Motor Possibility

Re: Motor Possibility

Postby krokodil » Sun Feb 24, 2019 1:08 pm

ConducTTor wrote:Also, does a dual motor setup require two decoders?


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Not necessarily, it depends on the motors (how much power is required) but I personally do not like the dual motor drives. Normally the motors are never equal, ie there is almost always a slip between the two drives, what reduces heavily the efficiency of the whole construction.
Some people were trying to equalize the performance of the two motors with two decoders but I never heard about big success stories.
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Re: Motor Possibility

Postby Tom Dempsey » Sun Feb 24, 2019 1:57 pm

In O Scale, the two motor set up is referred to as a "China Drive". It is extraordinarily unpopular and generally disdained by builders. The minimum fix is to connect the motors in series rather than in parallel as shipped. Only one decoder is required. Builders pretty much seem to universally go with a single horizontal drive. Faulhaber, et al, put out gear head motor setups. Very small, very powerful, very torquey, very small current draw, very cool running, very quiet, very expensive.
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Re: Motor Possibility

Postby ConducTTor » Sun Feb 24, 2019 2:18 pm

What is a gearhead motor setup?


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Re: Motor Possibility

Postby CFRiad » Sun Feb 24, 2019 2:32 pm

ConducTTor wrote:[...]what is your experience with twin motor drives?

ConducTTor wrote:Also, does a dual motor setup require two decoders?

krokodil wrote:Normally the motors are never equal, ie there is almost always a slip between the two drives, what reduces heavily the efficiency of the whole construction. Some people were trying to equalize the performance of the two motors with two decoders but I never heard about big success stories.

First rule of engineering: two identical systems are never identical. With dual motors, there are differences in the motors themselves and in the gear trains. If using separate decoders, there are differences in the decoders too.

With this out of the way, let's say that it can be done. It may be labour intensive and convoluted, but not impossible. Basically one needs to speed match the two trucks. It is easier if the two motors are on separate frames, such as an articulated unit. For a rigid-frame loco, I would use two decoders and build two temporary identical frames, each with a driven truck at one end and an idle truck at the other. Then I would speed match the two frames, making sure one truck runs in reverse.

There are quite a few websites demonstrating the principles of speed matching for consisting.

With that done, I would install the trucks in the loco and voilá, dual drive. It will never be perfect, there will be some slip, but within acceptable limits. Don't forget, it's a toy, not a life-critical piece of equipment.

Even on the prototypes, there is always a slip between the 4 or 6 motors. The locos with DC motors and pulse-width modulation drives have electronics that monitor the slip and regulate the current to individual motors. The machines with 3-phase asynchronous induction motors and variable frequency drives regulate themselves (sort of).
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Re: Motor Possibility

Postby Bernd » Sun Feb 24, 2019 3:24 pm

ConducTTor wrote:What is a gearhead motor setup?


I see all of your questions have been answered by the experts.

Here's what a gearhead motor looks like. Motor turns several thousand RPM. The output of this gearhead is 10RPM. It is impossible to stop the drive shaft. The numerous gears multiply the torque to such a high limit that you would probably break the teeth on a gear.


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I have a solution to the twin motor problem but TT scale is to small to implement an eddy drive clutch.
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Re: Motor Possibility

Postby krokodil » Sun Feb 24, 2019 3:40 pm

In US mostly the people used dummy units for many reasons.
As already mentioned two units never worked identically. The problems was always with the start-up, when one of the locos did not started at the same voltage ( what is almost always a case) the better engine started earlier and tried to pull the next locomotive(s) and the whole train. In the model world it is not easy to pull the not working locomotive (the gear is mostly blocking the wheelsl, and the motor does not get any "help" from the wheels to rotate). The running engine was mostly overloaded in this sequence, reached easily the stall current what was usually over 1-2 Amps. Additionally the starting voltage of each locomotive may vary even dynamically, depending on the wear off of gears, bearings and the brushes in motor. The new motor motor has a starting loss of about 1,7-2 V ( on brushes and commutators), all other contacts ( track-wheel, wheel-pickup) requir another 0,5-0,7 V, the mechanical losses can add another 2-3 V! Ie some locomotuves will start to move at around 3 V, other may require 5 V to start.
In DC world the power packs were normally designed for up to 5 average locomotives (2-3 Amps), in case of multiply units they often became overloaded and reduced the power into the tracks ie, practically it was never easy to run with multiple units. The people always tried such scenarios but it was never a very successful event.
On the real RR the situation is completely different. The gears are nonlocking ( no worm gears) and the motors in classic diesel locomotives are at least in the starting phase in parallel. Ie. If any of the locomotive starts to move, all other locomotives get this mechanical information from the wheels and the internal compensation circuits will control each motor in the locos.
In our club we have in scale 1 a triplex locomotive with 3 motors and analog sound module, the starting current is over 5 Amps. It is not easy to deliver over the tracks, wheels and power pickups that much power!
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Re: Motor Possibility

Postby CFRiad » Sun Feb 24, 2019 4:25 pm

krokodil wrote:As already mentioned two units never worked identically. The problems was always with the start-up, when one of the locos did not started at the same voltage ( what is almost always a case) the better engine started earlier and tried to pull the next locomotive(s) and the whole train.

This is very true for DC.

The problem can be mitigated to a satisfactory (not perfect) level in DCC. If DCC locos are speed matched correctly, they will run fine at all speeds. The only problem that remains with multiple DCC locos in consists is poor current pickup from the track. If, at low speed, one of the locos loses power momentarily, the consist jerks.

Since here we are talking about dual drives on one loco, both decoders can be powered in parallel from all pickups so they will both lose power at the same time or not at all.
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Re: Motor Possibility

Postby Bernd » Sun Feb 24, 2019 6:35 pm

Guess I better yank the motor and gears out of the Cow, the one without the cab, on the TR2 project since they won't play nice together.

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Video coming as soon as I make one of these two running together.

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Re: Motor Possibility

Postby krokodil » Sun Feb 24, 2019 8:22 pm

Bernd wrote:Guess I better yank the motor and gears out of the Cow, the one without the cab, on the TR2 project since they won't play nice together.

Image

Video coming as soon as I make one of these two running together.

Bernd


That is correct. Also do not forget - especially for analog layout, that there are isolated track segments where the trains should stop. This length is often designed for about 40 cm. ( earlier even shorter) that means also the power pickup of the traction part should be shorter than this distance.
Also the commercial manufacturers often ignore this fact. The long, multiunit sets collect from each axle power, ie they will not stop at all = shunt out the isolated segment ( probably in front of the signal).
There is also a challenge with such sets where the motor is in the middle of the set, the optimal solution ( slowly penetrates also into the heads of constructors) that power should be collected in each direction only from the front part of the train. That means the train will stop before the signal, even in the case when the motor is in the last part of the multiunit train ( ICE etc). This is not a simple task!
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Re: Motor Possibility

Postby CFRiad » Sun Feb 24, 2019 8:55 pm

Bernd wrote:Guess I better yank the motor and gears out of the Cow, the one without the cab, on the TR2 project since they won't play nice together.


Bernd, everything I read so far in this thread, makes me guess that you are running DC. If this is the case, your life will be much easier with a motorless Cow.
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