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Sep 09, 2019, 09:54 AM
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Fourdan's Avatar
Hi
To get the same Kv you have to use the same number of turns
Example for 7 turns dLRK and some kit, same Kv
a) 7 turns /tooth (or arm) two strands in // of thin wire (could be 0.30 mm diam)
b) or 7 turns /tooth (or arm) one strand (single wire) of thicker wire (could be 0.42mm diam)
Generally two strands are wound "together", soldered at the 2 endings
--
The number of strands could be 2, 3 .. more
It is difficult to "erase" the enamel at the ends if you have many thin wires
When I rewind myself I am using "single wire" 0.35 mm up to 1 mm (depending of the size of the motor)
Louis
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Sep 09, 2019, 11:27 AM
Registered User
I see. Thank you.

We'll for simplicity sake I will go for the same KV using the same 0.5mm wire with the same amount of turns, 14 in this case.

Cheers.
Sep 13, 2019, 12:30 PM
Registered User
Rewound stator with 13 turns of single strand 0.5mm wire and achieved a 561kv motor.

I believe that the original was wound using 2 strands of 0.5mm wire to achieve 1050kv.

Motor ran really well for my very first rewind. Tried it with a prop and it handled it well.

I'm now going to rewind it as per original using 2 strands and may go for 6 or 7 turns and see where that gets me.
Sep 13, 2019, 04:26 PM
Registered User
Fourdan's Avatar
Quote:
Originally Posted by Rhyell
Rewound stator with 13 turns of single strand 0.5mm wire and achieved a 561kv motor.

I believe that the original was wound using 2 strands of 0.5mm wire to achieve 1050kv.

Motor ran really well for my very first rewind. Tried it with a prop and it handled it well.

I'm now going to rewind it as per original using 2 strands and may go for 6 or 7 turns and see where that gets me.
Hi Rhyell
Comments
Original Cobra 2814 : 7t+7t Delta ?? Kv 1050 rpm/V Product KvWD = 7x 1050 = 7350
Rewinding 13t+13t Kv = 561 rpm/V Product KvWD = 13x 561 = 7293 (good agreement)
Louis
Sep 13, 2019, 06:37 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Parallel wound wires behave/count as one single wire. One, two, ..., hundred (litze wire) parallel, no difference.

Efficiency increases by cramming more copper in the slots, motor can handle more power:
Efficiency governs maximum power a motor can handle - RCG

(Re)winding and building motors, tips & tricks, checks & tests - RCG (sticky)

Calculating copper fill factor and Kv calculations/predictions
Turn Calculator 8.0 - RCG (sticky)

Prettig weekend Ron
Without a watt-meter you are in the dark ... until something starts to glow
e-flight calculators watt-meters diy motor tips Cumulus MFC
Last edited by Ron van Sommeren; Sep 14, 2019 at 04:35 PM.
Sep 14, 2019, 08:45 AM
Jack
jackerbes's Avatar
Quote:
Originally Posted by Rhyell
Rewound stator with 13 turns of single strand 0.5mm wire and achieved a 561kv motor.

I believe that the original was wound using 2 strands of 0.5mm wire to achieve 1050kv.

Motor ran really well for my very first rewind. Tried it with a prop and it handled it well.

I'm now going to rewind it as per original using 2 strands and may go for 6 or 7 turns and see where that gets me.
You are in danger of becoming a man knitter of motors! :>)

Now that you have a rewound motor and (I assume) a measured and accurate resulting Kv, you can use one of manuel_v's great Turn Calculator spreadsheets to get the Kv predictions for other turn counts. Or even for other windings and terminations too!

You can download most of the versions of Turn Calculator here: https://www.rcgroups.com/forums/show...&postcount=248

As an example the attached image is the Turn Calculator 5 predictions for rewinding a DAT-750 motor. Some of the older versions have more simple and easy to understand presentation, the newer versions have a presentation with more technical and useful information.

If you don't have M$'s Excel software you can always use the free LibreOffice Calc spreadsheet.

Jack
Sep 14, 2019, 11:20 AM
Registered User
Man knitter.... Hahaha.

My results were measured. I 1st tested a known good motor of 1250kv and the measured result was 1250.688kv

I have rewound the motor using 2 strands of 0.5mm wire and measured 1104kv. I was hoping for 1100kv and partially expecting 1050kv so all good.

Gonna test it on 4s in a moment.

Here's the no load results on 4s... Motor gets warm after around 20 seconds on max power. Around 35 to 40 degrees celcius. Is this a concern?
Last edited by Rhyell; Sep 14, 2019 at 11:43 AM. Reason: Added image
Sep 14, 2019, 04:47 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Note that Kv says nothing about max.power or max. current a motor can handle, or about max. torque. A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same Kv of 700rpm/volt.


Kv is the velocity constant, expressed in rpm/volt. Kv is not a rating, not a figure of merit (explanation).
A Kv=1000rpm/volt motor on 10volt and a Kv=2000rpm/volt on 5volt will give same (no_load)speed of 10,000rpm, provided the motors can handle the current and power.


It's all about what the motor wants to do versus what the motor can do.
Kv matches rpm and voltage, there's nothing more to it.
  1. Want
    Kv and voltage determine how fast motor wants/tries to run
    (rpm_noload = voltage Kv, or, in other words, Kv = rpm_noload / voltage.)
  2. Current
    RPM and prop determine torque, which in turn determines current
    (current = torque Kv, in SI units!, or proportional to Kv ).
  3. Can
    Max.current and max.power determine whether that battery/motor/rpm/prop combo can run without going up in smoke.
An engine tries to keep a constant torque: as load goes up, rpm goes down.
An e-motor tries to keep a constant rpm: as load goes up, torque must go up, and therefore current goes up.

Prettig weekend Ron
Without a watt-meter you are in the dark ... until something starts to glow
e-flight calculators watt-meters diy motor tips&tricks Cumulus MFC
Last edited by Ron van Sommeren; Sep 19, 2019 at 11:05 AM.
Sep 19, 2019, 10:33 AM
Jack
jackerbes's Avatar
Quote:
Is this a concern?
Unless you intend or need to fly at full throttle for a long time, it is OK if you have to limit the time there to avoid overheating.

I seldom use a lot of throttle when I fly so if will stabilize at a safe temperature and 75% or so I consider that a good setup.

If you can get the temperature readings in flight with an eLogger or something like that. Or land quickly and check for heat immediately. That way you will know what is actually happening in flight as compared to a static test.

Jack
Sep 19, 2019, 02:54 PM
Registered User
Cheers Jack.

I intend to fly it in a Caipirinha 2 and it previously flew at around 3 to 7amp cruising. With the airflow I'd imagine the cooling will be better than static tests.

Hopefully remaiden it this weekend.


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