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.

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

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

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

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

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

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

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

Your guessing is good, Tim

Wikipedia has a useful page: **LINK**

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

MichaelG.

.

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.

My understanding is that most brushless motors (from model aircraft engines to those in EV's) are 3 Phase?

Ian P

partly…

If you had a 3 phase variable frequency source and connected it to a 'brushless dc motor' – such as in a 'lectric bike or scooter, or an RC model, the motor would happily turn at a speed related to the supply frequency – in exactly the same way a generally understood 3 phase motor would. There is NO difference between these types of motors in function – Brushless DC motors tend to have one of it parts ( rotor or stator) as permanent magnets, while generally understood 3 phase motors use induction to excite the other half, is all. The phases are not switched in sequence ( like a stepper motor might be), but are supplied true 3 phase. That the 3 phase is generated by a PWM signal from a DC source is the only reason for the term 'Brushless DC motor', but is irrelevant from the motor's point of view – the current in the windings is a decent 3 phase sinus..

And Dave (SOD) – they are not DC motors by any stretch…the 'DC pulses they are fed' is misleading. Only the controller's source for the PWM generation is DC. The magnetic field within the motor is a 3 phase sinusoid, or close to , thanks to Di/Dt assimilating those PWM DC pulses into a current in the motor windings, the value of which varies as it would in an 'ordinary' 3 phase motor connected to 3 phase mains supply.

The controller can be regarded an electronic commutator because it's action in switching DC to the windings is similar to the way a mechanical commutator works. Like brushed DC motors, brushless motors contain permanent magnets.

Not sure how to interpret this at all…Many brushed DC motors do NOT contain permanent magnets – series type DC motors have none..There is no similarity between commutator action in a brushed motor and and induced motion from a rotating 3phase generated magnetic field –

Electronic commutation is more reliable and efficient because there are no brushes. There are other advantages too, making brushless motors best choice for many applications : I think it's true to say modern electric cars are all brushless, and no-one uses 3-phase motors.

It is true that modern electric cars use brushless motors, BUT THEY ARE ALL 3 phase and EVERYONE uses them…

Drifting badly from the OP's questions , but misinfo does not help other readers of the topic….