Lynch motor how it works

[simplystupid]

So how does this configuration achieve the translation from low number of turns into low rpm?

I wonder if someone really knowledgeable on the physics of motors can explain why this configuration works in the 3000rpm range.
Having dismantled motors from the age of 6 and built and rewound many since then,
my specific question is about this;

There is only one effective 'turn' between the relevant brushes supplying current.
There is also a lot of surplus copper, carrying current but not in the magnetic flux. In order to take the top/bottom paths of the rotor geometry.
More than actually in the flux. This would normally mitigate against high efficiency.

So in spite of a 1.5 Tesla or greater field, the back emf at 3000rpm is not going to be very high. Neither, it seems conventionally, would the torque.

One turn on a typical PM BLDC outrunner 'wye' or 2 pole BrushedDC, would need to operate at perhaps 200K or even more rpm!

So how does this configuration achieve the translation from low number of turns into low rpm?

Eight magnets and brushes is not the answer, the connected windings at any one rotor angle are all running in parallel. PM car starter motors now sometimes have 6 poles.

 

[major]

I believe you are mistaken about the armature winding configuration. It is a wave winding which puts all the coils (turns) in a series connected loop and then the brushes make it into a 2 path circuit. So instead of having one turn it has one half of the total turns in series between positive and negative.

Below is a diagram for a four pole wave winding. Sorry but I lost the source. I found it on google searching for wave winding armature. An eight pole winding would be similar but have 8 brushes. Interesting about this type of winding is that it can run on just 2 brushes, one positive and one negative, adjacent (one pole pitch apart).

 

[simplystupid]

thank you Major,
the Lynch motor works just fine and it must be I am mistaken about something going on.
I recognise your diagram as a starter motor example.
All motors with brushes that I have ever seen have the winding as a loop!?
I have done PM DC 'wave' re-windings, it is simply skipping over slots in the rotor. They are no different in the effect of the number of turns. The required torque ripple and other considerations dictate how a rotor is wound.
There are also windscreen wiper motors that have extra brushes that can be switched in at a different angle, giving hopeless efficiency but an alternative speed.
As you say, just running a four pole motor on two brushes at 90 degrees.
The other connected windings may be in series, but they are not at the right place at the right timing in the flux lines to do anything useful, for back-emf or torque. If you leave a winding 'open', the current switching causes such an abrupt spike as the field collapses that sparking occurs excessively and burns out the commutator very quickly. The motor still runs though, hence the curious look people take into the back of a drill while a dancing annular green flame consumes the segments.
I will have a close look at the original patent, if I can find the reference. So far I've only found the patents for additional water-cooled modifications and such like.
If you run a PM starter motor (with only a copper 'bus bar' for each turn) with no load, it will race away and eventually self-destruct. PM motors are inherently
self regulated, but only in the sense that they generate a back emf that eventually limits any further increase in speed with the same applied voltage.
Series wound field automotive starters usually have such diabolical bushes for bearings, the friction limits the top speed. Still vibrate and scream like hell though.

 

[simplystupid]

my missing punctuation -
'I recognise your diagram as a starter motor example.' Should be !!, as very unlikely. I saw this diagram recently in a round-up of motor types.
I can't get a clear picture of how the winding is connecting to the commutator from the image you posted.
A four pole motor would normally have two positive brushes opposite each other, then negative the same at 90 degree difference. As would the magnets be mounted, 2 North and South poles facing each other.

I have some pancake servo motors which are very similar to the Lynch in having a flat disc rotor with overlapping coils and a ring of magnetics sandwiching it.
They are low-inertia servo motors with only resin holding the coils. Likewise a wave winding with odd number of slots on commutator, but still following general rules of thumb about number of turns, magnet gap/strength for Kv etc.

 

[major]

my missing punctuation -
'I recognise your diagram as a starter motor example.' Should be !!, as very unlikely.

I don't understand. Starter motors, at least the series wound field type, are typically 4 pole lap wound due to the low voltage and have 4 path armatures. I have not personally examined the Lynch motor hands on so-to-speak, but have examined and studied other similar axial motors which are 8 pole wave wound constructed.

A four pole motor would normally have two positive brushes opposite each other, then negative the same at 90 degree difference. As would the magnets be mounted, 2 North and South poles facing each other.

The diagram in my previous post is of a 4 pole motor having 21 slots in the armature core, 21 single turn coils and 21 segments on the commutator. It shows the brushes at 90º spacing, one brush per pole, the negative brushes in line with the N poles and the positive brushes in line with the S poles.

 

[simplystupid]

Thanks again Major,
I'm going to make a smaller Lynch motor. Best way to find out is 'hands-on'. I have just the right parts around the bench and it will be more suited to an e-bike conversion - 100mm diameter/1HP output max.

This is a good link I think to 'wave winding'.http://www.vias.org/kimberlyee/ee_12_08.html

I've not studied a winding diagram for what I've been doing previously, but been aware of the odd number of commutator segments versus poles etc.
It maybe that the original patent is a bit skimpy on some of the details, but you are correct, it does indeed say 'wave wound'.

The Wiki entry has some things confused and the current path diagram may not be totally accurate, for example the rotor is most definitely not held together by magnetic forces.
It is impregnated with high-temperature resin, obvious from the post-mortem burnt out motor images on the internet.
And the fact the R version from Agni has a Kevlar band and cord fitted around the periphery of the rotor to hold it all together at high revs.

I thought of taking the geometry and making a 3 phase BLDC, with spinning magnet discs. But the weight would be much higher and it needs a sensored driver. Perforated magnet discs can be part of the structure in the original brushed concept. It is not unlike Hugh Piggotts' (of Scoraig) windmill disc generator designs of the early 1980's which I built.
It is a very neat and clever concept.

 

[simplystupid]

So have I finally understood how the Lynch motor works?

So the Lynch motor has 65 windings 'wave wound', with eight magnets and brushes. So eight 'poles'.
There are two (parallel?) series paths between a pair of brushes, so the result is 32.5 turns in each of those paths.
It doesn't matter that there are more brushes.
They are all timed to connect the same winding paths because they match the number of magnets, or poles.
The motor would run on one pair of brushes only, set at that same pole angle apart (360/8).
The purpose of the extra brushes is to provide greater current carrying capacity into the winding path, also making it less likely that contact would ever be lost between commutator and brushes.

Force = Current x Length x B-Field

I haven't got a Lynch motor to measure, so I'll guess from the photos dismantled.
The actual path of the current in the flux is about 40mm. It traverses this flux twice in the return to the outside of the rotor. In the wave winding that is 130 times. Don't know how to deal with the 'two paths'. Say 65 traverses then.
The torque is generated somewhere in the middle of this region, say at radius 65mm.
No idea what the transformer steel shims are doing to the flux, but say the neo magnets have 1.5T at their faces.

350N = 100A x 0.039 metre x 65 x 1.5T

At 1 metre radius, that would be about 22 Nm.

So is this the stall torque? Using whatever low voltage gave 100A.
Published torque constant depends on model/manufacturer/voltage etc..but all somewhere in the region of 0.07 to .25 Nm/A.
So not miles away from the rough approximate calculation.
How do the different voltage models deal with the construction using a fixed, or a very limited variation in, number of strips?

Is the published internal resistance of 30 milliOhm, the effective resistance with all eight brushes in contact?
The resistance of the complete loop is 4 times higher than this? 0.12 Ohm?
130 'sections' of possibly 150mm each tuning fork leg, is quite a lot of copper strip. About 20 metres, but it would only have a cross-section area of about 3.25 mm sq. for a resistance of 0.12 Ohm.
That's 14 gauge about. Which is only rated up to 20A or so.

 

[major]

Force = Current x Length x B-Field

I like to use Torque = Current * Flux, all vector quantities, so it is the cross product of Current and Flux.

Don't know how to deal with the 'two paths'. Say 65 traverses then.

Having a 2 path armature means that each coil sees 1/2 of the armature current.

The torque is generated somewhere in the middle of this region, say at radius 65mm.

Using my equation the radius falls out. You do need the dimensions to calculate the effective area of flux crossing the air gap.

No idea what the transformer steel shims are doing to the flux, but say the neo magnets have 1.5T at their faces.

The steel in the armature reduces the reluctance in the magnetic circuit so effectively increases flux over the case without the steel. But I do think the 1.5T is too high of a figure to use.

350N = 100A x 0.039 metre x 65 x 1.5T

At 1 metre radius, that would be about 22 Nm.

The torque is the same at 1 meter, 2 meters, etc. What you solved for was the force at a radius of 65mm and that is produced by a torque of 22Nm.

So is this the stall torque? Using whatever low voltage gave 100A.

Yes, but it is the torque at 100A at speed also (neglecting rotational loss).

How do the different voltage models deal with the construction using a fixed, or a very limited variation in, number of strips?

They seem to have only 4 models (with differing Kv). So they either use different bar counts, different diameters, different flux (magnets or magnetizing level), etc. Something has to change.

Is the published internal resistance of 30 milliOhm, the effective resistance with all eight brushes in contact?
The resistance of the complete loop is 4 times higher than this? 0.12 Ohm?
130 'sections' of possibly 150mm each tuning fork leg, is quite a lot of copper strip. About 20 metres, but it would only have a cross-section area of about 3.25 mm sq. for a resistance of 0.12 Ohm.
That's 14 gauge about. Which is only rated up to 20A or so.

Don't stick in that factor of 4. The armature resistance is independent of the brushes. Probe on 2 commutator segments one pole pitch apart and you have it. 32.5 coils in series and 2 of those series strings in parallel for calculation.

 

[simplystupid]

Excellent Mr Major, that was very helpful indeed!

I took one of my rewound 'wave wind' motors and added another two brushes.
No change in Kv or internal resistance. Ah, now I understand.
Difficult to say how the performance might have altered.
The timing was a bit off and I had to rotate the brush holder section a little(ie the motor has an aluminium 'cap piece' joined to the centre steel magnet cyclinder).
What should I have gained by adding the extra brushes?
The brushes all run cooler, because each is carrying only half the current?

I feel more confident that I can have a reasonable go at making my own 100mm diameter Lynch type motor.

 

[major]

Yep, the additional brushes help support the armature current. In the conventional drum shaped motors that allows a smaller commutator and shorter length, and likely lower cost. I'm glad I was able to help and good luck with your motor project.

major