On 4/30/2016 10:33 AM, Rob Brockway via Elec-trak wrote:
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.
Actually it would turn it off because when you use FW the armature current can go up a *lot*. If you had no field you would have infinite armature current (well, near infinite as there is some resistance) called a "short" which would blow the motor.
Also to keep in mind, the E12,15,20 motors have a field winding in series with the armature, so in a way they are shunt/series motors. When you go up a hill, that field is added to the normal field and the motor will slow down/torque will go up, and up you go.
HOWEVER if you're running in reverse odd things happen: First the series field is now the opposite polarity from the main field, so the tractor will go a bit faster in reverse. Likewise if you climb a hill in reverse the motor will behave as if field weakening is in effect because the main field will be countered by the series field which is opposing it. This I think is why field weakening is completely disabled in reverse, you could wind up with a field weak enough that the motor becomes essentially a shorted armature with all the heat and fun that entails.
Also when you have FW enabled and you are loading up the motor you're pulling more current through the armature which then starts to become heat (also known as the ultimate baloney detector) and waste heat is inefficient. Thus the E20 and E15 measure the current across the armature and when it starts to exceed the recommended max draw FW is dropped out. Good efficiency, keeps motor from overheating.
But really, I mow on the E20 in L2, speed 4 (full field, all three L1/2/3 resistors bypassed), cruise enabled. It would be kind of nice to be able to cruise in speed 5, I might sit down and wire up the E20 for that sometime. Fun stuff. D1 is too quick for mowing, and in D2 full speed on the road I am afraid I could bounce the tractor and flip it.
For me, speeds 5-8 are good for when I want to speed up on a straight-away, but then I go back to speed 4
Off to mow! C