Te He...... To be a fly on the wall in the shop when something goes wrong at least you would learn lots of new compound four letter words!
After many years in the military, Im sure i could probably even add a few new ones to somebodys repertoire
Probably so, but I am a sailor One of my friends who is driving a LONG way for the Memorial day 3-Day (aka Spring Regional) has told me that his boat may not be done in time. So I will be concentrating on Dupliex (someone else in my area already has a Foch) so that he can help me sink the Allies I will still post pics of Othar's new clothes
Although I still need to do some more work on getting the gyro to precess properly to provide the balance I am hoping for, I will try to post a few pics of what I have currently, and try to explain a little about some of the parts as I get time. the kitchen remodel is far enough along that I can take some time here and there to do fun stuff... like boat building As I previously mentioned, there was no real chance of getting an off the shelf motor small enough to drive the rotor, and still have the power to get it up to speed without frying itself trying. I even tried to make a few rotors that pressed over the outside of the can on a few different outrunners. It had a lot of promise, but ultimately it seemed like too much stress on the small motor bearings to carry the heavy rotor at high rpm, a task the bearings were never really intended for. That being said, I decided that it would save a ton of weight and space to build the motor into the gyro, if it were possible. I spent some time studying how the motors work, and found a few sites that have pretty helpful write ups and instructions on how to wind the motors yourself. The design I wound up using was an outrunner type motor, where the gyro rotor doubled as the "can" for the motor by carrying a flux ring and magnets in an internal bore.
I thought I had uploaded a few photos of individual components, but I guess I just thought about uploading them, so I will have to try to do it tomorrow and continue from there!
Please do. I have enough knowledge to grasp that you are talking about something really neat but not enough to really understand what's going on.
Here is a pic of, from left to right - 1) The Bottom side of rotor with flux ring pressed in and magnets in place, 2) Same as far left without magnets, 3) Flux ring before pressing and final machining .
This was an interesting part to make due to the flux ring. Oversimplified, the flux ring is necessary in a brushless motor to essentially intensify and "complete" the magnetic field generated by the magnets, which increases torque and efficiency. This allows using much smaller magnets than would be required without the ring. From what I can gather, a motor can run without one, just not as well as with one. The best material for the ring is one that is highly magnetic, and to be effective in any capacity, must have some magnetic properties (for example, aluminum won't work). Since the rotor must spin at a high rpm (I was looking for around 20,000), balance is essential. This means that concentricity between the flux ring and rotor are important, even more so when you figure in the weight of the magnets that will be attached to the flux ring. Proper clearance is also important (I wanted between .004 - .006" air gap between the magnets and the stator). To achieve all these goals, I built the flux ring first. Then I machined the rotor, but before cutting it off, I pressed the flux ring into it while it was still in the machine. After pressing I would machine the Internal diameter of the flux ring, which would make it perfectly concentric to the rotor. I machined a series of 12 flats around the I.D. of the ring, each just wide enough to cradle one of the flat magnets. This is not necessary, but it did provide an awesome method to make sure that the magnets were properly spaced around the flux ring. It also has the side effect of improving the magnetic field a bit over what a round ring would have. I decided to use only 6 magnets. Adding more would have made a more torque and efficient motor which would have run at a lover kv, but I only needed enough torque to get it up to speed, and I felt I really needed to be around 20,000 rpm. Everything is a trade off with a brushless motor design. If you do something to increase the kv, torque and efficiency suffer. If you do something to increase torque and efficiency, max rpm for a given voltage (kv) will be lower. More magnetic power means more torque - but lower rpm. The type of winding on the stator effects output as well. The number of winds and gauge of wire used for winding has an effect. Its all about deciding what you need from the motor and choosing the proper combination of several different variables to get it there. It took me 2 tries at winding to get the rpm I wanted. It's still draws more amps than I was hoping for, I may look into rewinding the stator again to try to improve that without killing the rpm.
That is crazy cool. But if it's brushless, apart from changing the number of poles wired, windings should not affect the kV value of the motor; they will affect how much torque the motor can produce at a given RPM, though. And current draw is proportional to torque exerted. The speed of the brushless motor is set by the ESC (and thus, by you).
I probably didn't communicate my intentions well when I attempted to explain the windings. Like I said earlier (much earlier.... - like months ago!), there isn't a one stop shopping place so to speak on all the info you need to build a brushless motor to meet a specific need, so I had to get what I could from a number of sources. This one does a pretty good job of breaking it down to and understandable level (very useful for me - my electrical knowledge is rather limited). http://www.flyelectric.com/ans.kv.html
I should also mention for those who are interested that there are a handful of different ways (or patterns) that the stator can be wound. Star, Wye, LRK, CDRom (pretty much a star pattern), and probably others. Each has its advantages and disadvantages, and each has a pronounced effect on motor performance characteristics. Most are only compatable with certain stator tooth counts and number of magnet combinations.
Windings (number of turns) directly affect the Kv. Kv units are rpm/volt, which is constant for the motor. The ESC just reduces the voltage to control the speed, it cannot change the speed that the motor will turn for a given voltage. Maximum torque (i.e., stalled condition) is more a factor of the rotor diameter and length but is of course also dependent on the magnetic force. Each time you add a turn of wire to a stator tooth, you increase magnetic force and decrease the Kv. Additional windings result in longer wire, increasing resistance, and sometimes smaller wire is required to allow fitting all the winds, further increasing resistance. Unfortunately, the only way to discover the performance of the motor is to make it, and then re-wind it when it is outside of your needed values. Which is only slightly less fun then sitting on the couch winding a motor and suddenly realize that you have lost count. I lost count of how many times that happened to me. Cannonman - I realize that you are trying to reach a pretty high rpm, which is easier done by using 6 rather than 12 magnets. However, to reduce current draw and only slightly reduce your Kv, you might try 12 magnets but with a Delta termination instead of a Wye termination. At least I'm assuming you used a Wye termination. That would beat rewinding with thicker wire to reduce the amps.
I agree that the number of windings (and number of poles) determine a motor's kV rating. But. To run a 3-phase DC motor, the ESC controls the frequency applied, which creates a rotating magnetic field on the stator, which determines the rotor's speed within the envelope provided by the voltage applied and kV rating. The ESC only cares about 2 things: the rpm produced and not exceeding its overcurrent setpoint. It does not directly vary the current to the motor. It maintains a speed and the load determines the current draw at that speed*. Torque is proportional to current. It's an induction motor, it has to obey the same laws that other induction motors do. * there is a %slip that increases with increasing load, but that's what drives the increase in current as load increases. I have a fair bit of experience with ESCs (up to 100kW)(notkV, kW) and until I get some VERY convincing evidence to the contrary, that's my understanding of it
It's a common misconception that the ESC varies frequency. The frequency of the PWM is fixed and the duty cycle changes, changing the voltage, while the uP determines when to change commutation of the winds based on position. Refer to Microchip AN885 for an explanation of BLDC motors and why they are considered synchronous rather than induction motors. In hobby controllers, only helicopter ESCs have a feedback loop to maintain a constant speed, in the rest of the ESCs the speed and current change based on load as described. 100Kw? Sounds a little anemic to me... I worked on some 60,000 Kw motors in the Air Force. Actually we had two of the 80K horsepower and and two 60K horsepower motors connected to a single shaft - but they had variable frequency drives
I didn't mean to hijack the thread, sorry. As an act of contrition and to get the topic back on small destroyers, I'll share one of my top secret projects. I just ordered a compact BB magazine 3D printed from Shapeways. It is a continuous spiral and should hold about 45 BBs, so it will require a short hook up tube to get the full 50. It's about 7/8" high and 1 1/16" in diameter. Now I have to wait 3 weeks, since I don't have a home 3D printer like Tugboat. Price wasn't bad, just over $10.
How thick are the walls of the magazine, and what material did you choose? They have several cool ones that I just can't print in (like metal!).
The material is white strong and flexible. Minimum thickness is 1/16" - from the outside edge of the spiral to the outside edge of the cylinder - but thicker in most places. Getting it printed in stainless steel would raise the price to $50. I would lose the weight savings I need for the destroyer, but it should be indestructible.
