Help! How to read Motor Ratings?

[JasonK]

I picked up an FT H pack which has 2 1106-4500kv motors in it. The store page indicates 2, 3, 4s on the included 3" props. When I got mine, I did the 'make sure it doesn't get to hot test and found that 2s was fine, however 3s with the given prop only took 10-20 seconds until it got to hot to touch. I ended up ordering some smaller props (2.5x3.5x3) and 3s still got to hot, just took 45-60 seconds instead of inside of 20 seconds.

I finally tried running the motor on 3s full open with no prop (so no load other then the motor bell) after 60 seconds, I could grab the motor, but it was right at the edge of 'can keep a hold of it' for 20 seconds. I didn't try a longer time as I didn't want to keep getting the motor to hot.

Am I miss understanding what the ratings mean and the rating only mean that they are ok to run, but with an expected duty cycle (for example, I wouldn't expect a quad to be pushing 100% very often or for very long and they typically have fairly high max thrust to weight ratios I remember seeing something like 8:1) that is well below 100%? or is there perhaps something else going on here?

 

[Merv]

I can see how you read the web site, it is not very clear.

Here is a copy & paste from the site.
Mixing great efficiency and power from the Flite Test gremlin class quad line-up. The Power Pack H features 1106 motors swinging a 3" prop. This combination allows for quiet efficiency on 2,3,or 4 cell batteries.

I believe the "This combination" refers to "Mixing great efficiency and power ". Not the 3" prop.

If you want to run 3-4S, you will need a smaller prop. Most all electric motors will run hot without any airflow. A prop is needed to cool the motor.

 

[JasonK]

sure... I tried a smaller prop which didn't seem to work even on 3s... are you saying I need an even smaller prop to get up to 3s?

I also did some digging and found this, which seems to be the same (or very similar) motor, which says 4500kv motor can do 4S for a 2in class prop, but 3S up to 3in.

Either way... I am starting to think that I don't have a combination that is going to get me what I need. Unfortunately my flight controller is only rate for 3-6s, so I don't think I can use these motors ;( I was hoping that the smaller props (which were listed on the gremlin section of the store) would work at 3s and still get me the thrust I needed.


I did try it with a smaller prop (2.5x3.5x3), but that only got me up to ~45-60 seconds until to hot.

 

[Merv]

... are you saying I need an even smaller prop to get up to 3s?

Yes.
You could try 2 blade 2.5x3.5

 

[JasonK]

yah... I don't know that that will even get me the thrust I need at that point. Was looking to be able to Vtol 250g with 2 of these motors. The thrust calculator spreadsheet says that only gets ~105g thrust at 0 speed for 3s on a 2in prop, where 2s at 3in gets ~193g. (not sure how good the sheet is for these tiny props).

The motors on 2s with the included props were fun on a dart until one of them vibrated its mounting screws loose and yanked out one of the wires out of the motor. I was testing out the 1 still good motor in more detail before replacing the bad one, trying to make sure I can get the behavior I needed