Help to figure out this brushless motor

[Futterama]

Hi forum,

I'm totally new here, I need a place to get help with my brushless motor.

I have this brushless RC motor: http://www.skyrc.com/index.php?route=product/product&path=39_48&product_id=141

I'm building an ESC for it and I'm trying to understand the motor and how it works. It is a bit different from the other brushless motors I have (1:10 RC scale motors) and there are some things I'm wondering about.

The goal is to add hall sensors to the motor and control it using a PIC microcontroller, MOSFET gate drivers and low RDSon power MOSFETs.

The motor data I could figure out that the manufacturer didn't give:

Rotor: 4 poles.
Stator: 12 slots.
Winding pattern: WYE.

The first thing that makes me wonder is when turning the motor by hand, it has 24 "positions" per revolution. Given the motor data above, I would expect it to only have 12 "positions" per revolution. What am I missing here?
I'm suspecting an inaccuracy in the rotor poles because when turning the motor by hand, one "position" at a time, I feel that the rotor jumps easy from position 1 to 2 but from 2 to 3, there is more resistance. From 3 to 4 it is easy again, and then more resistance again from 4 to 5. It's almost like position 1 and 2 should be one position and not two, then giving a total of 12 positions as I expected. Am I on to something here or is this totally normal? The power wires are left unconnected during this "test".

I also did the above test with 2 wires powered (from the lab supply through a power resistor to limit the current to 4A) and this is the same result, the motor can be placed in these 2 positions, but if I force it a few positions away, it will jump back due to the lightly magnetized coils.

I made another test on the motor. I hooked 2 power wires to my cheap handheld scope and turned the motor using a powerdrill at 1200rpm. The resulting BEMF waveform is attached. I does not look like a perfect sine as I would expect. Another evidence of a rotor defect?

I hope you guys can help me figure out this motor.

Please let me know if I should post some pictures of the motor internals.

Best regards,
Martin

 

[Futterama]

I did a quick test on a 1:10 scale motor, again spinning it to 1200rpm using a powerdrill and the waveform is much more like a sine.

 

[Miles]

Please let me know if I should post some pictures of the motor internals.

Hi Martin,

Yes, please do..

 

[Futterama]

Here you go

 

[Futterama]

I thought this was a forum with knowledge about brushless motors, I guess I was wrong with so many views and so few (none) answers to my question :-(

 

[Miles]

If I had anything to add, I would have..... As you say, it should have 12 cogging steps. I can't think of any explanation why this should be doubled.

It looks like it has two winding layers?

 

[Futterama]

Hi Miles,

The strange thing is that a friend also have one of these motors, not the same model but same "Beast" series from the same manufacturer and his also have 24 cogging steps.

I took 2 small steel balls and placed them on the rotor, and they seem to be attracted to the edges of what I think is the magnets. The magnetic field is probably more intense in the area between the magnets, maybe this is what causes the extra steps?

I will make myself a clear/transparent back plate from acrylic and mark the magnets on the end of the rotor, and then take a look at the magnet positions compared to the stator when turning the rotor.

I can't really figure out exactly how the windings are made because it is so hard to see, but I do believe it is WYE since the "wires" seem to meet where they are covered with shrink tube.

If no one here can help, where can I then go to find help? I'm thinking RC forums, but I don't want to post on every forum and spam the internet

 

[Miles]

If no one here can help, where can I then go to find help? I'm thinking RC forums, but I don't want to post on every forum and spam the internet

RC Groups.

LRK Torquemax

Engineering Tips

 

[major]

I made another test on the motor. I hooked 2 power wires to my cheap handheld scope and turned the motor using a powerdrill at 1200rpm. The resulting BEMF waveform is attached. I does not look like a perfect sine as I would expect. Another evidence of a rotor defect?

I think the waveform is due to the fact that you are viewing the generated voltage across two of the 3 phases. Those 2 phases are positioned 120 electrical degrees apart. So the waveform is the sum of the two, each being a rounded off trapezoid. The shape of the generated voltage with respect to rotor position or movement is dependent on the magnetization pattern on the rotor. I don't think it is evidence of a defect. But I am no expert on BLDC, just curious about motor puzzles

If you really wanted to see the true generated voltage pattern on the coil (or conductors), you could undo the wye node. But that would be a bitch. So you could string a single thin wire thru the air gap along the top of a stator slot and connect the scope across it. It would be like a half turn coil. The generated voltage would be low at 1200 RPM, but I imagine the scope could read it above noise. I bet then you'd see a rounded trapezoid. The wire could be very thin as there would essentially no current and without current, no magnetic force on it. Maybe a thin narrow piece of foil would work as the half turn coil (conductor) in the air gap.

 

[drebikes]

+1 to all major said

...and a few cents of mine. It does seem the waveform you posted is the BEMF between two phases, which is the motor is 12 slots (and not 12 poles), between phases it's normal to see 24 steps.

As to why it's not as sine-wavy as the other one, it's likely by design. The square-ish type of BEMF is more common to high cogging torque motors, for example this type could be found in lower speed, low airgap or with very strong magnets. The shape of the tooth tip can play a lot on the deformation of the BEMF as well as the saturation point for which the motor is designed. All this to say it's not unusual. I'm not sure how a default would look like, maybe try to measure between all 3 combos of phases and see if there are deltas on two combos versus the third (likely the common would be the faulty. A faulty rotor magnet would produce a BEMF with a "chip" from the normal value you would expect on all phase combos.

BTW, not having the numbers I can't really say, but I "like" the square-ish BEMF more. The other one isn't quite as sinus as it seems, it has a nasty peak which in practice means it needs quite high Vdc (at least 2x the amplitude of the BEMF) for probably diminishing benefits.

Could you pull an FFT of the waveforms?

 

[Futterama]

Hi major and drebikes,

Thanks for contributing. Miles, thanks for the links.

major, you are probably right about the waveform, and I will try to put in a thin wire as you suggested.

drebikes, I cannot make sense of this piece of your post:

It does seem the waveform you posted is the BEMF between two phases, which is the motor is 12 slots (and not 12 poles), between phases it's normal to see 24 steps

It's called slots in the stator, not poles, noted, thanks.
So it's normal to see 24 steps, ok.
Do you mean the BEMF is normal for a 12 slot motor?

FFT as in Fast Fourier Transform? I have no idea what that is other than it is some sort of calculation on a waveform. I can post the waveform values from the scope if you like. Remember my current scope is a "cheap" handheld device which came as a kit from Velleman (soldered and calibrated by myself) so it has it's limitations. A USB PC based 2ch 12MHz scope is on it's way in the mail (also Velleman, not a kit though).

I made a video where I have marked the rotor magnets on the rotor end, added a clear acrylic plate to support the rotor and turned the motor by hand so it is easy to see how the rotor reacts to the slots in the stator:

[youtube]iMAJtdaXp20[/youtube]

I have also attached the picture I took when playing with my balls... the balls of steel I mean... ah, you know what I mean, just look at the picture :lol:

 

[Miles]

The lowest common multiple of 12 and 4 is 12.....

There is quite a wide gap separating the magnets... It's about the same width as the heads of the teeth. That could be a factor.......

 

[Futterama]

I got a very thin wire routed in between two slots and the scope has no problems seeing the voltage, see attached waveform. I also attached the datafile (readable text format inside the zip).

 

[major]

I got a very thin wire routed in between two slots and the scope has no problems seeing the voltage, see attached waveform.

Cool. I think I was right about the previous trace being the combo of 2 phases. This wire traces the flux around the rotor as it sweeps past the stationary wire. You can see the interpolar region between the magnets near the zero crossing. The dip near the peak indicates a weaker zone near the middle of the magnet pole. I'm not sure why that is there or if it was intentional or if it is of much consequence. Or it just occurred to me that the dip might be due to the flux being directed by the stator teeth away from the wire at that particular position. Thanks for running the test. Good info. Got a picture of the wire or the set up?

 

[drebikes]

Wow, I really messed up that sentence, I had a colleague bugging me and lost my tracks. The FFT yes, it's the fast fourier transform. It easy to do the computation with an excel sheet, there are guides on the net how to. That said your test with a wire makes the FFT unnecessary, it's obvious the motor is well built. My guess is the dip in the middle of the bEMF max is by design - the thumb rule in motor design is at the max of the BEMF (phase-neutral) one would prefer to have a sort of plateau, thus to prevent the maximum equivalent to the "ideal" sinus. Usually it's a plateau, not a dip though and it is made by designing the back iron to saturate at precisely the right point. Maybe also major's guess is good, the magnet may be slightly smaller at the midpoint of the rotor pole.

 

[Futterama]

major, what do you want to see a picture of? It's just a very thin wire (OD 0.28mm) taped in place between two stator teeth and routed out of the motor.

 

[major]

major, what do you want to see a picture of? It's just a very thin wire (OD 0.28mm) taped in place between two stator teeth and routed out of the motor.

Picture's worth 1000 words But you explained it well. Thanks.

 

[Futterama]

So, I got my PC based 2-channel scope today and began to do some measurements of the sensored 1/10 scale RC brushless motors I have. Here are the waveforms, on one of them the motor turns clockwise, the other counter clockwise.

Since these motors have static timing (timing cannot be changed by rotating the sensor board inside the motor) I would have expected the peak BEMF to be exactly in the middle of the sensorsignal. I have read this document about matching the sensors to the phases, so I have measured all 3 sensors with the same phase-pair and know these are a match, but still there is some offset between the signals.

Link to document:
http://www.pmdcorp.com/downloads/app_notes/BrushlessSensorConfig.pdf

Can I assume that these motors have some static timing built into them that makes them deviate from the optimum timing to give the motors some specific characters?

The motors have 2 poles and 3 slots and are WYE wound.

 

[Miles]

Hi Martin,

This caught my eye, as I skimmed through the attached paper:

It is interesting to observe
that as the width of the pole is close to the optimum
width, the shape of the cogging torque changes. The
frequency of the cogging torque doubles at optimum
width. As the width is reduced lower than the optimum
width, the cogging torque starts to increase in
magnitude again and the frequency is back to the
frequency of the cogging torque at the original width.

This is referring to a non-uniform gap. Is yours even slightly non-uniform? It doesn't look like it is.

 

[Futterama]

It looks uniform to me but I really can't tell what it looks like under the "wrapping" around the rotor magnets.

 

[Futterama]

Or it just occurred to me that the dip might be due to the flux being directed by the stator teeth away from the wire at that particular position. Thanks for running the test. Good info. Got a picture of the wire or the set up?

Would it help to run the wire as a loop around the stator tooth to simulate a winding? I think I'll try that cause I'm quite curious about it.