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Contra-rotating motor set up help

Old 04-09-2010, 11:41 PM
  #1  
Hookpilot
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Default Contra-rotating motor set up help

Got a strange request: I've scratch built an RC scale British motor torpedo boat. It has operational torpedoes. The boat is 60 inches long. The torpedoes are 17.5 inches long and 1.5 inches in diameter, and I want to convert them to contra-rotating propeller systems. There are several contra-rotating brushless motors out there that will fit inside the torpedos (I.D. 30mm), but they all say I need to run these motors through an ESC. The torpedoes will not need speed control. Once launched out of the tube a magnetic reed switch will start the motors and run them at full power until they hit the beach or drain the battery. I don't need endurance here, just a lot of power. The question is - can I run brushless motors with a simple on/off switch without damaging the motors, or do I have to spring for two brushless ESCs? I'm after max speed for the torpedo. The boat will be moving at about 10 mph when it launches the torpedoes so I don't want to run over them before I have a chance to turn the boat. Any suggestions guys? See the boat in action at: http://homepage.mac.com/hookpilot then go to "Hobbies" to see build photos and a video of the boat underway.

Mike in Honolulu
Old 04-10-2010, 01:41 PM
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Dr Kiwi
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Default RE: Contra-rotating motor set up help

A brushless motor NEEDS an ESC to run... you cannot operate it in any other way (unlike a brushed motor which you can connect directly to a battery and run at WOT).
Old 04-10-2010, 03:03 PM
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Hookpilot
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Default RE: Contra-rotating motor set up help

Dr Kiwi:
Thanks for the info. I figured as much. I just found a thread here where someone is running a two motor contra rotating system off one ESC. How would I wire a single ESC to the 6 wires coming off the two motors to achieve an ON/OFF control scheme?

Mike in Honolulu
Old 04-11-2010, 09:23 AM
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Dr Kiwi
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Default RE: Contra-rotating motor set up help

They must have got lucky... typically, trying to run two brushless motors off one ESC is an exercise in frustration and futility... one or both won't start... they won't both run at the same rpm.......
Old 04-11-2010, 05:39 PM
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Hookpilot
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Default RE: Contra-rotating motor set up help

Dr. Kiwi:
I'm going to use 2, 15A ESCs for the AEO-RC CR23M motor setup. How do I run them together with a simple ON/OFF switch?
Old 04-11-2010, 09:36 PM
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Default RE: Contra-rotating motor set up help

ESCs can't be directly operated with a simple on/off switch. Just like R/C servos, ESCs require a very specific pulse train in order to operate.

An inexpensive solution might be to utilize a servo tester or servo adjuster. I've used one to operate brushless ESCs for testing motors. (So has Dr Kiwi.) You might be able to insert a switch in the signal line from the servo tester to the ESCs, adjust the servo tester for full throttle, then close the switch in the signal line to start the motors.

Mind you, I have never tried switching the signal line as I have just described.

Here is the servo adjuster I have used:
http://www.hobby-wing.com/e-sky-serv...-ek2-0907.html

- Jeff

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