Logged in users > User Diaries
Dual Rotor Toroid Core PMA build
MagnetJuice:
--- Quote from: CraigM on July 12, 2019, 02:59:37 PM ---I've seen photos of burned out coils in axial flux type stators. Does the burn out happen when the coil temperature exceeds the insulation temp rating, which in turn breaks down the insulation and causes a short within the coil?
--- End quote ---
Insulation breakdown and failure is always the cause of stators burnouts. When the insulation fails, the wires shorts out. Overheating of the wire because of excessive current flow through the coil is one of the main causes of this breakdown. However, many other factors contribute to insulation breakdown.
Other factors that contribute to the deterioration and breakdown of the wire insulation are moisture, impurities on the wire, wild temperature swings and vibration within the coil.
Turbines that are powered by the wind are going to have drastic temperature swings, from very cold to very warm. There is nothing that we can do to eliminate those temperature swings. However, there are good practices that can be implemented during the winding of the coils that can minimize the possibilities of stator burnout.
Make sure that the wire is clean and dry before winding the coils. Wearing clean gloves during winding eliminates the possibility of oil from the hands getting on the coils. Superglue (Cyanoacrylate) should not be used on the coils. That type of glue breaks down over time because it is not designed to resist high temperatures. Superglue will become chemical impurities after the high temperatures breaks it down and stays within the coil. Regular superglue breaks down at around 100C, and the high temperature superglue breaks down around 135C.
If the coils are not wound tight, moisture can get inside the coils. If those coils are to be enclosed in resin, the moisture remains trapped inside the coils and over time will contribute to the deterioration of the insulation.
The best way to avoid these problems is to make sure that the coils are tightly wound. If high temperature epoxy can be applied during the winding, that would be the best protection against moisture sipping inside the coil as well as minimizing damage from vibrations. There are a few high temperature epoxies suitable for this application. MG Chemicals 832HT is one of them. Tightly wound coils also contributes to the efficiency of the alternator.
If the coils are not going to be enclosed in resin, the exposed part of the coils should be coated with a thin layer of high temperature epoxy to keep moisture out and impurities from accumulating on the wire.
The alternating current flowing in the wire causes vibration within the coils. During every cycle, the AC current reverses the flow of electrons. That happens many times per second. If the coils are not wound tight, that causes a jerk in any loose wire and it will hit and rub the adjacent wire, weakening the insulation and with time contributes to its breakdown.
Some builders get in a hurry to wind their coils and don't pay much attention to these details. Following these good practices during winding of the coils can help to insure that the alternator lasts a long time.
Another cause of stator burnouts is the magnet rotor hitting the stator. Worn out or poor quality bearings with excessive play can cause the magnet rotor to rub the stator.
The magnet rotor can also rub the stator if the coils expand and increase in thickness because of high temperatures. If that happens and there is not sufficient gap between the magnet and the coils, the stator could get hit by the magnets.
For stators that are to be enclosed in resin, one way to help with the cooling is to drill through the center hole of the coils after the stator is cast. That will allow air to flow and remove some of the heat.
I hope this is helpful.
Ed
CraigM:
Hello Ed (MagnetJuice),
Yes, that was extremely helpful, thank you for providing the information.
I researched MG Chemicals 832HT and see how this epoxy is purposely manufactured for high heat applications and commonly used for potting electronic boards.
Once I have my single coil test complete and I have a better idea of wire size and number of turns required I'll be sure to use this during final coil winding. Probably will need to apply with a brush as each coil is wound.
I ordered a laser tachometer and currently working out how to spin the PMA at a desired RPM using a pulley and belt system on my cheapo' drill press.
Looking back I can see I should have used the 832HT when building the steel core. The core was tacked together with CA glue when winding and then I applied several layers of thinned epoxy to the surface of the core. The epoxy wicked into the core and after several applications if no longer wicked and started to pool on the surface. The epoxy I used has a service temperature of 300F but will that be high enough? Time will tell, experimental stamp on this.
Thank you,
CM
CraigM:
SparWeb,
Thank you also for your reply. You have consistently provided valuable information in my effort to make something of this build.
Much appreciation!
CM
CraigM:
Performed an open voltage test today. Please review and let me know if I did this correctly. Not really sure what this is telling me... if I did this correctly am I seeing close to cut in voltage for 24 volt system using 20 AWG and close to cut in voltage for a 12 volt system using 15 AWG? Also not sure what I want/need for RPM cut in, is 190 RPM a bit low?
What other information do I need? Assuming I need to know wire resistance (Ohms).
Single coil test – 190 RPM
20 AWG, 22 turns;
1.225 VAC X 8 coils in series = 9.8 VAC
9.8 VAC X 1.73 (star connected 1 WYE stator) = 16.954 VAC
16.954 VAC X 1.4 (AC RMS to DC conversion) = 23.74 VDC
17 AWG, 16 turns;
.895 VAC X 8 coils in series = 7.16 VAC
7.16 VAC X 1.73 (star connected 1 WYE stator) = 12.387 VAC
12.387 VAC X 1.4 (AC RMS to DC conversion) = 17.34 VDC
15 AWG, 12 turns;
.685 VAC X 8 coils in series = 5.48 VAC
5.48 VAC X 1.73 (star connected 1 WYE stator) = 9.48 VAC
9.48 VAC X 1.4 (AC RMS to DC conversion) = 13.27 VDC
Here's the test setup.
and useful chart I need to learn more about.
Thank you,
CM
MagnetJuice:
--- Quote from: CraigM on July 21, 2019, 07:50:42 PM ---I have to admit the electrical side of things is my weak point and what I need most to study and learn about.
--- End quote ---
If your weak point is the electrical side you are learning fast.
Your calculations are good with the exception of one small figure.
I am going to use the calculation with the 16 turns of 17 AWG wire.
Your calculations are good up to the 12.387 VAC.
The error you made is that the 12.387 VAC is not RMS voltage. It is Average voltage; therefore, it has to be multiplied by 1.57 to obtain your DC voltage. Then you have to subtract .7 volts per each rectifying diode. That will be a total of 1.4 volts for the two diodes on the bridge rectifier.
12.387 x 1.57 = 19.45 – 1.4 = 18 VDC
As far as deciding the cut-in RPM, first you have to decide what you are going to be using to power your alternator. VAWT or HAWT. I think SparWeb or someone with more experience in that area could help you with that better that I can.
By the way, that is a nice test setup.
Ed
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version