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



Dave,

I agree that the power loss is trivial.  Field power consumption at full field strength is about 68.4 watts.  One of the arguments for PM motors is the elimination of the waste of the field power.  When you drop the amps to 1 by adding a 19 ohm resistance you consume 36 watts (and generate 19 watts of heat in the resistor).  When you drop it to .5 amps it is 18 watts total.  The key is to stay in the efficient operating envelope of the motor.  The original card controller was design to do this, reducing the weakening at higher armature amp levels.  Obviously a solid state controller for voltage reduction would provide an opportunity to save power but not a lot.

I use field weakening when I have a steep climb followed by a flat area.  It allows me to go in a lower gear for the steep climb and then hit the button to field weaken on the flat.  Then I do the opposite when returning to dynamically brake down the slope.  I find the motor is less likely to lock one wheel than the brake.  I do not have it on my E-12 and find I have to do a lot more shifting.

Rob



From: David Roden <etpost drmm net>
To: elec-trak cosmos phy tufts edu
Sent: Saturday, April 30, 2016 12:55 AM
Subject: Re: (ET) Curtis controller mod - Field - Buck converter maybe

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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