MTH "Speed Control"

@SD60M posted:

Does this seem normal?  Effectively, only one set of wheels is really doing all the work??

Not really.  While it's true that one truck will start out first, simply because of the servo loop, in real life with all the wheels on the track, both motors are contributing to the pulling power.

If you don't believe that, just remove the motor without the tach and just put the motor mount back on to hold the truck in place.  Now that truck will free-wheel.  Tape the motor down inside to keep the weight and balance the same.  Try a pulling power test that way and with both motors working.  I think you'll see the light.

Not smart enough to do a scientific test...but, I coupled one of these engines to a heavy train...actually couldn't move the cars...wheels just spun around making no traction.  The "dead" truck...without the tach tape...did nothing...didn't even have any wheel slip.

Not sure what that means...but, I guess I just woke up to how this really works...or doesn't.

On my first (and only) MTH dual-motored diesel with speed control: if you look closely you can tell that one truck starts before the other.  When the second truck breaks loose and both motors have enough power to overcome the drag in the gears, the loco lurches to life at about 3 smph.  If I roll the thumbwheel down to 2 mph, it exhibits bucking and jerking and will sometimes stall outright.  I've taken it apart and nothing is really binding in either motor truck.  This is an FT with thin, small diameter flywheels that don't contribute much to the situation. 

MTH service claimed that some variation in motor performance is "normal" and at a minimum of 3-4 smph, it's operating within expected norms.  MTH uses worm gears that are "self-locking," so one truck cannot easily move the other, and the two motors can't really help each other, at least until both have broken the threshold of static friction.

When Lionel was struggling with the Odyssey "lurch" in the early 2000s, they came up with the idea of back-drivable gears.  Actually it wasn't a new idea, because all of their postwar diesels were made this way.  With back-drivable gears, the wheels can turn the motor (albeit witih some difficulty.)  So you get smoother starts, and both motors work in tandem even from a dead stop.  This is a desirable characteristic, and all of my dual-motored diesels came from Lionel after that.  

All MTH diesels have the same gear ratio, and similar motors.  Some others claim their MTH diesels will run smoothly at 1 smph so maybe mine is a "lemon."  But I was soured by the experience and more impressed by Lionel's meaningful design improvements to address their lurch problem.  My $.02.

I'll continue to demonstrate my ignorance...but, I thought that, somehow, the speed control feature was "transmitted" to both powered trucks.  That does not seem to be the case...maybe you guys said that.  I just know that providing some resistance to the set of wheels connected to the motor with the tach tape...causes that set of wheels to "speed up" or provide a counter resistance to the resistance the wheels are encountering.

To me it seems the other set of wheels is turning...but, if I provide resistance to that set of wheels...nothing speeds up or provides a counter resistance...the wheels are more likely to just stop because of the resistance.  So, granted, the wheels start turning at some point...once they overcome that static friction...but they just turn providing some assistance to the tach tape motor...they do not react to added resistance caused by the weight of the train.

I guess I am just a fool...I thought both motors, somehow, provided the speed control effect.  Well, of course, they do...but, not to the degree of the tach tape motor.

As mentioned above, once the engine was coupled to a very heavy train...it would not move it...the only set of wheels reacting to the resistance was the tach tape motored set.  The other set did nothing.

Nevertheless...thank you for your patience and detailed explanations.

@SD60M posted:

 ... The "dead" truck...without the tach tape...did nothing...didn't even have any wheel slip...

 

 

The "dead" truck is anything but dead. If you look at the motor wiring you will find that both motors are wired together, in parallel. Thus the same voltage is always being applied to both motors. When the control circuitry applies a higher voltage in response to the tach reading BOTH motors produce a higher torque. As John said, if you were to remove one motor the engine would indeed produce about half the drawbar pull.

In a perfect world, both motors would have positive speed control.  However, this not being a perfect world, it's doubtful that most people would pay the premium of adding the hardware for positive speed control of both motors.  You would have to have two complete tach sensing circuits and two motor driver circuits, one for each motor.  That would significantly raise the cost.  With two speed control circuits, you also have to balance them, the software would not be trivial.

I agree that there are instances where having a single motor with positive speed control is really a major pain. Uneven track, entering and exiting grades, curves, etc.  In those situations, it's not uncommon for one power truck to lose traction momentarily.  If that is the speed controlled truck, then the power is reduced to both motors, causing a slowdown as the speed controlled motor slips, nature of the beast.

@SD60M posted:

I'll continue to demonstrate my ignorance...but, I thought that, somehow, the speed control feature was "transmitted" to both powered trucks.  That does not seem to be the case...maybe you guys said that.

The only speed control method commonly used for O-gauge model trains that is somewhat spread between the trucks are those products using back-EMF.  The ERR cruise control products are the obvious example, and many if not most HO products also use back-EMF.  It's not a perfect distribution, but since the electronics is actually reading the back-EMF from the motor or motors during a brief time the drive electronics is not powering them, it does react to the speed of either motor.  The drive voltage is actually PWM (Pulse Width Modulation) and drives the motor with varying timing of on/off voltage.  When the voltage to the motors is off, they act like a generator and can sense the actual rotational speed of the motor.  Given that the motors are in parallel, it's not crystal clear to me exactly how effective that sensing is with parallel motors.  However, the proof is in the execution.  The ERR cruise works quite well, so obviously they're doing something right.

Ooops...I thought of something else to say.  The gentleman above said this, "The "dead" truck is anything but dead..."

As I had the engine coupled to the very heavy, immovable train...the motor with the tach tape attempted to function...resulting in wheel slip and not train movement.  The other truck...2 powered axles...did nothing...no wheel slip, no moaning and groaning...nothing!  Since the two motors are wired in parallel...I thought that set of wheels would have developed wheel-slip or ... something.  Instead it did nothing.  This is tantamount to the remove-the-motor-drill the gentleman (above) suggested...except...the motor was there...wired in parallel...and it still did nothing.

Okay...now, I am done.

@gunrunnerjohn posted:

The only speed control method commonly used for O-gauge model trains that is somewhat spread between the trucks are those products using back-EMF.  The ERR cruise control products are the obvious example, and many if not most HO products also use back-EMF.  It's not a perfect distribution, but since the electronics is actually reading the back-EMF from the motor or motors during a brief time the drive electronics is not powering them, it does react to the speed of either motor.  The drive voltage is actually PWM (Pulse Width Modulation) and drives the motor with varying timing of on/off voltage.  When the voltage to the motors is off, they act like a generator and can sense the actual rotational speed of the motor.  Given that the motors are in parallel, it's not crystal clear to me exactly how effective that sensing is with parallel motors.  However, the proof is in the execution.  The ERR cruise works quite well, so obviously they're doing something right.

My two bits,  ERR cruise commander preforms better in lashups  with single or two motored locos than Odyssey or DCS especially when starting or operating two motor locos at the minimum speed the locos were capable of.

 Some time ago I noticed this issue with one motor starting first and have an idea  I have not tried  but keep thinking about.   On DCS or Odyssey dual motor locos strings of diode pairs on the motor which starts first might bring the motors start voltage closer together.  I don't think this would work with ERR cruise commander boards, not that it is needed, as the diodes likely would likely interfere with back emf .  Would love to hear the results if anyone gives the diode thing a try before I get a round2it.  This may even have some value with non command two motor locos locos         j

@JohnActon posted:

Some time ago I noticed this issue with one motor starting first and have an idea  I have not tried  but keep thinking about.   On DCS or Odyssey dual motor locos strings of diode pairs on the motor which starts first might bring the motors start voltage closer together.

You're obviously free to try this, but I can only visualize it making things more uneven, not better.  If there is no mechanical issues, the motors starting slightly staggered when free-running is a non-issue when the locomotives is on the rails.  The motor with the speed control will almost always be the one to start first, just the nature of the beast.

Both motors are normally close enough and system works well for a toy train.  But an issue with truck friction/wear, dirty motor commutator or brush could cause an imbalance.  But rare.   The motors are coupled via the track and traction tires.  Even if one motor pulls 10% less then the other motor, the tach reader is counting revolutions of the FW (correlates to wheel dia turning based on the gearing) to do the calculation, and it will adjust voltage to motor until the imbalanced pair turns the wheel enough to meet the set speed.  As long as traction tires not slipping or the tach reader miss counting, the engine balances itself to give the correct speed.

What you might see is increased amperage as the engine requires more power to compensate for the extra friction, dirty motor, etc....   G