Model Electric Motors?

A couple of Ron's jars perhaps?

Sam

Sorry Ron, my 'resistance' was low

Sam

!

Well I think it works pretty darn well!

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

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.

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.

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.

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

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.

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

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

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

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.

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

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

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

Andrew

SoD: Not only has the maths left the track it seems to have burst through the perimeter fence and ended up in the slurry pit.

I'd agree with your calculation of the energy (Joules) stored in the inductor. The time constant of the inductor (L/R) is small (about 436 microseconds) compared to the on time of a 20Hz square wave (25 milliseconds). So the current waveform will be, for all intents and purposes,a square wave, the amplitude of which is V/R, ie, 357mA. The power dissipated will be I²R. That is 0.357²x7 = 0.892W, divided by 2 to account for the 50% duty cycle, thus 0.446W. That equates to 0.446J per second. The energy per cycle is 1/20 of that, ie, 0.022J. So still two orders of magnitude bigger than the energy stored in the inductor.

You could save a load of energy by reducing the duty cycle.

Andrew

And not for the first time!

Many thanks for helping me out. At school my best subject was skiving...

Dave

As Andrew said ... You could save a load of energy by reducing the duty cycle.

If adding capacitance to 'mop up and or reuse the BEMF' has no value, I'm tempted to ask - "How about the ON/OFF angles before TDC?"

They, I presume, are inherent in adjusting the duty cycle?

Sam

Dave, how did you measure the coil inductance? Assuming it was using the "L" range of a multimeter or an LCR bridge, what it measures is the inductance with a large air gap since the coils aren't energised. The reluctance of a gapped core is dominated by the gap. But when the coils are energised the gap is much smaller and the inductance much larger and the core material plays a much larger part (reluctance is proportional to reciprocal of gap).  

Edited By John Haine on 20/11/2018 10:32:54

Yes, the method is flawed. I used one of these general purpose component testers to measure inductance:

Not sure how it works. I believe it puts an oscillator and a known capacitor across the unknown inductance, measures the resulting frequency, and calculates the inductance from that.

Closing the magnetic circuit raises the measured inductance by 0.4mH.

Understanding that the gap makes a difference causes me to wonder about the effect a DC current has on inductance. However it calculates the answer, the tester only puts tiny voltages on the coil and I they're probably AC. I guess the inductance of the coil could be very different when 0.35A flowing is flowing through it and the core is heavily magnetised. I shall try and cook up a way of measuring the inductance of an energised coil. Be good if I could get something right!

Next time I plug an electric motor in and it 'just works', I'll think of the decades of engineering and study behind its apparent simplicity. Although at one level I understand exactly how Jason's motor works, a closer look reveals massive ignorance! It's yet another distraction, but I'm tempted to build one and do some serious measuring.

Dave

Nothing wrong with that, it's a simple way of measuring inductance. It will certainly be using AC, otherwise the frequency will be zero!

Even quite small gaps cause the inductance to fall significantly. For items like pot cores that come with ready ground gaps for power applications the gaps are on the order of a few tens of thou at most. It's why contactors chatter when there are small specks of dirt between the poles; even a few thou can have an effect.

The effect of DC on an inductor opens a can of worms. The simple answer is that the inductance is irrelevant given a DC current. The SoD original tests show that, after some time the current settles down to a DC level determined purely by the applied voltage and resistance of the coil. Inductance only comes into play when the current is changing.

A mixture of DC and AC currents is rather more complicated. The effect on the inductance, and the AC current, will depend upon the shape of the B-H curve for the core material, as the DC current will bias the operating point. There are two other possible effects of a DC current. One, the DC current may magnetise the core material to some extent. It's how permanent magnets are made - expose them to a strong magnetic field. To demagnetise you expose the material to an alternating magnetic field, with no DC bias. Two, at higher DC currents a magnetic field is produced that can saturate the magnetic core, ie, an increase in current does not produce a proportional increase in magnetic flux density. Effectively the inductance falls rapidly, although not quite to zero.

Small loose wound air cored inductors are pretty close to an ideal inductor. Once you start introducing cores all bets are off, and the inductor becomes far from ideal. Of the passive components inductors are the most difficult to understand and design, and often require on the bench refinement - aka trial and error!

I bet that lot will put people to sleep more effectively than counting sheep!

Andrew