Sometimes you simply need a better motor

Last weekend I upgraded one of my model trains - an old two car railbus ETA 515.

The original ones were built for the Deutsche Bahn starting in the 50ies of the 20th century - an e-drive by the way. A railbus operated by an electric motor supplied by an exchangable batterie set. Their service time is reported to have ended somewhere in the 90ies. That's about when I got the M?rklin H0 scale model. So I have had it about 30 years meanwhile.

M?klin was the brand having AC voltage at the tracks and thus using the universal motors. Changing the direction was solved using a relay which switched between two field windigs which were wound in opposite directions.

In an universal motor the field winding is connected in series to the rotor which is a DC-motor design having a commutator and the typical t-segments with the individual windings. As the same current passes the field winding and the rotor the polarity of the voltage does not matter. Look it up at Wikipedia if you need details.

Combined with the traditional spur gearhead these locomotives had a remarkable robustness. When dirt on the tracks occured the motor was de-energized (I=0) and simply rode over short problematic segments. They needed a higher current that the DC operated models though.

But control was of course purely analog. So knowing where the hot end of the solder iron is, I exchanded the relay against different generations of digital drivers as soon as I could afford them. Built my ones first and later switched to different commercial ones having tuneable parameters.

Here for my ETA 515 I ended up with latest one from M?rklin for use with these AC motors after having tried different ones. But whatever driver I used in the end the behavior was somehow disappointing. There seem to be two main reasons.

• First is: now having a driver and a motor whenever the supply is interrupted the electronics in the driver is shortcutting the motor. So it's no longer freewheeling but now has a current path closed and stopped abruptly.
• Second is - these motors need a relatively high voltage to acutally move at reasonable speed and due to the losses in the driver (rectifier + power-stage) the possible speed range was significantly reduced.

So I got me a kit to replace the AC-motor by effectivly a PM based DC-motor.

So here is the comparison.

Disassemble the railbus

The original AC motor

Its a three pole rotor in these old motors and a disc commutator

The new yoke having a permanent magnet

having the rotor inserted

It's a five pole rotor now and a drum commutator. Inserting the rotor was a little difficlt but I don't think they reduced the air gap but it's the permanent magnet which pulls at the rotor.

Add the motor shield

By the way gone is the ferrite used for EMC with the old motor. These might be inductors here - two of them.

Of course I also once again had to change the driver electronics form the one used for an AC motor to one having full-bridge power stage to control a DC-motor.

Result

It's perfect. Very smooth operation. Accelating slowly and reaching a resonable speed . So of course it's the electonics which is controlling the behaviour but in the end it's needed a better motor.

By the way I do own a model having been upgraded to a FAULHABER DC-flat motor. That of course is even better.