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Anyone used an Inverter with a Colchester Student

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Aidan Browne13/12/2010 22:48:08
15 forum posts
Hi,
 
I got the chance to get a Colchester Student Lathe 6" recently and I don't have 3 phase near the workshop. I have an inverter that I have used on my Ajax mill for the last year without fail and I was wondering about using it on the Colchester as well. 
 
The Ajax mill was easy to wire up simply changing from Star to Delta. But the Colchester looks a bit more complicated. For example it is a 2 speed motor and it has a circular set of connections. I'm attaching a photo. If anyone can help I would be very grateful.
 
Thanks,
 
Aidan.
 
Image of electrical connection
 
http://i1135.photobucket.com/albums/m628/scruffyneck/Image0064.jpg
 
.

Edited By Aidan Browne on 13/12/2010 22:56:56

Terryd13/12/2010 23:23:52
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1936 forum posts
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Hi Aidan,
 
Is this what you meant to do?
 

 
With all due respects and hoping not to insult your intelligence but You should use the 'Insert Image' icon at the top of the dialogue box when composing your message in order to insert the actual picture.
 
 
Best regards
 
Terry
Steve Garnett14/12/2010 00:03:58
837 forum posts
27 photos
Posted by Aidan Browne on 13/12/2010 22:48:08:
Hi,
 
I got the chance to get a Colchester Student Lathe 6" recently and I don't have 3 phase near the workshop. I have an inverter that I have used on my Ajax mill for the last year without fail and I was wondering about using it on the Colchester as well. 
 
Well the first thing to check is the rating of the inverter - because all the Colchester Students I've come across have 3HP motors, and if your inverter isn't comfortably rated for that then you have a potentially expensive damage possibility. But that may be the very least of your worries...
 
 
The Ajax mill was easy to wire up simply changing from Star to Delta. But the Colchester looks a bit more complicated. For example it is a 2 speed motor and it has a circular set of connections.
 
The real problem here is that it's a dual-speed motor, and generally these are a complete no-no with inverters. Even if you manage to sort out the wiring so that both speeds are available with a delta configuration, then switching between them whilst the inverter is running will almost inevitably blow it up, simply because of the massive back emf from the windings at switch-over. Normally an inverter controls a motor without any disconnection at all, and this is the only way they can work, realistically.
 
So your only real chance of success with the existing inverter is to pick one speed for the motor, and wire it in delta for just that - if it's possible, and you can figure out the connections. Alternatively (and I think that this is what others have done), buy a new single-speed 3ph. 3HP 240v motor, and use that, because you are far less likely to have problems with it. Your other alternative is to purchase a new inverter just for the lathe, and get one of the 240v single phase to 415v 3-phase ones. Then arrange the switching so that you can't alter the motor speed by switching when the inverter is running. This can be arranged to be failsafe, but it requires a clear understanding of what's going on, and at least one relay. Perhaps not for the faint-hearted, because...
 
The one thing you learn early with inverter experiments is to be very, very careful - mistakes are easy to make, and are invariably not that cheap to correct, especially on anything over 2HP.
 
So I'm sorry that this isn't entirely good news, but I'd rather be the bearer of that than have you blow up an inverter that's working fine as it is, I think.
Hugh Gilhespie14/12/2010 09:17:15
130 forum posts
45 photos
Hi Aidan,
 
I have also very recently aquired a Student 1800 and had the same problems that you are facing. I had a static converter running my Bridgeport but now that I had two machines needing 3-phase supplies I decided to bite the bullet and put in a 'proper' 3-phase supply.
 
I bought a 7.5 HP 240 to 415 volt digital inverter and a matching output filter from Drives Direct. This is used in 'plug and play' mode and I have installed four 16 amp 5-pin sockets to give me 3-phase supply points where I want them. I had a bit of discussion with DrivesDirect about the inverter rating as in theory you should use an inverter rated for three times the power of the largest motor you will use, i.e. a 9HP inverter for the 3 HP Colchester lathe. However, the digital inverters are rated for a short period of overload and my system works without any problems.
 
The advantages - as I see them - for this system are that you don't need to make any alterations to the machines, just plug them in and run them. I also have found that the Bridgeport motor runs a lot quieter than when it was powered by the static converter, seems to have a bit more toque too. You also get all the accessories like power feeds on the mill and coolant on the lathe working in their normal way.
 
The disadvantages are the cost, these inverters are not exactly cheap, and you don't get all the sexy stuff like variable speed, soft start, jog etc. You will also need a hefty mains supply to the inverter, think cooker outlet. Mine is rated for 60 amps with a 32 amp breaker. Even though the breaker is the 'slow open' type for inductive loads like motors, it still often trips when I turn the inverter on. I have to hold it closed until the inverter finishes powering up then it's fine for the rest of the session.
 
As I said, I wired up the 440 volt system myself but if you are at all uneasy about this then I strongly recommend getting a proper electrician to do the install. 440 volts is a LETHAL voltage.
 
Hope this helps.
 
Regards
 
Hugh
 
 
Nicholas Farr14/12/2010 10:10:31
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Hi,  
 
      Posted by Hugh Gilhespie on 14/12/2010 09:17:15
      As I said, I wired up the 440 volt system myself but if you are at all uneasy about this then I strongly recommend getting a proper electrician to do the install. 440 volts is a LETHAL voltage.
 
You may find that even 240 volts is a LETHAL voltage also. But there again you may never know if its lethal to yourself.
 
Regards Nick.
Andrew Johnston14/12/2010 11:02:57
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Technically, at these relatively low voltages, it's the current that causes a problem, not the voltage per se. To get a shock you also need a reasonably conductive path to ground, or another phase; no path, no shock.
 
Regards,
 
Andrew
Hugh Gilhespie14/12/2010 13:38:35
130 forum posts
45 photos
One question for the electrical whizerati. With my setup, i.e. inverter supplying 415 volt, 3-phase with no neutral wire, will the ELCB in the 240 volt feed to the inverter still work if there is a short of 1 (or more) phases to earth on the 3-phase side?
 
I have a worrying idea that it won't work and that the 3-phase output is effectively a floating supply like you would get from a transformer.
 
What do the experts think?
 
Regards
 
Hugh
Les Jones 114/12/2010 15:12:27
2234 forum posts
153 photos
Hi Hugh,
                  The answer to your question depends on the design of the inverter. If the inverter has the neutral point connected to earth then the input RCD will not trip if there is a short to earth on one or more phases (Or if you touch one phase.) In this situation you would receive a shock (240 volts) (Or the inverter would be damaged in the case of a short to earth.) If the output was floating a short to earth on one phase or you touching one phase would not trip the input RCD. The difference with the first situation is that you would not receive a shock or the inverter would not be damaged. If one phase was shorted to earth and you touched another phase you would receive a 440 volt shock. Again the input RCD would not trip. The way an RCD works is to detect the difference in the current in the neutral and live (3 lives in the case of 3 phase.) If there is no short to earth then the current in the neutral will be exactly the same as the current in the live. (Or the vector sum of the current in the three lives.) I think it would be a good idea to put a three phase RCD on the output of the inverter. NOTE even an RCD will not protect you from touching two phases or live and neutral.
Les.
Steve Garnett14/12/2010 22:02:55
837 forum posts
27 photos
Posted by Hugh Gilhespie on 14/12/2010 13:38:35:
 
I have a worrying idea that it won't work and that the 3-phase output is effectively a floating supply like you would get from a transformer.
 
What do the experts think?
 

 
Well I think that in all cases, you are correct, because as you say, there is no neutral point on the three-phase side. All of the incoming supply has to be converted to DC, and then chopped at a high frequency into the three outputs and between the input and the output, there has to be complete isolation. You can purchase an artificial neutral generator that some people use with 415v inverters to create a neutral potential, but what that would do in terms of using it with an RCD is unclear - but I suspect that it might not work too well.
 
And, without any proper ground reference on the 3-phase side, a three-phase RCD isn't going to do you any good at all. On top of that, whilst the current waveforms flowing in the three-phase outputs generally look like reasonable sine waves, the voltage waveforms certainly don't - and therefore the conditions under which an RCD would normally work aren't met - with who knows what consequences?
 
What it comes down to is that with all of these systems, a degree of common sense is required with the outputs. First off, stop regarding them as a replacement mains supply, because as inverters, they simply don't behave like this. They are designed to take complete control of the motor, and any means of disconnection between the inverter and the motor is a potential source of trouble - especially if it disconnects itself whilst the motor is running. So ideally you have a solid screened connection between the inverter and the motor, and the screen is connected to the primary side earth at the inverter end, and the other end of the screen is insulated. The machinery, and specifically the motor frame should also be connected by a separate connection back to the protective earth point, so that the machinery is independently grounded. And when you've done all this properly, how are you going to get a shock from it, unless you deliberately do something stupid? Well basically, you aren't going to.
John Olsen14/12/2010 23:02:48
1189 forum posts
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I think for the Direct Drives setup you would need to talk to them about the capabilities. They don't give a great deal of detail about how their digital inverter and filter works, but it does sound like it is intended for applications where motors are switched on and of downstream of the inverter.As such, it may be possible to fit an RCD on the downstream side, but remember RCDs only help with certain types of faults
 
The usual inverter is very much intended to be connected to one motor, and should not have any switching or plugs and sockets in between the motor and the inverter. There will generally only be one motor conected to a given inverter, although I did drive an electric car thirty years ago that had two identical motors on one inverter...that gave a  rather nice inherent limited slip differential effect. Still, those guys had designed the inverter and were pioneers in the field, so they knew what they were doing. I would not think that an RCD between the inverter and the motor would do any good.
 
For either type of inverter it is probably better to rely on the inverter itself for the  downstream protection. Certaly the usual single motor type is protected for most faults, and considering the way it is applied, the Direct Drives type mist be too
 
On the lethality of different Voltages...well, it takes a high enough Voltage to overcome skin resistance, and 230 is plenty for that, depending of course on how sweaty you are, and whereabouts on your body you make contact. 12 Volts has been known to do the job is some cases. When they are doing surgery they have to worry about millivolts, eg the difference in earth potential between two different outlets can be enough to kill the patient. Meanwhile really high Voltages do not necessarily kill outright, but the flash burns and so on will not make you very happy. So whether or not it was the current or the  voltage that did the damage is a bit academic, especially since you won't get one without the other, and the body resistances involved are pretty nonlinear anyway.
 
regards
John
 

Hugh Gilhespie15/12/2010 09:28:06
130 forum posts
45 photos
Thanks to everyone for the responses and apologies to Aidan for the thread drift.
 
As I thought, it's not a simple matter. I don't know how 3-phase ELCBs operate but it occurs to me that if they work in a similar way to single phase devices there may be a problem. Without a neutral wire, I can only assume that earth leakage is detected by an imbalance between the currents in the three phases. My Bridgeport uses one of the three phases to supply a transformer to give 110 volts for the power feeds so that when a power feed motor is operating, there will be an imbalance between the currents in the three phases that would cause the ELCB to trip.
 
I am sure that Steve is right about applying common sense to this wiring. I have been very careful about making sure that  the earth connections are all sound and all the wiring between inverter / filter and the sockets is in conduit so no loose wires anywhere. However, the only earthing point I have used is the one on the inverter output, maybe I should run a separate earth wire to a 'real' earth on the input side?
 
I think I will ask Drives Direct about fitting an ELCB - they are very helpful and if I get a useful answer I will post the details.
 
Regards, Hugh
KWIL15/12/2010 09:39:56
3414 forum posts
66 photos
Hugh, As this is an electrical installation and not merely a part replacement, did you get Part P "authorisation" from your local Planners?
Steve Garnett15/12/2010 10:29:30
837 forum posts
27 photos
Sorry for the length of this, but there are important points and distinctions to be made...
 
Hmm... as far as the voltages are concerned, it should be remembered that it's the volts that jolts, but the mills that kills; in other words the voltage you feel is pretty much irrelevant - I have given myself a 25,000v shock before now (CRT EHT supply) and all it does, apart from producing a huge spark and making you jump, is to produce a small yellow spot on the skin, which soon fades. But there is virtually no current involved in this at all, and that's why you survive.
 
But the moment you get a current somewhat in excess of 50 mA flowing through the body, then fibrillation is likely to occur. And under damp conditions, you don't actually need that many volts across you for this to happen. But strangely, if the current gets up to about 5A you might survive, because of bone conduction effects. Wouldn't actually want to try this though - the internal burning is significant! But it is the reason that in America, when they were using electric chairs, they had to go through a carefully set out series of different operating currents before they could officially declare somebody dead.
 
One somewhat perverse point about this is the whole low-voltage building site transformer thing (the 110v ones) seems to be inherently wrong - often you have wet conditions, and inherently twice as much current available. Okay, it's completely isolated and that's a good thing - but it would have been safer just to isolate on a 1:1 basis!
 
Okay, the inverters. Yes it's quite possible to make an inverter which has ground-referenced outputs, and that may be what some people have, if they bought one of the posh, expensive 415v versions. It's also possible to fit socking great filters to the outputs, which effectively get the voltage waveforms on the outputs to look like sine waves. If you have one of these systems, then to all intents and purposes it looks like 'genuine' three-phase again, and an ordinary RCD would work with it. I would still contend though that for a motor controller, if you have all of this stuff wired correctly and importantly, no plugs and sockets on the three-phase side, then you don't have a situation that would warrant one, simply because of the considerably increased likelihood of damaging the inverter, and losing control of a rotating motor.
 
Why considerably increased? Well, there's one thing I forgot to mention yesterday that is significant, and that was a bit remiss of me. It is that most of the inverters have two bits of internal switching that will cause havoc with an RCD; they have a braking mechanism that can switch in a socking great resistor (sometimes external ones are provided for), and even worse, use a system that injects DC into the motor windings when running to achieve additional braking, and on a good system (like Lenze ones), effectively lock the shaft when it's still. The chances are that if the control system braking changes don't trip the RCD when they occur, the DC injection certainly will. And that trip will occur exactly at the point that you really don't want it to happen.
 
All manufacturers of inverter systems for single motors provide wiring diagrams with them. I've never seen one with an RCD on the secondary... They will all tell you that you get better control of the motor with shorter cables, and the recommendation to screen them is widespread. As far as I'm aware, the only systems that can safely run multiple motors are the fixed-frequency ones that effectively replicate the supply conditions (as I mentioned above), and these can do so because they aren't actually motor controllers as such - and that's an important distinction to make. Chances are that they will even cope with switched-speed motors, but only if you stick to the recommendation to rate the inverter at 3x the motor HP; this will give you massive active devices within the inverter that would actually be able to stand up to the reverse EMF without destroying themselves.

Edited By Steve Garnett on 15/12/2010 10:30:23

Nicholas Farr15/12/2010 11:17:44
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2962 forum posts
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Hi,
      Posted by Steve Garnett on 15/12/2010 10:29:30
      Hmm... as far as the voltages are concerned, it should be remembered that it's the volts that jolts, but the mills that kills; in other words the voltage you feel is pretty much irrelevant - I have given myself a 25,000v shock before now (CRT EHT supply) and all it does, apart from producing a huge spark and making you jump, is to produce a small yellow spot on the skin, which soon fades. But there is virtually no current involved in this at all, and that's why you survive.
 
But the moment you get a current somewhat in excess of 50 mA flowing through the body, then fibrillation is likely to occur. And under damp conditions, you don't actually need that many volts across you for this to happen. But strangely, if the current gets up to about 5A you might survive, because of bone conduction effects. Wouldn't actually want to try this though - the internal burning is significant! ...........
 
For reassons I'll not go into here, foolheartedly I once was doing some electrical work many many years ago, in a my home with the system live. I was being extremly cautious and extremly carefull, consentrating on what I was doing. Suddenly my ex wife had come  into the room and spoke; all consentration was lost in a instant, and 240v was traveling from one hand to the other. I can confirm that that when gripping somthing in your hands with electricity flowing through, you cannot let go. I had to throw my whole self away to disconnect myself from a potential fatality. Luckly I suffered no ill affects whatsoever, just shaken up for half hour or so. However I did have to then fix the kicthen ceiling downstairs where my foot when though the only floor board that I had up while doing the work. I have never worked on anything live since.
   Moral of the story, don't mess with electricity.
The 110v transformers that you mention Steve, are usually 110v split across ground potential, therefore you should never be exposed to more than 55v in the unlikely event of a fualt, of maintained equipment. I belive 55v is an acceptable level in our wet climet conditions to be reasonably safe.
 
Regards Nick.

Edited By Nicholas Farr on 15/12/2010 11:35:21

Steve Garnett15/12/2010 20:09:08
837 forum posts
27 photos
Posted by Nicholas Farr on 15/12/2010 11:17:44:
The 110v transformers that you mention Steve, are usually 110v split across ground potential, therefore you should never be exposed to more than 55v in the unlikely event of a fualt, of maintained equipment. I belive 55v is an acceptable level in our wet climet conditions to be reasonably safe.
 
 
The lowest recorded potential I'm aware of that has caused death is 32v. Most of the deaths caused with relatively low voltage AC occur with a potential of between 100 and 250v apparently, but I'd say that was hardly surprising, since this covers the vast majority of the world's domestic supplies!
 
But yes, treat the stuff with respect. And if in any doubt, do as we've mentioned before on this forum - use SIDE. Switch off - Isolate - Dump (residual charge) Earth anything you have any doubt about at all before working on it. This is particularly relevant with Inverters, because these can retain a charge for anything up to 3 minutes after switching off and unplugging - as most of them warn you. I've never got a belt off one, but I've measured a few residuals, and often it's true about the self-discharge time. And the larger the Inverter, the longer it can retain it.
Andrew Johnston15/12/2010 20:40:16
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6237 forum posts
676 photos
Steve,
 
That's interesting; I also had in mind that the lowest recorded electric shock fatality was 32V. I also thought it was a DC supply, as used on farms in the USA which were not connected to the grid.
 
So, either it's true, or it's a well established myth!
 
Regards,
 
Andrew
KWIL15/12/2010 21:27:01
3414 forum posts
66 photos
The worst situation is when the potential is applied hand to hand [ie across the heart].
 
5KV hand to leg caused me to be monitored for fibrillation for an extended period, earthing wands with a large hook fit into the SIDE scenario quite nicely!
Jon15/12/2010 23:46:04
1000 forum posts
49 photos
Posted by Steve Garnett on 15/12/2010 10:29:30:  It is that most of the inverters have two bits of internal switching that will cause havoc with an RCD; they have a braking mechanism that can switch in a socking great resistor (sometimes external ones are provided for), and even worse, use a system that injects DC into the motor windings when running to achieve additional braking, and on a good system (like Lenze ones), effectively lock the shaft when it's still. The chances are that if the control system braking changes don't trip the RCD when they occur, the DC injection certainly will. And that trip will occur exactly at the point that you really don't want it to happen.
 

Think i am going to have problems in this respect.
I already have trouble starting a 1 1/2hp single ph motor 1 in 3 blows a 13a fuse but never the 20a MCB B type Starbreaker.
 
I now need to run a 3hp 415V motor plus coolant pump etc, non are dual voltage. Took a flyer 6 weeks ago since i was in direct drives area and collected the single to 3ph 220V outputs which they will glady exchange for the 415V output.
30a MCB C type feed 4mm cable ok or do you reckon i will still have trouble?
 
Lastly the electric co changed from 100A fuse to a 65A a few years ago, is this the norm? Hob draws 32A, mill will throw 20A MCB forget the welder slightest touch lowest setting spend more time resetting fuse. C type cure it?
 
 

Andrew Johnston16/12/2010 00:07:28
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6237 forum posts
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Definitely not the norm, at least in the UK. As I understand it the normal household incoming fuse is at least 100A these days. When I bought my bungalow 10 years ago the incoming supply was rated at 60A; the surveyor marked this down as an issue that would need to be sorted out.
 
When I upgraded to a three phase supply the electricity board automatically installed 100A fuses per phase.
 
Where are you based Jon?
 
Regards,
 
Andrew
Steve Garnett16/12/2010 00:48:10
837 forum posts
27 photos
Posted by Jon on 15/12/2010 23:46:04:

Lastly the electric co changed from 100A fuse to a 65A a few years ago, is this the norm? Hob draws 32A, mill will throw 20A MCB forget the welder slightest touch lowest setting spend more time resetting fuse. C type cure it?

 
Andrew's spot-on about this being a little strange - they regressed back to a 65A???
 
If a type C MCB doesn't fix the problem (they will trip somewhere between 5-10 x the rated current), then something is severely wrong. And it may well be with the input arrangements on your inverter - often there are variations possible, and this can be all it takes to prevent all sorts of tripping nasties. On a domestic supply, you'll probably not want to try a type D, because that will get you into difficulties with most installations from a compliance point of view - in other words, it could cause the company fuse to blow quite easily, because a serious short overload with these can be up to 20 x the breaker normal rating.
 
The first thing to ask you is whether this happens just when the inverter is switched on, but not the motor. If it is, then you have definitely got an input wiring issue that needs to be addressed. Some inverters provide alternative input wiring instructions, and these aren't always the clearest in the world, so it's worth checking this carefully.
 
If, on the other hand, you are just expecting to switch the motor on immediately, then I can't say that I'm entirely surprised that this happens - you will introduce a pretty large surge current with a hard start, and an inverter can actually make this worse, simply because it's not 100% efficient, and is adding to the load. What most people find in this instance is that programming a soft start in makes all the difference - ramp the motor up over a second or two, and that surge almost doesn't happen.
 
Have to say though that most of the breaker trouble I've ever experienced has been with rotary converters of one sort or another - they really can't avoid a really large surge, simply because of the way they work. Sensibly set up inverters generally seem to get around this quite well.
 
 

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