New to brushless

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Steve Mansell
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New to brushless

Postby Steve Mansell » June 3rd, 2017, 7:06 pm

Hi,
Moving from IC to leccy, and will have a few new things to learn.
The many forums I have visited have a wealth of info, but it's knowing where to look!
First question:
I am under the impression that small dc type motors often control their speed by "wasting" energy through a resistor. The energy source sees a constant demand, irrespective of the motor speed.
Does the same apply to Brushless out-runners, or do you reduce your demand with a lower throttle setting.

If the demand is reduced, does the motor see a constant voltage, and only reduce its current requirements, or do the voltage and amperage both reduce.

Hope you can help.
Cheers steve

Steve Perry
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Re: New to brushless

Postby Steve Perry » June 5th, 2017, 5:07 pm

The voltage is switched on and off very fast with no limiting of anything.
There is a video on youtube that explains how they work https://www.youtube.com/watch?v=OZNxbxL7cdc

Steve Perry
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Re: New to brushless

Postby Steve Perry » June 5th, 2017, 5:17 pm

I should have mentioned that the output is AC not DC, another video showing what is actually inside them https://www.youtube.com/watch?v=22CpRl2_TRI

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » June 5th, 2017, 11:44 pm

Steve,
Thanks for the links. So it's basicly "pulsing" the motor, but to fast to be noticable. Bloody clever!

Is there a formular for estimating flight times if I know the following:

Battery voltage
Battery capacity
Battery C rating

motor maximum watts
motor maximum amps

possible 80% throttle take off
flight average 40% throttle.

I knew I should have paid attention at school!

Cheers
Steve

Steve Perry
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Re: New to brushless

Postby Steve Perry » June 6th, 2017, 5:27 pm

There are 3 things needed, the capacity of the battery, the full load current of the motor, and the C rating of the battery.

Here is an example

Battery capacity 4400mah

Motor full load current 70 Amps

Battery C rating 60

First convert the mah of the battery to Amp minutes, 4400mah = 4.4 amps for one hour, muliply by 60 to give minutes = 264

264 Amp minutes divided by the 70 Amps full load current of the motor = 3.77 mins at full chat or 7.54 mins at 1/2 throttle

Next the C rating, this is how much load can be put on the battery without damaging it, and how hard it can be charged at.

Full load is 70, divide this by the battery capacity in amps 4.4 Amps to give the minimum required C, 70 / 4.4 = 15.9, so the example battery with a C rating of 60 is fine.

Charging, divide the actual C rating of the battery by the 4.4 amps of the battery, 60 / 4.4 = 13.6 Amps safe charge current and is called the 1C charge rate.

The battery may be marked as "Charge 4C" or similar, this means that you can multiply the safe charge current by this to give the maximum charging current that you can use, in this case it would be 4 x 13.6 = 54.4 Amps, unless otherwise marked treat all Lipo's as charge at 1C.

Do not run Lipo's down below 3.3 volts per cell to prevent damage, and when not it use store them at 3.85 per cell or at the voltage advised by the manufacturer.
Last edited by Steve Perry on June 7th, 2017, 6:23 pm, edited 1 time in total.

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » June 6th, 2017, 11:19 pm

Steve,
Just the explanation I was looking for. So it's not a "black art", and whats more amaizing is that I understood it after going over it a few times!

Many thanks
Steve

Steve Perry
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Re: New to brushless

Postby Steve Perry » June 7th, 2017, 7:10 pm

More about ESC's

First a warning, Futaba are infamous for the throttle needing to be reversed !

Esc's do not arm until the throttle is in the idle position, with the stick in the mid position at switch on they enter programing mode.

The first thing is once the battery is connected is that the motor must be treated as actually running even if the prop is stationary until the battery is disconnected !

There are two schools of thought as to how to keep the motor safe while connecting the battery and doing the normal activity's in the pits -
The first is to have the stick at idol with a throttle cut switch in the idle position, optional elastic band hooked over the stick to hold it at idle. ( I use a switch on the top of the tx with the safe position towards me, that way if I put the tx down the switch will not get knocked into the armed position ).

The second method is to keep the stick at full chat so that the ESC does not arm, elastic band to keep the stick in the up position is mandatory with this method !

The BEC built into ESC's is low powered and only good for a few normal servos, you will probably want to use a stand alone UBEC or SBEC, some folk use a separate battery for the radio rather than share the motor battery.

Programing (if required) is best done with a programing card rather than using the stick waggle method, see the manual for the ESC.

Wires, only use silicon covered wires of the same size if you need to make extensions, only extend the motor wires not the battery ones, the battery ones can be extended but more Low ESR capacitors need adding as close to the ESC as possible.

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » June 7th, 2017, 10:15 pm

Steve,
A wealth of knowledge in that noggin of yours!
I had been wondering about safety once the batts are connected, and decided to use the same switch on the tranny that I used for ignition cut on my petrol stuff, that way it is habit, and I don't have to learn something new. The esc I have didn't have any instructions, but I have ordered a programming card for it (the motor was also sent without any literature). Apparently Hobbyking are known for being "light" on instructions or manuals, but there is quite a lot of advice on various forums, from helpful people such as yourself.

Many thanks again
cheers
Steve

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » September 8th, 2017, 11:32 pm

First the bad news-
My wife saw how much the motor, ESC, data logger, watt meter, large soldering iron, 400w charger, and 2x5000mah 6s cost!

I mentioned it was still no where near the cost of the show tickets I've ordered for our anniversary. I think I've got away with it!

Now the good news-
Used EC5 connectors on the batts, series harness, watt meter, and ESC. 5mm bullets on ESC/motor.
I made a mount out of 15mm ply and bolted it to my workmate, set it all up, programmed the ESC with a YEP card, following the instructions from the YGE site.

Tranny on, reciver on, flight batts connected (ESC is opto, and no BEC, so seperate rx batt).

First gentle run with no prop - nothing! Reprogrammed the throttle "stop" and "full power" and bingo - a bit scary at first, but runs good.

Bolted a 26/8 wooden scimitar prop (from propguy) and at 1/4 throttle the workshop nearly took off!

Chuffed as punch with my first attempt at electrics.

When it comes to installing it all in my new creation, I am going to make up an arming link out of some EC5s, and mount it under a scale hatch on the cowl. That way I can power the rx and servos, and arm the motor just before t/o and dis-arm immediately after landing.

The extra cable length due to the data logger, series harness and arming link means I may have to install a capacitor bank at the ESC, due to the possibility of high voltage spikes, but still doing google research on that.

Once the plane's finished (I'm a slow builder, 'cos I live near a couple of good pubs) I may post an abridged build thread.

Thanks for all the good advice on the forum,

Cheers
Steve

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » September 15th, 2017, 12:45 pm

Ran another test today with a smaller prop.
24x8 wooden Scimitar – WWI from Propguy
12s (2x6s) Overlander 5000mah
Rotomax 50cc brushless
YEP 180a HV ESC
4 runs up to full throttle, each run approx 30 seconds
Batts before test 50.48v
Batts after test 46.93v
Average amps 65.78a
Peak amps 69.19a
Average watts 2740.0w
Peak watts 2957.1w

Although the spec for the motor states 5300 watts and 120a max, I have a feeling that's a bit over optimistic.
The motor "felt" happier on this prop, but that's just a feeling I had. It was definatly cooler this time.

Apart from seeing what other props people are using to give me a guide, how can I tell when I have the correct combination in the set-up to prevent damage (or meltdown!)

Cheers
Steve

Nigel Cox
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Re: New to brushless

Postby Nigel Cox » September 15th, 2017, 5:40 pm

You can put your data into this site https://www.ecalc.ch/ and try different prop size/pitch and other variables to see what the calculated power will be without risking any damage

Barrie King
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Re: New to brushless

Postby Barrie King » September 15th, 2017, 7:01 pm

Steve, on the HK site it gives the motor spec as 10s with 44v max, so maybe it does not like the 12s you are feeding it?

Barrie

GordonTarling
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Re: New to brushless

Postby GordonTarling » September 16th, 2017, 12:04 pm

Steve - I have the same motor that'll be going in a Tiger Moth. I've been running it on 10S with a larger prop than you and it's been fine during testing. What determines the real limits of any motor is heating, as long as it's staying under 100C, then it will be fine. However, do NOT engage in extended testing on the ground, but run for a minute or so to get some readings, shut down and test the temperature. An IR gun type thermometer is useful for this, but try and get a reading from the windings and not from the outside of the case. Also, remember that the motor will normally be running a little cooler in flight as long as it's getting some cooling air. From the figures you've given above, you're well within limits.

Steve Mansell
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Re: New to brushless

Postby Steve Mansell » September 16th, 2017, 5:41 pm

Thanks all,
Although the readings seemed ok, on 12s the 26x10 and 26x8 both sounded awful! souded like they were ripping the air. Big volt drop (39v on 12s) on the 26" props. Tip speed may be the sound issue. at 172 kv I think 10s will be a better power source. It will be going into a 1/4 scale Alb. Ciii.
It's strange that the spec on the website says 10s, but the box states 12s. For the type of flying i do, a slower, low pitch, big prop will be better, so its a case of trying 10s with the props I have, and just keep an eye on the current load.

Further investment will have to be a tacho and IR thermometer.

Cheers all
Steve

Steve Perry
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Re: New to brushless

Postby Steve Perry » September 16th, 2017, 7:45 pm

You need to use less cells for a bigger prop and can use more cells on a smaller prop, this keeps the current down.

GordonTarling
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Re: New to brushless

Postby GordonTarling » September 17th, 2017, 10:36 am

10S or 12S makes little difference to the motor - current draw is determined by the load you put on the motor. For the record, I tested mine on a 27 x 12 prop on a 10S 5500mAh pack and current draw was (I think) around 65 Amps. The 'ripping' noise you are hearing is not because the prop tips are going supersonic, but probably because the prop is stalled when there's no airspeed - judge it when it's in the air and not on the ground.

Steve Mansell
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Joined: March 1st, 2012, 11:59 pm
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Re: New to brushless

Postby Steve Mansell » September 18th, 2017, 12:28 am

Thanks all,
Better get building then!
I'll update when Iv'e (a) learnt some more; (b) built and flown the plane; and (c) got the best combo.

Although it would delay the Alb build, I think I may get something that I can fly as a test bed, (big, slow, and fairly cheap), without risking a full scale build.

Thanks again
Steve


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