite a wide usable speed range and drive the lathe direct

Ian P

Even a petrol engine from a scrappy can suck up the juice. The conversion to leccy, from utilising a good, efficient diesel generator, would likely not be cheaper (but certainly a lot easier) than a conversion with an ICE.

As I see it, it is all about a good consistent frequency sine wave input to the VFD.

I think Bazyle's point is very valid as there are some excellent 3kW brushless DC motors available together with their speed controllers from vendors such as Vevor. These are intended for electric scooters but I'm sure they can be repurposed to drive a machine. They are designed to work from a battery, which at 72V, is relatively safe to "play" with. Lithium battery packs can be obtained in various capacities depending on how long you need to machine for and extra ones can be added if needed. Lithium batteries also have very low self discharge so won't need "topping up" except possibly once every 6 months or more which favours occasional use. I've used a couple of DeWalt Lithium battery packs from my BLDC motor hand tools to power a watchmakers lathe to make it truly portable and independent of mains.

As Bazyle mentioned a big machine doesn't necessarily need a big motor if one doesn't need to remove metal at maximum rate. However a BLDC motor can produce huge amounts of torque, even if only for short periods, which can improve some machining processes.

What's not to like about BLDC motors but I'm sure someone will feel the need to enlighten me.

CS

Looks like preference 3 on the OP's original post rules out Clive's suggestion.

Given a choice I choose non-brushed motors for all the projects I get involved with and an induction motor with VFD would normally be the best choice for this application but it depends on what the existing off grid electrical system is.

Ian P

Ian P

Not necessarily. If an electric scooter/bycycle gets crashed or goes wrong it often ends up being thrown away rather than being fixed. Old Lithium batteries lose their capacity so may get dumped but are still viable. You just need more to wire up in parallel.

You could use a VFD and a ex Nissan Leaf 300V battery but that would be scary.

A 2kW motor and controller at £107 is equivalent to a tankful of fuel for the car you could use collecting salvaged stuff.

CS

If the supply frequency varies, the speed selected on the VFD is likely to vary as well

As I see it, it is all about a good consistent frequency sine wave input to the VFD

VFD's don't care much wrt the input frequency - the same VFD works in the UK and the USA ( 50Hz - 60Hz) and most will work from 400Hz input as well - and down to 40 Hz. All VFD's rectify the input AC to a DC high voltage bus from which the variable AC output is generated. If you go too low in input AC frequency, the internal Electrolytics are to low in value to keep the DC bus ripple low enough for the VFD to deliver rated output. If the other way, to high in frequency, the electrolytics tend to heat up due to the internal resistance.

Anywhere from 40Hz to 100Hz is quite safe for the average low power (say 1.5KW or so) VFD. A bigger deal is input voltage - a 220V Rated input VFD should not be fed with less than 200VAC , and then typically only at 60-70% of rated load...Likewise feeding the VFD with 250VAC would be possible, but brave, and heavily dependant on the voltage rating of those electrolytics - generally they are 400VDC, and @ 220VAC input they will see near 320VDC or so. 250VAC in will place near 350VDC on the caps.

You would be in possession of a really bad genset if it exceeded those limits for typical homeshop lathe loads...

By the way, an AVR on a genset does not guarantee clean output power. The rotor inductance is huge, and large sudden changes in load - turning on the compressor for example - take many 10's, even 100's ,of AC cycles to stabilise - Di/Dt will never allow the current in the rotor to change fast enough to control at that level. Even slower to react is the engine driving the genny - most cheapies engine speed are governed by air pressure against a paddle in the engines cooling fan airstream.

A VFD set for slow start will keep the genny happy too...

Edited By Joseph Noci 1 on 22/10/2022 13:47:16

I think a cheap VFD is well worth a punt in this case, especially if the lathe already has a 3-phase motor.

The way VFDs work should make them fairly tolerant of dirty power. They start by rectifying the mains to charge a large DC capacitor, which will cover a multitude of sins on the AC input. Would have to be tried on a particular generator to be sure, but I think it will work provided the genny stays reasonably close on voltage: I don't think frequency variations will matter. Spikes are the deadly enemy.

Look for AT1 and AT2 VFDs on Amazon or ebay; at the moment these seem to be affordable without too many complaints! Fairly basic and easy to set up compared with their more sophisticated brethren. Other types available. It's often worth talking to suppliers but I'm not sure in this case - the generator is a massive unknown, and I'm not sure anyone will guarantee their VFD will be OK with it. Going off-grid means taking your own risks.

Dave

Just so I understand what is going on ... the term Brushless DC Motor seems to me (I'm guessing) to be shorthand for a Brushless 3 phase motor driven from a DC supply using some sort of Variable Frequency Device - perhaps bult into the motor package.

Am I right? If not, please explain!

Thanks, - Tim

Your guessing is good, Tim

Wikipedia has a useful page: **LINK**

https://en.wikipedia.org/wiki/Brushless_DC_electric_motor

MichaelG.

Thanks for that, Michael. As long as I know (now) that the motor is not in reality a DC motor it makes sense. Life is full of words and expressions which are in reality confusing shorthands.

And how about the too-common term 'pre-prepared'? - Or even 'pre prepared in advance' ...?

Cheers, Tim

.

For what I would claim to be proper usage …

please see my recent post on page_3 of this: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=168196&p=3

MichaelG.