Gary,

You're way out of my league in terms of motor power, but may I suggest you look at 'soft start' devices:

**LINK**

https://uk.rs-online.com/web/c/automation-control-gear/electric-motors-motor-controllers-peripherals/soft-starts/

MichaelG.

Yeah i’m with Andrew, not sure what the dilemma is. Why would there be an inrush current ? It’s not like starting a compressor etc.

Iv’e run a Student 1800 for years from a 2.2kw inverter, with no issues.

God alone knows where you managed to pay £1400 for a 3kW VFD – they must have seen you coming. Get a true professional quality drive and save the best part of a grand. (But you'll also need an EMC filter).

Mark – no urban myth. Most VFDs specifically tell you not to switch motors under load, particularly vector controlled drives. You say this every time it comes up – perhaps you should write to all the manufacturers and tell them you know better. I know, I know, you have one of the few brands that DOES allow it but exhorting people to ignore the specific instructions regardless of the model won't be doing them any kind of a service.

As pointed out, the VFD starts the motor off at zero speed with a controlled torque and actually very little power draw. Ignoring losses, the electrical power is torque times speed – do the math.

Murray

Might be easier to just buy another cheap VFD, like the one linked to by Murray to solely run the lathe. Do away with the Colchester switchgear and get the advantages of Soft start and infinite speed control etc.

That’s how i run mine, works great, i have a pendant with forward, reverse, inch and speed controls..

My mill is powered by another seperate VFD.

Your drive parameters can’t be too far away if the motor starts and runs, they may be far from optimum and could probably be improved. The main inrush will be when the inverter is switched on to charge the DC link capacitors. A 5hp inverter will start and run a 5hp motor but if the start up ramp time is too short you may get a current limit error. Starting the lathe needs to get the whole mass of drive train from motor to chuck moving and unless you have infinite power this will take time. A direct on motor hits the drive train with a savage jolt but a VFD will spin things up smoothly and is much more sympathetic to the mechanicals, although the lathe will have been designed to take the start forces it has to be helpful to have a gentle start.

Mike

Sounds to me more like a problem you would have with a phase convertor, not a Variable Frequency Drive… terminology problems maybe causing misunderstanding because there's no way a VFD rated for 5HP will struggle to drive a 3HP motor unless you have the motor wired for the wrong voltage.

However a phase convertor needs to be set correctly to be able to start and run different motors of different power outputs correctly (you are basically switching in and out different value capacitors to allow for the changing loads).

I've no time now to investigate further, but I'm guessing you have One Of These 415v output converters.

I struggle a bit with their product information since they moved over to ebay only sales info.

However, from that ebay shop;

PLUG & PLAY MODE

Another SPECIAL feature of this converter is the ability to run it in Plug and Play mode,

In this mode you can take the 415 Volts output of the converter and connect it to your machine so that the existing machines START/STOP buttons and switches will still work to control the machine as if it was connected to a factory 3 phase supply,

You need to take into account the motors starting requirements when sizing up the converter for Plug and Play mode, normally a 3 phase motor will require at start up a surge of 3 to 8 times the normal running power and the size of converter needs to be large enough to provide this power,

Unless the machine has a very high start load for example in the case of a planer machine because of the cutter block's inertia you can go for a 3 x overload rating, as an example the main spindle motor on a 2HP Bridgeport Mill will require a 6HP surge to get it going,

All this range of DIGITAL Plug and Play Converter allow you to take a 150% overload for 30 seconds so the 6HP starting requirement of the 2HP mill can be accommodated by this 5 HP converter,

As the output from this unit is a DIGITAL output you can only use it to power normal motor/switch loads and not any items that contain electronics,

If you need to provide power for an item with electronics of any type you will need to fit a SINE WAVE FILTER(See ) to the output, once you have this fitted then you can power machines with electronics in them and even running a CNC machines is possible,

Hence it seems that when used in your current Plug & Play mode, downstream switching is OK, but you need to take starting surge into account.
I'm sure it's quite a sophisticated unit, which can also be programmed as a normal single machine inverter, but does have the extra facility to connect directly to a standard 3 phase machines input, albeit at a price.

I'm guessing that most folks on these have little experience of one of these inverters, but the vendor claims to offer telephone support.

Bill

But since Gary already has one, it has the advantage of allowing a series of 415v 3 phase sockets around the workshop, so anything can be plugged in using its original switchgear; subject to the caveat of electro mechanical switch gear, rather than electronic gubbins that require sine wave supplies.

Also allows more than one single voltage motor per machine, i.e. lathe drive + suds pump + Low Voltage (415v transformer, MEM type) light.

Personally I've gone down the route of an inverter per machine and 240v 3 phase outputs, but that in itself has caused problems, such as the 415v motor on the Centec power feed etc.
I do have a plan to dive into the windings and convert to delta with a soldering iron, just not had the time thus far.

Bill

Just a thought, are you running the motors direct from the VFD output or via the sine wave filter?

Andrew

Right Gary i am i think the only one on here that can give the honest true answer, that 5hp Invertor not a VFD is way under powered.

I had exactly the same scenario 8 years back to power an M300 2.2kw motor. I did buy the exact same invertor you have and it lasted 1hr 25mins, bye bye £600.

Remedy was to get the 10hp invertor, i paid £900 now well over £2.2k.

Whats happening the startup current is more like starting at least a 9hp motor, add on what ever the gear is in can be like trying to pull away in a car in 5th!
Even if you can get it running by givng it a push start, slightest cut put on will see the invertor constantly ramping up to a certain point where it will slow down. Thats with clean up cuts.

Same problem with a Transwave Convertor think 5hp? maybe more, except that was much worse, not even a menial cut 0.3mm could be put on in mid range speed without it deafening you and spindle slowing to a halt constantly.

Touch wood 7 1/2 years on the 7.5kw invertor is still running and used daily for several hours at a time, plus power the whole workshop. No need to remove and plug in each machine, theyre left permanently plugged in to 3ph sockets and can run several machines at same time.