[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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.
>
>
> _______________________________________________
> Elec-trak mailing list
> Elec-trak cosmos phy tufts edu
> https://urldefense.proofpoint.com/v2/url?u=https-3A__cosmos.phy.tufts.edu_mailman_listinfo_elec-2Dtrak&d=CwICAg&c=c6MrceVCY5m5A_KAUkrdoA&r=kRHnlt3cKuWr4wuU8rx6ayYoAVctaoS9Z3cUPIkQn-k&m=739b-QPUKY6qBprHtBtDMu05ulewq17Wn181jK7twIk&s=2YEYQs4SM2em8A4iwJ7eMGmwKbtJ7zy9FZdG4kLcwq0&e=
>
> _______________________________________________
> Elec-trak mailing list
> Elec-trak cosmos phy tufts edu
> https://urldefense.proofpoint.com/v2/url?u=https-3A__cosmos.phy.tufts.edu_mailman_listinfo_elec-2Dtrak&d=CwICAg&c=c6MrceVCY5m5A_KAUkrdoA&r=kRHnlt3cKuWr4wuU8rx6ayYoAVctaoS9Z3cUPIkQn-k&m=wIZhxx3PUH0NzQN-JVYBhPSVrTjYobv4usuLGDwxpNs&s=N6u4xEQS1H-YTs12UvIGEVqPOmCXNlcXdgNzQ3Y4CvI&e=
>
_______________________________________________
Elec-trak mailing list
Elec-trak cosmos phy tufts edu
https://urldefense.proofpoint.com/v2/url?u=https-3A__cosmos.phy.tufts.edu_mailman_listinfo_elec-2Dtrak&d=CwICAg&c=c6MrceVCY5m5A_KAUkrdoA&r=kRHnlt3cKuWr4wuU8rx6ayYoAVctaoS9Z3cUPIkQn-k&m=wIZhxx3PUH0NzQN-JVYBhPSVrTjYobv4usuLGDwxpNs&s=N6u4xEQS1H-YTs12UvIGEVqPOmCXNlcXdgNzQ3Y4CvI&e=