Solved BL-Motor making strange sound

[Romf]

Hi,

after a crash of my Tiny Trainer the "nose" of the propeller (Spinner?) and the moving part of the motor (outrunner) had scratches. But when I turned it, it seemed to run perfectly straight.

After repairing the TT I made a motor test and found that the motor sometimes(!) makes a strange sound that sounds as if the prop would touch part of the foam (which it doesn't do!).

By "sometimes" I mean that when I increase the throttle the noise seems sometimes to kick in and stay there independent from the rpms. Other times the sound does not appear and the motor spins up to its maximum power totally fine.

The Motor and BEC are the ones from the Powerpack A (German version from Graupner): ULTRA 2804 2300KV brushless Motor CCW and ULTRA 20 A SBEC BL HELI S 2-4S XT-30

What could be the cause of this phenomenon? Should I try to fly the plane or is complete destruction imminent?

- Robert

 

[DamoRC]

Given the intermittent nature of the noise I would first check the bullet connectors from the motor to the ESC, one of which might have been damaged during the crash. I is possible that one of the connections is damaged or broken. Give each connector a tug to see if its corresponding wire is loose or comes out - test the three female and three male connectors separately. Vibration can affect when the connection is good and bad leading to the motor behaving well sometimes and poorly at other times.

The next think could be a damaged bearing - in my experience the motors sound as if they have gravel in them when the bearings are worn or shot. The bearing can get damaged during a crash.

DamoRC

 

[Hai-Lee]

Without seeing, examining, and hearing the motor I can only guess but to me it sounds like one of the following should be checked for.

The CAN of the motor has been jarred loose slightly and the noise is the can trying to rotate on and gouge the motor shaft.
One of the motor bearings is damaged and a (Ball?) bearing is occasionally jumping out of alignment and grinding itself against the other bearings and the housing.
A bearing has had foreign matter forced into it due to impact and the noise is the grinding of the bearing and the foreign matter.
A loose or damaged motor CAN magnet grinding on the stator/windings.
A foreign object or dirt between the motor CAN and the stator/windings,
A bent or bulged motor shaft.
A bearing has been actually bent or lacks adequate lubrication!

The above list is not comprehensive but it gives you a few things to check for.

Have fun!

 

[d8veh]

yes, check all the connections, then the solder joints where the motor wires solder to the ESC. Many of these issues, that look and sound mechanical are caused by electronics. The import thing is whether the motor makes full power. If it doesn't and you've checked the other things, you have probably damaged the ESC's electronics, in which case you have to get a new one. Hobbyking is a good place to get motors, ESCs, receivers and servos - all cheap and reliable. Free shipping if you order something like 50 euros. Don't forget to click on the EU warehouse for every item. You can buy stuff from the other warehouses, but you get additional shipping costs.
https://hobbyking.com/en_us/hobby-king-20a-esc-3a-ubec.html

 

[Romf]

Hi,

@Hai-Lee it seems you were right with your guess about the broken bearing. When I took the motor apart I found that one of the two bearings only turns with some (variable) resistance.

So I will see, whether I find replacement parts for the motor...

- Robert

 

[Romf]

Hi @d8veh,

eventhough I found a broken bearing which probably can be replaced, I wonder if I should buy a replacement motor anyway. This one got damaged on the second crash already...

So I wonder what replacement I require. The motor has two parameters kV (2300) and a 4 digit number (2804). While the kV value is explained everywhere on the internet, I haven't found really anything about the other number apart from some vague mentioning of the turns of the wires...

Do you know a source of information about that topic?

- Robert

 

[Hai-Lee]

Hi,

@Hai-Lee it seems you were right with your guess about the broken bearing. When I took the motor apart I found that one of the two bearings only turns with some (variable) resistance.

So I will see, whether I find replacement parts for the motor...

- Robert

As you are going to buy a new motor anyway, (a wise move by the way(, Keep the other motor as spare parts. I assume that it still has a good bearing a reasonably good CAN and a pretty straight motor shaft.

Sourcing a new bearing can be difficult so it might be time to start an RC "Junkyard"

Just what I do!

Have fun!

 

[Chappie66]

Hi @d8veh,

eventhough I found a broken bearing which probably can be replaced, I wonder if I should buy a replacement motor anyway. This one got damaged on the second crash already...

So I wonder what replacement I require. The motor has two parameters kV (2300) and a 4 digit number (2804). While the kV value is explained everywhere on the internet, I haven't found really anything about the other number apart from some vague mentioning of the turns of the wires...

Do you know a source of information about that topic?

- Robert

2804-2300kv is what you will want to look for when replacing. Form that part number I find:
https://www.graupner.com/ULTRA-2804-2300KV-br-ushless-Motor-CCW/S7048/

https://www.tomtop.com/p-rm2122.htm...MIlLf71a_r3gIVTrjACh07jQ6lEAYYASABEgKXh_D_BwE


These are just a couple possibilities...

 

[d8veh]

There are a whole range of motors, ESCs and propellers you can use to get the same power. It's really complicated, and I'm still trying to get my head around it all. I have a $10 tiny motor that can make more thrust than others twice the size. The important thing is that the motor, propeller, battery and ESC match each other.

One of these motor sets will work well in the Tiny Trainer with a 3S battery and a 10x4.5 propeller. Probably cheaper than just the bearing for your damaged motor. The bearings are probably the same too:
https://www.ebay.co.uk/itm/30A-cont...131531&hash=item5213c6627b:g:qmwAAOSweW5VLi5J

Generally, a motor with a high kV number will spin faster, so it'll make more power for its size. Using more cells in the battery is equivalent to a higher kV. A 3S battery will make a motor spin 50% faster than a 2S one, i.e. the speed is directly proportional to voltage. Using a higher number of cells also pushes more current through the battery, which makes even more power. You regulate the current with the propeller. The smaller the propeller, the faster the motor spins, and the faster it spins, the lower the current. The higher the current, the more thrust you get.

Putting that all together, whatever motor and ESC you have, you need to make sure that the current isn't too high for either of them. You have to refer to their specifications to see how high you can go. If your current is too high, you put on a smaller or lower pitch propeller to spin faster, which reduces the current.

Rather than try and figure out everything, it's easier to use combinations that are recommended to work. It helps if someone has done the necessary measurements of the thrust and current.

Here's a lighter combination that has amazing thrust (around 1kg) for its size:
Any 20A ESC, 3S battery, 6x4 propeller and this motor:
https://hobbyking.com/en_us/multist...e-racing-motor-1808-2600kv-ccw.html?wrh_pdp=3

 

[Romf]

2804-2300kv is what you will want to look for when replacing. Form that part number I find:
https://www.graupner.com/ULTRA-2804-2300KV-br-ushless-Motor-CCW/S7048/

https://www.tomtop.com/p-rm2122.htm...MIlLf71a_r3gIVTrjACh07jQ6lEAYYASABEgKXh_D_BwE


These are just a couple possibilities...

After reading a bit about the motors I assume, that the "2804" part of the type is not directly related to a certain technological property. In its description of the motor I found the stator to be specified as "2204". When I search for this number I get way more hits in motors and I assume, that these should work (when KV etc. are the same)

- Robert

 

[d8veh]

I would make the same assumption.

 

[kilroy07]

The nomenclature referrers to the physical dimensions of the motor.
Whereas your 2804, the bell should be 28mm wide and 4mm long.

The kv is how many revolutions you should get (in an ideal world) per volt.
Higher Kv motors spin faster so you usually need a smaller prop or you are going to over tax the motor or ESC and let the magic smoke out (ask how I know!)
Another way to think of it, is the slower motors have more torque (big V8) where the faster motors have higher revs (turbo 4 banger.)

So, in the example, you could have two motors both the same physical size, but because of the number of windings (wraps of wire around the armatures) they could have different Kv values and thus work best with different sized props.

Long story short, stick with the props that were included in your motor kit(s) or are recommended or, get yourself a amp tester to check out different motor/prop combos.

 

[Romf]

Hi @kilroy07,

ok, so my strategy should be to find a motor with (about) the same kV and the same recommendation of props as my old motor and then check either via data sheet or via amp tester whether they don't overload my ESC?

And the other numbers are only concerning the physical dimensions? Which have to fit, too, of course but I have some flexibility here ...

The bearing, by the way is only about 3,-€, so I will try to fix the motor when it arrives.

- Robert

 

[quorneng]

Romf
Be careful of the motor numbers. Not all manufacturers use the same notation. For some it is the size of the stator inside, for others it is the outside of the bell. The only sure way for the same kV is to compare their 'physical' dimensions and current limits.

 

[kilroy07]

find a motor with (about) the same kV and the same recommendation of props as my old motor

Yea, depending on you initial set up you could have loads of headroom for the ESC as long as you are in the ball park (ie. starting with a 5x4.5 prop and trying a 5x5. etc.)

Just don't be like me where I thought, well this 2205 (2300Kv) was spinning that 5" prop just fine on a 2 cell battery, let's get MORE power from it by slapping on a 6" Tri-blade on it AND go to 3 cells!! (all this on that woefully inadequate 12amp ESC they used to provide in the "F pack").
It flew like a mad man... for awhile... then not so much... (we need a magic smoke emoji)

 

[kilroy07]

Not all manufacturers use the same notation.

This is true! Others just equate their motor to a gas/glow engine (which doesn't help me at all!)

I used a cheap chinese knock off like this;
https://store.flitetest.com/eflite-power-meter-efla110/p19704
It's helped a LOT understanding the loads different props put on the motors.

 

[DamoRC]

Yea, depending on you initial set up you could have loads of headroom for the ESC as long as you are in the ball park (ie. starting with a 5x4.5 prop and trying a 5x5. etc.)

Just don't be like me where I thought, well this 2205 (2300Kv) was spinning that 5" prop just fine on a 2 cell battery, let's get MORE power from it by slapping on a 6" Tri-blade on it AND go to 3 cells!! (all this on that woefully inadequate 12amp ESC they used to provide in the "F pack").
It flew like a mad man... for awhile... then not so much... (we need a magic smoke emoji)

Yep. The extra blade and extra inch can make a huge difference. For my Tigercat I took a motor that pulled 31 amps on a 12x4 prop and used. 9 x 6.5 tri-blade and it pulled 38 amps. There is a load factor estimate you can calculate to compare prop loads : D^3 x P x Sqrt (N-1) where D is the diameter, P is the pitch, and N is the number of blades. It's not perfect but it can tell you ballpark if the load created by one prop is going to be equivalent to another.

DamoRC

 

[Romf]

Romf
Be careful of the motor numbers. Not all manufacturers use the same notation. For some it is the size of the stator inside, for others it is the outside of the bell. The only sure way for the same kV is to compare their 'physical' dimensions and current limits.

Yes, that explains all this mess of names...

Ok, so do I understand you correctly in that you recommend to choose a motor with the same KV, the same physical dimensions AND the same current limits?
Problem is, that not all manufacturers provide those...

I figured, that my ESC, having 20A should have some headroom compared to the 12A ESC in the US version of the Power Pack A and therefore choosing a motor with the same KV and the same recommendations of props (I have bought several ones ...) should be relatively save.

btw. Instead of a power meter, can I use a current clamp to measure (or at least estimate with adequate precision) the current and power?

- Robert

 

[quorneng]

Romf
You can use a clamp meter but just beware of their accuracy is you are running very close to say the ESC's limit. You can over load a motor or battery quite a bit for a short time but the electronics in the ESC will fail in seconds if over loaded.

If you have set of numbers/letters for a motor do a Google search. Many of the big suppliers like Hobby King will have details of the motor or their version of it on their web site.
Note that motor weight is a useful guide. For the same type of brushless motor with the same kV its weight will pretty accurately determine its performance.