David,
I'm just lumping all DC-DC converters together. Yes, a buck converter would be a good approach, I just figured many people may not know what that is, so I was just calling it a DC-DC converter (since it is a type of DC-DC converter).
The field resistors are only rated at 10 Watts? Ahhh, I didn't realize there was so little power being dissipated across them. If that's the case, then you're right, weakening the field with an inline resistor isn't all that wasteful. In general, I just don't
like the practice of using a resistive voltage divider to reduce the amount of voltage your load gets, since it can be an inefficient practice (such as the toaster resistors). But, I didn't think about the fact that the current through the armature is significantly
greater than the current through the field coils, so a voltage divider on the field coils isn't going to be as costly overall.
Ok, I'm back to thinking that it would be fine to just use a "turbo" switch that lets you put a resistor (or variable resistor) into the field circuit for weakening. That would be simple to do, and apparently not that inefficient.
Off-hand, I can't think of any other losses that would be introduced due to weakening the field. The main downside is that you have less torque. But if you only use it when you're driving around (not doing anything active like tilling), that shouldn't be a
big problem. IMO at least.
Mike
________________________________
Michael S. Briggs, PhD
UNH Physics Department
(603) 862-2828
________________________________
________________________________________
From: David Roden <
etpost drmm net>
Sent: Saturday, April 30, 2016 12:55 AM
To:
elec-trak cosmos phy tufts edu
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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