Model Electric Motors?

[Anonymous]

Posted by SillyOldDuffer on 19/11/2018 18:02:13:

Always happy to have my maths corrected!

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 19/11/2018 20:09:02:

Posted by SillyOldDuffer on 19/11/2018 18:02:13:

Always happy to have my maths corrected!

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.

…

Andrew

And not for the first time!

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

Dave

[Sam StonesParticipant @samstones42903]

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

[John HaineParticipant @johnhaine32865]

Posted by SillyOldDuffer on 19/11/2018 18:02:13:

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.

……..

Dave

 

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

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 20/11/2018 10:32:13:

Posted by SillyOldDuffer on 19/11/2018 18:02:13:

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.

……..

Dave

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

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

[Anonymous]

Posted by SillyOldDuffer on 20/11/2018 12:56:56:

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

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

[Ron LadenParticipant @ronladen17547]

Impressive Andrew, you obviously know your stuff, unfortunately I didnt understand any of it but I am still awake..

Ron

[Sam StonesParticipant @samstones42903]

And I now have considerable reluctance to ask where you'd stick a capacitor.

[JasonBModerator @jasonb]

Well I think I can say that Lecky is now finished, all seems to run very smoothly with a good run on when the power is turned off and will happily tick over at a nice slow speed or go like the clappers.

You will see at the end that I have brought this 110 year old design upto date.

[Ron LadenParticipant @ronladen17547]

That really is very nice indeed Jason, you have made a lovely job of it, I,m still amazed at how fast it runs. When its ticking over I could listen to it for ages, its a bit like an old clock ticking away in the background, something very soothing about it.

Ron

[Sam StonesParticipant @samstones42903]

Congratulations Jason.

The motor’s speeds are impressive, and so is yours in bringing the undertaking to completion so quickly.

How clever too, the reversing mechanism.

This will make a good first project for those not greatly into steam power.

Not yet, anyway.

Regards,

Sam

Back to winter temperatures and rainfall in Melbourne.

[Neil WyattModerator @neilwyatt]

If you spin it backwards, will it charge your phone?

N.

[JasonBModerator @jasonb]

Silly Boy

[Ron LadenParticipant @ronladen17547]

Have you started on the Bipolar motor Jason or saving it for the future.

[Joseph Noci 1Participant @josephnoci1]

Posted by Ron Laden on 03/12/2018 19:32:05:

Have you started on the Bipolar motor Jason or saving it for the future.

Is that the one that needs a Shrink before it can start?

[JasonBModerator @jasonb]

Not yet Ron, too many other engines nearer the top of the list

[I.M. OUTAHEREParticipant @i-m-outahere]

Any hints as to what they are ? By the time i type this you will probably have finished two of them 😁

[JasonBModerator @jasonb]

Look at my first post a d the linked video

[Carl Wilson 4Participant @carlwilson4]

Could you run it from a solar cell in a solar engine kind of arrangement?

[JasonBModerator @jasonb]

I would have thought so. I have got a solar trickle charger so will give it a try if the sun stays out.

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