How do I go about this?

[JohnW2]

This was discussed at some length here Fridge motor ... Looking at it again, I'm not clear where 3-phase, or more, at the motor or supply, in ericmark's #22, comes into the discussion, but no doubt you will enlighten me!

It's a good example of the intriguing way in which discussions (not just here - virtually everywhere) can move from tangent to tangent, such that the 'origin' of the discussion quickly becomes 'an obscure matter of ancient history' - it really often can be just like "Chinese Whispers"

• This thread started with the OP asking about how he could learn more about 'electrics', noting that he had problems with (and a dislike of) maths. Early on (#11) i suggested that the amount of maths needed in relation to domestic electrics was pretty trivial, and that far more important was a good understanding of basic electrical principles. In reinforcing that, I cited an anecdotal experience from decades ago when a so-called 'electrician' tried to tell me crazy things about the need for an additional RCD which illustrated that he didn't have the slightest clue about very basic electrical principles.​

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• eric then jumped in and said that that 'electrician' may have had more of a point had we been talking about 'voltage-operated' earth leakage circuit breakers ((VOELCB, or ELCB-v), even though they were already seriously obsolete by the time of that anecdotal experience.​

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• In #22, eric then broadened that issue (the fact that VOELCBs had given way to RCDs a very long time ago) by producing a list of things that had changed since he was first educated/taught about them. One of those observations was ".... go back 20 years, and with domestic we never saw a three phase motor, but today in my house I have at least 3, a fridge/freezer, a freezer, and a washing machine, possible the drier also has one?".​

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• Quite a bit later (#50), after a lot of interesting, but irrelevant today, discussion about VOELCBs, you picked up on that statement from eric, expressing your surprise that he had talked about 3-phase domestic appliances.​

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• I then explained what I thought he was getting at - not domestic appliances fed with a 3-phase supply but, rather appliances with a single phase supply that 'converted; that to provide a polyphase supply for the motor.​

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• The subsequent exchanges between you and I are, as they say, then 'a matter of history' .​

As I said at the start, I find these happenings pretty intriguing - but they do, at least for me, have the advantage that I learn about, and am able to discuss, a much wider range of things (maybe ones I'd never heard of, or thought about) than would be the case if discussions always remained 'on topic'.

The important thing, from the POV of OPs asking questions, is that those tangents do not start muddying the water until after the OP's questions(s) have been dealt with as best as they can be - which I would say is, more often than not, what usually happens.

Kind Regards, John

 

[fixitflav]

Good summary

• In #22, eric then broadened that issue (the fact that VOELCBs had given way to RCDs a very long time ago) by producing a list of things that had changed since he was first educated/taught about them. One of those observations was ".... go back 20 years, and with domestic we never saw a three phase motor, but today in my house I have at least 3, a fridge/freezer, a freezer, and a washing machine, possible the drier also has one?".

Does the introduction of 3 (or more) phase motors affect the operation of RCDs? That's what I was unclear about.

 

[JohnW2]

If numbers are imaginary, do they exist?

I'd probably have to ask my 'littler' daughter that one, since she's the one with a degree in Philosophy

Slightly more seriously, mathematicians and some engineers etc. would probably say that they 'exist', in the sense that they can deal with them in much the same way as 'real numbers' - but I'mnot so sure about the rest of the world's population!

Having said that, 'fictitious concepts' such as this can be very useful - as, for example, the concept of 'sucking' (as in 'vacuum' cleaners) or the concept of 'holes moving' in semiconductors

 

[JohnW2]

Good summary ... Does the introduction of 3 (or more) phase motors affect the operation of RCDs? That's what I was unclear about.

Not at all. As I said,the discussion jumped dramatically, several times, and 3-phase motors (in domestic appliances) only got into it as part of eric's list of things which had 'changed' since his initial electrical education. Absolutely nothing to do with RCDs (or VOELCBs, let alone the OP's question!).

 

[fixitflav]

Not at all. As I said,the discussion jumped dramatically, several times, and 3-phase motors (in domestic appliances) only got into it as part of eric's list of things which had 'changed' since his initial electrical education. Absolutely nothing to do with RCDs (or VOELCBs, let alone the OP's question!).

OK thanks

 

[fixitflav]

mathematicians and some engineers etc. would probably say that they 'exist', in the sense that they can deal with them in much the same way as 'real numbers'

Definitely, I was of course joking. Complex number theory is a very interesting and useful branch of maths, with applications in several fields in science and engineering, AC theory for one.
Are you aware of Euler's formula, e^(i*x) = cos(x)+i*sin(x)? Three apparently separate areas of maths, exponentials, imaginary numbers and trig functions turn out to have this surprising connection.

 

[JohnW2]

Definitely, I was of course joking. Complex number theory is a very interesting and useful branch of maths, with applications in several fields in science and engineering, AC theory for one.

Sure -and, in turn, I was essentially joking in my reply - particularly in relation to my 'philosopher' (actually now a barrister !) daughter. However, as we are agreed, imaginary numbers are not only of interest and value to mathematicians but also to those in other fields, of which electrical engineering is an important one.

Are you aware of Euler's formula, e^(i*x) = cos(x)+i*sin(x)? Three apparently separate areas of maths, exponentials, imaginary numbers and trig functions turn out to have this surprising connection.

I was aware of that formula, but have never had reason to use it - but, as you say, it represents an interesting coming together of a number of mathematical concepts (getting back to the OP, way beyond anything needed in relation to 'domestic electrics'!).

To the electrical engineer, or other people involved with 'wave mechanics' etc., I suppose that imaginary (particularly 'complex') numbers provide a useful way of dealing with 'vectors', in a manner which makes them more mathematically 'manipulable', the alternative probably being a lot of trigonometry!

Kind Regards, John

 

[fixitflav]

I suppose that imaginary (particularly 'complex') numbers provide a useful way of dealing with 'vectors', in a manner which makes them more mathematically 'manipulable', the alternative probably being a lot of trigonometry!

Imaginary numbers are a subset of complex numbers (as are real numbers). By convention, electrical engineers call them vectors, but complex numbers don't obey the rules of vector algebra, though there are similarities. If you need to work with complex numbers, Mathcad is excellent.

 

[JohnW2]

Imaginary numbers are a subset of complex numbers (as are real numbers).

Unless definitions have changed since I was at uni (not impossible in 50+ years!) a 'complex number' is merely the combination of a real and imaginary number in a single expression.

By convention, electrical engineers call them vectors, but complex numbers don't obey the rules of vector algebra, though there are similarities.

I'm not quite sure what you mean by not obeying the rules of vector algebra. A quantity which has both magnitude and direction can be expressed as either a complex number (a 'single expression') or, separately, as a magnitude and angle (relative to some reference). The complex number approach would seem (to me) to have the advantage that it can usually be handled, 'algebraically', as a single expression, whereas one otherwise essentially has to deal with magnitude and direction separately, primarily by using trig. Is that not the case?

 

[fixitflav]

Unless definitions have changed since I was at uni (not impossible in 50+ years!) a 'complex number' is merely the combination of a real and imaginary number in a single expression.

I don't know about "merely", but that's right

I'm not quite sure what you mean by not obeying the rules of vector algebra. A quantity which has both magnitude and direction can be expressed as either a complex number (a 'single expression') or, separately, as a magnitude and angle (relative to some reference).

You can add (or subtract) components of 2 vectors, and same with real and imaginary parts of 2 complex numbers, so they're similar in that respect. But there's no such thing as vector dot and cross products with complex numbers.
Also eg calculating impedances and doing circuit calcs works well with complex numbers, but not with vectors (as defined by mathematicians). I think, anyway, it's a long time since I was much into this as well.

 

[ebee]

Some of that is slightly going over my head, I am not an ex-Uni type I`m afraid but I do grasp some of what you two are saying too.

 

[fixitflav]

Some of that is slightly going over my head, I am not an ex-Uni type I`m afraid but I do grasp some of what you two are saying too.

Yes, I think we've thrashed this to death, sorry to bore some on here!