|Aug 08, 2007, 11:02 AM|
Twin Electrics: some food for thought and success
I've been reading around several forums and, as always, the really good guidance and alternative points of view far outweighs the bad. But there has been some stuff that concerns me on this subject. I'll save my other current rants about cramming as many watts of power into a small parkflyer airframe so it goes 120 mph -- I particularly got a kick out of the guy with a Corsair that was doing the scale equivalent of 1500 mph -- for another day. Today, I wanted to pass along some GREAT information on twins. I have found it to be very beneficial.
I've come across recent posts advocating stuff like using a separate channel for each motor on an e-power set-up for "true twin control;" others saying that using two different speed controls from two different manufacturers at different amp ratings really is no issue; arguing you can still maintain a twin model in flight just fine on one motor if one has ESC cut-off -- yeah if you know how to fly on only rudder and to only turn into the running motor. Ugh. And, one that I often personally struggle with -- balacing the ease of installing separate batteries in each nacelle rather than sourcing from one battery or parallel set in the fuselage providing energy to both motors. So, I wanted to pass along this piece below from Joe Ford at Castle Creations(firstname.lastname@example.org) who essentially relates the wisdom of the big boss, Patrick. It's a good quick read...
Brushless Multi-Motor Power Systems
If you’ve ever heard a multi-motor aircraft fire up, there’s no way to describe the feeling it gives you...even if you’re only a spectator. The sound of the props beating the air just screams cool. Since I began working at Castle I’ve received literally hundreds of inquiries on the necessary components, speed controller programming settings, and wiring arrangement needed to be successful with this type of system.
The most frequent question I receive is “Can I run two brushless motors off a single esc?”. We highly recommend using a separate controller for each motor. While you can run two motors off a single controller in some situations, there are several things one must keep in mind. As I cannot come close to explaining these reasons as well as Patrick, I’ve taken the following excerpts from his posts on RCGroups.
First, the motors must be very closely matched– within 1% or so on Kv. If they are not perfectly matched, timing from the controller is averaged between the difference on the two motors (this happens because the back-EMF pulses average when the wires are paralleled on the motors. This will cause higher surge currents to the motors and will tend to increase the temperature of the controller and motors. They will not always start correctly. If they don’t start correctly (for example, one motor starts, and the other does not) the controller can be damaged. If two motors are run, they must be started on the ground, and there should be no attempts at restart in the air. If one motor fails to start, the power should be shut down immediately. If the user runs two motors on a single controller, the controller should be derated by 30% (for example a 45A controller should be used in an installation which draws no more than 31 amps on both motors). If two motors are run on a single controller they must be the same type and wind, and must be well matched. The motors should be started on the ground, and should not be restarted in the air. High inertia loads (for example, large props on small motors through gearboxes) shouldn’t be attempted in a dual motor installation.
Generally, motors with kvs that are closely matched are also the more expensive ones...food for thought when deciding whether or not to attempt such a setup. The reason for this is that there is much less variance in these motors. In most instances, motors will not start properly and will not run efficiently...leading to heat in both the motors and the controller used, as well as a lower power output. Whether you’re willing to “roll the dice” so to speak, is entirely up to you. I personally prefer the tried and true method of using one controller per motor.
When using one controller per motor, there are several things that one must do to ensure proper motor operation. Let’s start with the power system components required. While we have had some reports of the Thunderbirds being used successfully in this application, it is not recommended. They do not have all the recommended programming options, and long term effects of use in this manner are not known since they were only released a short time ago. The Phoenix line has been used for quite some time with excellent results, and the plethora of programming options make them ideal for multi-motor applications.
There are three things to remember when selecting the controllers to be used. First, make sure you use controllers that have the same amp rating. There are several differences between controllers, even when made by the same manufacturer, that can cause issues. The most notable of these are resistance values and overall design. Second, be sure to use controllers that are the same generation. Here there are many more issues of concern that can cause improper motor operation. Different generations of controllers can use different processors, FETs, resistors, etc. Obviously this can vastly affect operational performance from one motor to the other. Generally, our hardware does not change very often. If you purchase the controllers within 3-4 months of each other they are more than likely the same generation. Finally, make sure you are using controllers that have the same firmware...the reasons should be obvious.
In addition to power system requirements, there are also specific settings that need to be programmed into the controllers in order to assure proper operation. First, you MUST disable current limiting...COMPLETELY. Do not set it for insensitive or you may end up paying the price. There is no risk involved if you know your full throttle current is at or below the maximum current limit of the controllers. In short, this will ensure that the motors keep running no matter what. Having one or several motors shut off in flight and the remaining motor(s) continue running will put a serious dent in your ego...not to mention your wallet. Second, you must use fixed throttle. This gives the ESC control over the endpoints instead of calibrating each time you power up to the transmitter endpoints. It will ensure that all motors turn at as close to the same rpm as possible. Third, set the controllers for soft cutoff. This ensures all motors will continue to run when they are at the voltage cutoff point. If you use hard cutoff, one motor will cut off before the other(s) and the pack voltage will recover, increasing the rpms of the remaining motor(s) and sending you into a death spiral. The final requirement is to program the controllers with the same soft start option. This way, all the motors will start in unison.
Wiring is the final hurdle to overcome and we’ll be airborne. A common misconception of the Phoenix controllers is that you must disable one (or more) of the BECs (Battery Eliminator Circuit) in order to keep them from interfering with each other. This is not the case. The Phoenix BECs were designed to be run in parallel with no conflict...this means you can gang several BECs together to run even more servos and other devices. Retracts, gun turrets, lights, bomb bays, you name it.
Make sure you test the setup on the ground prior to putting it in the air, obviously. Run the motors at ½ throttle while simultaneously exercising all the servos. Do this for approximately 1 minute. If you hear the motors cut out momentarily you know the BECs are having a hard time with the setup.
Now, back to the wiring. Use a “y” connector (or several if necessary) to parallel the speed controllers, and plug the single connector into the throttle channel on your receiver. The absolute most important thing to remember is to run ALL of the controllers off a single battery pack, no matter how many motors are in the setup. You can parallel several packs together to handle the current draw if needed. This will ensure that all motors are receiving the same voltage, and will not hit the cutoff voltage before the other(s).
That’s it...you’re good to go. Get out there and make everyone in the perimeter flock jealous towards you. As always, happy landings!
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|Aug 08, 2007, 04:26 PM|
Thanks for the posting. Its very timely for me as I am just considering venturing into twin motors for the first time and its nice to have a concise summary like that to work from. I tend to use Hyperion Titan ESCs. If anyone knows of any quirks with these ESCs re twin motor set-ups I would appreciate some advice.
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