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Dual Control Stick Trim Switches Without Relays

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nuckolls.bob(at)aeroelect
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Posted: Fri Apr 17, 2015 1:51 pm Post subject: Dual Control Stick Trim Switches Without Relays

At 09:19 2015-04-17, you wrote:

 	  Quote: 	
		  



The pilot's trim switches require 4 wires, the copilot's 5.

Ray Allen documentation includes includes wring diagrams for 

installation without using relays.  See WIRING SCHEMATIC FOR STYLE 1

http://www.rayallencompany.com/RACmedia/instructionsG2grip.pdf

Quote from Ray Allen's pdf:

 > The single pole double throw switches in the G205 stick grip are 

 each rated for 5 amps, so they can be wired directly to your Ray Allen servos.

  I could not find any mention of arc suppression in Ray Allen's 

 documentation.  Either arc suppression is built into their servos 

 or else the motor current is small enough that they are not 

 concerned with inductive voltage spikes.  I know that the elevator 

 trim servo in my RV-12 can be operated with a 9 volt transistor 

 radio battery (for testing purposes).  The pitch trim circuit, 

 designed by Van's Aircraft in the RV-12, does not include arc 

 suppression or relays.


	


    When driving a PM motor (i.e. the Ray-Allen actuators

    or similar), no arc suppression is necessary . . .

    for a couple of reasons. The first is that the

    inductive component of these little motors (or

    any PM motor) is small compared to that of say

    a relay of the same voltage and current demand.



    Secondly, the effective voltage impressed across that

    inductance is a small fraction of applied (i.e.

    bus) voltage. This is because the rotating armature

    generates a counter emf component that opposes the

    applied voltage.



    For example, If you know the motor's resistance

    . . . let's say 28 ohms and that it draws say 100

    milliamps under full load with 14 volts applied

    we can deduce the following:



    0.1 amps x 28 ohms equals 2.8 volts. This means

    that counter emf generated by the rotating windings

    is 14 - 2.8 or 11.2 volts. While rated at 14 volts

    in this particular service the motor actually RUNS on

    2.8 volts!



    If you study the opening circuit voltage across a

    spreading set of contacts controlling a PM motor,

    you will find it to be VERY UNLIKE that which

    occurs across the battery master switch.



    The energy in an inductor is proportional to

    L x I(squared). Given that the value of current

    for the trim motor is on the order of 100 mA vs.

    say 700 mA for the battery contactor, all other

    things being equal (inductance), then the energy

    released to arc the contacts will be 7 squared or

    about 1/50th smaller for the motor than for a

    battery contactor. Now stack the ratio of inductances

    on top of the analysis and the energy gets small still

    by perhaps another factor of 10.



    Ergo, one can expect satisfactory service life

    from the Lilliputian switches controlling a

    motor that wants to be a hydraulic pump driver

    when it grows up.



   Bob . . .

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user9253

Joined: 28 Mar 2008
Posts: 1938
Location: Riley TWP Michigan

Posted: Fri Apr 17, 2015 3:15 pm Post subject: Re: Dual Control Stick Trim Switches Without Relays

Thanks for that information Bob.  Very interesting.

Joe

Do not archive

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Joe Gores

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