What gauge wire

[Shaun Walsh]

I intend to add an electric power pod to a glider using this motor

https://hobbyking.com/en_us/d2826-10-1400kv-brushless-motor.html

The question is, what gauge wire do I need to connect it to the ESC?

Thanks

[Engine Doctor]

I dont know the theoretical way to work out what wire is needed but would simply use the same gauge wire that is fitted to your ESC.. Always worked for me .

[Shaun Walsh]

That was my initial thought, I am using an ESC rated to 40A burst current so presumably the wiring is sized to for that, just wondering if I could get away with something a bit thinner as I have to drill a hole for the wiring to pass through in a moulded nylon fuselage and wanted to keep it as small as possible. I guess it' best to play safe and use either the same gauge as fitted to the ESC or the motor whichever is thinner

[Nigel R]

Playing it safe is usually better than electrical fire!

I would go for 12AWG.

For really short lengths, 14AWG is likely to be perfectly ok.

The connectors will be larger than the wire size anyway, so your hole size is kind of set by those.

Incidentally -

That is a 200W motor - a 40A ESC is complete overkill.

Edited By Nigel R on 07/09/2018 10:55:18

[Shaun Walsh]

Will go for 12 AWG then.

The 40A is the short burst rating, its rated for 30A continuous also cooling is not fantastic so didn't want it to get too hot.

[Dave Hess]

12g is the theoretical correct size for a long length and if you want to run at 40A continuous, but for a short length, like is typically used in a model aeroplane 14g will easily be enough and it's a lot easier to solder. It's also lighter. I think 12g is overkill for what you've described.

Edited By Dave Hess on 07/09/2018 11:49:19

[PatMc]

Posted by Shaun Walsh on 07/09/2018 11:13:43:

Will go for 12 AWG then.

The 40A is the short burst rating, its rated for 30A continuous also cooling is not fantastic so didn't want it to get too hot.

Shaun, I think the realistic max continuous current for this motor is more like 15A - 18A. The product description rates it at 205W, which presumably will be when using 3s.
IMO a 25A ESC would be adequate & any extension wires need only match those already on the ESC.

A few questions - what model do you have ?
What's the size prop does the clearance allow ?
Do you intend only flying it with the power pod or will you be flying it off a tow line/bungee or slope soaring as ?
If only flying with the power pod will it be on continuous/most of the time or to gain altitude then glide only ?
What size battery will you be using - capacity & cell count ?

[Shaun Walsh]

Posted by PatMc on 07/09/2018 21:19:50:

Posted by Shaun Walsh on 07/09/2018 11:13:43:

Will go for 12 AWG then.

The 40A is the short burst rating, its rated for 30A continuous also cooling is not fantastic so didn't want it to get too hot.

Shaun, I think the realistic max continuous current for this motor is more like 15A - 18A. The product description rates it at 205W, which presumably will be when using 3s.
IMO a 25A ESC would be adequate & any extension wires need only match those already on the ESC.

A few questions - what model do you have ?
What's the size prop does the clearance allow ?
Do you intend only flying it with the power pod or will you be flying it off a tow line/bungee or slope soaring as ?
If only flying with the power pod will it be on continuous/most of the time or to gain altitude then glide only ?
What size battery will you be using - capacity & cell count ?

Hi Pat

In reply to your questions;

The model is a Graupner Cumulus 2800 from the 1970s, I recently managed to get hold of a genuine unused Graupner motor pod kit from eBay.

The motor specified with the kit is an OS10 which puts out about 200W and uses a 7x4 prop. On YouTube there is a video of an electric converted Cumulus, the person who posted it kindly provided some information on motor etc.

https://www.youtube.com/watch?v=fFnHcy4eRPA

Regarding prop clearance, there should definitely be clearance for a 7" prop and if set up as per the video in pusher configuration with the motor above the COG there will probably be clearance for an 8" prop.

I have 2 motors I could use, one is the 1400Kv in the link posted above which would be used if there is only clearance for a 7" prop, the other is a 1000Kv motor which i could use if there is room for an 8" prop both are nominally 200W.

The power pod is removable , just unplug the wings disconnect the motor and it should come right off if i want to slope or bungee launch (I originally flew it off a bungee or a winch in the 1970s).

The battery is a Turnigy Graphene 3S 2200, it fits in the nose after a bit of minor surgery and the COG is spot on with it in, all up weight is 1300g without motor and pod, as per the original design weight makes you realise how much lighter modern rc gear is.

[Shaun Walsh]

Oh, and the motor will be used to climb then glide and the capability to go around again when I inevitably mess up a landing approach

[Focae]

The output wiring from the ESC to the motor does not have to be the same wire size as the input wiring from the battery. The input wires have the continuous current.....the output wires have a switched load so the current is less. I have never had any problems using anything more than 16awg with a 40A ESC. Thinner cable is much easier to route to the motor and weighs less.

[PatMc]

Hi Shaun, I actualy came across your thread in RCG whilst looking for something else, the video on that is a few days previous to the one linked above.
I remember the Cumulus well it's a really nice glider.
I used to compete in the IOM glider week back in the '70s & one of the locals flew one in the thermal comp. IIRC it wasn't long after the first trial run of a model glider world chamionship had been held in the USA & won by a Scandanivian (Finnish ?) contestant flying a new Cumulus.
The chap in the IOM comp brought his model in for a spot landing but stood just a few yards in front of the spot. The model's glide flattened due to ground effect in the approach at about ankle height. The pilot jumped over it but caught the right wing with his left foot, planted his right foot on the left wing then finished the job by bring his left foot down on the right wing !!!
Hop - bang - crunch - crunch - expletive deleted.
He did turn up next day to fly the Cumulus sporting a pair of new wings - his mate, the LMS owner, had them in stock.

Back on topic : I was going to suggest that you might want to try a double Rx solution. Phil Green, the single channel guru, has/had a Veron Impala that he flew as a slope soarer with a Rx & Nihm battery for rudder/elevator control in the fuselage but had the option of electric power conversion by fitting an over-wing pylon complete with a motor, ESC, small Rx & lipo.
However since you've already sorted the cg using the EP battery I doubt that you'll now want to consider this.

Personaly I'd go with the 1400 KV motor but I think that power level with a 7" prop might give a steady but lethargic climb rate.
OTOH if you can get the clearance for an 8" prop I think that would make a huge difference.
I have used & logged the data for a Graupner 8 x 4.5 folder on this E-Power 1400 KV motor using 3s lipo. Although it's a heavier motor the results with your 1400KV Turnigy should be in the same ball park.
The battery used was past it's peak at 10.27V under load, current 19.42A , power 199W & 12480rpm.

Of course you could always chop of the nose, fit a bigger, lower KV motor, 10" -12" prop for a proper climb followed by a better glide due to the cleaner lines.

That's what I did with my Graupner Amigo II.

[Mike Blandford]

Posted by Focae on 08/09/2018 09:41:05:

The output wiring from the ESC to the motor does not have to be the same wire size as the input wiring from the battery. The input wires have the continuous current.....the output wires have a switched load so the current is less. . . .

I'm not sure I agree with this!

Suppose that at full throttle you have 30A in the input wiring.
Now you have 3 wires going to the motor, let us call then A, B and C.
For a third of the time you have 30A going out on A and in on B, the next third of the time you have 30A out on B and in on C and the last third of the time you have 30A out on C and in on A.

Yes, the average current in each wire is less, but the current, when it does flow is the same as in the input wiring.

Now suppose you reduce the throttle to have only 15A in the input wiring. The ESC has done this by only driving the output for part of the time. I think you will find you actually have 30A out on A and 30A in on B, but for only half of the first third of the time, and similarly for the other 2 thirds of the time. So the output wires are actually carrying more current than the input wires!

Mike

[Shaun Walsh]

Thanks for the reply Pat,

Ideally I want to try to keep the Cumulus looking as original as possible so no cutting the nose off to fit a motor , I have a classic VW Type 3 in original condition and when I see what some people do to classic Beetles it makes me shudder inside. If it wasn't for the fact that our site is silent flight only it would be getting an OS10 on the pylon (perhaps with a micro servo operating the throttle). I contemplated installing the 1400 Kv motor as a tractor with a standard 7x4 prop (definitely no room for an 8" tractor prop) but decided that an 8" would be better and therefore it had to be a pusher and if I mount the motor over the COG I should be able to add or remove the pylon without needing to mess around with adding or removing weight from the nose it also means I can use a folding prop to reduce drag..

It proved remarkably easy to get everything in the cockpit, 2 small servos at the back with the ESC beneath them, I had to remove the front stiffener and install a battery tray for the lipo and the receiver takes up no space at all with the canopy on you wouldn't know it wasn't full of 1970s 27MHz electronics apart from the lack of a visible aerial.

All I need now is the time to finish it, roll on retirement!

[Focae]

Posted by Mike Blandford on 09/09/2018 00:19:22:

Posted by Focae on 08/09/2018 09:41:05:

The output wiring from the ESC to the motor does not have to be the same wire size as the input wiring from the battery. The input wires have the continuous current.....the output wires have a switched load so the current is less. . . .

I'm not sure I agree with this!

Suppose that at full throttle you have 30A in the input wiring.
Now you have 3 wires going to the motor, let us call then A, B and C.
For a third of the time you have 30A going out on A and in on B, the next third of the time you have 30A out on B and in on C and the last third of the time you have 30A out on C and in on A.

Yes, the average current in each wire is less, but the current, when it does flow is the same as in the input wiring.

Now suppose you reduce the throttle to have only 15A in the input wiring. The ESC has done this by only driving the output for part of the time. I think you will find you actually have 30A out on A and 30A in on B, but for only half of the first third of the time, and similarly for the other 2 thirds of the time. So the output wires are actually carrying more current than the input wires!

Mike

Mike, the output current can never exceed input current. A certain gentleman named Kirchoff determined that in one of his Electrical Laws. ‘The sum of the output currents must equal the input current’. You will also never have the same current in two legs of the motor wiring. Outrunner motors are essentially three phase ac motors. The rotating field is created my rapidly switching dc to the legs in turn. The voltage and therefore current in these legs will never be instantly at the dc voltage because the motor windings, being inductors will cause the voltage to rise relatively slowly creating a sinusoidal wave in each leg. Because the windings are 120 degrees apart, you never have all of the current in one leg being opposed by an equal and opposite current in another and you can never have all of the current flowing in one leg. Also consider this, how thick are the wires that form the winding? They are essentially just a continuation of the wires that connect them to the esc. If you did have all off the current flowing in one leg for longer than the ‘average’ switched time, the smoke escapes, usually at the winding because it is thinner than its supply wires.

[PatMc]

Shaun, if you wanted to use a tractor 7" prop you could use 4s lipo to increase the power by 33% over the 3s figure. I'm fairly certain that the current would still be inside the motor limit on a 7x4 & that it would give higher rpm than an OS10 on the same size prop.
A Graphene 4s 1500 weighs about the same as a 3s 2200 so the cg wouln't be changed. Downside is that the rpm would almost certainly be too high for safe use of a folding prop. However even with a fixed prop the overall drag from the pod/motor/prop combo would be less than the same with an OS10. And of course fixed props are a lot cheaper than folders. If going this route my preference would be an ic 7x4 or 7x6 depending on the current draw.

Incidentaly I think the OS Max10 was contemporary with the Cumulus not the OS 10FSR which means that the ic power would have been nearer to 120W than to 200W.

[Shaun Walsh]

Posted by PatMc on 09/09/2018 12:17:12:

Shaun, if you wanted to use a tractor 7" prop you could use 4s lipo to increase the power by 33% over the 3s figure. I'm fairly certain that the current would still be inside the motor limit on a 7x4 & that it would give higher rpm than an OS10 on the same size prop.
A Graphene 4s 1500 weighs about the same as a 3s 2200 so the cg wouln't be changed. Downside is that the rpm would almost certainly be too high for safe use of a folding prop. However even with a fixed prop the overall drag from the pod/motor/prop combo would be less than the same with an OS10. And of course fixed props are a lot cheaper than folders. If going this route my preference would be an ic 7x4 or 7x6 depending on the current draw.

Incidentaly I think the OS Max10 was contemporary with the Cumulus not the OS 10FSR which means that the ic power would have been nearer to 120W than to 200W.

That's a good point Pat, I hadn't thought about increasing battery voltage to reduce required prop size. However there are just two problems, I have a supply of 3S 2200 lipos and the club I belong to, as well as being silent flight only, limits batteries to maximum of 3S, not entirely sure why (only been a member for a year, not good form to join and immediately start questioning the rules) but we fly from the local school playing fields and one boundary is formed by a road with houses the other side so presumably it's an issue of potential noise complaints (thinking 6S powered EDF jets) and limiting model mass/velocity in the case of an uncontrolled excursion beyond the boundary.

The only output data I could find rated all the OS 10s the same as about 200W, if the original design was for a glow engine of around 120 W that gives me hope that it should fly reasonably well, certainly the one in the video link seems happy on 200W.

[Nigel R]

Even an fsr would be around 150w. unless you allowed it to wind up and really scream. All those ic quoted outputs are in optimal conditions and high revs with nitro and no silencer.

[Shaun Walsh]

"high revs with nitro and no silencer"

Ahhh I remember those days, open exhausts or venturi "silencers" and high nitro fuels, never did me any harm, I said "IT NEVER DID ME ANY HARM" , what do you mean don't shout, I'm not shouting, you're whispering.

Seriously I wonder how much hearing loss is associated with being too close to screaming engines back in the 60s and 70s, sorry, this is seriously off topic, I shall stop now.

[PatMc]

If you're limited to 3s then I reckon the pusher 8" prop would be preferable to a 7" tractor. The prop diameter is just as important as the input wattage in determining the right power.
Many people think that a low powered glider is a nice gentle flyer but don't appreciate that it's also taking up a lot of low level space for a relatively long period from the launch. They're often not good company in mixed discipline club sites.
Another disadvantage of slow climb rate is that it's not suited to reaching an area of thermal activity before it dissipates from the time it's spotted .

Hope I'm not painting too bleak a picture as I think the Cumulus will be fine on the power you propose as the videos already demonstrate.

If you're interested in comparing power levels of old engines this site has an very comprehensive list of engine test results.

[Mike Blandford]

Posted by Focae on 09/09/2018 10:05:14:

Mike, the output current can never exceed input current. A certain gentleman named Kirchoff determined that in one of his Electrical Laws. ‘The sum of the output currents must equal the input current’. You will also never have the same current in two legs of the motor wiring. Outrunner motors are essentially three phase ac motors. The rotating field is created my rapidly switching dc to the legs in turn. The voltage and therefore current in these legs will never be instantly at the dc voltage because the motor windings, being inductors will cause the voltage to rise relatively slowly creating a sinusoidal wave in each leg. Because the windings are 120 degrees apart, you never have all of the current in one leg being opposed by an equal and opposite current in another and you can never have all of the current flowing in one leg. Also consider this, how thick are the wires that form the winding? They are essentially just a continuation of the wires that connect them to the esc. If you did have all off the current flowing in one leg for longer than the ‘average’ switched time, the smoke escapes, usually at the winding because it is thinner than its supply wires.

"the output current can never exceed input current" No, but there are capacitors on the ESC input that provide the "extra" current for the current pulses.

"Outrunner motors are essentially three phase ac motors" No, they are DC motors with electronic commutation.

The ESC connects one of the three motor wires to the positive supply, a second to the negative supply and reads the induced voltage on the third wire to time the commutation.

Yes, the inductance will limit the current rise, but actually sufficiently slowly that the current is essentially constant.

"you never have all of the current in one leg being opposed by an equal and opposite current in another" Yes you do as it is a DC motor, I just described how the commutation is done.

"how thick are the wires that form the winding?" About the same thickness as the wires from the ESC. You may look at the windings and see quite thin enammelled wires, but, for ease of winding and to get more copper in place, the windings are made of multiple, parallel thin wires.

Mike

[Shaun Walsh]

Posted by Mike Blandford on 09/09/2018 14:40:32:

Posted by Focae on 09/09/2018 10:05:14:

Mike, the output current can never exceed input current. A certain gentleman named Kirchoff determined that in one of his Electrical Laws. ‘The sum of the output currents must equal the input current’. You will also never have the same current in two legs of the motor wiring. Outrunner motors are essentially three phase ac motors. The rotating field is created my rapidly switching dc to the legs in turn. The voltage and therefore current in these legs will never be instantly at the dc voltage because the motor windings, being inductors will cause the voltage to rise relatively slowly creating a sinusoidal wave in each leg. Because the windings are 120 degrees apart, you never have all of the current in one leg being opposed by an equal and opposite current in another and you can never have all of the current flowing in one leg. Also consider this, how thick are the wires that form the winding? They are essentially just a continuation of the wires that connect them to the esc. If you did have all off the current flowing in one leg for longer than the ‘average’ switched time, the smoke escapes, usually at the winding because it is thinner than its supply wires.

"the output current can never exceed input current" No, but there are capacitors on the ESC input that provide the "extra" current for the current pulses.

"Outrunner motors are essentially three phase ac motors" No, they are DC motors with electronic commutation.

The ESC connects one of the three motor wires to the positive supply, a second to the negative supply and reads the induced voltage on the third wire to time the commutation.

Yes, the inductance will limit the current rise, but actually sufficiently slowly that the current is essentially constant.

"you never have all of the current in one leg being opposed by an equal and opposite current in another" Yes you do as it is a DC motor, I just described how the commutation is done.

"how thick are the wires that form the winding?" About the same thickness as the wires from the ESC. You may look at the windings and see quite thin enammelled wires, but, for ease of winding and to get more copper in place, the windings are made of multiple, parallel thin wires.

Mike

I thought it was a simple question, obviously I was wrong

[Focae]

Sorry Mike but you have a been grossly misinformed about how brushless motors and esc’s work. This is really getting beyond the scope of the OP’s question but here we go anyway.....

Brushless motors as used in rc are 3 phase ac motors with the windings 120 degrees apart and connected in ‘star’ configuration. The 3 phase ac power is created by the esc which converts the dc battery input into a sine wave like output by pulse width modulation (pwm)using mosfets. The speed of the motor is controlled by varying the pwm ouput frequency. The output voltage (and ultimately current) is controoled by the width of the pulses. The higher the frequency, the faster the motor turns. Although the pwm output is created by switching the dc on and off, the output becomes more like a sine wave due to the inherent time delays in the esc circuitry and because of the motor’s inductance.

The capactitors on the esc input are there to smooth the ripple that is created in the supply dc due to the switching of the output. They do not provide any additional ‘kick’ to the output. They protect the esc from self induced spikes.

You are correct about motor multiple strands but you misunderstood what I was trying to say as I didn’t explain very well.....typically, an esc’s output wires will be thicker than the wires at the motor it is being connected to.

To get back to the OP’s question, the current on any one of the output wires will be less than the total current if measured at the dc input. A good guide to wire size is to use the same as the wires that exit the output of the esc to the motor.

[Dave Hess]

Posted by Shaun Walsh on 09/09/2018 14:56:55:

I thought it was a simple question, obviously I was wrong

Some things just aren't simple. You need third order differential equations and Laplace transforms to figure out what's actually happening. We discussed this to the death on ebike forums and still couldn't make it easy.

This is the way that I rationalise it. You have power coming into the ESC in two wires, then that power (actually a little less because of ESC efficiency) is diverted up and down three motor wires. That means that the phase wires must be handling less power than the battery wires by a factor of 2/3, so they can always be a little thinner than the battery ones. That's in relation to the wires melting.

The current in any one phase wire at any one time can be something like three times as high as the battery current, but it's pulsed, while as battery current is continuous. It's made more complicated by the current pulses alternating their direction after each pulse in each phase wire, so it goes forward and then back. I guess for efficiency, you would want the motor wire to be as thick as possible because of those high instantaneous currents, so maybe it's a nice compromise to have phase wires the same as the battery wires.

If you want a laugh, have a look at this, it doesn't give the answer you want, but does explain somethings, and I found it very entertaining:

 

Edited By Dave Hess on 09/09/2018 16:15:24

Edited By Dave Hess on 09/09/2018 16:17:21

[Focae]

You are on the right track Dave except where you say that the current in one phase can be three times the input. Kirchoff’s First Law says that all the currents leaving a circuit must equal the currents entering it. As there is only one input supply, the battery, no current in one leg can ever exceed the battery supply current.

You are right that the calculations in a three phase inductive ac circuit aren’t simple and for our motors are further complicated because we aren’t using a true sine wave. It is easier to accept that the manufacturer of most low cost esc’s have already determined the wire size for you. There is little point on substantially increasing the wire size that they have already soldered on for you.

[Dave Hess]

Posted by Focae on 09/09/2018 16:58:04:

You are on the right track Dave except where you say that the current in one phase can be three times the input. Kirchoff’s First Law says that all the currents leaving a circuit must equal the currents entering it. As there is only one input supply, the battery, no current in one leg can ever exceed the battery supply current.

You are right that the calculations in a three phase inductive ac circuit aren’t simple and for our motors are further complicated because we aren’t using a true sine wave. It is easier to accept that the manufacturer of most low cost esc’s have already determined the wire size for you. There is little point on substantially increasing the wire size that they have already soldered on for you.

That's true, but isn't the current in the phase wires supplemented by charge in the capacitors, plus, there are other strange inductive effects? I don't think you can use Kirchoff's law in this situation for instantaneous current, but it must average out over time somehow.

Ebike controllers all regulate the current. Some can regulate either the phase current or the battery current. I can't see how they can measure the phase current, so i guess that they use some sort of algorithm that calculates it from the battery current, which is measured by the voltage drop through a shunt. Some have the default settings with the phase current three times the battery current, others about 1.5 times. That's where my estimate came from.

Edited By Dave Hess on 09/09/2018 17:28:29