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Old Aug 03, 2005, 12:33 PM
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mjdee14's Avatar
Nashville TN, USA
Joined Jun 2004
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CDR Motor "Growls"

I have a GBv 15t 24g motor on TP730 lipos, CC10 esc, running a GWS 8040 sf prop...every once in a while it growl like it's rubbing or catching on something..(nothing is rubbing).. Motor has no shorts...gets good beeping tone from esc on bat plug in....

I broke the prop and put a smaller one on it...and have not heard the sound since...

Is the growl or abrupt stop what a motor does if overpropped? I have the same wind on annother motor but am using the GWS DD prop and have never had that happen...was the SF prop just too much for it...??

Also....what is the correct timing for an outrunner or GB motor? I believe the GB website says standard....but then I read most outrunners houls be advanced....?? Any rule of thumb?

Anyone have any thoughts?

Mike
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Old Aug 03, 2005, 01:25 PM
Flying motor mount master
fly_boy99's Avatar
San Jose, California, United States
Joined Oct 2004
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I've heard something similar on some of my home wound motors. It basically comes down to the bearings were not securely fastened to the shaft.


Quote:
Originally Posted by mjdee14
I have a GBv 15t 24g motor on TP730 lipos, CC10 esc, running a GWS 8040 sf prop...every once in a while it growl like it's rubbing or catching on something..(nothing is rubbing).. Motor has no shorts...gets good beeping tone from esc on bat plug in....

I broke the prop and put a smaller one on it...and have not heard the sound since...

Is the growl or abrupt stop what a motor does if overpropped? I have the same wind on annother motor but am using the GWS DD prop and have never had that happen...was the SF prop just too much for it...??

Also....what is the correct timing for an outrunner or GB motor? I believe the GB website says standard....but then I read most outrunners houls be advanced....?? Any rule of thumb?

Anyone have any thoughts?

Mike
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Old Aug 03, 2005, 01:58 PM
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podavis's Avatar
Louisville KY
Joined Oct 2004
1,069 Posts
I suspected bearings too on my high RPM cdr's, until I heard exactly the same noise from a new high RPM Feigao 130XXX on a low ratio gear box. That tells me it's the prop resonating. Two questions to ask are does it only happen at top RPM and is the prop held on with a propsaver. The only way I can think of getting rid of of it for a particular prop is to tighten down the connection to the shaft with a big washer to try tune the props resonance above the top RPM of the motor. 15,000 RPM equals 250 Hz and this is consistent with the growl I heard. It was also very loud, and the large surface area of the prop makes it a good 'speaker'.

POD
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Old Aug 03, 2005, 03:49 PM
igo
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Joined Apr 2004
981 Posts
okay, here's my recent observations on cdr noise as applied to ducted fans. i've been running a cdr motor with a bronze bushing in it for about 2 years. even with moly grease every now and then, the fit between the shaft and the bushing became more noticeable and the noise was getting louder at full throttle on only a 2 cell pack. so a few days ago, i made a cdr motor with 2 ball bearings. so now the motor runs much quieter. what i'm trying to say it that ball bearing do make for smoother/quieter operating motors. (nothing entirely new but worth mentioning IMHO of course...)

so, does anyone know of a micro ball bearing manufacturer that make 2mm ID and 4mm OD bearings?
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Old Aug 03, 2005, 04:15 PM
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Mid-Michigan
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What it may well be is a resonance of the motor mechanics driven by the switching frequency of the ESC. This is not all that uncommon. But radial looseness in the bearings is just as likely, and needs to be investigated.

Boca Bearings will have what you need. http://www.bocabearings.com/main1.aspx?p=docs&id=26

Very nice do deal with, too. They specialize in the hobby field, and don't object to you buying $2.00 worth per order.
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Old Aug 03, 2005, 08:55 PM
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Nashville TN, USA
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Guys..

Thanks for the replys...it is not at full throttle...I don't think it's the prop...and a couple of times it actually shut the motor down. I ran the same motor tonight with a different prop gws 8040DD instead of the gws 8040 sf...I did not get the noise and when I get it, it's a low rpm....what gets me it abruptly stops the motor....but it ran alright on the smaller prop. This motor has bearings...not bushing....tried a different shaft and bell...maybe the bearings are bad....

Mike
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Old Aug 03, 2005, 09:04 PM
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Louisville KY
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Yup, I agree it's not the prop. You might try pushing the bearing firmly onto the end of a pencil. Rotate it between your finger tips while pushing it onto the pencil. I found a bearing that had a 'gritty' feel when I did this.

POD
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Old Aug 03, 2005, 09:38 PM
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Ohio, USA
Joined May 2002
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I have also experienced the "growling" when overpropping a GBv motor. The motor sounded fine at low and high throttle, but growled and overheated at mid-range throttle settings. Switching to one size smaller prop cured the problem.

I believe the "growling" is caused by the ESC losing sync and thus failing to operate the motor smoothly.

- Jeff
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Old Aug 03, 2005, 09:59 PM
Lance Armstrong- 7 times!
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Madison, Wisconsin
Joined Sep 2001
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Overpropped. If you want to turn an 8x4 prop, get a GWS HD 8x4 and put 21 winds of 26 guage on your motor. Runs great. The "growling" is a pain- if you use just one less wind (20) it growls, but at 21 it doesn't.

If you want to keep your 15 turns, try a GWS HD 7x3.5. Even that might be too much prop- that setup is better with 17 winds.
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Old Aug 04, 2005, 08:15 AM
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Mid-Michigan
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Which gets back to what I suggested earlier - the switching frequency of the ESC could be hitting a harmonic of the coil/magnet frequency, (i.e. how many times per second does each coil see a magnet pass by). In Bob Boucher's excellent book on brushed motors, he describes this exact phenomenon in the context of commutation frequency and the switching frequency of the old-fashioned "frame-rate" drive amps (50Hz.) that were available in the early days of electric. Astro Flight actually sold one of these for the low-end market. I have one, and sure enough it will make a motor "growl" at certain (rather low) RPM. I can also get this to happen by using my primitive variable-voltage DC supply, which is just a Variac with a rectifier. There is quite a bit of 60Hz. ripple in the output, and that will make a brushed motor "growl".

Because this phenomenon is coupled to what servo engineers call the reflected inertia of the rotating parts, as well as the aerodynamic drag, just changing the prop may indeed make it go away.

edit:

Wait a minute - what I just said only applies to brushed motors, you're using brushless, so the switching freq is a variable. But it still can be related to the reflected inertia. But I'll have to think it through for a while to come up with a coherant explanation that will fit in a coupla paragraphs. Meanwhile. change the prop, or change the wind, as was suggested. What Galloping Gimp said is very close - the ESC is having trouble maintaining the correct commutation advance, due to a resonance with the rotating mass and aerodynamic load.
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