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Home Workshop > Members > Forum > Information Area > Information > Inverters and dual speed motors

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Inverters and dual speed motors

Re: Inverters and dual speed motors

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.

I think this is a very clear definition of what has been worrying me re: modifying inverters; I don't think some people are sufficiemntly aware of the dangers of the voltages involved. I don't want to discourage folk from tinkering, or knock others who are trying to pass on the benefit of their experience, but the problem with electrics is that you can go from a 'dangerous' condition to a downright lethal one in a matter of milliseconds. You don't have any period of degrading safety that you might encounter in other areas of engineering. For example, if you are forcing a machine beyond it's cutting capability, you generally get some feedback it's not happy (and therefore not safe) and you can back off a bit. Maybe I'm more electrical safety conscious than I need to be, but a good few years ago, I lost a relative, a professional electrician, who made the mistake of forgetting just how dangerous the stuff is...

Another issue is that when you are being trained up on these sort of systems, it's normally in an environment where there is someone else around to switch the power off if you do something stupid and inadvertently connect yourself to something you shouldn't. A person working at home will probably be by themselves, and we all know what H&S think of 'lone workers', judging by the tomes of procedures that have to be produced to cover these circumstances.

Finally, I will always remember a safety lecture I got in the machine shops at college. The instructor asked the class what the most dangerous tool in the workshop was. Some went for the lathe, others the milling machine, I favoured the shaper. The instructor said none of these, and pointed to the 3 phase supply instead. His argument was that if you got entangled in a machine, you might lose a finger or a hand, or an arm if you were unlucky. You might even bleed to death whilst waiting for the ambulance, but the 3 phase supply could kill you in under a second.

(Just as a point of interest, in case anyone thinks I'm a 'elf & safety nerd (which I'm not) I have been chased around my workshop by a wood lathe faceplate and 1000mm workpiece. I thought it was secure on the left hand mandrel, powered up, did the job, then switched off. To my horror, the momementum of the work was sufficient to unscrew the faceplate from the mandrel, which then shot, frisbee style around thw workshop, at about 200rpm, bouncing off the walls and suchlike. Definitely needed to go for a lie-down after that little bit of excitement had stopped.)

Dave Pack
75 Posts
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Re: Inverters and dual speed motors

I can only echo what Dave has said - 415v 3 phase is VERY unforgiving, not like ordinary 240 volt mains which will give you a jolt and make your fingers tingle. A friend who was trained on industrial electrics in the Sheffield steelworks is always at pains to point out the "extra" lethality of 415v 3 phase - and the extra need for a very good earth connection. Physics tells us that high voltage/high frequency electricity will pass along the surface of a conductor and not through the body of it, but I think we are talking very much higher frequencies than encounterd in an inverter - but it is a long time since I studied such things so check it out.  And I think the same caution applies to static or rotary converters using capacitors - unless they have a means of automatically leaking the charge after use, contact with the terminals can be very disheartening, especially when everything is "switched off". There are those who contribute to this website who know a damned sight more than I do about electrics of this nature - what is their view in regard to "amateur tinkering" ? How does it affect any life insurance policy you might have? worth thinking about.

"He who knows nothing is closer to the truth than he whose mind is filled with falsehoods and errors"
Thomas Jefferson

Peter Smithurst
193 Posts
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Re: Inverters and dual speed motors

I think static converters are a good deal more dangerous than conventional/real three phase. Any one phase of real three phase is only 240V above earth, you need to get a phase to phase shock to get the 415V and the chances are, touching the first phase will be enough to discourage you from touching the second. With a static inverter in particular, you very likely have 415V above earth on a single conductor and that is the major risk being discussed. The same may or may not apply to a RPC depending on the design, but probably it will.

On the specific (ancient) static inverter I have, the capacitors will discharge almost instantly because of the design,  but that is certainly not the case with an electronic Inverter which usually carry significant warnings to leave them for a period before (dis)connecting them. It may well also apply to some static and rotary converters, it will obviously depend on the specific design.

I was also asked what was the most dangerous nut on the machine and was told the correct answer was the one holding the controls at the time of the accident :-/
No, I'm not guiltless either, anyone who's ever really used machines for a while is bound to have some 'events'. I had a grinding wheel come off much as David's faceplate, the inverter was supposed to 'coast to halt', but because of a change in the firmware actually came to an instant stop, though fortunately only from half speed as I happened to have a ramp programmed. Certainly got my attention 'though!

- Richard -

Richard Shute
Who dies with most toys, wins
399 Posts
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Re: Inverters and dual speed motors

On the grinding wheel, chuck incidents, this can be a major problem when fitting DC injection braking to woodworking machinery to comply with PUWER regs which states machines must stop in 10 secs.  Still have a rather large dent in Garage door from chuck key incident and a pair of scorched (they actually caught fire) overalls from a 9" grinder.
Generally on workshop incidents the issue of lone working seems to be a major problem, there are times that if I had been hurt it could have been many hours before I was found. I have no answer even my mobile phone doesn't receive much signal where I live.From experience it's not the risks that you have thought about and minimise that get you it's the unexpected.

Ashley (Remap Volunteer look us up  The best volunteering opportunity for practical people)

Ashley Slater
503 Posts
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Re: Inverters and dual speed motors

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