Dual motors, Dual BECs (Tutorial)

Discussion in 'Electronics' started by LukeWarm, Jan 4, 2012.

  1. LukeWarm

    LukeWarm Top Gun

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    I am an electronics technician. I saw the video at the top of the forum on hooking up dual motors. If you don’t know why you do it, REMOVING ONE RED WIRE IS THE SAFE THING TO DO. BUT, if you are using the same battery (or two batteries Y-connected together), you do not need to pull the one red wire. The only reason to do that is if you are powering the ESCs with separate batteries; if there is a difference between the batteries, they will try to equalize each other through those little wires. Five or ten amps going through those little wires will melt them and destroy both BECs.

    Powering the identical ESCs with the same power source (separate batteries Y-connected together, or one battery) prevents this and always gives each motor equal power. That is what DAVE suggested on the video. DAVE’S WAY OF CONNECTING THE BATTERY(s) IS THE BEST WAY, but with his way, the little red wire does not need to be disconnected because differences between power sources are no longer possible. All the little red wire does is power the receiver (and the servos and gyros the receiver controls).

    One big benefit of leaving it connected is: if you have two 3 amp BECs, you now have 6 amps available to power the receiver. The two amperages are in a parallel circuit, so they add. To do this you must have identical ESCs and motors, getting power from the same power source. In other words, you can supply the receiver with two power sources, but they must be identical; Otherwise, do not take the chance. The exception to the rule is; expensive BECs have a switching regulator, they will fight each other if used in parallel and get very hot. Manufactures are working on a solution. Only BECs with a linear regulator can be used in parallel.

    This will be hard for a lot of people to understand, but if you understand this, it’s easy to answer most receiver power questions. On the receiver, all red wires are on the same line (connected together), all black wires are on the same line. The white wires are control, and they each have their own independent circuit.
     
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  2. rodrigo

    rodrigo Top Gun

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    excellent, this is a great news for poor Boxcar.. I will draw +15grs between BEC , cables and plugs.

    one question: do you think if a BEC problem could be the origin of my sunday deadly flat spin?
    more info in this thread:
    http://www.rcpowers.com/forum/showt...ential-motors-and-flat-spin-expert-pilot-help

    I do not know where the source of the problem and I need to resolve it before my twin Bronco first flight:
    both ESC are ok (GH-25A)
    motors ok (rctimer 1800kv)
    receiver ok
    welding in Y harnees from battery OK
    last measures of current, 19-20 A at full throttle each motor, 41A near the battery (both motors at full throttle
    battery , turnigy nanotech 3s 1550mah 30C (According to calculations 46.5A max)

    I will appreciate any help
     
  3. LukeWarm

    LukeWarm Top Gun

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    If there was no power going to the receiver, the motors should turn off. If the motors continued to run, the BEC is not the problem unless you've over loaded it. With an over loaded BEC, servos chatter and are weak. Do you think you over loaded BEC?

    If you have identical ESCs and motors with one power source, leaving both little red wires connected will double your BEC amperage.

    Battery Elimination Circuit Load Rules, see post #1
    http://www.rcpowers.com/forum/showthread.php?10202-Do-I-need-a-BEC&p=103711&highlight=#post103711
     
  4. Foam and Tape

    Foam and Tape Cadet

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    Actually your results will vary, depending on the type of ESC's you use. When overloading some of my BEC's with random full deflection movement the servos will jump from one position to the next and the motor may stay at whatever throttle setting you left it at until the bec can reset which takes a while as the twitching servos all trying to return to center draw more current causing more amp draw from the bec trying to recover from the last session.
     
  5. rodrigo

    rodrigo Top Gun

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    about overloading de BEC, I think it was complying with the servos´s rule : 3A BEC, 2 alileron servos, 2 flap servos, 1 elevator servo all microservos, 5 servos working (flap at level , they should not have been working at the time)... plus 2 servoless retract but these are turned off when in its final position.

    maybe when I try to recover the plane it may have overloaded the BEC. The Foam and Tape post could be explain why I don´t have any motor reaction until I decided to drop controls and turn off the motors.

    For sure I will use your config, Luke . I use same ESC with 2A BEC

    anyway, the origin of the spin was unclear, it was too far away to see if one motor fail but the motor noise was there, the receiver should be working because I cut the throttle when I lost the sight, maybe 10 m before the impact, besides I'm using a two way rx and tx module and the module announces when it loses contact with the receiver (This finally happened when It was on the ground) ...could be a gust the origin of the spin ?

    thanks for your help
     
  6. LukeWarm

    LukeWarm Top Gun

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    Scott give me some more information so I added it to post #1. In this 3rd section of post #1, the new stuff is shown in red below.

    One big benefit of leaving it connected is: if you have two 3 amp BECs, you now have 6 amps available to power the receiver. The two amperages are in a parallel circuit, so they add. To do this you must have identical ESCs and motors, getting power from the same power source. In other words, you can supply the receiver with two power sources, but they must be identical; Otherwise, do not take the chance. The exception to the rule is; expensive BECs have a switching regulator, they will fight each other if used in parallel and get very hot. Only BECs with a linear regulator can be used in parallel. Manufactures are working on a solution.
     
  7. JettaManDan

    JettaManDan Administrator

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    well give me some clarification if you don't mind....your topic is suggesting external BEC's....or does this apply to the normal 20-40 amp ESC's with a BEC installed internally?

    ....my example is this...on a 2 motor setup i just built...i have 2 motors and 2 30 amp esc's that have their own 3amp power supply for the radio reciever built into them...i'm using one battery to power both ESC's and then the motors in turn....i de-pinned one of the red power wires from one of the ESC's so the power from one only is running the Rx....undertsanding the use of parallel and series electrical devices even i was a bit questioning why i was doing it. If the power supplys are equal the voltage would be the same but the available draw for the system is now 6 amps..but not feeding 6 amps until it is called upon. I'm running 6 micro servo's at one time (but 3 are not really being used in flight...cargo door..rudder...nose wheel steering) so i'm not too worried about power draw from the one 3amp source. But as long as the 2 esc's are run in parallel then there should be no issue just having both the power wires go in and run the system. Am i correct? One battery...2 esc's in parallel and 2 motors running the same prop and same RPM...

    let me know your thoughts Luke...

    Dan
     
  8. LukeWarm

    LukeWarm Top Gun

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  9. LukeWarm

    LukeWarm Top Gun

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    Scott directed me to the castle wed site when we were discussing this. They said their external BECs that have a switching regulator will fight each other if used in parallel, they are working on a solution for this problem. The Castle ESC's with a BEC installed internally (that I saw) have a linear regulator and said nothing about a parallel connection being a problem. Linear regulators are the cheaper of the two and are used in integrated BECs, so you should be safe. When you plug in the Y connection (with the battery plugged in), you should not see a spark. Also check the BEC's heat sink, if it quickly heats up, there is a conflict. A short test should not be a problem.

    RCPowers policy is "support the rule that there are no exceptions to" so their rule is "never use two power supplies for the receiver". A person like you that understands the concepts well enough to understand the exceptions, knows what it takes to do things safely. To use two power sources at once to supply power to the receiver, the voltages have to be exactly equal, otherwise when you connect them, you might puff a battery or burn up the BEC. You must also have a separate better connection between the batteries so that they do not equalize through the receiver; this could fry the receiver.
     
  10. JettaManDan

    JettaManDan Administrator

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    ok..for now i'll be safe and just run the one power source..i made up my own Y harness with one of the red wires cut..so now i don't have to cut the wire on the ESC directly...seems to be working well....thanks for the help!
     
  11. LukeWarm

    LukeWarm Top Gun

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    I never cut that wire. lift the lock with a razor blade and the pin pulls right out of the connector.
    This is my first video. I hope it gets the job done.



    OK, OK I used a straight pin instead of a razor blade, sue-me.
     
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  12. JettaManDan

    JettaManDan Administrator

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    I don't like to de-pin personally becasue it will never go back in and stay tight i've found...i bought a Y harness to use for this plane only so i cut one of the wires in the harness and taped it off..real easy and accomplishes the same goal..and it then doesn't matter which esc is into which side of the Y..one will always be depowered....but either way works well...
     
  13. LukeWarm

    LukeWarm Top Gun

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    If you are you running dual batteries, one battery is a lot lighter;
    Single battery- 1 each, 3cell 20c 3000mah will handle 60 amps (212 grams) at $24.00
    Dual batteries- 2 each, 3cell 20c 1600mah will handle 64 amps (267 grams) at $40.00
    The wiring for dual batteries weighs a lot more also.

    With a single battery, you can put your jet on a diet and get rid of over 2 ounces of weight.
     
  14. JettaManDan

    JettaManDan Administrator

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    I'm gonna run one battery..but for some reason when i went out for my 1st flight one of the ESC's didn't arm...so now i'm gonna put your theory to the test Luke and run a new Y harness and not cut one power wire...so i'll be running 2 ESC's in parallel into the ESC and out to the 2 motors and use only one battery..and see if it works or smokes ;)
     
  15. LukeWarm

    LukeWarm Top Gun

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    Play with it for a second on the ground, then check the temperature on the BEC. It is the little board on the ESC that the red receiver wire comes from. Do this correctly, and even if there is a conflict, i don't believe you will fry anything.
     
  16. rodrigo

    rodrigo Top Gun

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    I will be waiting for the result to do the same in my twin...thanks for your sacrifice JMD
     
  17. JettaManDan

    JettaManDan Administrator

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    will do..i have a laser temp gun i can use for it to check..

    heh..no worries..i'm trying it out this weekend hopefully..will report back for sure...
     
  18. JettaManDan

    JettaManDan Administrator

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    1st quick test was today for an update..nothing more than pulling my modded Y harness out that had one of the power wires clipped..and throwing in a Y harness without a wire clipped..so both ESC's running parallel to the receiver..and both my motors worked again..they still seem a tad out of sync but no biggie. Running both motors at about 1000rpm for about 60 seconds and checking with my temp gun neither ESC went above 75 degrees. So as long as the Rx is ok this is going to work just fine. Of course it is 30 degrees out and windy so no chance for a maiden. But think i am ready now.
     
  19. phoenix_md

    phoenix_md Airman

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    Can anyone advise how differential thrust is achieved using dual motors for yaw? I imagine it has something to do with mixing the rudder into throttle but wouldn't this require two separate channels for throttle?
     
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  20. ScottLott

    ScottLott Administrator

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    Instead of using a Y-splitter to attach the two ESCs to the receiver, use an onboard mixer running from the throttle and rudder channel. You'll have to pull the red wire from BOTH ESC's to prevent feedback from hitting the onboard mixer, meaning you'll have to use an external BEC also.
     
    phoenix_md likes this.
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