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



Yes, using a DC-DC converter to vary the field strength would be 
preferable over 
the load resistor approach (essentially a voltage divider). DC-DC 
converters do
have some losses, but they are relatively small - so it would largely 
eliminate
any inefficiency associated with field weakening. Using a DC-DC converter 
to 
weaken the field just adds a little complexity compared to tossing a 
resistor
in series. Back when the Elec-Trak was designed, converting low voltage
DC to high voltage DC was not an easy task. 


Mike

________________________________
Michael S. Briggs, PhD
UNH Physics Department
(603) 862-2828
________________________________

________________________________________
From: John <JohnReinhard rochester rr com>
Sent: Friday, April 29, 2016 8:11 AM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) Curtis controller mod - Field - Buck converter maybe

So, I think you are implying that a buck converter / SMPS (Switch Mode
Power Supply) would be better than the load resistors, to drop the
voltage into the field coils.  Field current is not that high (I think -
anyone know the rated range ?).

Ebay shows 200W  DC-DC converter rated 15 Amps at 8 to 55V in and 1 to
36V out - continuously adjustable - for $10.35 and free shipping from
Hong Kong.

I think the E-traks would want a unit rated 72V max in (due to surges &
back EMF & etc) but maybe protection devices could be installed ahead of
the buck converter, circuit wise.

As soon as I replace the FET's in the old, burned out Curtis Controller
I have, I could try one.

John

On 04/29/2016 07:52 AM, Briggs, Michael wrote:
> In case anyone wants a long-winded explanation of the physics behind why 
> field weakening lets the tractor go faster....
>
> When I first learned that the Elec-Trak achieves its highest speeds by 
> weakening the field (which I had never heard of a motor doing), I was 
> confused about how it worked. The torque on a loop of wire of area A 
> carrying current I in a magnetic field B is IABsin(wt), where w is the 
> angular frequency that the motor is spinning at. So the torque would be 
> maximum when sin(wt) is +/- 1, so the max torque would be IAB. So, the 
> torque is proportional to the field strength.
>
> So it seemed odd that  weakening the field would allow the motor to spin 
> faster, if there is less torque. So, back when I first heard about this, 
> I had to did a little more thinking about it. In case anyone is 
> interested, here's my explanation of why it does this.
>
> It all has to do with the induced current. The  magnetic "flux" through 
> a loop of wire is the amount of magnetic field passing through the loop 
> - an analogy would be holding a hula hoop outside in the rain, we could 
> speak of the "rain flux" through the hoop. The rain flux would depend on 
> how hard it's raining (R), the area of the loop (A), and the angle 
> between them (cos(theta), where theta is measured between the rain and a 
> vector perpendicular to the hoop - so the rain flux is maximum when the 
> hoop is held horizontal), RAcos(theta).  The magnetic flux would 
> similarly be BAcos(theta). If the loop is spinning at a rate w, then 
> theta would be wt, so the flux is BAcos(wt).
>
> If you change the flux through a loop of wire, it induces a current in 
> the wire. You could alternatively think of it as inducing a voltage 
> across the loop (called "back EMF"), which is equal to the number of 
> loops (n) multiplied by the change in flux, which would be the 
> derivative of the flux, thus the back EMF is nwBAsin(wt).
>
> Notice the w that is now part of the stuff in front of the trig 
> function. So the maximum back EMF is nwBA, thus proportional to the 
> magnetic field strength (B) and the angular velocity the loop (armature) 
> is spinning at. As a motor spins faster, that back EMF gets bigger, 
> opposing the battery voltage - and that is what ultimately sets a limit 
> on how fast the motor can spin. So since that back EMF is proportional 
> to both w (speed) and B (field), if you make the field smaller, it will 
> allow the motor to spin faster before the back EMF gets high enough to 
> keep it from going any faster.
>
> The problem is that the field strength is reduced by just throwing a 
> resistor in series with the coil - and any time you toss a resistor in 
> series with the stuff you want to power, you are wasting some power 
> across that resistor, accomplishing nothing other than producing heat. 
> So, it's less efficient.
>
> If the field strength could be reduced without just wasting energy 
> across a resistor, then it shouldn't be any less efficient.
>
> Mike
>
>
>
>
> ________________________________
> Michael S. Briggs, PhD
> UNH Physics Department
> (603) 862-2828
> ________________________________
>
> ________________________________________
> From: Robert Troll <roberttroll hotmail com>
> Sent: Thursday, April 28, 2016 11:12 PM
> To: elec-trak cosmos phy tufts edu
> Subject: Re: (ET) Curtis controller mod
>
> So ideally the field weakening should only be used occasionally, not 
> when doing active things (mowing, etc.).
>
> Mike
> ________________________________
>
> My understanding, and i think it is stated in the manual, is that field 
> weakening is only to be used occasionally when torque is not required 
> (working) as a short burst of speed. As weakening the field causes the 
> motor to spin faster, use more power, loose torque, and generate more 
> heat. It does say that "whenever possible the speed control should be 
> maintained in the recommended DRIVE position for most efficient 
> operation". Drive position is straight 36v, non weakened field.
>
> It is a extra feature, not something intended to be used as a normal 
> operating mode.
>
>
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