Model Electric Motors?

[Ron LadenParticipant @ronladen17547]

Sounding good Jason, what material have you used for the bobbins and excuse my ignorance but what is ERW tube..?

Ron

[John HaineParticipant @johnhaine32865]

I wonder how much of the battery energy is converted to mechanical power compared with the energy stored in the magnetic circuit every cycle that's presumably wasted in the contact arc when it opens? An interesting curiosity but a bonkers way to make a motor!

[JasonBModerator @jasonb]

Ron, I have used black Acetal rod which machines quite nicely. ERW Electric Resistance Welded which is a seamed steel tube, not the nicest steel to machine but it has it's uses.

John, it is really just an electric toy so no need for efficiency. When I get round to making the early bipolar motor that this thread started about that should be more efficient but again I'd be happy to just have something that runs for display only. If I wanted performance then I would not start with such an old design.

[John HaineParticipant @johnhaine32865]

Sure, Jason, I realise that, I was just musing.

[Tim StevensParticipant @timstevens64731]

John Haine – Those who are 'model engineers' with an interest in steam models are in no position to point a finger at the 'efficiency' of an electric motor, even an inefficient one. Just read the test figures for steam loco models quoted in ME and very few are above 2% – ie 98% of the power is wasted. Just remember that in the days when steam 'ruled' the most efficient engines were the guns on battleships – and most of their power, too, just flies out of the barrel.

Cheers, Tim

[JasonBModerator @jasonb]

With all the parts made I did a quick lash up of the wiring and tried it on the 6V battery.

Needs a few tweaks and tarting up but at least the basic mechanics all work. It almost runs on a 1.5V AA cell so hopeful that once complete that will be all it needs as the 6V is a bit too fast though speed can be regulated by altering how long the spring is in contact with the conrod.

[Ron LadenParticipant @ronladen17547]

Excellent Jason, it certainly gets a move on at full throttle but I,m really impressed at how controllable it is, I didnt think the speed would be that adjustable but it clearly is.

Great stuff.

Ron

[John HaineParticipant @johnhaine32865]

Posted by Tim Stevens on 17/11/2018 20:48:27:

John Haine – Those who are 'model engineers' with an interest in steam models are in no position to point a finger at the 'efficiency' of an electric motor, even an inefficient one. Just read the test figures for steam loco models quoted in ME and very few are above 2% – ie 98% of the power is wasted. Just remember that in the days when steam 'ruled' the most efficient engines were the guns on battleships – and most of their power, too, just flies out of the barrel.

Cheers, Tim

I'm not that interested in steam either TBH.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ron Laden on 18/11/2018 13:15:13:

Excellent Jason, it certainly gets a move on at full throttle but I,m really impressed at how controllable it is, I didnt think the speed would be that adjustable but it clearly is.

Great stuff.

Ron

.

+1

Very impressive performance.

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

That's an excellent model. I'm not sure the motor's efficiency is the problem, more its effectiveness.

Bad for efficiency are the bearings, arcing, and a likely mismatch between the motor and the battery (too many volts), all of which could be improved easily. The timing is certainly off as well, but that's an adjustment. The magnetic cores could be improved with more suitable materials than mild-steel. In favour of higher efficiency the moving parts are light and there are few of them. In comparison a reciprocating steam engine is obliged to waste lots of energy moving the weight of its piston and the valve gear, all of which parts suffer high friction.

Be interesting to measure this simple motor's efficiency – I think it might be surprisingly high.

Not such a good story when considering effectiveness. The motor pulses power only once per revolution and it requires a flywheel, which limits its application. However, I think the main problem with this form of electric motor is not that it's fundamentally poor, but rather the number of ways in which it can be improved on! Arranging more electromagnets around the shaft while also making the shaft do the switching minimises the number of moving parts and bearings whilst improving electrical and magnetic efficiencies at the same time. The same rearrangements make the motor smoother too.

I should get off my soapbox and go and make one…

Dave

[John HaineParticipant @johnhaine32865]

There's still a fundamental problem with these motors, that energy is stored in the magnet when the armature is fully pulled in and inductance at its highest, which has nowhere to go when the "commutator" switch opens and is therefore wasted. Would be interesting to see if there's a way to recover it

[Sam StonesParticipant @samstones42903]

A couple of Ron's jars perhaps?

Sam

[Sam StonesParticipant @samstones42903]

Sorry Ron, my 'resistance' was low

Sam

[Neil WyattModerator @neilwyatt]

!

Well I think it works pretty darn well!

The 'scramble' winding isn't up to your usual standard though Jason!

[Michael GilliganParticipant @michaelgilligan61133]

Jason has sparked some interest with this little model [sorry for the inevitable pun]

Whilst I wouldn't presume to set the Rules, I think it would make an excellent subject for an 'Efficency Competition' … maybe something along the lines of 'who can get the most mechanical work out of two AA cells ?'

Any interest ?

MichaelG.

[JasonBModerator @jasonb]

Neil, the first couple of rows went well then it started to jump a bit as the LH wind went over the RH and just got worse. Luckily it will be covered when finished.

[John HaineParticipant @johnhaine32865]

Wasn't there a competition a few years back at the London show, to get the highest lift of a defined weight from either an AA battery or a tealight? Mind you there's more energy in a nightlight than an AA cell, about 460 kJ for a tealight vs. ~3 Wh = 10,800 Ws = 10.8 kJ for the battery. Maybe a competition for the next show? Get Duracell to sponsor? Perhaps a birthday candle rather than a tealight.

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 18/11/2018 15:56:03:

There's still a fundamental problem with these motors, that energy is stored in the magnet when the armature is fully pulled in and inductance at its highest, which has nowhere to go when the "commutator" switch opens and is therefore wasted. …

This forum has ruined my life! I hadn't thought of that, and now you've got me wondering about the relationship between inductance, current and stored energy in electromagnets. My gut feel is that the amount of energy stored in the magnet is small. But that feeling's completely untrustworthy because I don't understand the relationship and will have to swot up. I seem to waste awful amounts of time on interesting diversions when I should be doing something useful like hoovering.

Dave

[John HaineParticipant @johnhaine32865]

The energy stored is just L*I^2/2. Half the product of inductance and square of current.

I seem to remember from my electrical machines course that for efficiency you try to design the machine so the inductance is constant as it rotates, but that was in the dim and distant.

[Anonymous]

We mustn't interrupt the hoovering, so I'll save you the time; the energy stored in an inductor is:

E =½ LI²

It can be significant, more than enough to cause some excitement. One of the tests automotive electronics has to pass is the dreaded load dump. The scenario is the alternator running at speed and charging the vehicle SLI battery at high current. Then the battery gets disconnected. The energy in the alternator windings has to go somewhere. Basically the output voltage rises until current flows and dissipates the energy. If I recall correctly from my work designing controllers for US heavy duty trucks (24V systems) the peak voltage for the test waveform was up to 200V and the energy to be dissipated over hundreds of milliseconds could be tens of joules.

Andrew

SLI = starting, lighting, ignition

[Ron LadenParticipant @ronladen17547]

I hope you are keeping up with all of this Jason..

Edited By Ron Laden on 19/11/2018 16:30:24

[JasonBModerator @jasonb]

Indeed I am

Funny enough that self same 6V battery is the one that I use for the hit & miss engines that have an ignitor rather than a spark plug. They work by the ignitor initially completing a circuit including a low tension coil and when that circuit is broken a whopping big spark is produced as the stored energy tries to jump across the gap.

Just splashed some Satin black paint onto the "cast" parts so may have it ready for Andrew to have a play with next weekend.

[SillyOldDufferModerator @sillyoldduffer]

I hope some of this is of more general interest. Here's a picture of the first 'real' thing I for which my mini-lathe was useful. I turned the cores, made the adjusting knobs, the terminal screws, and some threading and knurling. I tried to wind the coils on the lathe but that was a disaster – 150 rpm was too fast and there's no clutch.

The device is a telegraphic Morse sounder as featured in cowboy films. A dot is a fast double click, and a dash is a slow double click – it doesn't beep nice tones like you get off the radio.

The magnetic part is similar to Jason's motor. A real electrical engineer would have worked out the number of turns & wire gauge needed to pull the magnet against the spring at a particular current and voltage. I guessed!

Now I move on to thin ice because my maths is dreadful! Here goes:

The two sounder coils in series have 3.05mH of inductance and 7ohms resistance.

The magnet operates reliably at 2.5V and the coils draw 350mA.

So using John/Andrews formula, on each release my sounder would lose:

E = ½ x 0.00305 x 0.35^2 = 0.0002J

If my sounder magnets were switching at 20 pulses per second – equivalent to 1200rpm – it would waste 4mW a minute due to inductive loses.

This is small compared with the ohmic loses in the copper wire. With a 50% duty cycle,

E = ½ x 0.35^2 / 7 = 0.875W

As each click takes 0.875W, or 2.5V * 0.35A, 20 clicks per second at 50% duty cycle would burn 8.75W continuous.

The efficiency of my sounder is much lower than Jason's motor because the sounder does no useful work apart from making loud clicks. Jason's motor could be made to lift a weight, which would be an efficient use of the energy. However, as a machine my sounder is more effective because it actually does something useful with the energy.  Assuming that is you are a fully trained 19th century telegrapher…

Always happy to have my maths corrected!

Dave

 

Edited By SillyOldDuffer on 19/11/2018 18:04:17

[Sam StonesParticipant @samstones42903]

Phew!

Now we’re really cooking.

Although I had tongue in cheek when I asked …

does the back EMF work 'for' or 'against' the solenoid motor's performance?

… AND … (for authenticity) suggesting a couple of Ron’s jars (sorry again Ron, I meant Leyden’s Jars).

… I can't get my head around it.

So where, if at all, would Jason connect (extra) capacitance?

Great job Jason!!!

Sam

[Anonymous]

Posted by JasonB on 19/11/2018 16:54:00:

Just splashed some Satin black paint onto the "cast" parts so may have it ready for Andrew to have a play with next weekend.

Better remember to pack the Marigolds then………..

Andrew

[Author]

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