Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Tom Eccles]

Hey Sweeper,

Thanks for the address, there is some very interesting "gear" on there but I think it is rather larger than the variable speed g/box I have lurking somewhere in my tiny mind.

So far (like many a good designer) I know where I am (input shaft) and I know where I want to get to (output shaft) I just need a map to get me there in the most power efficient way.

I just have this idea that the motor would be most efficient running at rated revs. As you will have seen from the thread there are some terrific ideas out there, every reply has had me thinking again.

Great stuff!

Cheers
Clegg

[malcolmfrary]

Probably the reason that a set up using an electric motor will normally have just an electric controller comes down to cost and reliability. 
In many full size applications involving large deisels, rather than have a gearbox and clutch, there is a generator and electric motor.  Making a mechanical arrangement to allow the motor to just run at its peak efficiency has probably never really been feasible on a cost basis, bearing in mind that most electric motors (talking conventional permanent magnet, brushed) do have a wide spread of power/rpm, probably much wider than IC.

[sweeper]

Hi there Clegg,

I've been sitting here contemplating your "design" parameters. Using methods from way back in the grey cell area (i.e. as a young student), if you want to keep your motor running at the rated revs but be able to control the prop, how about the idea of a magnetic clutch?
By varying the excitation of the magnet you are able to allow slippage in the drive ,without doing any nasties to the clutch, as there are no clutch plates.
A simple version would be easy enough to construct with the connections to the magnet winding done through two slip rings.
I understand that some of the submarine guys use a similar idea on their models to avoid punching holes through the hull (but use it locked motor to prop).
Just a random thought on the subject.
Regards,

[bogstandard]

Right Clegg,
Now that we have got all the ideas out of the way, whether they be simple or complicated, all are going to cost either in time to make or commercial purchase costs more than the cost of a good speed controller.
If you are just out for a good efficient system that will allow you to run for about four to five hours from a reasonable sized battery, forget about the commercial motors that are available. People seem to think that these are the only ones available and like blind sheep they buy them from the local model shop thinking these are the only ones available and then complain when they get only an hours running from an 8AH battery with their tug or warship etc running round like a fast electric.
You should be looking at more efficient motors, and these aren't expensive, a lot of the time they can be free. I have used 850 size motors that can turn a 4" scale prop with direct drive and pull less than 500MA on full load, that will give me 6 hours running at full load from a 12volt 3AH gel cell. on a very large model. Almost all the motors I use are 24volt running on 12volt, and for years our club has been using motors like this, and they all sail for a full session, or a few sessions on one charge of a small gel cell, and no one has ever complained about not being able to sail at scale speed or above.
So look at your motor in the first instance, is it efficient.

John

[Tom Eccles]

Hi sweeper,
I think we have a touch of the "great minds thinking alike" syndrome.
I have been thinking how to transfer (controllable) power without a mechanical clutch and magnetics was something that wandered through my mind.
It seems that all we need now is an electronics genius and we may be heading somewhere!
We have a couple of submariners in our club, and an AGM next Monday. I will do a bit of brain picking.
Thinks........I wonder if Floating Wombat and/or FLJ are reading this........

Thank you very much for your input

Cheers
Clegg

[Tom Eccles]

Hi there bogstandard,

Thanks for your input.
I probably didn't make clear what I am trying to acheive here. I am NOT knocking an ESC (god knows I rely on them enough!) I am having a sort of brains trust about alternatives to the ESC controlling the motor as it implies in the title of the thread.

I am trying to think outside the accepted box and pick the brains of all who have been kind enough to contribute to the thread. You never know, something may come of this and, if nothing else, it takes minds away from the more accepted ways of doing things.


Thanks again for your input, EVERY view is welcome.

Cheers
Clegg

[sweeper]

Hi Clegg,
Would respectfully suggest that it isn't an electronics whizz you need, the solution to this problem is to do some slide rule bending to evaluate the field strength required, the number of poles and then manufacture two discs very accurately - one holding the field coils and the other holding the ferrous pieces. You would need to be able to produce these with some very close tollerences (air gap of a couple of mm max). The closer you can get the discs, the more efficiently the magnetic flux can be transmitted. Large airgaps are a big no-no in any magnetic system. I suppose that, for comparison of operation, the nearest thing I can think of is the operation of a synchronous motor (interaction between stator field and rotor).
I've just been rooting around through some old (and I mean old) textbooks. Yes, there is an illustration of such an animal but the application is for operation in an engaged/disengaged mode.(The book is circa 1937).
Looking at the practical considerations of the whole idea, I would think that the biggest problem would be the arrangement of weakening the field to obtain slip between the two plates of the clutch while maintaining sufficient field strength to continue drive.
An interesting little problem.
Regards,
                                                                                                                                                                                                                                                                                                 

[malcolmfrary]

The mechanical arrangements are fine for use with a mechanical power source, such as an I/C engine, where self starting from a stop is a problem, as is reversing. 
With electric motors, there is a vast speed range which is controlled very efficiently and effectively by modern PWM controllers.  When you consider how much operating time you get off one set of batteries, any alternative has to provide an improvement in running time at the same performance from the same battery, otherwise it's just an interesting oddity.  All the proposals so far use power to lose power whereas a PWM controller just applies as much power as is required to do the job. 
This one reason why railways tend to use a generator/electric motor arrangement instead of a mechanical arrangement of a clutch/gearbox/hydraulic torque converter etc when they use diesel engines.  Some designs may have a gearbox to ease starting/low speed control, but a boat has a built in torque converter/clutch in the for of a propeller in water.

[Bee]

The earlier pulley discussion focussed on the belt system. That system is also used on the Boxford variomatic lathe drive.  More common (for modellers) systems used friction wheels in 2 arrangments. These have been used to give wide speed control for IC engines (especially diesel)  before good throttleable glow engines were designed. A 'famous' one is the Model Engieer's 060 Loco desingned by Edgar Westbury in the '40s. Strangely it used a bronze cylinder casting instead of aluminium because there was a war on. I digress.

The above used a large diameter wheel on the engine shaft. A slotted shaft at right angles crossed the face of this wheel. A friction wheel keyed to this shaft can slide along the shaft and makes contact at a variable diameter of the disc. When passed over the center of the disc it reverses. Input and output can be reversed, but it is potentially inefficient.

Method 2 has a long cone on the motor shaft. The output shaft has a similar cone, reversed, so the two cones run parallel with a gap between them, narrow diameter of one next to large diametor of the other. A friction wheel is interposed to transfer drive from one to another and moved to vary the ratio.

Two more methods are seemingly more awkward but are the two actually marketed commercially nowadays in medioum power applications up to 200HP
The first is I think German. Take a shaft with an eccentric on it, with follower that produces an oscillating liniar motion. This motion is used to drive a pawl that engages a toothed wheel and drives it round one notch at a time. (you may know some winches work like this) The toothed wheel moves round slowly but in jerks. If levers are introduced that make the oscillation amplitude variable more than one tooth can be engaged making the output turn faster. Replace the pawl/wheel witha roller clutch giving an 'infinitely toothed wheel' and you get smooth rotation.

The second is deliberately lossy. Imagine a car differential but use one wheel as the input shaft, lock the normal input , and the result is that the second wheel rotates at the same speed as the input but in the opposite direction. If you allow the 'normal input' shaft to rotate controlled by a brake then the output shaft slows down and with the brake right off the output rotates in the same direction as the input ( I think, brain starting to hurt)

Both these could be made in meccano to experiment.

[Bee]

Just remembered another one, proposed rather unsuccessfully for bicycles.
The large chainwheel of the bike (the one the pedals are on) is replaces by a disc with several little toothed wheels, say 6, round the outside. The chain goes round the outside of these little wheels so is sort of going round a hexagon and must give an uneven drive. Anyway a mechanism allows the small gears to move in towards the center of the disc so the effective diameter the chain goes round reduces. You need a system like the bike derailier to take up the slack in the chain. Didn't take off.

[bogstandard]

Maybe you could try a fluid coupling, it is a type of slipping clutch as used on auto transmission on cars. It is a very simple system using two paddled rotors in a casing separated by a fluid.
There are fluids out now that change properties when a current is passed thru them, the more current, the thicker the liquid goes, the more power is transmitted as the fluid thickens. Infinitely variable just by varying the current.
A very basic system of this was used in old cassette decks, when you opened the lid it opened very slowly. It was just a paddled rotor running in a very thick liquid.

John