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Mar 31, 2020, 03:49 AM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
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Build Log

FreeRCHobby MP120 motor rewind


Hi All, I very much hope you are doing ok in this COVID-19 pandemic?

Here in NZ we are in lockdown
Although I'm still very busy with work (I'm in a role supporting health technology), I thought I would take the opportunity to learn about motor rewinding I have been meaning to rewind my MP120 motor for a very long time and apart from the challenge of getting materials, this seems like a good time to start this project.

In many ways this is a continuation of this motor thread that I started (surprisingly ) three years ago: https://www.rcgroups.com/forums/show...-%2828-Pole%29

The motor is from here: http://www.freerchobby.cc/e_products...ic-Car-39.html

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It comes from FreeRCHobby wound as a dLRK delta-terminated motor with windings wound from 150 parallel strands of 34AWG enameled wire. At least I think its 34AWG, measuring these tiny strands is pushing the resolution of my caliper! If indeed it is 34AWG this makes the windings a mere 3.0mm2 in total winding cross-sectional area, which would make the advertised spec of 250 maximum Amps (I've only ever run it up to 150A and 8kW) a bit of a stretch The copper when removed weighed in at 415gms.

I have measured this motor's KV many times and at many loads. At no load it sits at a KV=92. With a light load it sits at 84, pulling down to 81 with an 8kW load. My project aim is to wind the motor to a KV=27, which will allow my Ver#2 aviation project to run a larger prop and higher battery Voltage.

A video of the current (old) project is here for your amusement
(the video is confusingly called Version 2, but that refers to the video not my project)

Electric flight at Drury (Version 2) (7 min 15 sec)
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Mar 31, 2020, 04:06 AM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP

Stripping the windings:


The MP120 is a very easy motor to disassemble, although with the rotor-bell removed it really is a 'spaghetti' mess of fine wires:

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Blobs of epoxy are used to keep all that spaghetti in the right place. I found a micro-gas-torch perfect for softening this adhesive enough to allow the windings to be pulled away from the hammerheads:

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This left a relatively unscathed stator:

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And 415gms of copper that is now only really good for steam-punk art
Apr 02, 2020, 03:32 AM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP

Test/learn winding


To continue -

Borrowing from Ronaldo's nice idea of throwing on a test wind for KV checking, I thought that would be a great way to enter the world of "man-knitting", to use a term coined by Jack(?).

I wanted to try winding with a wire of 'reasonable' diameter, so hurriedly (day before our country locked-down) bought the largest winding wire that my local electronics store stocked - 1.25mm diameter (16AWG/1.3mm2). I like the idea of winding LRK and using Manuel's excellent Turn Calculator worked out I would need the winding to be 13 turns with a Y termination to realise my KV = 27 goal.

Winding the motor took three evenings and I was quite pleased with how the wire sat, even with a much lesser copper fill than I want the final wind to have. On testing I found I had made a silly error (ESC could not 'fire') in winding terminations. I was able to remedy this with some snip-and-solder corrections.

Happy to say theory and reality are in the same room Going from the original 4 Turns dLRK, to 13 Turns LRK gave an unloaded KV of 55. Loading lightly (16 Amps) gave a KV result of 52.

The Turns Calculator predicted: a KV of 58 unloaded and 53 lightly loaded - so very close Unfortunately I could not get my Flier ESC to run with the newly rewound motor in Y (it would only run terminated at the higher delta KV), so was unable to see if my 27 KV goal was realised. I'm thinking of building a VESC or Lebowitz ESC, so not overly concerned by this. And not too surprised, as the Flier ESC firmware is not exactly robust.

Photos of the new test wind:

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Motor running. Meter is displaying Hz with two magnets activating a hall-effect counter.
97.6Hz/2 pulse per RPM = 48.8Hz => 2928RPM @ 53.3V => 54KV
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Apr 03, 2020, 12:37 AM
Registered User
Trent Collins's Avatar
interesting project. What wire diameter will you use for the final design?
Apr 03, 2020, 05:40 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
Hi Trent,

Well the tongue-in-cheek answer to that is: "as much as I can cram in"

But before answering, I need to back-peddle a bit. I re-discovered Christian's fantastic Drive Calculator program. Putting the ex-factory parameters into the Wire & Stator Calculator sections provides the data:

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First some some corrections on previous statements I made...

If I use a wire diameter of 0.18 (AWG 33), which I'm assuming ignores insulation, the calculated Rm Delta/WYE is very close to what I previously measure. That is: calc = 31.5/10.5 mOhms vs. meas = 29.6/10.4 mOhms. Also the weight is within co'ee too: 346gms vs. 414gms (with tails).

As you can see from the calculator, it has suggested that a max current burst of 965Amps is permissible. Not sure what that translates to in constant current, but I imagine the factory spec of 250Amps is not such a stretch as I first thought?


So, back to your question Trent...


This could be a good option
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Over twice the fill. Although max burst current greatly reduced (reduced copper cooling surface area I guess?)

Or going to a thicker wire with less parallel conductors
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Hmm, some thinking to do...
Apr 04, 2020, 12:11 AM
Registered User
Skylar's Avatar
Quote:
Originally Posted by argo-2
I have measured this motor's KV many times and at many loads. At no load it sits at a KV=92. With a light load it sits at 84, pulling down to 81 with an 8kW load.
How do you measure Kv? I'm asking because I find it intriguing that you say Kv varies from no-load, to lightly loaded and with 8kW load.

Kv is a constant and should not vary if measured correctly. Maybe we are talking about two different things here. The Kv I'm referring to is the RPM per volt speed constant.

Edit: I see in post #3 you show how your 'Kv' is determined. I don't have a problem with that, except that Kv should only be measured at no-load. If RPM is measured with a load, the result of your calculation shouldn't be called Kv, but a loaded RPM. Or call that what you want, but to call it Kv, is confusing at best.
Last edited by Skylar; Apr 04, 2020 at 01:25 AM.
Apr 04, 2020, 04:11 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
Quote:
Originally Posted by Skylar
Edit: I see in post #3 you show how your 'Kv' is determined. I don't have a problem with that, except that Kv should only be measured at no-load. If RPM is measured with a load, the result of your calculation shouldn't be called Kv, but a loaded RPM. Or call that what you want, but to call it Kv, is confusing at best.
Hi Skylar, hope you are well in these challenging times?

We may have had this conversation already three years ago, unfortunately its been a long time between 'drinks' and my memory is greying year by year Yes Kv...Motor velocity constant, back EMF constant. And yes agree my bad, should only be used for an unloaded motor/generator And also as it speaks to velocity ("v"), a small v should be used as you rightly typed it.

I incorrectly used Kv as a personal shorthand to anticipate the RPM of the motor in a loaded situation, with varying Voltages. That is - if I know the motor turns at a certain RPM for a given Voltage when loaded, I can anticipate the RPM at a different Voltage with the same load (give or take). But for correctness going forward I will use "RPM/Volt with a load of Prop x" Of course then I'll probably get someone saying, so you mean "KV"

I certainly do not want to inject confusion! Thanks for pulling me up on this
Apr 04, 2020, 04:28 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
Quote:
Originally Posted by argo-2

So, back to your question Trent...

This could be a good option
Attachment 13353425
Over twice the fill. Although max burst current greatly reduced (reduced copper cooling surface area I guess?)

Or going to a thicker wire with less parallel conductors
Attachment 13353481

Hmm, some thinking to do...
And it seems to be my weekend for making mistakes. Realised yesterday after I had posted the above, that the Wire & Stator Calculator assumes dLRK windings (a winding on every hammerhead vs. LRK - a winding on every second). This is actually a good thing for my design, it means I have twice the area I first thought to shove wire into Still more thinking to do, but I think if I divide the: "Number of turns chosen" by two, this should make the fill correct. Something did not sit right for me with the Ampere capability between original motor and calculated results, this realisation addresses that
Apr 05, 2020, 07:14 AM
Registered User
Fourdan's Avatar
Hi Argo
France (french cocks, also locked at home) is saluting New Zealand (all blacks)
From your testings, my rapid analysis :

Original dLRK 4 turns/tooth delta := Kv 92 rpm/V ; product 4x92 = 736
Your rw LRK 13 turns delta := Kv 54 rpm/V ; product 13x54 = 702
ratio dLRK/LRK products = 736/702 = 1.048
All seems perfectly coherent

I am following you
Louis
Apr 05, 2020, 10:37 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP

Hello to you too Louis


Hi Louis

New Zealand returns the salute to France
Sadly not a lot of ruby being played anywhere right now

Thank you for your quick analysis, a nice quick and simple "sanity check". A handy tool for the motor-rewinders' workshop, I think!

Take care,
Brendan.
Apr 08, 2020, 02:14 PM
Registered User
Fourdan's Avatar
Hi Brendan
If you plan to try/test a VESC , please post your results and comments
Best regards
Louis
Apr 09, 2020, 11:32 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
After a lot of messing around with my ESC settings I got the rewound motor to run terminated in star. Resultant Kv was 31 rpm/V. Checking the new delta/star Kv ratio: 54/31 = 1.74

The ESC had trouble starting up in star. However, once a certain rpm was reached gate-triggering seemed to settle and it ran up to WOT nicely.

Motor-unloaded battery current was 1.8Amps @ 54V.

I fitted my cut-down prop as a load and although harder to get rotation, once running the motor turned fine (only 4Amps battery current) with this light load.

So far, so good
Apr 09, 2020, 11:34 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
Quote:
Originally Posted by Fourdan
Hi Brendan
If you plan to try/test a VESC , please post your results and comments
Best regards
Louis
Sure will! A very nice open-source ESC design, tuned over the years by Benjamin and his contributors.
Apr 10, 2020, 04:25 AM
Registered User
Fourdan's Avatar
Quote:
Originally Posted by argo-2
After a lot of messing around with my ESC settings I got the rewound motor to run terminated in star. Resultant Kv was 31 rpm/V. Checking the new delta/star Kv ratio: 54/31 = 1.74
Motor-unloaded battery current was 1.8Amps @ 54V.
So far, so good
Hi Brendan
Very good
Please give a summary of your no load precise results for 13 turns LRK

a) Delta, no load
Kv = 54 rpm/V
R phase-phase ? mohm
Io @ 2 voltages (could be 27 V and 54V) and no load rpmo

b) Star, no load
Kv = 31 rpm/V
R phase-phase ? mohm
Io @ 2 voltages (could be 27 V and 1.8A @ 54V) and no load rpmo
1.8A @ 54 V = 97W around rpmo = 1670 rpmo seems to me very good (low iron losses)

Louis
Apr 11, 2020, 10:08 PM
Maiden>Confident>Cocky>Crash!
argo-2's Avatar
Thread OP
Quote:
Originally Posted by Fourdan
Hi Brendan
Very good
Please give a summary of your no load precise results for 13 turns LRK

a) Delta, no load
Kv = 54 rpm/V
R phase-phase ? mohm
Io @ 2 voltages (could be 27 V and 54V) and no load rpmo

b) Star, no load
Kv = 31 rpm/V
R phase-phase ? mohm
Io @ 2 voltages (could be 27 V and 1.8A @ 54V) and no load rpmo
1.8A @ 54 V = 97W around rpmo = 1670 rpmo seems to me very good (low iron losses)

Louis
Hi Louis, I hope you are still well.

I may have inferred that I have made more progress than just a test-winding, sadly not so. Sourcing supplies in NZ is very difficult right now, maybe the same in France? This means there is no point in taking Rm measurements at this time. A question: is Io affected by winding cross-section?

At any rate, I have however made more measurements with the 13-turn test winding in star, with different Voltages. These are:
Batt V | Io | rpm WOT | Kv
53V | 2A | 1647 rpm | 31 rpm/V
26.5V | 1.5A | 830 rpm | 31 rpm/V
21.6V | 1.4A | 677 rpm | 31 rpm/V

I could not go any lower in Voltage and still have correct motor rotation.

Regarding a final wind, I am happy with LRK 13-turns terminated in star. I am considering winding with 3 x 1.5mm diameter strands in parallel. This will certainly be physically challenging, but could give a nice fill.

What do you think Louis about 3 x 1.5mm? Here is the calculated result:
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And original winding calculated:
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Twice the copper with a 3 x 1.5mm winding


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