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Sustainability of under powered VFD on Initial motor starting

Will my VFD cope in the long Run????

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Absolute Beginner08/10/2018 21:42:45
103 forum posts

Hi All,

I have a 5HP Drives Direct VFD which I currently use in a plug and play mode to power my Bridgeport Mill. No problems at all. I have recently purchased a "new to me" Colchester lathe a Student 2500 and have wired my VFD in a ring main configuration to provide power for it, No they will not be run together. Yes my lathe now seems to run ok but the convertor is somewhat underpowered at 5HP to cope with the initial start up inrush current on the Colchester lathe at 2.2KW.

I just wondered if anyone else had ever done the same, and if my VFD will continue to cope with the initial inrush current requirements of the lathe? The option to purchase another VFD is an option but having initially spent around £1400 on the original 5HP device I want to try and get some use out of it. Yes I can purchase another VFD but understand I will need a 10HP device at around £2400...eek

So maybe I will have to if the 5HP fails, hope it lasts a bit??? Just wondered if anyone else had any experience in doing the same or else any alternatives as to a cheaper solution?

Kind regards


Michael Gilligan08/10/2018 21:48:56
20182 forum posts
1053 photos


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



Andrew Johnston08/10/2018 22:10:21
6601 forum posts
701 photos

Surely the VFD has electronic soft start capability built in? So it should be more than capable of starting, and running, a 3hp motor.


Absolute Beginner08/10/2018 22:11:51
103 forum posts


Now why didn't I think of that, thats a link in the right direction, again. Thanks


John Olsen08/10/2018 22:14:58
1250 forum posts
94 photos
1 articles

The VFD does a soft start by ramping up the speed slowly. In a case like yours there might be merit in setting that to be even slower than normal, but in any case inrush current is not likely to be a problem. The problem would come if you fully loaded the larger motor, in which case it would take more current than the VFD is rated for. Provided it is set up to limit the current, it should protect itself in that situation too. The really important thing is that you must never switch the output side of a VFD while it is working, that is apparently a really good way to kill them.

But are your numbers right anyway? 2.2 kW is about 3HP, and would seem low for a surge for a 10HP motor, which at full load could take about 7.5kW, plus a bit more since it will not be 100 percent efficient.

To enlarge on the starting surge induction motor behaves electrically much like a transformer, and at starting it is like a transformer with a shorted secondary. If it is switched direct on line, then there is naturally a very large current flow, particularly if the switch happens to make at just the wrong point in the cycle. VFD's avoid this by ramping up the voltage and the frequency relatively slowly. Really large motors often have a starter that adds resistance into the wound rotor to reduce the maximum starting current. This can't be done with the squirrel cage rotor usual on smaller motors.


Alan Waddington 208/10/2018 22:16:08
523 forum posts
88 photos

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.

Mark Rand08/10/2018 22:16:49
1272 forum posts
28 photos

The VFD is rated at 170% of the motor power. and it'll have a short term overload capacity of at least 150% of that. If it direct-on-line starts the lathe ok now, it will carry on starting it ok. If the VFD has a maximum load power configuration option, it will start the motor using that defined maximum output, which will cause the motor to start a bit more slowly (and the VFD to change tone as it starts).


Don't pay attention to the urban myths about switching between VFD and load unles it is specifically prohibited in the VFD manual.

Edited By Mark Rand on 08/10/2018 22:19:33

Absolute Beginner08/10/2018 22:23:43
103 forum posts

HI All,

In plug and play mode the VFD just supplies a nominal 415V at 50Hz, so unfortunately some of the benefits usually associated with VFD's is lost. The device I have is rated at 5HP, although it can cope with a 50% overload for 30 secs or so. My Colchester Lathe has a 2.2KW motor approx 3HP which when switched on utilising the VFD in a plug and play mode will need many times this, so the soft start protection is not least I think this is the case...hence the strain of the VFD?


not done it yet08/10/2018 22:39:43
6809 forum posts
20 photos

If you follow the direct drives options, there is either soft start (using the inverter front panel for the usual VFD options) or plug and play, using only the existing starter (giving instant full power to full speed). The former mode of operation is advisable, I would think.

Muzzer08/10/2018 22:52:44
2904 forum posts
448 photos

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.


Absolute Beginner08/10/2018 23:02:48
103 forum posts

HI Murray,

unfortunately the drive you link to is only 230v, not 415v and is incapable of running all the control switchgear. Hence the need for a higher cost plug and play device in this instance that coverts single phase 230V to 3 phase 415V and has inbuilt EMC filtering and sine wave filtering to produce a cleaner PWM sine wave capable of operating contractors and relays etc so that the lathe can be controlled from its intended original controls.

Because of this as per my original post the 5HP unit, I am guessing is put under considerable strain starting the 3HP motor....

Alan Waddington 208/10/2018 23:43:09
523 forum posts
88 photos

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.

Peter Bell09/10/2018 07:47:28
376 forum posts
164 photos

Like Alan my son has abandoned the 440v switch gear etc on his Harrison lathe and connects the VFD direct to the motor and enjoys full control and jog etc from the VFD by using the existing manual F&R switches to control the VFD .

A separate VFD runs his mill with similar functions, think both are off fleabay but he managed to find a few at the right price which simplifies programming.


Mike Poole09/10/2018 08:40:42
3335 forum posts
73 photos

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.


Chris Evans 609/10/2018 09:00:20
2056 forum posts

My Bridgeport mill has run happily from a cheap £90 Chinese VFD for five or six years. Prior to this I tried running it from a Transwave static inverter but the mill would only run for a few minutes before tripping a contactor. The 3HP lathe still runs from the static inverter but start up is vicious and sudden with no soft start. One day I will drag the lathe from by the wall remove the motor get someone to find the star point and wire it for 240V through a VFD. Doubt I will spend more than a couple of hundred pounds so maybe I should move the job up the roundtuit list.

Nick Taylor 209/10/2018 09:22:26
102 forum posts

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).

Neil Wyatt09/10/2018 09:50:15
19033 forum posts
734 photos
80 articles

I am confused!

You say "inbuilt EMC filtering and sine wave filtering to produce a cleaner PWM sine wave capable of operating contractors and relays etc".

How are you running the contactors etc. off the VFD? Does it provide a separate constant voltage feed for them (separate from the motor drive)?

If you use a VFD with relays and contactors they won't work properly until the voltage is ramped up sufficiently, which could cause all sorts of problems.

If you want to run a lathe like that using all its switchgear you would normally just use a 240 -415 converter, not a VFD.

I really can't see the point of keeping all that complex switchgear if you have a VFD. I would buy a more affordable VFD, rewire the motor for delta. If you want to use the existing switches to operate the VFD for a more convenient or 'authentic' experience, that shoudl be perfectly feasible.

On power, as has been stated by others the rating of a VFD relates to the motor it can operate, inrush current is irrelevant because that's not the way a VFD operates. If a 5Hp VFD couldn't start a 3HP motor then I'd be in big trouble running a 0.5Hp motor off a 0.5Hp VFD.

As for switches between VFD and motor, the best advice for here is 'only do this is your manual says you can' as most entry level inverters advise against this but with the complexity of the manuals it is easy to miss the warning against doing so.


peak409/10/2018 10:33:13
1713 forum posts
183 photos

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;


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.


Neil Wyatt09/10/2018 14:16:41
19033 forum posts
734 photos
80 articles

If you have what Bill has linked to, it should be fine.

Used like that it is acting as an inverter only, not as a VFD so the comments about switchgear don't apply.

My only thought is by using it that way you are missing out on all the benefits offered by soft starts and speed control.


<Edit - do make sure you have a hard wired power supply with the required 30A capacity>

Edited By Neil Wyatt on 09/10/2018 14:18:00

peak409/10/2018 17:46:13
1713 forum posts
183 photos
Posted by Neil Wyatt on 09/10/2018 14:16:41:


My only thought is by using it that way you are missing out on all the benefits offered by soft starts and speed control.


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.


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