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HomeAircraftHelicopterCAD - Engineering - Technical › Dynamo tail drive?
01-12-2009 10:26 AM  9 years agoPost 1
astro-g

rrApprentice

Wellington, New Zealand

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Has anyone tried using two motors, one as a dynamo attached to the tail-drive output, and electrically linked to the other in the tail, in place of a flexi-drive system for a scale model?

Beating the air into submission

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01-12-2009 02:16 PM  9 years agoPost 2
papatango

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Toronto, Ontario, Canada

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You just missed the huge debate about the tail-motor-on-boom.....

You're better off putting a battery in the fuse, with a separate esc running to a small outrunner brushless hidden in the tail, or directly on the tail.

For 60 3d birds, a 4s 650-750kv motor set up at 7-9000 rpms was holding like a champ in hard 3d. On .90' birds, 4-6s on on 120 tailblades at 8000 rpm seems to be fine.

Search the forum for "3DD", Taildrive, Kora 25, hacker 30-12xl.

You'll find loads of info on this topic, and even more in the AP & Minicopter sections.

Cheers,

Patrick

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03-11-2009 03:25 AM  9 years agoPost 3
DKNguyen

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Not to mention it's also really inefficient...you might as well power the tail drive motor straight from the main battery.

Converting power from the battery/fuel to rotation with the main motor/engine has some losses, then using another motor to convert it back into electricity has a HUGE amount of losses, and then sending it to the tail motor...nothing is 100% efficient and you lose power at every stage.

It's a little less true for fuel helicopters since you don't have a main battery to draw power from directly for the tail motor, but you can draw the rotation energy directly with a belt or TT.

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03-13-2009 12:41 AM  9 years agoPost 4
astro-g

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Wellington, New Zealand

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I did say 'in place of a flexi-drive system'

I wouldn't even contemplate this if shaft drive was practical.

But a good electric motor should be ~90% efficient, either as a motor, or as a dynamo.
With adequate wiring I see no reason for it to be worse than the mechanical losses of a complicated gear/belt system, or a flexishaft.

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03-13-2009 08:58 AM  9 years agoPost 5
DKNguyen

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So ideally you'd need to get two identital motors: one for the tail and one for the generator so that the tail motor loads the generator motor down properly for maximum efficiency from both.

But to get that efficiency you are going to need to find a way to spin that generator motor at it's optimal speed and keep it there. You'd have to find a motor that is simultaneously matched to the tail rotor operation but also able to generate the same amount of power when turned at engine RPMs (after gear reduction or whatever you plan).

I'd say the efficiency lies more around 80% than 90%. 90% would have to be a good, large, well built brushless motor under the right conditions and overly optimistic in my opinion (personally I consider 80% optimistic too but I'm conservative and like to assume 70% efficiency). Between the two motors the efficiency would be more around 64% which is pretty low. don't know how efficiency drive shaft system are though, but at least for electrical systems that's awful.

Along with the 3-phase rectifier and smoothing capacitors to turn the generator output into DC-ish output, you might need to take additional care that the ESC and tail motor does not get fuddled up due to beat frequencies and noise.

WHat makes the shaft drive impractical? And, as you probably know, there would be no tail rotor in autorotation.

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03-14-2009 07:37 AM  9 years agoPost 6
astro-g

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Wellington, New Zealand

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Why even involve an esc in this?

I was contemplating getting 2 3phase motors, possibly identical, or maybe 1 of slightly higher KV than the other, and linking them directly, phase to phase.

When the nominated generator is spun at any significant speed, the nominated motor should spin in exact synchronisity.

This train of thought was triggered by a build-log of a very large scale model, I think it was a blackhawk, where the builder had considerable trouble the routing the drive to the tail, attempting to use a flexi-shaft (too-wide a bend radius) a shaft-drive, with custom gear box (to big).
and two different belt-drive systems, before settling for a direct-drive electric motor, powered by a battery in the main fuse.

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03-14-2009 07:57 AM  9 years agoPost 7
DKNguyen

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That's a good idea if the motor stays in synch, but that's a big assumption. An ESC waits for the motor to pass the proper position before commutating. In this case, the generator motor will blindly commutate whether or not the tail rotor has reached the proper position.

The energy produced from the winding of the generator will magnetize the same winding in the tail. This will cause the tail to accelerate towards the next pole. THe more energy the generator supplies, the more force, acceleration, and speed it has. But it won't be the same as the generator because of mismatching between the motors, and the fact that the tail motor will not convert all of of the energy from the generator into motion.

If the generator is too weak, the generator will commutate after the tail motor does. If the generator is too strong, the tail motor will commutate first. You could size the generator to produce exactly the right amount of energy to make up for the efficiency losses in the tail motor so they would move at the same rate, but mismatches will still exist and you'd be hard press to find two motors that are exactly the same, let alone two motors of different sizes that are exactly the same in every way, except one is capable of higher torque by exactly the right amount.

It's like running two brushless motors off of one ESC...don't count it.

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03-17-2009 01:55 AM  9 years agoPost 8
astro-g

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Wellington, New Zealand

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you know the resistance of a motor changes, depending on what its doing right?

Imagine a DC motor, spinning unloaded, with 5v supply, and its drawing, say, 50mA

Now If I start applying braking force, that motor is doing more and more work, the voltage stays the same, but the power to do that work must come from somewhere, you can see the current draw increase. obviously the effective resistance has gone down.

The same thing applies to 3phase motors.

The generator is going to put out x volts, at y Hz regardless.
the motor is going to draw as much power as it needs to keep itself in synch with that signal. if there is no load, thats going to be quite small, if there is a 120mm blade pulling 15 degrees of pitch that could be quite a lot.

Beating the air into submission

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03-17-2009 08:29 AM  9 years agoPost 9
DKNguyen

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Yes, I know how the BEMF causes the resistance measured at the terminals of a running motor to appear higher than the actual internal resistance, and what you say is not true. THe motor does not draw as much as power as it needs to stay in synch. It draws as much current as it needs to develop the voltage drop across the internal resistance to balance out the BEMF at the current RPM with the supply voltage. The current in the winding makes a magnetic field and *IF* the winding happens to be near a magnet, then torque will also be generated. If the winding is not near magnet like in miscommutation, you basically get a stall-like condition.

And this leads us to this core of a direct generator-motor setup won't work:
A motor powered by a fixed voltage source will slow down when torque is applied. Yes, it will draw more current to produce that torque, but it will slow down. No amount of current draw is going to speed up the motor. Motor speed depends on voltage and torque (but not current even though the two are directly proportional), and motor torque depends only on current.

Your generator is going to be spinning at a pretty constant speed isn't it? Being driven by the bottomless pit of energy that is the engine? THe generator is not going to slow down with the tail rotor. It's going to be spinning at the same RPM, commutating at the same speed, outputting the same voltage. When the tail motor becomes loaded and slows down because it's input voltage is not increasing to compensate, it's going to require a lower commutation rate but the generator is going to be commutating at the same speed as it was before. Miscommutation is inevitable.

Yes, a higher generator kV than the motor kV would compensate for this...but only at a single motor torque value. As soon as the motor torque changes, guess what? THe motor speed changes because the input voltage has not changed, but the generator commutation rate stayed the same.

Blind commutation is used, but only for sensorless startup. And when it is used, the torque conditions across the entire range of operation where blind commutation is used must be known (for example, a fan just starting up in static air requires almost zero torque and is easily predictable).

Or try thinking about it this way:
If your reasoning was true, there would be no sensored brushless ESCs. In fact, sensorless brushless ESCs wouldn't even need the "sensorless" BEMF detection at all to commutate the motor- they could just commutate the motor blindly. The motor would always smoothly follows the commutations of the ESC as long as the ESC output enough current (which also completely ignores the effect that voltage has).

Test it out if you wish, but you are going to see jerky rotation and overheating (from miscommutation sending current being the wrong windings - windings not moving near a magnet and generating no BEMF thus producing stall level currents). You need an ESC (and all associated 3-phase rectifier stuff)if you are going to get this to work properly.

I will admit that my initial suspicions about a direct generator-motor connection were more intuitive than cold-hard fact, but after your challenged me to think about it some more, the setup's shortcomings have become much more obvious to me, and I hope to you too.

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