Quote:
Originally Posted by dcb
Just curious, what do you suppose the resistance of that bit of nichrome is? And it is in series w/the field when engaged?
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I measured the resistance of the Nichrome a while back, and if memory serves it is about .09 ohms. This gives about 10% increase in speed compared to non-FW. Some people use a short length of small diameter (e.g. a foot or two of #6) battery cable with similar results.
This is a mild amount of Field Weakening. I have considered adding another contactor in the FW system to use half of the existing resistor (.045 ohms) for more speed, but I am concerned that the extra amperage across the armature may burn out the brushes if used too much.
The resistor is in parallel with the field only - part of the current goes through the resistor, the rest goes through the field; all the current still goes through the armature.
As to the speed/FW relationship, here is a dyno run for the Citicar from a while back. The dynamometer shows that this motor puts out 10.6 hp /46 lb ft torque (417 amps @ 44.1v input) - thin green line. Field weakened motor puts out 11.3 hp and about two lb ft above stock curve - thick green line. The end of the thin green line is the RPM limit when FW is not turned on.
The strangeness in the beginning of the FW curves is a dyno data error.