Motor comparisons [CA120 et al.]

Miles

100 TW
Joined
Mar 16, 2007
Messages
11,031
Location
London UK
CA 120-70
http://www.hobbyking.com/hobbyking/store/__14427__Turnigy_CA120_70_Brushless_Outrunner_100cc_eq_.html

14427P1.jpg




RotoMax 150
http://www.hobbyking.com/hobbyking/store/__25413__Turnigy_RotoMax_150cc_Size_Brushless_Outrunner_Motor.html

rotomax150-main.jpg
 
CA 120-70
Kv : 150 rpm/V
Resistance: 0.004ohm
No load current: 13A @ 20V
Weight: 2730g

No. of teeth: 24
No. of poles: 28
Stator diameter: 106mm
Stator length: 30mm
Lamination thickness: 0.35mm

Can diameter: 118mm
Can length: 58mm

Winding: 3T

Miles said:
Revised data (from toolman2) for the CA120 presently being offered by HobbyKing

Wt 2.73 kg
Lamination thickness 0.5mm
Kv 164 rpm/V
Kt 0.058 Nm/A
Rm 0.005 ohms
Km 0.82 Nm/√W
Specific Km 0.3(Nm/√W)/kg


RotoMax 150
Kv: 150 rpm/V
Resistance: 0.011ohm
Inductance: 18.0uH
No load current: 5.2A @ 51.8V
Weight: 2530g

No. of teeth: 24
No. of poles: 20
Stator diameter: 101
Stator length: ?
Lamination thickness: 0.2mm

Can diameter: 110mm
Can length: 65mm

Winding: 8T
 
Both motors listed as 150 rpm/V

CA 120-70 has lower resistance and higher no load current. RotoMax has higher resistance and lower no load current.

Reasons for the difference:

- Depth of slots
- Fill factor
- Pole count
- Lamination thickness
- ?
 
Here are my random thoughts on the compairison:

I am almost willing to bet they share the same stator. all the other dimensions seem to be identicle.

ca120 is wound with three turns stock & has very low resistance.

I would guess they put a few more turns on the rotomax & dropped the pole count to up the rpm's into "prop range" thus raising the resistance a bit.

edit: I see they list it as 8 turns.

I have re-wound the CA-120 a few times....you can easily get the kv down in the 50 range....no load draws on 12cells drop to less than 1 amp.
 
Hi Todd,

I think they may be different stator geometries - compare the photos. The CA 120-70 seems to have deeper slots. If that's so, it's another factor in its lower copper losses but higher parasitic losses. In any case, they have different lamination thicknesses, unless the CA 120 has/is to be been upgraded.

What is the stator diameter and stack of the CA-120-70?
 
:oops: I guess i shold look a little closer before yapping on.....interesting none the least.

for the record, here are the measurments:
106mm dia.
30mm stack
laminations are (edited to stop confusion if possible) .35 ish off from a damaged lamination.

P8300089.jpg


P8300088.jpg
 
I'd bet that no-load power is lower on the rotomax due to higher inductance. And it is due to higher turn count. And the only way to maintain same KV at higher turn count is smaller magnets. And this explains why it is cheaper.
Looks like the only RC motor of this size that has a practical use due to acceptable inductance and power losses.
 
circuit said:
I'd bet that no-load power is lower on the rotomax due to higher inductance. And it is due to higher turn count. And the only way to maintain same KV at higher turn count is smaller magnets. And this explains why it is cheaper.
Don't forget that the pole count is also a factor! Together with the thinner laminations this must make a big difference.

As they are from different sources (or were...) I'm not sure it's necessary to explain the price difference in that way.
 
Miles said:
circuit said:
I'd bet that no-load power is lower on the rotomax due to higher inductance. And it is due to higher turn count. And the only way to maintain same KV at higher turn count is smaller magnets. And this explains why it is cheaper.
Don't forget that the pole count is also a factor! Together with the thinner laminations this must make a big difference.

As they are from different sources (or were...) I'm not sure it's necessary to explain the price difference in that way.
I can't see how lamination thickness in this particular application (200μm vs. 350μm) can affect waste energy by more than few %.
Teeth count is the same, only magnet pole count differs, which gives you different electrical phase frequency, but PWM carrier is still the same and it all (ok, most of it) counts mostly to inductivity of the windings.
 
circuit said:
I can't see how lamination thickness in this particular application (200μm vs. 350μm) can affect waste energy by more than few %.
Eddy current losses go up with the square of the flux frequency and the square of the lamination thickness..... 28 poles @ 0.35mm vs 20 poles @ 0.2mm

circuit said:
Teeth count is the same, only magnet pole count differs, which gives you different electrical phase frequency, but PWM carrier is still the same and it all (ok, most of it) counts mostly to inductivity of the windings.
Higher pole count will give a lower Kv, all else being equal. Right?
 
Miles said:
circuit said:
I can't see how lamination thickness in this particular application (200μm vs. 350μm) can affect waste energy by more than few %.
Eddy current losses go up with the square of the flux frequency and the square of the lamination thickness..... 28 poles @ 0.35mm vs 20 poles @ 0.2mm
Less poles = bigger poles = same amount of lamination. So I'd leave pole count out of this equation.
OK, let's calculate Eddy. 20/35 = 0.57. Square that and we get 0.33. OK, I agree, based on your statement of square law, thinner lamination leads to three times lower Eddy losses. BUT! How big part of the whole power loss is caused by Eddy? 5% would be my guess. So you lower the no-load losses like by 2% or so.
We all see that no-load losses of these motors differ more than by 2%. And more than by 200%. Probably close to 1'000%.
To me this is just a marketing trick.

Miles said:
circuit said:
Teeth count is the same, only magnet pole count differs, which gives you different electrical phase frequency, but PWM carrier is still the same and it all (ok, most of it) counts mostly to inductivity of the windings.
Higher pole count will give a lower Kv, all else being equal. Right?
Wrong. It depends on winding scheme. For example there is a huge difference between DLRK and ABC schemes.
 
I added the CA-120 dimensions in the earlier post...


Circut,
please expand on this thought:
For example there is a huge difference between DLRK and ABC schemes.

I havn't yet re-configured a test motor for ABC but geometricly, it looks like it would produce a higher kv as the ellectrical transactions to produce a a single rpm is lower on an ABC motor.

I do know lowering the pole count will make the motor spin faster for any given voltage.

you have any examples that show this?
thanx.
 
Thud said:
I added the CA-120 dimensions in the earlier post...


Circut,
please expand on this thought:
For example there is a huge difference between DLRK and ABC schemes.

I havn't yet re-configured a test motor for ABC but geometricly, it looks like it would produce a higher kv as the ellectrical transactions to produce a a single rpm is lower on an ABC motor.

I do know lowering the pole count will make the motor spin faster for any given voltage.

you have any examples that show this?
thanx.
I was reading a lot on this topic before rewinding my CA80100. At the end I started drawing flux vectors, doing FEMM and 3D modeling... I no longer have the drawings and don't have the time to draw it once again, but there was a good website somewhere - will drop a link when I find it. It clearly shows that you can't simply change the pole count and assume that your motor will spin fine. There is no direct relation between pole count and KV ratio, as the reduction ratio (between mechanical and electrical RPM) not only changes non-linearly, but also may change the direction. And there are only few teeth/pole combinations that actually work. Especially for ABC scheme, which is usually used in induction motors.
For example, the best scheme for CA80100 is DLRK, as the teeth/pole ratio is not suitable for ABC.

This page looks similar to what I was reading:
http://translate.google.com/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.powerditto.de%2Fexperimente.html
 
These are the most useful references:

http://www.powerditto.de/Kombinationstabelle.html
http://www.powerditto.de/bewicklungsrechner.html

http://www.emetor.com/edit/windings/
 
Miles said:
These are the most useful references:

http://www.powerditto.de/Kombinationstabelle.html
http://www.powerditto.de/bewicklungsrechner.html

http://www.emetor.com/edit/windings/


Yes, that is the one. Thank you.


Anyone has thoughts on inductance of the Rotomax motor? Based on turn count and that CA120 is about 5μH/phase, this one should be around 36μH/phase, giving some better time for cheap controllers, at last.
 
sorry to have missed the excitement till now, i have a bare stator for a ca120 for the measuring up if needed, and it may be time to measure one with std windings for inductance.
i have a good function generator and a 2 channel usb cro, the gen has 50ohlm output impedance but i have seen the waveform and can get a good comparison (rather than an actual inductance number) i could compare the max voltage seen to a 130kv or 180kv 80-100 that IS known?
any idias on how -maby a 20khz square wave into windings and look for some volts during the switching etc? arlo is "asking" about this also so ill go ahead if folks know how to do it half properly?

by the way the 20 pole one may be a hacker a200? and i nilly bought one for $1300 a while back.
 
nope, i get 80 lams total and a 30mm length so its about .37mm, and i checked 4 motors, all bought at different times.
i spose theres paint etc so maby they are .3mm?

are you sure your not smoking crack thud?
 
toolman2 said:
im real keen to hear about anything thinner that anyone has seen, etc.
Hmmm. My assumptions were based on 0.35mm lams but Thud measured his as 0.2mm.... Maybe they upgraded at some time? If so, I would have expected a significant difference in the no load current at higher speeds......

See: http://www.endless-sphere.com/forums/viewtopic.php?p=631657#p631657
 
not so much crack as rum & long hours.....
I just pulled the band-aid off my much abused stator for a more scientific test than a eyeball & calipers on the end of the stack to an actual measurment......I stand corrected. :oops:
I get 0.34mm
P9050093.JPG
 
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