Re: (ET) Curtis controller mod - Field - Buck converter maybe

["David Roden" <etpost drmm net>]

On 29 Apr 2016 at 17:46, Briggs, Michael wrote:

> using a DC-DC converter to vary the field strength would be preferable
> over the load resistor approach ... 

I may be missing something, but I don't understand why you'd want to use a 
DC:DC converter.  Isn't a DC:DC converter usually used to produce a higher 
or lower fixed DC voltage from a given DC input?  It seems to me that we 
don't really want that here, since the GE motor's field requires a 
variable 
voltage <= the battery voltage, 36 volts.

Maybe I'm just using a different term for the same thing, but it seems to 
me 
that what you want for more efficient FW on the ET is not a DC:DC 
converter, 
but - as the subject here suggests - a buck converter.  As it turns out, 
that's exactly what the Alltrax DCX300-ET controller uses.

That said, from what I can see, the amount of energy lost in the GE FW 
resistors is trivial compared to what you're losing in the armature 
resistors.  Look at the relative size of the resistors.  IIRC the field 
resistors are rated at 10 watts, so the average power dissipated in them 
can't exceed that amount.  I'd guess that the typical running power of the 
motor is about 1500 watts, so unless I'm missing something, we're talking 
about - at most - a 0.7% loss in the FW resistors.

I'm not a motor expert, but my understanding has for some years been that 
the reduced efficiency during field weakening is caused more by losses in 
the motor than by losses in the field control method.  But I can't find 
any 
reference for that online right now, so maybe I'm remembering it wrong.

In any case, if you're going to use a PWM controller or buck converter or 
whatever for only one part of the motor, it makes more sense to use it for 
armature control.

> Back when the Elec-Trak was designed, converting low voltage DC to high
> voltage DC was not an easy task. 

I guess that depends on what you mean by "easy task."  

Switchmode power supplies were, if I'm not mistaken, developed in the mid- 
or late 1960s, mostly thanks to the US space program.  So semiconductor 
based DC:DC converters certainly existed by the time GE was making the ET. 
 

However, I suspect that the price of high power semiconductors at the time 
made them too costly for a more-or-less mass market product like our ETs.

BTW, if they'd been willing to spend the dough, GE absolutely could have 
used a solid state speed controller in the ET.  The earliest General 
Electric SCR speed controller manuals I've seen bear a copyright date of 
1967.  So GE should have had plenty of solid state controller expertise in 
house when they designed the ET.  

I suspect again that they went with the cruder resistive design to keep 
the 
cost of the already-expensive ET somewhat closer to competitive.

In fact GE could have used a transistorized speed controller design, 
though 
again the cost might have been prohibitive.  In 1968 Motorola released an 
app note for their MP506 germanium (!) transistor, showing a practical 36v 
300a controller design.  


David Roden - Akron, Ohio, USA

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