motor control

OK, one for the electrical boffins......

I understand the fundamental differences between series and shunt-wound DC motors and permag DC motors. I also am aware of the range of electronic controllers available for use with permag DC motors powering locos.

What I would like to know is whether a group of permag DC motors connected in series/parallel configuration like in a full-size 'old school' diesel-electric or straight electric loco/EMU or tram with drum controller would exhibit similar performance as series-wound motors in that application.

I'm happy to experiment a bit on this but wondered whether someone has already been down that road before I start buying motors, resistors and control switches.

Grateful for any feedback.

Connecting permag motors in series and parallel will give you 'low speed' and 'high speed' options, but as permag motors are more like shunt wound motors with a relatively constant field strength you won't get the full benefits.

What Roger B says.

A permnent magnet motor will will run at a speed proportonal to the voltage across the windings and draw a current proportional to the load (torque) applied. The apparent voltage across the widings drops with load due to brush gear and winding resistance which is why they slow with loading.. Running in parallel with fixed, rated, voltage will give full speed and maximum torque. Putting the same two motors in series across the same supply will in theory give half the speed and half the maximum torque (twice the internal resistance that limits curent) so 1/4 power. In practice the motor resistance is too low to limit the current (motor on full voltage can draw much more current than rating if overloaded) so in series you can still pull full current without overheating the motor so can normally get full torque at half speed so half power. Practically it is just a simple two speed system. With modern electronics a PWM controller will give a much better solution.

Robert G8RPI.

Thanks gents for the quick reply. Sounds like it would work but is perhaps not the best solution. I like the sound of getting full torque at half speed at minimal risk of overheating, and yes it would be a simple 2-speed system. With the introduction of resistors that could be switched out it would provide more speeds but the penalty would be a lot of energy wasted as heat through the resistors.

I know that trams having 4 motors usually had them connected in parallel as pairs, with the pairs then being connected either in series or in parallel. This gave them 2 controller settings at which they could run without additional resistors so giving a full torque low speed setting for hill climbing and a higher speed setting for level track. That is the sort of thing I was wanting to emulate. The controller settings that incorporated resistors were only used in accelerating to a point where the series or parallel operation could be sustained without tripping the over-current breaker. In practice they spent little time in operation with resistors in circuit so there was not a lot of lost energy. TIme to try a few experiments I think.

I've built a PWM controller long ago for model trains, found it difficult to find a frequency that did not emit objectionable humming from the motor in operation. Ended up running at around 550Hz from memory. Most other model train controllers I built were standard transistor types. They all worked fairly well.

Geoff P