Real life maths problems for electrics students

[anneh]

I'm trying to help students studying to become electrical (and mechanical) engineers with their maths. I want to give them questions that relate to real life things they're likely to want to do as electricians - like working out how much cable, pricing the materials for a wiring job... Not only because this is the only way they will stop shouting and throwing things at each other in order to think about the questions but because I want to be teaching them stuff that is obviously going to be something they will use.

I've already been reading this forum and getting a few ideas of some electrical calculations from posts, but if there are any electricians who could spare a few moments to tell me ways in which they use numbers in their work, it would help a lot.

 

[Lectrician]

Transposing formuale

Ohms law - Volts=Current x Ohms (V=IR)

Power - Watts=Volts x Current (P=VI)

Volt drop - mV fig from regs table x length in meters x current drawn all devided by 1000 to give the answer in volts (not milli volts).

Thats just a few.

 

[anneh]

Thanks, Lectrician. Unfortunately formulae without concrete examples turn them off: they are 16-18yr old lads that just want to be doing practical hands-on stuff all the time. I'm trying to learn about the practical electrical stuff so I can give them examples that make them see why they would need to learn the maths. So the formulae are nice and obviously electrical-related but when would you use them at work? If you are working out what kind of fuse to use or something?

 

[Lectrician]

A 9.8kW shower.

What size cable??

Use forulae to find out load in AMPS - (P=VI)

so 9800w/230v=42.6amps.

Select cable that can carry this current.

Check that the selected cable will drop more than the 4% allowence for volt drop. (4/100 * 230v=9.2v max)

Find price from wholesalers per meter for cable, and the price for a switch capable of switching 42.6amp.

 

[breezer]

do you mean something like:

Breezers freezers supermarket is being built. (bit like iceland)
It will have 3 isles
and freezers all around the walls
each wall freezer is 1.5 m wide x 1.5m long
each isle freezer is 1.5m x2m long
internal floor dimensions are 21m deep x 15m wide
assume no freezers are required across the front
one freezer must be left "missing" at the back for a fire exit door
there are 3 tills at the front one in line with each isle
each appliance requires one 13a socket
each freezer draws 0.2A when iddiliing but when running draws 3A


Questions

if all freezers are switched on at same time how much current will they draw.when
running
ididling
how many sockets will be requred
how many back boxes required
how many chocolate icecreams were sold on Thursday
if each socket is fixed by two screws, (so needs 2 rawl plugs) screws come in boxes of 50 how many boxes required
rawlplugs come in strips of two, 6 strips to a box how many boxes required

i have not mentionmed things like cable size, length required etc

oh and the shop can not open as its dark (some one forgot to quote for the lights)

but you get the idea, its maths that will be required for wiring a shop, as electrical engineers they will need to know quantites req etc, there are more things you can add/ change perhaps others would like to add?

 

[Lectrician]

you have 10 light fittings in your home. 8 of them have 100watt lamps, 2 have 60watt lamps.

What is the total wattage you are using with all lamps on??

(100watt x + (60watt x 2)=920watt

The fuse protecting the circuit is rated at 5amps, how many more 100watt lamps could be installed before the fuse would blow??

(P=VI) : 230volt x 5amp=1150watts

1150watts - 920watts = 230watt

230watt / 100watt = 2.3 (so only 2 more lamps could be used)

 

[Lectrician]

Good one Breezer

Breezers Freezers - LOL

Reminds me of a college assignment that one!

 

[anneh]

Fantastic. Thanks so much. I tried to pose a problem involving security lighting, but I'm a mathematician, not an electrician so I don't know the right questions to ask about hmm power draw and stuff and I suspect that my circuit was somewhat unrealistic and missing in essential details. Any further ideas are equally appreciated.

 

[ban-all-sheds]

Thanks, Lectrician. Unfortunately formulae without concrete examples turn them off: they are 16-18yr old lads that just want to be doing practical hands-on stuff all the time.

I worry about their capacity to become competent electricians...

I'm trying to learn about the practical electrical stuff so I can give them examples that make them see why they would need to learn the maths.

This sort of thing?:

An immersion heater rated at 240 V, 3 kW is to be installed using twin with protective conductor p.v.c. insulated and sheathed cable. The circuit will be fed from a 15 A miniature circuit breaker type 2, and will be run for much of its 14 m length in a roof space which is thermally insulated with glass fibre. The roof space temperature is expected to rise to 50°C in summer, and where it leaves the consumer unit and passes through a 50 mm insulation-filled cavity, the cable will be bunched with seven others. Calculate the cross-sectional area of the required cable.

Calculate the volt drop for the case above. What maximum length of cable would allow the installation to comply with the volt drop regulations?

You'll find those, and others here:

http://www.tlc-direct.co.uk/Book/4.3.9.htm

http://www.tlc-direct.co.uk/Book/4.3.11.htm

http://www.tlc-direct.co.uk/Book/3.7.2.htm

http://www.tlc-direct.co.uk/Book/3.7.3.htm

and probably elsewhere in the book. I guess you should sort out the legalities of using that material.

If these people are also studying electrical/mechanical engineering, can you get some concrete examples from those subject lecturers?

 

[L.Spark]

Well everything here is all installation design stuff which is great fun, if they don't enjoy this I actualy have to agree with you bas, because when it comes to electrical science, I very much doubt they will enjoy that.

 

[whatif]

If they are anything like the mechanical and electrical engineers I have worked with the last few years they will have to be able to create graphs and email them to their bosses and CC them to everyone else. An ability to use excel for everything (lists, tables, notices, etc.) except calculations also seems to be the way ahead.

It tends to be a "good" electrician who does all the calculations and planning, leaving the degree educated engineer to administrate and manage.

Is it different at your place?