TT scale trains and models

There are motors listed in part 2 of NorthWest Short Line online catalog.
http://www.nwsl.com

I also did some tests on the Mitsumi M15N motors.
The no load current at 12 V is about 60 -70 mAs. In pure DC ( filtered DC) under load the current rises up to 120 mA at 12 V. This current remains almost stable to 20 V but the motor is getting warm.
Under PWM the situation changes especially in the most used mid range.
At 12 V PWM the current is very difficult to measure, so I measured the overall power to the PWM controller (the input of the PWM controller is 10-20 VDC). The controller itself without a motor requires about 40 mA.
When the motor starts under the same load the overall current is about 100 mA. Ie the current is lower as in DC (100-40 mA under load!)



The most critical portion is the midrange. The overall current does not rise too much about 120 - 130 mA but the temperature of the motor does. ( duty cycle 30-80%)



At 12 V the motor is getting warm ( under the same load) in about 2-3 minutes. This is caused by the Back-EMF generated by the motor when the PWM pulse is off. This power must be discipated either in motor or in electronics (DCC decoder) or in both. At 12 V PWM the BEMF is about 6V ( see the marked portion).
At top speed in PWM the motor gets into the similar condition as in DC, and it runs normal at normal temperature. You can touch the motor surface.

The situation dramatically changes when the motor gets 20 V PWM pulses. (some DCC control units driven from 16 V AC transformers). In this case the motor in midrange (30-80% duty cycle) heats up to over 60 C degrees in less than a minute - one cannot touch the motor surface. I did not wanted to destroy the motor, therefore I do not have any information how long can work in this condition.

Here is my testpad. The gear itself provides the basic load ( Today I used only the basic mechanical load). There is also a small 3 V motor what can be used as electrical load ( the generated electricity can be shunted by resistors).



From my point of view it is quite good motor, fully acceptable for TT models. The specified speed is for about 8 V.
In DCC ( as any similar motor) the thermal overload can be critical. If the decoder has a thermal protection it will probably shut down when in track is over 14 V and running longer in mid speed range. The PWM frequency in tests was 230 Hz.

One more test. This time for temperature. The motor runs for 5 minutes at half speed with about 120 mA constant load.
Start. End. Temp.
DC. 6 V. 120 mA load. 26C. 39C
PWM. 12V, 50 % duty cycle. 27C. 48C. Hot
PWM. 20V 50%. Duty cycle. 27C. 58C. Very hot

The temperature is measured on the motor surface, the motor is running on open air (not like in most locomotives encapsulated in metal blocks.
The temperature is measure in Celsius degrees.

Very sophisticated setup Krokodil. I like your test bed setup.

Bernd

Bernd wrote:Very sophisticated setup Krokodil. I like your test bed setup.

Bernd

Thanks Bernd.

It is some cheap mechanizm sold here for school children to build different vehicles. It has a small 3 V motor and all kind of outputs, like the main axle for wheels but also some arm to lift something. It is cheap and is excellent for testing motors. I can even add a nonlinear load ( resistors to shunt the motor, where the load will increase with the speed).