Field Weakening for Efficiency

[TomEV]

From your description of a Field Shaping Device - are you envisioning something like a SEPEX motor/controller, but with the ability to feed more than a few amps to the field? This may be doable with a pair of off-the-shelf controllers. The complex part would be synchronizing the outputs to provide the appropriate speed/power.

The FW resistor doesn't get very warm at cruise power settings. It is only bypassing perhaps 10 amps at steady state 25 MPH. It gets red hot if I accidentally leave it turned on when accelerating from a dead stop, however!

I like the idea of adding or reducing field strength, but 'stealing' power from the field to put back into the batteries probably wouldn't amount to much - the losses will always be greater than the gains. But could be better than the 100% loss of a resistive dump load.

Along the multi-controller stream of consciousness, I've thought about adding a second controller to provide regenerative braking. The mechanical parts/contactors to make it transfer to regen mode seem to be a bit complicated, as is regulating the voltage before the regen controller comes on line so the controller FETs don't get zapped.

[robchalmers]

Sounds a lot like what I do in RC racing. Back in the of Brushed motors we'd retard the timing and re-shape the field using Neodyniums on the outside of the can for more torque on the tighter tracks. Yet for the longer, faster open track when we were limited by gearing I'd advance the brush timing and neo the field mags for a higher rpm. the difference in run time between a 'close track' motor and and 'open' on a medium circuit? 5-10% on NiMhs -'bout 5% on LiPos.

Good luck mate

[TomEV]

I also experimented with brush advance. The original setting in the Citicar was zero, and I tried a 10 degree advance at first. It didn't work out well - used more power and was slower to accelerate.

I tried again with a 5 degree advance - Better acceleration, about 2 MPH faster, with about the same efficiency. It is currently at 5 degrees advance - no room to put holes in for 7 1/2 or 2 1/2.

I imagine it'd take some big magnets or interpoles to change the field shape... Do the RC motors run interpoles?

[rmay635703]

I often wondered if just making points within the field that you could shunt from would be better

AKA find the half way point on your field winding and wire up a terminal, this would mean your full current would go through the field and half wouldn't be wasted.

Cheers
Ryan

[TomEV]

I have a small series motor from an old sander. I may try out the half-field (or 3/4 field) method to see if it changes anything.

The label on the motor says 120VAC only, but also seems to run OK with DC.

[robchalmers]

Originally Posted by TomEV

Do the RC motors run interpoles?

Nope, sadly not.

Have you tried retarding the timing? earlier contact earlier push/pull.
I also found it would keep the temps lower - probably what helped the eff by reducing the heat related wind resistance build-up.

[TomEV]

Originally Posted by robchalmers

Have you tried retarding the timing? earlier contact earlier push/pull. I also found it would keep the temps lower.

Problem with retarding the brushes is that it reduces the top end speed. Without field weakening, at zero degrees my top speed was 32 MPH. At five degrees advance, top speed is 34 MPH. At ten degrees, it goes back to 32 MPH, accelerates slower and is much less efficient. I'd imagine that at -5, it might be more efficient, and perhaps a bit more acceleration, but the top speed would be around 30 MPH.

For this motor at 48 volts, (rated for 36 volts, 98 amps) five degrees advance is a good number. It will probably need a bit more advance when I increase the pack voltage next year.

[rmay635703]

Retarded brushes also tend to arc and burn much more easily, not really worth it.

[TomEV]

If I give it full throttle in reverse from a dead stop up a steep hill, the motor will make arcing noises - probably because the brushes are retarded five degrees from the reverse perspective. Fortunately I rarely park on a hill, much less in a location where I need to use full throttle to get out of the parking spot.

[MetroMPG]

I experienced that (arcing noise, and smell, if I recall) with the ForkenSwift when playing with brush timing. Didn't have to drive 50 feet to know it needed to be corrected