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Inverters and dual speed motors

Inverters and dual speed motors

I have followed with interest the recent postings under the heading of “Colchester Bantam 240V help” initiated by Neil Grainger, in particular the inputs from Bill Todd.
To digress for a moment, on my own Bantam I did a motor swap and fitted a Mitsubishi 240v inverter, programmed to give 2800RPM at 100Hz.
Back to my question:
I currently have 3 machines, each fitted with a dual-speed, star configured, motor (Bridgeport BRJ Mill, BCA Jig Borer and Smart and Brown Model L plain lathe).
I attempted to run the S & B lathe from my Transwave 3HP static converter, but could only get it to run in low speed. It just wouldn’t entertain running up to top speed. I have not yet tried the B’Port or BCA on the Transwave converter.
After reading the Bantam posts I thought maybe there is scope for applying Bill Todd’s method of retaining the original dual-speed motors and using a step up transformer and 415vin/415v out inverter.
Bill indicates that the Siemens MM150 will accept a single phase 415v input. Now I happen to have a couple of brand new Mitsubishi and one used IMO Jaguar inverters sitting on the shelf gathering dust. All are 3-phase 415v in/415v out. None appear to be capable of accepting a 415v single phase input.
Before I consider motor swap, or purchase of further inverters, I would like to explore the possibility of using my existing inverters with a step up transformer to power my dual-speed motors.
I have read posts in several of the Yahoo Group forums (Colchester, Bridgeport etc.) which suggest that a 415v 3-phase inverter can be fed from 415v single phase supply; the live/neutral inputs being connected to L1/L2and L2 then bridged to L3. Also, a de-rate of approximately 50% must be applied.
Has anyone tried and had any success with this method?
Does anyone know of any alternative method of utilising my existing inverters, apart from having 3-phase supply laid on?
Any help/advice will be much appreciated.

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Re: Inverters and dual speed motors

None appear to be capable of accepting a 415v single phase input.

Hi Leonard,

Might be worth checking with the manufacturer but, the only real difference internally (on the smaller units) will be the size of the smoothing capacitor (3ph needs a smaller cap) often the VFD will be perfectly OK if just de-rated slightly (ie. draws less current max current in your normal operation - which is most of the time on a small machine tool) . Some units have phase loss detection, which would stop the fun unless it can be disabled.

One very important point (apologies, if you know this)  - do not switch the motor with the VFD running (i.e. if it's outputting any current) .  So, download the manual and check if the VFD has an instant stop option  (not the normal decelerate - they often have a number of STOP modes )  then you can configure a protection switch that'll stop the VFD before you can change speed and so it does not get damaged.

None appear to be capable of accepting a 415v single phase input.Before I consider motor swap, or purchase of further inverters, I would like to explore the possibility of using my existing inverters with a step up transformer to power my dual-speed motors.

415 volts is far more dangerous than 240 - if you are at all unsure of what you are doing  - DON'T - find someone who knows

Well if they were my inverters, I give them a try - if you can find a small control transformer  ( a few 100 VAs is all you need to test) that'll give you ~415ac out wire it up to a motor (unloaded) and try it. The VFD will either power up properly or display a phase loss fault (TILT wink ) it will not be harmed by a loss of phase if it's not powering the motor.

Bill

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Re: Inverters and dual speed motors

Hi Bill,
Thanks for the advice. If Iget a phase loss fault would you then recommend I link L2 and L3, as others have suggested?
Len

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Re: Inverters and dual speed motors

Leonard Brandon wrote:

Hi Bill,
Thanks for the advice. If I get a phase loss fault would you then recommend I link L2 and L3, as others have suggested?
Len

It might work, but it'd have to be a pretty dumb phase loss detection system that would be fooled by a simple link. smile

In a typical VFD the inputs meet a three phase rectifier (six diodes instead of four ) which turns the 400-500vac into ~600-750vdc  stored on the power caps(s) (so don't touch the DC terminals, even if been un-powered for a week) The critical bit is the ripple current in this capacitor, since this will heat it and shorten its life. The ripple current is a proportion of the load current, so de-rating, or just ensuring that maximum power is not drawn continuously  (which it rarely is on a lathe ), will ensure the capacitor will live a long and happy life.

Bill

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Re: Inverters and dual speed motors

Hi Guys,

You do not need any kind of trasnformer on the input stage to run a 400v inverter from single phase input. You do need to arrange some form of volatge doubling at the DC stage of the inverter. Yes this can take the form of increasing the AC side, but it is much easier to use the capacitors on the DC smoothing side. Here the inverter designers mostly help you, at least in my experience, because instead of using expensive 1000uf capaitors they use banks of two 470uf. This gives you the opportunity to feed the neutral to the mid point of the banks and just take the line through the rectifier. The result is an adequate DC stage, which is all the inverter is interested in.

The use of 400v inverters is extremely useful in two circumstances: multi-speed motors which would be a nightmare to reconfirgure for 240v running and motors over 2.2kw, which is normally about the biggest 200v inverter made.I've done this twice, once for each reason, and not had any problem. The over 2.2kw installation though does need it's own supply from the distibution board in my workshop, and would benefit from a type C MCB.

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Re: Inverters and dual speed motors

Hi Nick,

Yes, that would do it*  but it does require opening-up the VFD and modifying things  (warnings above apply!)  which many people would avoid  (not you obviously wink ).

* [ for those that don't know: This is how a typical (older)  PC power supply works when it is switched to 110vac input ]

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Re: Inverters and dual speed motors

Hi Bill,

Yes in the past I have opened up the device and modified it. Older devices are easier because generally there is more space. I have however seen an add on for sale from New Zealand, which looks interesting, indeed I might even copy it for sale in the UK and save the postage, or is (that pocket?). I don't know what the NZ device does, but it seems to me that a couple of diodes and capacitors could form the external doubling stage with the resultant, albeit poorly smoothed, DC fed into the normal VFD input. The inverter won't care since they only ever monitor the volts arriving at the DC  stage, by which time it's own capacitors will have aided the smoothing. I've priced the bits at under £40, plus a suitable heatsink, which could be a pukka job or a lump of ally plate. I haven't tried this approach yet, but there's no reason to suppose it won't work.

Nick

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Re: Inverters and dual speed motors

Have n't read all this thread  so this may not be relevant but I have run an Emco 2 speed motor from a 1 ph. 240 volt in , 3 ph 240v out inverter for the past 5or six years. Despite being an electrical engineer I was pretty rusty having spent my time in technical sales. All I did was brought all six inputs of the motor out to a temporary terminal block and then briefly injected 3 phase into each set of windings separately and measured the effect in terms of power and speed and then injected the 3 phase into both sets of windings in the configuration which gave me top speed at max. power. All the local winding and motor experts  told me it would run hot , at low power , out of balance and burn out.
It actually runs very cool,very smoothly, is balanced (checked with a scope), gives good power and has n't burned out (yet anyway). Mind you I could n't tell you the details of the motor (could n't get them ). I did of course lose the lower speed by switching but with an inverter this did n't matter--might be worth experimenting with the motor if this is easier as opposed to modifying an inverter.

Alan. Everyday is a schoolday.

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Re: Inverters and dual speed motors

Drives Direct sell an inverter that  outputs 415 volt 3phase and does not require characterising to a particular motor
From a single phase input mine will run both my lathe and m mill at the same time

Lathe: Emco V13   dual speed motor 1.8/2.2 kw with built in brake (that actually uses one of the motors windings as an auto transformer to develop the voltage necessary to lift the brake disk and hold it off against strong springs)

Mill: Alexander master Toolmaker with 1.1kw (1.5 Hp) dual speed motor

No amount of stopping /starting /speed switching of either machine seems to have any effect on the other machine when both are in operation.
The only parameter settings that Drives Direct recommended was to set the 3ph output up tp 420v to help pull that lathe motor brake off  and it certainly does after a very slight hesitation 
Thats a pretty tall order for an inverter - a 2 speed brake motor + another 2 speed motor !

Noise wise my inverter has a fan controlled by a thermister attached to the cooling fin stack so in the winter it only really comes on when I start one of the machine tools

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Paul Churchill

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Re: Inverters and dual speed motors

I would speak to the guys at Transwave and they will supply rotary convertor which is what I use, I run dual speed motors in 3 Hardinge lathes and in a Deckel mill from a 3 phase ring main suppled by one of these. I just switich it on when I go into the shop and casn run up to 3 machines at once without problem.
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Re: Inverters and dual speed motors

Hello Leonard,
You have not said what is the output voltage from your Transwave static converter, I am assuming it is 415V, you have virtually nil chance of getting a 415V motor to run on 240V especially if it is 2 speed. If the converter is 415V, try running an 'idler' motor and then connect the recalcitrant S&B to take its supply in parallel with the running idler. The idler could be one of the other machines running off load, for a test. Adding the idler in this way turns your existing static converter into a rotary phase converter. That's all there is in the box of a bought one, plus a bit of switchgear.

All the other advice offered is entirely valid, if you decide to take the inverter route my personal preference would be for the internal doubler method as Nick has suggested, I've done a few that way and it has been fine. Some models of inverter are more difficult to get into than others, but it doesn't seem to follow any particular trend so it has to be on a case by case basis - one IMO was dead easy, another was a PITA as the PCB was glued to the heatsink.

To put a slightly different slant on Bill's warning, 415V ac is indeed a bit nastier than 240V , but 650V odd DC is very nasty indeed. There should be discharge circuitry in the box to dissipate the charge on the capacitors, but you should check the DC rail voltage with a suitable DC voltmeter before poking you fingers in there.

One of the biggest problems with all this is that there are so many different possible solutions any one of which is potentially valid, there isn't a single 'Right' answer. It depends on what hardware you have to hand, how much you want/can afford to spend, how keen you are to poke about in the electrics, etc... the list goes on

- Richard -

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Re: Inverters and dual speed motors

To put a slightly different slant on Bill's warning, 415V ac is indeed a bit nastier than 240V ,

A bit! wink
Put simply , the 'danger', ( i.e. the likely-hood of a fatal amount power flowing through your heart)  increases with the square of the voltage (V^2/R where R is your skin resistance)  so 415vac is three times more dangerous than 240vac : 650v is about 7.5 times  (some would argue DC is less dangerous but, having had many shocks of both types, I can tell you they all hurt)

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Re: Inverters and dual speed motors

All 3 phase inverters (VFD will work on a single phase input. However the ripple current in the DC link reservoir caps will be substantially high than when the inverter is supplied from a 3 phase supply. This may only be a problem if the inverter if fully loaded in which case the DC link is likely to droop below the under voltage detection limit and so may trip the drive. There is also the question of the heating effects in the reservoir caps due to the high ripple current which reduces the life expectancy of these parts. However on many machines such as lathes the motors may not experience full load unless serious amount of metal are being removed or very high speeds are required. Also in the "domestic" application many machines are not running 24/7 or anything like the load duty cycles that may be experienced on production machines or industrial applications. So fitting an inverter of a higher 3 phase rating than is needed by the motor, say by 100%, would be sensible. Alternatively external capacitors can sometimes be fitted if there is access to the DC link through terminals or one is prepared to get inside the inverter. However as has already been stated the voltages and stored energy of the DC link are lethal so only experienced "electricians" should tackle this. Inverters can use an idler motor similar to that used on a rotary converter and for the same purpose which is to provide a degree of stored energy, especially if it has a flywheel, as a 3 phase induction motor can also act as a sort of generator. This will enable the inverter to drive a motor subjected to intermittent high loads although care must be taken to limit the rate of any speed changes to prevent the inverter tripping or folding back ie the inverter must be programmed for soft start and stops. Quite often inverters will trip, through overvolting, if the rate of deceleration is too high unless a braking resistor is fitted.

Although it is important not to disconnect the motor from an inveter whilst it is running I'm not sure that if an idler motor were permenently fitted to an inverter  that a second motor could be connected through the normal machine 3 phase on/off switch. Maybe I should try this when I got a spare inverter I don't mind sacrificing.

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Re: Inverters and dual speed motors

Two points:

1) As an ex-electrical/electronics engineer, I have to say I get slightly concerned when I read some of the suggestions for hacking around inside 3 phase equipment. OK if you have 20+ years experience of this sort of thing, but I would hate to think of a newbie trying it!

2) An alternative suggestion, when I looked at the costs of rotary/static inverters, it actually worked out cheaper to get the electricity board to run 3 phase into the house. I dug the trench to keep costs down, then got the 3 phase extended to the workshop. Makes life a lot easier!  I just use 3 phase in, 3 phase out inverters for speed control and soft start. (Stops the workshop lights dimming when I power up - was always a bit unnerving.....)

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Re: Inverters and dual speed motors

Hi Dave,

Since I'm one of the people you're 'having a go at' over encouraging people to try voltage doubling, I guess I'd better respond.

Much of what I read about engineers and electrics worries me. The guy who uses one inverter for two machines for example: how does he arrange for proper control from each workstation? Not I hope by interrupting the inverter output. The people who use static and rotary converters without thinking about it properly: why would you do that and miss out on the soft start of an inverter? as well as the implied lack of understanding of electrics basics. Off-the-shelf soltuions are only used by those with limited understanding, that worries me as much as them doing more advanced work without full knowledge. That doesn't mean that those who have a proper interest and respect for electricity shouldn't be advised of the options. How else will they find them?

3 phase installation is an expensive one-off cost, even if it outweighs the long term cost of alternatives; when do you have it installed? After you've got one machine? two machines? etc. Different perhaps if you're starting with six machines, but not many of us do that.

All of my machines are copmpletely re-wired to suit their environment, and I would always encourage others to do the same. All controls are at control voltage, or operated after the inverter has been disabled via control voltage interlock (Holbrook multispeed motor changeover for example). In the 1950s and 60s, the period from which most of my machines originate, the electrics were, almost literally, shocking. I always start from scratch and would encourage everyone to do the same. That doesn't mean that I don't isolate from earth those inverters that have nasty leakage issues that trip the house RCCB (the machine is earthed but the inverter isn't, just make sure it is inaccessible - why would you want to touch it except for initial programming through it's plastic keys?). Take care, is the essential motto. In the same way invading the interior of the inverter and altering it's arrangement is fine, if you take the appropriate precautions. I don't think I've ever suggested it's a cake walk for the novice, but everyone starts somewhere. I hadn't opened an inverter, until I opened my first!

By the way, just for the record I have no formal electonics, and only limited electrical training, most of which post dated this sort of work. I do have a physics A level understatding of electricity and I do think about what I do. In my opinion no amount of training can substitue for proper thinking about the possibilities and risks, but you can't write that every time you submit an comment on electrics.

Nick

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Re: Inverters and dual speed motors

Nick well said. I agree completely. I have had one machine running from a static converter and I didn't think it ever ran well, it's now been converted. I can only small advantages and many disadvantages in having a rotary or static converter instead of an inverter.  I always rewire my machines so in most cases the electrics are in better condition than the machine. I try and use low voltage for the controls too, in most cases the inverter does this for you. Inverter manuals can be forbidding don't let them confuse you most of the functions you will never use so don't be afraid to play with the functions.   In most cases my inverters have been housed in steel enclosures and I add a small fan to keep them cool.  From experience with by Colchester Bantam and an inverter I'd say it's better than the two speed motor frankly I hardly use the geared headstock unless I want slow speed and lots of torque.
Do watch Ebay for inverters, there appears to be no such thing as a single phase input inverter only 3 phase ones that can be fed from single phase this way you only use part of the diode/capacitor bank the manual will tell you which terminals to use. It is important to get 240 volt input if you want 240 volt 3 phase out, if you want 415V 3 phase out you will have to seek the expensive inverters that do this from 240V or modify a 415V input one as Nick suggests. Last comment most manuals for inverters can be downloaded from many sources (I Google the inverter model that usually works), find the manual before you buy that will either inspire you to proceed or confuse you completely !
Cheers Ashley

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Re: Inverters and dual speed motors

Bill,
electric shocks do indeed hurt, I didn't intend to suggest otherwise; rather to shift the focus to an aspect that had not been commented on. As I'm sure you are aware, for a normal 3 phase system each phase is 240V above ground so you need a double fault to get a 415V shock and be touching both phases at once - fairly unlikely. The situation is different if using a step up transformer on the input to an inverter where a single terminal would be 415V above ground which I suppose was your perspective, although it could be arranged to be centred to earth so the worst would be 207V.

On a practical note, possible risk of current passing the body is a lethal manner greatly reduced, even in the event of a shock, if you keep one hand in your pocket. That is not in any way to suggest all other precautions should not be made, but for example if you probe something with a tester of some sort to check it is safe, if there is a fault or you get it wrong, if you have one hand on the tester and the other on the chassis, you get a shock stright across the chest. That's as bad as it gets. With one hand in your pocket the risks are greatly mitigated.

Although warnings and safety are serious issues, if people are so intimidated by references to instant death that they never dare to work on any of these devices how will they ever learn? I did not mean to trivialise the danger of electric shock and I don't disagree that if you don't understand what you are doing, keep your fingers out, but disseminating some understanding is what we are hoping to do here.

- Richard -

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Re: Inverters and dual speed motors

although it could be arranged to be centred to earth so the worst would be 207V.

Good idea Richard - I'll amend my diagrams  - Now, why didn't I think of that?

Although warnings and safety are serious issues, if people are so intimidated by references to instant death that they never dare to work on any of these devices how will they ever learn?

smile We're both on the same page here Richard  - with just a few elementary precautions and a bit of guidance,  working with any voltage can be perfectly safe.

I suggest these precautions  before working on any electrical item (perhaps you could add to the list)

Earthing:  Earth everything that is supposed to be earthed properly back to a good ground point- don't skimp!

Isolate:  Make sure the power is isolated properly either with one or more isolation switches (don't rely on the machines NVR contactors) Or by pulling the plug  out.

Discharge: While I do not recommend you try to discharge the storage caps in a VFD, you must be aware that some things can store charge for a long time after being switched off  (think of it like a spinning saw blade). NB. I like to have a  test-lamp handy (see below)

Test: Make sure you have suitable test instruments (these don't need to be expensive - a properly rated test lamp or neon screw-driver is often all you need)  Check terminals before touching .

Then...  Stand well back before applying power smile

[EDIT]

I was going to add a link or picture of a typical test-lamp, but these don't seem to be sold any more !  sad

Basically, these are a low wattage bulb with a series surge limiting resister mounted in safe box with probes - My one (which must be 60 + years old)  is rated 150-500v - it'll glow at 50v)   is perfect for checking that terminals are dead and discharging suppressor caps etc.  has a 200v 15w bulb in series with a 4000ohm resister and a fuse

http://www.homeworkshop.org.uk/components/com_agora/img/members/87/test-lamp.jpg

Bill

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Re: Inverters and dual speed motors

Bill,
I was surprised at your earlier comment that ac could be more dangerous than dc, it is a comment that is directly at odds with what I have previously been told so I did a little research. In the context we are discussing here, you are indeed quite correct; at lower voltages (and in this context that means less than something like a fewkV) the risk is mostly from heart fibrillation caused by the ac aspect of the shock.

At higher voltages the risk can become greater from 'deep tissue burning' caused by muscular contraction and the inability to let go of the conductor due to DC. Also, and perversely, it seems you are more likely to die from a shock of around 750V than from one of 10kV, not that I'd want to volunteer to demonstrate.
It is also the current, not the dissipated power that is significant. When I was given various warnings about 'more dangerous DC' I was working with high DC voltages, but the distinction was never made, clearly a significant omission.

These give a succinct description that is echoed elsewhere:
http://www.physics.ohio-state.edu/~p616 … rrent.html
http://www.allaboutcircuits.com/vol_1/chpt_3/2.html

One additional consideration to add to your list:
Make sure things, especially motors are secure.
I had a motor on the bench and fortunately clamped securely down. In switching capacitance values testing the start-up characteristics for an RPC, the motor instantaneously switched direction of rotation. Had I not clamped it to the bench, it would have jumped off and been a significant mechanical as well as electrical hazzard.

A more general comment is to think about what happens if 'it' goes wrong. ie manage the risk. It is the opposite to most people's modern common experience which assumes everything must be safe and risk free because it is 'required' to be. That has made people very unthinking and blase in their outlook too much of the time.

- Richard -

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Re: Inverters and dual speed motors

it seems you are more likely to die from a shock of around 750V than from one of 10kV, ...

Indeed a 10,000v DC  shock is what the paramedics will give you to save your life smile
Defib units are tested with a 10mm spark gap (~11,000v)

I had a motor on the bench and fortunately clamped securely down.

No no no  I've never had a motor chasing me around the workshop floor, swinging a torque wrench randomly, until it wound its own power lead in knot and pulled the plug out of the wall  :LOL:

Bill

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[The "clean" wor...]

[Crane 1]

Crane 1

[General view]

[Over the y...]

Over the years

[A few of my pro...]

[Emco Maxim...]

Emco Maximat Standard

[Lathe]

[avatar]

avatar

[]

[Tom Senior...]

Tom Senior M1 Mill

[Fitted with a D...]

[Year Total...]

Year Totals

[Total visits la...]

 

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