Help

[Jim Stevenson]

Guys I need a bit of help here.

A long time ago I got a set of plans for a Free Flight He 162 from " Mikes Flying Scale Models" webpage.

The plans are great and the bulk of the build is complete. BUT, the beast is designed for the "Rapier" series of rocket motors and there are continuous problems getting them. This has made me determined to produce a ducted fan version. I'm still going to complete the original and have ideas to make my own rocket motors.

However I am now at the stage where I have fabricated a jet pipe and compressor for the EDF and recently got a eflm30180mdfa motor which fits into the engine nacelle on the 162. So far so good. The snag is that I have no idea how to wire it to the battery pack and since this is to be a free flight model I don't even know if it will run without the ESC. Even discovering that it is a 7.4v motor meant looking up the model it is normally used in and seeing what replacement battery pack is sold for it.

SO! can anyone tell me how to wire this up, will it run without the ESC or will that harm the motor?

I'll drop a pic of the framework onto a post as soon as I have organised an album

Thanks in anticipation.

Jim

[John Privett]

Heck, just googled the motor, and it's a 11750kv motor - so it's going to be running at something like 70,000 or 80,000 rpm! But I guess for a small ducted fan that's what you need.

As for running it, you do need an ESC or something equivalent. This is a brushless motor, and with brushless motors the ESC has two distinct roles. Most obviously it controls the speed of the motor, but less obviously it controls the switching of the battery power between the three motor windings via the three leads. The latter is the job on an old-fashioned brushed motor that is done by the brushes and the commutator.

One of the members of my club is a free-flight flyer and specialises in small ducted-fan models. His latest is electric and employs a small controller that does the job an ESC would normally do - so they are available. This connects - as you'd expect - between the battery and the motor, and has, if I remember correctly, 3 adjustable pre-set pots. These govern the delay in startup (allowing you time to close the hatch after connecting the battery), also the motor duration and possibly speed, or maybe how gradually the motor speed reduces at the end of the run. I don't remember all the details. I also don't know where he got the controller from, I can ask if you want?

[Tim Mackey]

Or, use a regular ESC and couple it to a cheapo "servo tester" - have the control knob protruding from the model and turn it up to desired power before chucking it off?

[John Privett]

Tim, for a free-flight model he'd only want the motor to run for a few seconds - I'd guess in the order of 10 or 20 seconds max. Otherwise he'd need a huge area to fly it in and/or a very long walk to retrieve it afterwards. And he'd also want something to switch the motor off, rather than waiting until the battery runs down and damages itself in the process.

I think "lightweight" is also the order of the day. The models I've seen powered by those Rapier rocket motors (like a Jetex on steroids!) probably weigh less ready-to-fly than the cheapo servo tester pictured.

[Tim Mackey]

You're right John - sorry - didn't think that through too well did I

Edited By Tim Mackey on 17/10/2014 21:36:26

[John A H]

I think Derek Knight has been working on producing a small EDF as a direct replacement for Jetex/rapier motors. Not sure if it is on sale yet, SAMSMODELS will know details of when it will be available. He does an ESC that has a timer on it where you can set the power, startup delay and run time and I think it's set so the power decays rather than shuts down abruptly to help in the transition to the glide. I think they normally run on a small 2s 350mah Lipo.

You can e-mail or ring SAMSMODELS and they are normally happy to discuss with you.

[Steve Hargreaves - Moderator]

I'm guessing small & light will be basic requirements too.....

I'm sure there was something about these sorts of times in one of Nigel Hawes' Fly Electric columns recently.....a small pcb mounted unit that you could set for power & flight duration via small pots. Maybe a PM or email to Nigel might elicit some info....

I also found this which might be of use....

[Phil Green]

Posted by Steve Hargreaves - Moderator on 18/10/2014 12:30:28:

I also found this which might be of use....

unforch thats for brushed motors Steve

[Steve Hargreaves - Moderator]

Found this too....

[Steve Hargreaves - Moderator]

Hopefully heading in the right direction though Phil.....

[Phil Green]

Your biggest electric free-flight problem is already solved - that is, stalling the motor on landing - no problem with an EDF!

[Jim Stevenson]

Great feed back guys!

But, What I had intended was to fly this with some Spanish (El Cheapo) cells and accept that they will be defunct at the end of every flight. That way I get gradual run down of power for reasonable landing and the possibility of indoor flight. There is no R/C unit to be fitted so I don't need to split current etc, as the motor will be the only powered unit in the aircraft.

I have no idea if the concept will work, or if the cells will actually run the motor, or even if they will damage it! However at the power rating of the motor it will soon suck them dry. So I will hopefully not need a GPS tracker

Therefore I do not need a speed controller or cut out (weight saving) or a LiPo battery weighing three or four times more than the rest of the outfit.

The wingspan is only 11.5 inches so space is a major problem but, Hey I'm doing this as a challenge. Idid notice the comment that the ESC provides switching as provided by the brushes in a "normal" motor. How certain are you of this as if its a linear motor there should not be any switching simply two emf fields?

Soon as I get a handle on this board I'll drop a couple of pictures on but up to press I have not loaded an album up. It even took me three days to find the replies so apologies for the delay.

Regards

Jim

[Phil Green]

Posted by Jim Stevenson on 18/10/2014 19:44:41:

Great feed back guys!

But, What I had intended was to fly this with some Spanish (El Cheapo) cells and accept that they will be defunct at the end of every flight. That way I get gradual run down of power for reasonable landing and the possibility of indoor flight.

Have you thought about using a supercap instead of a battery Jim?  for a short motor run?
No problem discharging to zero volts, gradual reduction in power, instant charge... sounds ideal?

Value? well C=FV so say 10 Farads at 5 volts is 50 Coulombs, maybe 3 amps for 10 seconds or so?

Cheers
Phil

Edited By Phil Green on 18/10/2014 20:15:26

[Dave Hopkin]

Hi

A brushless motor works the same way as a brushed motor except that the stator holds the windings NOT the Rotor, the Rotor has an array of fixed permanent magnets - in a brushed motor power is "switched to each winding as the rotor rotates via the commutator - in a brushless motor there is no commutator, instead power is supplied to each stationary winding in turn in a three phase cycle, the ESC does the conversion from a steady DC supply to a switched 3 phase DC supply to each set of windings in turn - hence creating a constantly changing magnetic field in the stator that causes the permanent magnets to be repelled and hence rotate the motor

If you power a brushless motor with a direct un-switched supply you will simply burn out one winding

The clue is the wires from the motor a brushed motor has two connections, a brushless has three

[John Privett]

Posted by Jim Stevenson on 18/10/2014 19:44:41:

Idid notice the comment that the ESC provides switching as provided by the brushes in a "normal" motor. How certain are you of this as if its a linear motor there should not be any switching simply two emf fields?

100%, Jim. Look at it simplistically. The battery has two wires, the motor has three. You need a <<something>> to connect your two-wired battery to the three-wired motor. And that <<something>> is usually an ESC.

Also I don't see how this could be a linear motor. A linear motor is not like this at all, and is used in things like maglev trains for providing propulsion in a straight line, not for rotating a motor shaft.

[Peter Beeney]

Dave,

With the greatest respect of course, but just to be ever so slightly pedantic here, the ESC is an inverter, this means it changes DC to AC. Thus the current into the speed controller is DC, the current out to the motor is AC. Also any two or more motor windings are conducting the same current at the same time.

It’s an easy little mistake to make though, it’s been done a few times, and in the event it’s only really a tiny technical detail, the motor still works ok.

I don’t intend to make any criticism at all, rather only to just make it slightly easier to understand how the power train works…

PB

[John Olsen 1]

I wonder whether the electronics from the e flite Mig 15 would be suitable? That whole plane weighs under 3 ounces and uses a 2S lipo, 7.4 Volts. Of course it also includes the radio receiver and a three axis stabiliser unit. All the bits used are available separately, at a price of course. But then, when I look at the flying fields these days, having radio control of the model seems like a good idea. Few places have the kind of space for free flight as we used to know it.

John

[Jim Stevenson]

I seem to have opened a can of worms on this one. But let's start replying to your comments.

Phil, I had not thought of a supercap and it sounds like a great idea. It will also be easier to mount inside the airframe. Thanks

Dave, The implications from your post is that the brushless motor must be run through a separate set of "electronic" brushes and that the three wires are only to supply power to individual windings to create the continuous movement of the rotor. Please tell me if I am reading your comments correctly. Thank you.

John P, You have got it spot on. All I need is a power supply which is ungoverned and as far as I was aware the "extra" wire was to allow the power to the motor to be varied. I realize that I lack the specific knowledge about these motors so I have asked for help and the forum members have responded very well indeed. Your query about linear motors is best answered like this: Take your straight centre conductor and curve it into a circle. Feed it with power and put your vehicle with the other windings over it and it runs in a circle. Move the vehicle to the inside of the circle at 90Deg to the circle and you have your rotor and your stator. Does that make sense?

Peter, I was under the impression that ALL electric motors were DC. Traction voltage (trains, trams, etc) is in the I've range and so need regular feeder stations which could be done away with if AC was used with the relevant saving in costs.

John O, You are absolutely right. I just hate to buy something then destroy a fully functioning model to get the bits I want. As far as space to fly goes for free flight I can achieve that inside a sports hall have a look at the videos on this page **LINK** It's not what I intend but it shows you the possibilities.

Once again thank you guys. Armed with these suggestions I can now ask some more questions of others who may be able to help, but, please keep responding so that I can keep learning.

Regards Jim

[Jim Stevenson]

Since the previous post was a bit of a marathon I thought I should show you where I am at the moment with the build.

Regards

Jim

[Dave Hopkin]

@ Peter - Yep sorry you are right I said "3 phase DC" as bit of a contradiction in terms there!!

@ Jim - In a sense you are correct, the ESC does the work the Commutator would do in a brushed motor, in that it energises the windings in sequence to create rotation

And for Info there are both AC and DC electric motors (of numerous types) - AC Motors are using in many domestic appliances use them - Washing Machines for example

[Jim Stevenson]

Hi Dave

Was doing my research when your post landed. I was confusing the methods of driving the rotor. Looks like you can get circular track DC or AC linear motors but "Brushless" is a different animal, So it seems I need a ESC. It looks like the reason for this is that in larger scale motors the ESC is built into the motor, a bit like the inverter in a car alternator, but when you come down to model aircraft sizes it is larger than the motor and needs to be separate.

That clears up the requirements for the motor and the idea of a supercapacitor solves the weight problem so this looks doable. All I need to do now is work out what bits to order then get impatient waiting for them to arrive. I'll also need to build another 162 as the one pictured will not convert to EDF. Still, building them is a major part of the hobby for me.

Regards and Thanks

Jim

[John Privett]

Posted by Jim Stevenson on 19/10/2014 11:56:09:

Dave, The implications from your post is that the brushless motor must be run through a separate set of "electronic" brushes and that the three wires are only to supply power to individual windings to create the continuous movement of the rotor. Please tell me if I am reading your comments correctly. Thank you.

John P, You have got it spot on. All I need is a power supply which is ungoverned and as far as I was aware the "extra" wire was to allow the power to the motor to be varied.

Jim, I think you've now "got it", but just to confirm...

You've read Dave's comments correctly. The three motor wires connect to each of the three windings, although there are subtly different ways that can be done in the motor.

So, as you now realise, the "extra" wire is not there to govern the speed. All three wires must be used, and the power is switched between them to make the motor turn - the switching being one of the jobs of the ESC. Indeed, often the three wires don't have any colour-coding, and even when they do they have no particular significance. You connect the three wires on the motor to the three wires on the ESC - in any combination. If the motor then runs the opposite way to what you want you just swap over any two of the wires.

[Dave Hopkin]

If you want to avoid using an ESC totally and go with the Supercap idea then you will certainly need a brushed motor of which there are thousands to choose from.... their typically high RPM would be ok for a ducted fan too I guess

[Peter Beeney]

Jim,

Just to briefly lob some more mud in the water, an inverter converts DC to AC, but a rectifier converts AC to DC. Therefore the car alternator has an inbuilt rectifier, which is actually only a diode bridge, this simply ‘steers’ both halves of the alternating waveform produced by the rotational movement to go in the same direction. The terms DC and AC have always been confusing when relating to motors, but the innards of any motor, the bits creating the magnetic field, are always AC. Generally speaking, it’s more difficult to convert DC to AC than it is to convert AC to DC.

As it happens, I’ve always felt that the term Electronic Speed Controller is slightly something of a misnomer anyway, Electronic Commutator might be more suitable. The motor doesn’t necessarily need the speed control angle, but it certainly cannot dispense with the commutator bit. This is an essential part of the motor, they both operate in a closed loop system, the controller continually providing the switching sequence and the motor continually backwards supplying the timing points to the controller for this sequence. The speed is separately controlled by the applied voltage, although the same components are used to do this.

I don’t follow this stuff much nowadays, but I have noticed recently that a ‘Super’ SuperCap is being developed, it’s being credited with some real capacity. However, it seems that this is closely related to electric cars, they will operate in conjunction with the batteries, so it seem unlikely they will not be of much use for modellers, I imagine they will physically be a bit big as well. But, as with all things, I’m sure they will soon be made in smaller sizes, or maybe they already are…

Hope this is of some interest.

PB

[Jim Stevenson]

John, I knew I had only a sketchy idea of how the motors worked so I asked for help and clarification and got it. Now I need to work out how to use the info to fit the components into the airframe. Since all of the components are tiny it will not be too difficult.

Dave, as you can see I'm not against an ESC. I simply could not see the point in one. Now, with a better understanding of the components and their functions I am happy to work from there. Do you think the Supercap will function as a power supply in place of a battery?

Once more Thanks guys

Jim