

Mar 22, 2004, 12:57 PM  

Star or Delta  that is the question
Hi guys,
I sorry for a long posts, but I want to give you my conclusions I got from the calculations I did. If you don't want to go through all the details, please jump to the conclusion post. But please, read at least the assumptions I made, so we can discuss on the same level. I did some thinking about star (wye) and delta connection  which one is better, or maybe another way: what the difference in performance is.. It's common opinion, that Star is for torque, Delta is for speed. Before I present my calculations and conclusions, I want say it at loud: "I'm not and Alpha and Omega". I might be wrong. If so please correct me as soon as possible. But please don't just say: "You are wrong"  show me "where I'm wrong". Let's start with assumptions. 1. We used optimum wire diameter and number of turns. Optimum, that means we will be running the current through the coil that will go up to the core saturation level. That means the more power we want from the motor, the more current we need to run, that the less number of turns we have to use. That goes up to the limit, where ESC will loose a sync. So let's assume we used number of turns = n for optimum current = Imax. 2. We run the motor under the optimum load, that means no pole is saturated (the current in the single coil in no grater than Imax) 3. ESC is running square wave (not a sin wave) so the motor behaves a little different than industrial alternating current synchronous motor. ESC is switching power between ABC in the way that one wire has plus, other has minus and the last one is open (not connected). So at any moment only two wires carry the current and one is neutral. For static calculation (like a snapshot) let's assume the A is plus, B is minus, C is open. See the picture below for reference. Continued on the next post.... 
Mar 22, 2004, 12:58 PM  

Star or Delta  that is the question
So let's analyze Star connection:
The total current going through the motor is Imax (green line) going from A to O (center) and then to B. The power of the motor = U * Imax The torque from a single pole = I * n (n = some constant factor from number of turns) The motor's torque is sum of torque from two poles = Imax * n + Imax * n = 2 * Imax * n Because power = Torque times rpm then: RPM = (U * Imax ) / (2 * Imax * n) = U / (2 * n) About the efficiency: for a single pole copper loss = R * I^2 So for Star configuration: Copper loss = 2 * R * Imax ^2 Efficiency = 1  (loss / power), so for Star configuration: Copper efficiency = 1  ( 2 * R * Imax^2) / (U * Imax) = 1  (2 * R * Imax / U) Now it's time for Delta: The total current going through the motor is sum of Imax (green line) going from A to B and 1/2 Imax (red line) going from A to C and then to B. The power of the motor = 1.5 * U * Imax The motor's torque is sum of torque from all three poles = Imax * n + 1/2* Imax * n + 1/2* Imax * n = 2 * Imax * n Because power = Torque times rpm then: RPM = (U * 1.5* Imax ) / (2 * Imax * n) = 1.5*U / (2 * n) About the efficiency: Copper loss = R * Imax ^2 + 2 * R * (1/2 * Imax)^2 = 1.5 * R * Imax ^2 Efficiency = 1  (loss / power), so for Star configuration: Copper efficiency = 1  ( 1.5 * R * Imax^2) / (1.5 *U * Imax) = 1  ( R * Imax / U) Continued on the next post.... 
Mar 22, 2004, 01:00 PM  

Star or Delta  that is the question
Conclusions.
It is what I expected, and all my experiments confirmed that. Remember that we are trying to get the most of the power from the least of weight and size or torque with the minimum current. Maximum power: Delta connection can deliver 50% more power than Star before the stator is saturated. Maximum torque: Maximum torque of Star and Delta is the same, but will take 50% more current to run (giving 50% more rpm than Star). Torque from current: If you run both motors on the same current (For the same power) Delta will have 50% more rpm than Star, but Star will deliver 50% more torque than Delta. Loss: Copper loss is 1.5 times higher in Star than Delta, so the copper efficiency of Delta is much better. How does it translate for the speed and static thrust? Here it is: For maximum power from the weight and size use Delta. It can deliver 50% more power than Star, and at maximum power it will deliver the same torque at 50% higher rpm. But it doesn't mean you can use the same prop size. Because the prop will be running higher rpm, you have to use smaller prop, so the thrust will be the same. For the best torque to power ratio use Star. In example 10 x 4.7 in star will give the same static thrust as 8 x 4.7 in Delta, but Delta will take 1.5 more amps from the battery. Continued on the next post.... 
Mar 22, 2004, 01:01 PM  

Star or Delta  that is the question (The last one)
Going beyond the power limits.
What If we want beyond the efficiency level and oversaturate the stator a little. Let's say in that condition Stator will deliver 50% more magnetic field strength from 2 times more current. In Star configuration if we run 2 * Imax we will oversaturate both poles so the torque would be 50% more. In Delta connection we will oversaturate one pole only, and other two will go to the saturation limit so the torque would be 75% more. I still don't know what is the saturation level in our small motors and how we are doing running 5A through 24 turns where the single pole has 1.6mm x 5mm iron core area. But for sure with Delta configuration we are on the safer side than with Star. RysiuM 
Mar 22, 2004, 01:03 PM  

I'm already using 5x5x1 N45 magnets with less than a .5mm gap and as far as decreasing or increasing windings there are limits. With these 20mm stators I can get 16 to 18 windings max. After that I get the bell rubbing on exposed windings. On the minimum side. I've gone down to 8T windings in a wye configuration and the relative thrust is still less than the 14T winding delta. This whole experiment is to get a cheap upgrade to the stock EDF50s with the hopes of the A10 flying faster and better. All CDRom tests have been using the 7.4v 2S1P Kokam 1500s.
I have a stock EDF50 hooked up to a 9.6v 8 cell NiMH and I compare the output of that ducted fan to that of the CDRom modified ducted fan. It's a relative compare by placing my hand behind the unit as I have no measuring device constructed yet. Bob 
Mar 22, 2004, 01:16 PM  

Thanks rysium that helps some. I was composing my last post while you were posting. For some reason when I tried to highlight the [B]"Stop using delta"[B] quote from KreAture it didn't work and it looked like I was making that statement. I think I have to manually input the bold commands, we'll see with this post.
Bob 
Mar 22, 2004, 01:25 PM  
USA, NY, Jamestown
Joined Jun 2000
2,908 Posts

removing the magnet ring
rysium gotta thank you for the advice to use acetone to soften the adhesive for the magnet ring. Worked like a charm. I tried heating the bell with a paint stripping gun but it did not work and was hard to handle when hot. Soaked the bell overnight in acetone and magnet ring came out (in pieces) very easily.

Mar 22, 2004, 01:26 PM  

Rysium,
I think you are compareing apples with oranges (sorta). You are compareing a Y wind pulling a total of Imax current to a Delta wind pulling a total of 1.5*Imax current (Imax + 1/2 Imax) To be totally fair you would have to compare the same winds pulling the same TOTAL CURRENT. anyone can get more torque with more current. Toby 
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