Conference vmszoo::rc

    variable timing motors allow you to adjust the "angle" or timing
    of the commutator/armature/magnets by rotating the end bell which
    holds the brushes.  this affects when the armature is powered, relative
    to its position with respect to the maxium magnetic "flux" of the
    motor magnets.  the affect of this adjustment is to increase motor
    torque, sacrificing maximum rpm, or to increase maximum rpm sacrificing
    torque.  the power requirments/efficeincy of the motor is also affected
    by this adjustment, but at the moment i can't remember if advancing
    increases current draw at load, or decreases it.  In general, the
    stock unadjustable motors are timed for maximum efficiency (best
    rpm at load with a 6 minute plus motor run) and do not require
    adjustments.  you really only need an adjustable motor if you have
    a specialized application for it, or you are racing and need the
    absolute best performance for your gear ratio/weight combination.
    in such a case, timing can make a difference, but proper tuning
    requires a lot of empirical experimentaion.
    

    OK gang, who knows how to adjust the timing on an RC car's
    electric motor ?  I am aware that changing the timing for
    better preformance will have a negative effect on motor life,
    are there any other drawbacks ?
    
    			Still trying to reach the ol' learnin' curve,
    
    				Roger
    

    
    By increasing the timing (turning the endbell oposite of motor
    rotation) you will get more rpms and higher top end but your torque
    will decrease (less punch).  Also run time will decrease as current
    consumption goes up.  Decreasing the timing (turning the endbell same
    as motor rotation) will decrease your rpms and also increase your
    current consumption.  By experience you should never decrease the
    timing beyond the 0 deg mark or the point the motor was shipped at.

    Electric motors with adjustable timing usually have a set screw
    which, when loosened or removed, allows rotation and/or removal of
    the end plate of the motor (the part of the motor which carries
    the brushes, and the bearing for the rear of the armature shaft).
    Timing is adjusted by rotating this end plate a small amount in
    either direction, and then re-setting the set screw.  When you
    change the timing of a motor, you are changing when the armature
    windings are energized, with respect to their position relative to
    the magnets in the motor.  Someone else will have to tell you
    which direction (relative to the rotation of the motor under
    power) affects which aspect of motor performance.  All I remember
    is that if you rotate the end plate 180 degrees, you have reversed
    the direction of motor rotation.

    There are two things that happen when timing an electric motor.
    When you retard the timing (turn the end bell clockwise) you increase
    the amount of torque the motor can provide, and decrease the R.P.M.'s.
    Advancing the timing (turn the end bell counter clockwise) will
    increase the R.P.M.'s and decrease the torque. Timing will also
    change the amount of run time of the motor. Advancing will give
    you less run time and retarding will give you the maximum run time.
    
    Now, there are different types of timed motors from high torque
    to high speed and many combinations of both inbetween. A high torque
    motor would allow you to run a larger pinion gear without damaging
    the motor. On a high speed motor a small pinion gear is definately
    needed or you will never reach the max RPM you set the timing for.
    the best thing to do is experiment. Set the timing, select a pinion
    gear, and run your car around a track or parking lot for three minutes.
    Next check the temp. of your motor. If it is too hot (that is if
    you can't keep you hand on it) try going to a smaller pinion gear
    or retard the timing. If the motor doesn't feel too hot you can
    do the opposite. This will give you max. performance without damaging
    the motor.
    
    Some things that damage the motor are.
    - carbon build up between the brushes and comutator.
    - dust build up in the motor housing
    - torquing down the motor. This means that your overstressing the
      motor so it can't get to its max timed RPM. This causes a lot
      of arching, carbon build up, and most deadly HEAT.
    - Heat  causes magnets to loose their magnitisim.
    - worn brushes, berrings, springs
    I'm sure there are more but these are the major ones
    
    Remember to keep your motor clean (I clean mine after every 3 minute
    race), and don't overstress it and you will get a long life out
    of it.
                                                        
    Hope this helps
    
    Den
    

    Maximum torque for a motor is when the magnets are perfectly aligned
    with the brushes. Whether you advance or retard the timing makes
    no difference, your RPM's will increase and your torque will decrease.
    The torque required from the motor is more or less dependent on
    the friction of the car, so that for any given speed, you will require
    more current.
    
    Adjusting the timing off from perfect alignment is no way to increase
    run time. Many racers do it to fine tune their run time to the length
    of the race, getting some extra speed. Ideally, in a race situation,
    you want the battery to go dead just after the finish line.
    
    						John.

    
    Here's my reasoning on why advancing the timing on an electric motor
    will increase RPMs...
    
    As the motor turns, a winding passes through a particular part of
    the magnetic field for some percentage of it's revolution.  Only
    when it is under the magnetic field will that winding produce any
    torque (force, actually).
    
    Ideally, you'd like the brushes to energize a winding at the exact
    instant that the winding enters the magnetic field, and stay energized
    until it exits the magnetic field.  In this way, the wire will produce
    force over the maximum possible rotation.
    
    However, the coil inductance introduces a time lag between the time
    the brush contacts a winding segment on the commutator and the time
    the current reaches it's peak in the winding.  In this time lag,
    the winding has already moved a bit in the magnetic field without
    producing any force.  This is wasteful and silly.  So, by rotating
    the brush with respect to the fixed magnetic field, you will start to
    energize the winding before it actually enters the magnetic field.
    Optimally, the winding should have peak current in it just as it
    fully enters the field.             
    
    Actually, I don't believe that these motors have very large inductance,
    since they have so few turns, but on the other hand, I DO see a
    significant RPM increase by advancing the timing.
    
    I'm damned if I know for sure why advancing the timing chews up
    the commutator more quickly, but I know that it does.
    
    Ed.

< Note 641.10 by LEDS::ACCIARDI "I Blit, therefore I am..." >
<    I'm damned if I know for sure why advancing the timing chews up
<    the commutator more quickly, but I know that it does.
    
Just a guess but it could have something to do with the fact that if the
commutator tries to switch power from one winding to the next at a point when
the previously conducting winding is trying to "cut" through the magnetic field
(and therby trying to generate a current of its own) the collapsing field of the
coil being turned off will cause arcing due to the "inductive kick". It's kind
of like the SCR circuits which try to turn off the power of an AC line at the
point where the voltage crosses the zero line. Any place else will cause a
"zap". The motor is probably designed to have the commutator switch when there
is the least inductive current in the old winding. I'd guess that the shortened
life of the motor is caused by excessive arcing. 

Dave 

    The reason RPM increases when you advance timing is that the average
    field seen by the armature DECREASES.  In a dc motor, the no-load
    speed increases when the field strength decreases because the back
    EMF (Volts per RPM) decreases.  The no-load speed is the speed at
    which the back EMF almost equals the terminal voltage.
    
    V(terminal) = EMF + I(armature) * R(armature)
    
    The back EMF is proportional to   RPM * Field flux
    
    
    I would say that increasing the RPM this way is not a positive thing.
    It would be better to change gearing and take advantage of torque
    at a lower motor RPM than to reduce the torque by advancing the
    timing.  You paid for strong magnets, so use them
    
    
    CHarlie
    

    
    
    	I just called Trinity Silver DOT on how to time RC motors.
    The correct method is to run the motor using an amp meter and
    adjust for lowest current. That's all there is to it!